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Literature Search: Databases and Gray Literature

The literature search.

A collection of six databases that contain different types of high-quality, independent evidence to inform healthcare decision-making. Search the Cochrane Central Register of Controlled Trials here.

European database of biomedical and pharmacologic literature.

PubMed comprises more than 21 million citations for biomedical literature from MEDLINE, life science journals, and online books.

Largest abstract and citation database of peer-reviewed literature and quality web sources. Contains conference papers.

World's leading citation databases. Covers over 12,000 of the highest impact journals worldwide, including Open Access journals and over 150,000 conference proceedings. Coverage in the sciences, social sciences, arts, and humanities, with coverage to 1900.

Subject Specific Databases

Over 4.5 million abstracts of peer-reviewed literature in the behavioral and social sciences. Includes conference papers, book chapters, psychological tests, scales and measurement tools.

Comprehensive journal index to nursing and allied health literature, includes books, nursing dissertations, conference proceedings, practice standards and book chapters.

Latin American and Caribbean health sciences literature database

Gray Literature

Clinical Trial Registries

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literature search and bibliographic services

BJPsych Advances

Article contents

Defining the clinical question

Scoping search, search strategy, sources to search, developing a search strategy, searching electronic databases, supplementary search techniques, obtaining unpublished literature, conclusions, how to carry out a literature search for a systematic review: a practical guide.

Published online by Cambridge University Press:  01 March 2018

Performing an effective literature search to obtain the best available evidence is the basis of any evidence-based discipline, in particular evidence-based medicine. However, with a vast and growing volume of published research available, searching the literature can be challenging. Even when journals are indexed in electronic databases, it can be difficult to identify all relevant studies without an effective search strategy. It is also important to search unpublished literature to reduce publication bias, which occurs from a tendency for authors and journals to preferentially publish statistically significant studies. This article is intended for clinicians and researchers who are approaching the field of evidence synthesis and would like to perform a literature search. It aims to provide advice on how to develop the search protocol and the strategy to identify the most relevant evidence for a given research or clinical question. It will also focus on how to search not only the published but also the unpublished literature using a number of online resources.

• Understand the purpose of conducting a literature search and its integral part of the literature review process

• Become aware of the range of sources that are available, including electronic databases of published data and trial registries to identify unpublished data

• Understand how to develop a search strategy and apply appropriate search terms to interrogate electronic databases or trial registries

A literature search is distinguished from, but integral to, a literature review. Literature reviews are conducted for the purpose of (a) locating information on a topic or identifying gaps in the literature for areas of future study, (b) synthesising conclusions in an area of ambiguity and (c) helping clinicians and researchers inform decision-making and practice guidelines. Literature reviews can be narrative or systematic, with narrative reviews aiming to provide a descriptive overview of selected literature, without undertaking a systematic literature search. By contrast, systematic reviews use explicit and replicable methods in order to retrieve all available literature pertaining to a specific topic to answer a defined question (Higgins Reference Higgins and Green 2011 ). Systematic reviews therefore require a priori strategies to search the literature, with predefined criteria for included and excluded studies that should be reported in full detail in a review protocol.

Performing an effective literature search to obtain the best available evidence is the basis of any evidence-based discipline, in particular evidence-based medicine (Sackett Reference Sackett 1997 ; McKeever Reference McKeever, Nguyen and Peterson 2015 ). However, with a vast and growing volume of published research available, searching the literature can be challenging. Even when journals are indexed in electronic databases, it can be difficult to identify all relevant studies without an effective search strategy (Hopewell Reference Hopewell, Clarke and Lefebvre 2007 ). In addition, unpublished data and ‘grey’ literature (informally published material such as conference abstracts) are now becoming more accessible to the public. It is important to search unpublished literature to reduce publication bias, which occurs because of a tendency for authors and journals to preferentially publish statistically significant studies (Dickersin Reference Dickersin and Min 1993 ). Efforts to locate unpublished and grey literature during the search process can help to reduce bias in the results of systematic reviews (Song Reference Song, Parekh and Hooper 2010 ). A paradigmatic example demonstrating the importance of capturing unpublished data is that of Turner et al ( Reference Turner, Matthews and Linardatos 2008 ), who showed that using only published data in their meta-analysis led to effect sizes for antidepressants that were one-third (32%) larger than effect sizes derived from combining both published and unpublished data. Such differences in findings from published and unpublished data can have real-life implications in clinical decision-making and treatment recommendation. In another relevant publication, Whittington et al ( Reference Whittington, Kendall and Fonagy 2004 ) compared the risks and benefits of selective serotonin reuptake inhibitors (SSRIs) in the treatment of depression in children. They found that published data suggested favourable risk–benefit profiles for SSRIs in this population, but the addition of unpublished data indicated that risk outweighed treatment benefits. The relative weight of drug efficacy to side-effects can be skewed if there has been a failure to search for, or include, unpublished data.

In this guide for clinicians and researchers on how to perform a literature search we use a working example about efficacy of an intervention for bipolar disorder to demonstrate the search techniques outlined. However, the overarching methods described are purposefully broad to make them accessible to all clinicians and researchers, regardless of their research or clinical question.

The review question will guide not only the search strategy, but also the conclusions that can be drawn from the review, as these will depend on which studies or other forms of evidence are included and excluded from the literature review. A narrow question will produce a narrow and precise search, perhaps resulting in too few studies on which to base a review, or be so focused that the results are not useful in wider clinical settings. Using an overly narrow search also increases the chances of missing important studies. A broad question may produce an imprecise search, with many false-positive search results. These search results may be too heterogeneous to evaluate in one review. Therefore from the outset, choices should be made about the remit of the review, which will in turn affect the search.

A number of frameworks can be used to break the review question into concepts. One such is the PICO (population, intervention, comparator and outcome) framework, developed to answer clinical questions such as the effectiveness of a clinical intervention (Richardson Reference Richardson, Wilson and Nishikawa 1995 ). It is noteworthy that ‘outcome’ concepts of the PICO framework are less often used in a search strategy as they are less well defined in the titles and abstracts of available literature (Higgins Reference Higgins and Green 2011 ). Although PICO is widely used, it is not a suitable framework for identifying key elements of all questions in the medical field, and minor adaptations are necessary to enable the structuring of different questions. Other frameworks exist that may be more appropriate for questions about health policy and management, such as ECLIPSE (expectation, client group, location, impact, professionals, service) (Wildridge Reference Wildridge and Bell 2002 ) or SPICE (setting, perspective, intervention, comparison, evaluation) for service evaluation (Booth Reference Booth 2006 ). A detailed overview of frameworks is provided in Davies ( Reference Davies 2011 ).

Before conducting a comprehensive literature search, a scoping search of the literature using just one or two databases (such as PubMed or MEDLINE) can provide valuable information as to how much literature for a given review question already exists. A scoping search may reveal whether systematic reviews have already been undertaken for a review question. Caution should be taken, however, as systematic reviews that may appear to ask the same question may have differing inclusion and exclusion criteria for studies included in the review. In addition, not all systematic reviews are of the same quality. If the original search strategy is of poor quality methodologically, original data are likely to have been missed and the search should not simply be updated (compare, for example, Naughton et al ( Reference Naughton, Clarke and O'Leary 2014 ) and Caddy et al ( Reference Caddy, Amit and McCloud 2015 ) on ketamine for treatment-resistant depression).

The first step in conducting a literature search should be to develop a search strategy. The search strategy should define how relevant literature will be identified. It should identify sources to be searched (list of databases and trial registries) and keywords used in the literature (list of keywords). The search strategy should be documented as an integral part of the systematic review protocol. Just as the rest of a well-conducted systematic review, the search strategy used needs to be explicit and detailed such that it could reproduced using the same methodology, with exactly the same results, or updated at a later time. This not only improves the reliability and accuracy of the review, but also means that if the review is replicated, the difference in reviewers should have little effect, as they will use an identical search strategy. The PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) statement was developed to standardise the reporting of systematic reviews (Moher Reference Moher, Liberati and Tetzlaff 2009 ). The PRISMA statement consists of a 27-item checklist to assess the quality of each element of a systematic review (items 6, 7 and 8 relate to the quality of literature searching) and also to guide authors when reporting their findings.

There are a number of databases that can be searched for literature, but the identification of relevant sources is dependent on the clinical or research question (different databases have different focuses, from more biology to more social science oriented) and the type of evidence that is sought (i.e. some databases report only randomised controlled trials).

• MEDLINE and Embase are the two main biomedical literature databases. MEDLINE contains more than 22 million references from more than 5600 journals worldwide. In addition, the MEDLINE In-Process & Other Non-Indexed Citations database holds references before they are published on MEDLINE. Embase has a strong coverage of drug and pharmaceutical research and provides over 30 million references from more than 8500 currently published journals, 2900 of which are not in MEDLINE. These two databases, however, are only available to either individual subscribers or through institutional access such as universities and hospitals. PubMed, developed by the National Center for Biotechnology Information of the US National Library of Medicine, provides access to a free version of MEDLINE and is accessible to researchers, clinicians and the public. PubMed comprises medical and biomedical literature indexed in MEDLINE, but provides additional access to life science journals and e-books.

In addition, there are a number of subject- and discipline-specific databases.

• PsycINFO covers a range of psychological, behavioural, social and health sciences research.

• The Cochrane Central Register of Controlled Trials (CENTRAL) hosts the most comprehensive source of randomised and quasi-randomised controlled trials. Although some of the evidence on this register is also included in Embase and MEDLINE, there are over 150 000 reports indexed from other sources, such as conference proceedings and trial registers, that would otherwise be less accessible (Dickersin Reference Dickersin, Manheimer and Wieland 2002 ).

• The Cumulative Index to Nursing and Allied Health Literature (CINAHL), British Nursing Index (BNI) and the British Nursing Database (formerly BNI with Full Text) are databases relevant to nursing, but they span literature across medical, allied health, community and health management journals.

• The Allied and Complementary Medicine Database (AMED) is a database specifically for alternative treatments in medicine.

The examples of specific databases given here are by no means exhaustive, but they are popular and likely to be used for literature searching in medicine, psychiatry and psychology. Website links for these databases are given in Box 1 , along with links to resources not mentioned above. Box 1 also provides a website link to a couple of video tutorials for searching electronic databases. Box 2 shows an example of the search sources chosen for a review of a pharmacological intervention of calcium channel antagonists in bipolar disorder, taken from a recent systematic review (Cipriani Reference Cipriani, Saunders and Attenburrow 2016a ).

BOX 1 Website links of search sources to obtain published and unpublished literature

Electronic databases

• MEDLINE/PubMed: www.ncbi.nlm.nih.gov/pubmed

• Embase: www.embase.com

• PsycINFO: www.apa.org/psycinfo

• Cochrane Central Register of Controlled Trials (CENTRAL): www.cochranelibrary.com

• Cumulative Index of Nursing and Allied Health Literature (CINAHL): www.cinahl.com

• British Nursing Index: www.bniplus.co.uk

• Allied and Complementary Medicine Database: https://www.ebsco.com/products/research-databases/amed-the-allied-and-complementary-medicine-database

Grey literature databases

• BIOSIS Previews (part of Thomson Reuters Web of Science): https://apps.webofknowledge.com

Trial registries

• ClinicalTrials.gov: www.clinicaltrials.gov

[email protected]: www.accessdata.fda.gov/scripts/cder/daf

• European Medicines Agency (EMA): www.ema.europa.eu

• World Health Organization International Clinical Trials Registry Platform (WHO ICTRP): www.who.int/ictrp

• GlaxoSmithKline Study Register: www.gsk-clinicalstudyregister.com

• Eli-Lilly clinical trial results: https://www.lilly.com/clinical-study-report-csr-synopses

Guides to further resources

• King's College London Library Services: http://libguides.kcl.ac.uk/ld.php?content_id=17678464

• Georgetown University Medical Center Dahlgren Memorial Library: https://dml.georgetown.edu/core

• University of Minnesota Biomedical Library: https://hsl.lib.umn.edu/biomed/help/nursing

Tutorial videos

• Searches in electronic databases: http://library.buffalo.edu/hsl/services/instruction/tutorials.html

• Using the Yale MeSH Analyzer tool: http://library.medicine.yale.edu/tutorials/1559

BOX 2 Example of search sources chosen for a review of calcium channel antagonists in bipolar disorder (Cipriani Reference Cipriani, Saunders and Attenburrow 2016a )

Electronic databases searched:

• MEDLINE In-Process and Other Non-Indexed Citations

For a comprehensive search of the literature it has been suggested that two or more electronic databases should be used (Suarez-Almazor Reference Suarez-Almazor, Belseck and Homik 2000 ). Suarez-Almazor and colleagues demonstrated that, in a search for controlled clinical trials (CCTs) for rheumatoid arthritis, osteoporosis and lower back pain, only 67% of available citations were found by both Embase and MEDLINE. Searching MEDLINE alone would have resulted in 25% of available CCTs being missed and searching Embase alone would have resulted in 15% of CCTs being missed. However, a balance between the sensitivity of a search (an attempt to retrieve all relevant literature in an extensive search) and the specificity of a search (an attempt to retrieve a more manageable number of relevant citations) is optimal. In addition, supplementing electronic database searches with unpublished literature searches (see ‘Obtaining unpublished literature’ below) is likely to reduce publication bias. The capacity of the individuals or review team is likely largely to determine the number of sources searched. In all cases, a clear rationale should be outlined in the review protocol for the sources chosen (the expertise of an information scientist is valuable in this process).

Important methodological considerations (such as study design) may also be included in the search strategy. Dependent on the databases and supplementary sources chosen, filters can be used to search the literature by study design (see ‘Searching electronic databases’). For instance, if the search strategy is confined to one study design term only (e.g. randomised controlled trial, RCT), only the articles labelled in this way will be selected. However, it is possible that in the database some RCTs are not labelled as such, so they will not be picked up by the filtered search. Filters can help reduce the number of references retrieved by the search, but using just one term is not 100% sensitive, especially if only one database is used (i.e. MEDLINE). It is important for systematic reviewers to know how reliable such a strategy can be and treat the results with caution.

Identifying search terms

Standardised search terms are thesaurus and indexing terms that are used by electronic databases as a convenient way to categorise articles, allowing for efficient searching. Individual database records may be assigned several different standardised search terms that describe the same or similar concepts (e.g. bipolar disorder, bipolar depression, manic–depressive psychosis, mania). This has the advantage that even if the original article did not use the standardised term, when the article is catalogued in a database it is allocated that term (Guaiana Reference Guaiana, Barbui and Cipriani 2010 ). For example, an older paper might refer to ‘manic depression’, but would be categorised under the term ‘bipolar disorder’ when catalogued in MEDLINE. These standardised search terms are called MeSH (medical subject headings) in MEDLINE and PubMed, and Emtree in Embase, and are organised in a hierarchal structure ( Fig. 1 ). In both MEDLINE and Embase an ‘explode’ command enables the database to search for a requested term, as well as specific related terms. Both narrow and broader search terms can be viewed and selected to be included in the search if appropriate to a topic. The Yale MeSH Analyzer tool ( mesh.med.yale.edu ) can be used to help identify potential terms and phrases to include in a search. It is also useful to understand why relevant articles may be missing from an initial search, as it produces a comparison grid of MeSH terms used to index each article (see Box 1 for a tutorial video link).

literature search and bibliographic services

FIG 1 Search terms and hierarchical structure of MeSH (medical subject heading) in MEDLINE and PubMed.

In addition, MEDLINE also distinguishes between MeSH headings (MH) and publication type (PT) terms. Publication terms are less about the content of an article than about its type, specifying for example a review article, meta-analysis or RCT.

Both MeSH and Emtree have their own peculiarities, with variations in thesaurus and indexing terms. In addition, not all concepts are assigned standardised search terms, and not all databases use this method of indexing the literature. It is advisable to check the guidelines of selected databases before undertaking a search. In the absence of a MeSH heading for a particular term, free-text terms could be used.

Free-text terms are used in natural language and are not part of a database’s controlled vocabulary. Free-text terms can be used in addition to standardised search terms in order to identify as many relevant records as possible (Higgins Reference Higgins and Green 2011 ). Using free-text terms allows the reviewer to search using variations in language or spelling (e.g. hypomani* or mania* or manic* – see truncation and wildcard functions below and Fig. 2 ). A disadvantage of free-text terms is that they are only searched for in the title and abstracts of database records, and not in the full texts, meaning that when a free-text word is used only in the body of an article, it will not be retrieved in the search. Additionally, a number of specific considerations should be taken into account when selecting and using free-text terms:

• synonyms, related terms and alternative phrases (e.g. mood instability, affective instability, mood lability or emotion dysregulation)

• abbreviations or acronyms in medical and scientific research (e.g. magnetic resonance imaging or MRI)

• lay and medical terminology (e.g. high blood pressure or hypertension)

• brand and generic drug names (e.g. Prozac or fluoxetine)

• variants in spelling (e.g. UK English and American English: behaviour or behavior; paediatric or pediatric).

literature search and bibliographic services

FIG 2 Example of a search strategy about bipolar disorder using MEDLINE (Cipriani Reference Cipriani, Saunders and Attenburrow 2016a ). The strategy follows the PICO framework and includes MeSH terms, free-text keywords and a number of other techniques, such as truncation, that have been outlined in this article. Numbers in bold give the number of citations retrieved by each search.

Truncation and wildcard functions can be used in most databases to capture variations in language:

• truncation allows the stem of a word that may have variant endings to be searched: for example, a search for depress* uses truncation to retrieve articles that mention both depression and depressive; truncation symbols may vary by database, but common symbols include: *, ! and #

• wild cards substitute one letter within a word to retrieve alternative spellings: for example, ‘wom?n’ would retrieve the terms ‘woman’ and ‘women’.

Combining search terms

Search terms should be combined in the search strategy using Boolean operators. Boolean operators allow standardised search terms and free-text terms to be combined. There are three main Boolean operators – AND, OR and NOT ( Fig. 3 ).

• OR – this operator is used to broaden a search, finding articles that contain at least one of the search terms within a concept. Sets of terms can be created for each concept, for example the population of interest: (bipolar disorder OR bipolar depression). Parentheses are used to build up search terms, with words within parentheses treated as a unit.

• AND – this can be used to join sets of concepts together, narrowing the retrieved literature to articles that contain all concepts, for example the population or condition of interest and the intervention to be evaluated: (bipolar disorder OR bipolar depression) AND calcium channel blockers. However, if at least one term from each set of concepts is not identified from the title or abstract of an article, this article will not be identified by the search strategy. It is worth mentioning here that some databases can run the search also across the full texts. For example, ScienceDirect and most publishing houses allow this kind of search, which is much more comprehensive than abstract or title searches only.

• NOT – this operator, used less often, can focus a search strategy so that it does not retrieve specific literature, for example human studies NOT animal studies. However, in certain cases the NOT operator can be too restrictive, for example if excluding male gender from a population, using ‘NOT male’ would also mean that any articles about both males and females are not obtained by the search.

literature search and bibliographic services

FIG 3 Example of Boolean operator concepts (the resulting search is the light red shaded area).

The conventions of each database should be checked before undertaking a literature search, as functions and operators may differ slightly between them (Cipriani Reference Cipriani, Saunders and Attenburrow 2016b ). This is particularly relevant when using limits and filters. Figure 2 shows an example search strategy incorporating many of the concepts described above. The search strategy is taken from Cipriani et al ( Reference Cipriani, Zhou and Del Giovane 2016a ), but simplified to include only one intervention.

Search filters

A number of filters exist to focus a search, including language, date and study design or study focus filters. Language filters can restrict retrieval of articles to the English language, although if language is not an inclusion criterion it should not be restricted, to avoid language bias. Date filters can be used to restrict the search to literature from a specified period, for example if an intervention was only made available after a certain date. In addition, if good systematic reviews exist that are likely to capture all relevant literature (as advised by an information specialist), date restrictions can be used to search additional literature published after the date of that included in the systematic review. In the same way, date filters can be used to update a literature search since the last time it was conducted. Reviewing the literature should be a timely process (new and potentially relevant evidence is produced constantly) and updating the search is an important step, especially if collecting evidence to inform clinical decision-making, as publications in the field of medicine are increasing at an impressive rate (Barber Reference Barber, Corsi and Furukawa 2016 ). The filters chosen will depend on the research question and nature of evidence that is sought through the literature search and the guidelines of the individual database that is used.

Google Scholar allows basic Boolean operators to be used in strings of search terms. However, the search engine does not use standardised search terms that have been tagged as in traditional databases and therefore variations of keywords should always be searched. There are advantages and disadvantages to using a web search engine such as Google Scholar. Google Scholar searches the full text of an article for keywords and also searches a wider range of sources, such as conference proceedings and books, that are not found in traditional databases, making it a good resource to search for grey literature (Haddaway Reference Haddaway, Collins and Coughlin 2015 ). In addition, Google Scholar finds articles cited by other relevant articles produced in the search. However, variable retrieval of content (due to regular updating of Google algorithms and the individual's search history and location) means that search results are not necessarily reproducible and are therefore not in keeping with replicable search methods required by systematic reviews. Google Scholar alone has not been shown to retrieve more literature than other traditional databases discussed in this article and therefore should be used in addition to other sources (Bramer Reference Bramer, Giustini and Kramer 2016 ).

Citation searching

Once the search strategy has identified relevant literature, the reference lists in these sources can be searched. This is called citation searching or backward searching, and it can be used to see where particular research topics led others. This method is particularly useful if the search identifies systematic reviews or meta-analyses of a similar topic.

Conference abstracts

Conference abstracts are considered ‘grey literature’, i.e. literature that is not formally published in journals or books (Alberani Reference Alberani, De Castro Pietrangeli and Mazza 1990 ). Scherer and colleagues found that only 52.6% of all conference abstracts go on to full publication of results, and factors associated with publication were studies that had RCT designs and the reporting of positive or significant results (Scherer Reference Scherer, Langenberg and von Elm 2007 ). Therefore, failure to search relevant grey literature might miss certain data and bias the results of a review. Although conference abstracts are not indexed in most major electronic databases, they are available in databases such as BIOSIS Previews ( Box 1 ). However, as with many unpublished studies, these data did not undergo the peer review process that is often a tool for assessing and possibly improving the quality of the publication.

Searching trial registers and pharmaceutical websites

For reviews of trial interventions, a number of trial registers exist. ClinicalTrials.gov ( clinicaltrials.gov ) provides access to information on public and privately conducted clinical trials in humans. Results for both published and unpublished studies can be found for many trials on the register, in addition to information about studies that are ongoing. Searching each trial register requires a slightly different search strategy, but many of the basic principles described above still apply. Basic searches on ClinicialTrials.gov include searching by condition, specific drugs or interventions and these can be linked using Boolean operators: for example, (bipolar disorder OR manic depressive disorder) AND lithium. As mentioned above, parentheses can be used to build up search terms. More advanced searches allow one to specify further search fields such as the status of studies, study type and age of participants. The US Food and Drug Administration (FDA) hosts a database providing information about FDA-approved drugs, therapeutic products and devices ( www.fda.gov ). The database (with open access to anyone, not only in the USA) can be searched by the drug name, its active ingredient or its approval application number and, for most drugs approved in the past 20 years or so, a review of clinical trial results (some of which remain unpublished) used as evidence in the approval process is available. The European Medicines Agency (EMA) hosts a similar register for medicines developed for use in the European Union ( www.ema.europa.eu ). An internet search will show that many other national and international trial registers exist that, depending on the review question, may be relevant search sources. The World Health Organization International Clinical Trials Registry Platform (WHO ICTRP; www.who.int/ictrp ) provides access to a central database bringing a number of these national and international trial registers together. It can be searched in much the same way as ClinicalTrials.gov.

A number of pharmaceutical companies now share data from company-sponsored clinical trials. GlaxoSmithKline (GSK) is transparent in the sharing of its data from clinical studies and hosts its own clinical study register ( www.gsk-clinicalstudyregister.com ). Eli-Lilly provides clinical trial results both on its website ( www.lillytrialguide.com ) and in external registries. However, other pharmaceutical companies, such as Wyeth and Roche, divert users to clinical trial results in external registries. These registries include both published and previously unpublished studies. Searching techniques differ for each company and hand-searching through documents is often required to identify studies.

Communication with authors

Direct communication with authors of published papers could produce both additional data omitted from published studies and other unpublished studies. Contact details are usually available for the corresponding author of each paper. Although high-quality reviews do make efforts to obtain and include unpublished data, this does have potential disadvantages: the data may be incomplete and are likely not to have been peer-reviewed. It is also important to note that, although reviewers should make every effort to find unpublished data in an effort to minimise publication bias, there is still likely to remain a degree of this bias in the studies selected for a systematic review.

Developing a literature search strategy is a key part of the systematic review process, and the conclusions reached in a systematic review will depend on the quality of the evidence retrieved by the literature search. Sources should therefore be selected to minimise the possibility of bias, and supplementary search techniques should be used in addition to electronic database searching to ensure that an extensive review of the literature has been carried out. It is worth reminding that developing a search strategy should be an iterative and flexible process (Higgins Reference Higgins and Green 2011 ), and only by conducting a search oneself will one learn about the vast literature available and how best to capture it.


We thank Sarah Stockton for her help in drafting this article. Andrea Cipriani is supported by the NIHR Oxford cognitive health Clinical Research Facility.

Select the single best option for each question stem

a an explicit and replicable method used to retrieve all available literature pertaining to a specific topic to answer a defined question

b a descriptive overview of selected literature

c an initial impression of a topic which is understood more fully as a research study is conducted

d a method of gathering opinions of all clinicians or researchers in a given field

e a step-by-step process of identifying the earliest published literature through to the latest published literature.

a does not need to be specified in advance of a literature search

b does not need to be reported in a systematic literature review

c defines which sources of literature are to be searched, but not how a search is to be carried out

d defines how relevant literature will be identified and provides a basis for the search strategy

e provides a timeline for searching each electronic database or unpublished literature source.

a the Cochrane Central Register of Controlled Trials (CENTRAL)

d the Cumulative Index to Nursing and Allied Health Literature (CINAHL)

e the British Nursing Index.

a bipolar disorder OR treatment

b bipolar* OR treatment

c bipolar disorder AND treatment

d bipolar disorder NOT treatment

e (bipolar disorder) OR (treatment).

a publication bias

b funding bias

c language bias

d outcome reporting bias

e selection bias.

MCQ answers

1 a 2 d 3 b 4 c 5 a

Figure 0

FIG 2 Example of a search strategy about bipolar disorder using MEDLINE (Cipriani 2016a). The strategy follows the PICO framework and includes MeSH terms, free-text keywords and a number of other techniques, such as truncation, that have been outlined in this article. Numbers in bold give the number of citations retrieved by each search.

Figure 2

This article has been cited by the following publications. This list is generated based on data provided by Crossref .

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How to undertake a literature search: a step-by-step guide


Undertaking a literature search can be a daunting prospect. Breaking the exercise down into smaller steps will make the process more manageable. This article suggests 10 steps that will help readers complete this task, from identifying key concepts to choosing databases for the search and saving the results and search strategy. It discusses each of the steps in a little more detail, with examples and suggestions on where to get help. This structured approach will help readers obtain a more focused set of results and, ultimately, save time and effort.

Keywords: Databases; Literature review; Literature search; Reference management software; Research questions; Search strategy.

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Research and Writing Guides

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The top list of academic research databases

best research databases

Whether you are writing a thesis, dissertation, or research paper it is a key task to survey prior literature and research findings. More likely than not, you will be looking for trusted resources, most likely peer-reviewed research articles. Academic research databases make it easy to locate the literature you are looking for. We have compiled the top list of trusted academic resources to help you get started with your research:

Scopus is one of the two big commercial, bibliographic databases that cover scholarly literature from almost any discipline. Beside searching for research articles, Scopus also provides academic journal rankings, author profiles, and an h-index calculator .

Search interface of Scopus

Web of Science also known as Web of Knowledge is the second big bibliographic database. Usually, academic institutions provide either access to Web of Science or Scopus on their campus network for free.

Web of Science landing page

PubMed is the number one resource for anyone looking for literature in medicine or biological sciences. PubMed stores abstracts and bibliographic details of more than 30 million papers and provides full text links to the publisher sites or links to the free PDF on PubMed Central (PMC) .

Search interface of PubMed

For education sciences, ERIC is the number one destination. ERIC stands for Education Resources Information Center, and is a database that specifically hosts education-related literature.

Search interface of ERIC academic database

IEEE Xplore is the leading academic database in the field of engineering and computer science. It's not only journal articles, but also conference papers, standards and books that can be search for.

Search interface of IEEE Xplore

ScienceDirect is the gateway to the millions of academic articles published by Elsevier. 2,500 journals and more than 40,000 e-books can be searched via a single interface.

Search interface of ScienceDirect

The DOAJ is very special academic database since all the articles indexed are open access and can be accessed freely of charge.

Search interface of DOAJ database

JSTOR is another great resource to find research papers. Any article published before 1924 in the United States is available for free and JSTOR also offers scholarships for independent researchers.

Search interface of JSTOR

PubMed is the number one resource for anyone looking for literature in medicine or biological sciences. PubMed stores abstracts and bibliographic details of more than 30 million papers and provides full text links to the publisher sites or links to the free PDF on PubMed Central (PMC)

literature search and bibliographic services

Literature Search Service

Hours: 8 a.m.-5 p.m., Monday - Friday Phone: 217-545-2113  Email: [email protected]

Research Support and Consultation

Expert literature search service.

Literature Search Request Form The Medical Library provides Literature Search Services to support SIU students, staff, faculty, and residents’ educational and research needs.  Library faculty will: •    Develop a robust search strategy based on the information provided by the requestor •    Provide the search string and results via a word document and/or .RIS file for bibliographic management programs ​​​​​​ Library Faculty will not: •    Synthesize data or otherwise interpret results.   •    Complete literature searches for course assignments; such requests require a search consultation as part of the library’s educational mission. Requesters should submit a fully formed research question with as many details and keywords as possible. If the search request is too vague or broad, library faculty may ask the requester to revise and resubmit the request and/or require a meeting to further develop the research question. For additional information on how to formulate a research question, please visit Responsible Literature Searches and PICO LibGuide .  Searches conducted on fee-based databases (i.e. PsycInfo, Embase, etc.) will incur charges to recover the costs of online access and the downloading of search results. These searches must be approved for payment in advance by the department fiscal officer or paid by personal check. The Medical Library currently offers three types of literature searches, each with varying deliverables and turn-around times. 

*Due to the in-depth and time-intensive nature of this work, library faculty can only work on a limited number of literature searches at a time. The service may not be immediately available if we have already reached our maximum capacity. In such cases we will contact you with an estimated start date.

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literature search and bibliographic services

A literature search is a systematic, thorough search of a range of literature (e.g. books, peer-reviewed articles, etc.) on your topic. Commonly you will be asked to undertake literature searches as part of your Level 3 and postgraduate study.

It's important before undertaking any research to fully understand the shape of the literature in the area. Literature searching can be broken down into a series of iterative steps. You may want to revisit some of these several times throughout your search.

Planning your search

What to search for: keywords and phrases.

Start the process by clarifying the research question you would like answered. Your next step is to use your research question to help you identify keywords. The language and terminology of your subject area will help you to identify the most effective words for your search.

You can also identify keywords by looking for background information on key areas within your topic online as this will give you ideas for synonyms and other words commonly used.

The following activity can provide further guidance: Choosing good keywords .

Where to search: Library Search, Databases and Google Scholar

Now that you have your keywords you need to decide where to search. Library Search is a good starting point, particularly for unfamiliar topics, to provide background information and lead to further sources. 

No two databases include exactly the same content. It is therefore advisable to search several databases to make sure you do not miss a key paper on your topic. If you are unsure where to search, the selected resources for your study page will help you find the most relevant databases. 

You may also like to use Google Scholar, which will search a wider set of resources, including items not available through the OU Library. There is more guidance on the Access eresources using Google Scholar page. This also shows how to add the "Find it at OU" button to Google Scholar search results.

Search techniques

Once you have your keywords you will need to combine them. You can use the helpsheet on Advanced search techniques as guidance. You may also find the following useful:

The Library online training session Smarter searching with library databases .

This activity on Filtering information quickly .

Further reading:

Byrne, D. (2017).  Developing a researchable question .  Project Planner . 10.4135/9781526408525. Sage Research Methods.

Byrne, D. (2017). Reviewing the literature .  Project Planner . 10.4135/9781526408518.&nbsp. Sage Research Methods.

Evaluating information

It’s important to evaluate the literature you find for quality and relevance. The PROMPT criteria will help with this. You can access this activity on evaluating the quality of information (requires login) for further guidance.

Organising information

When conducting a literature search recording the information you find in an organised manner is essential. Literature searches require you to read and keep track of many more articles than you would read for an assignment. You may want to try using a bibliographic management tool to help organise the references you have found. The library page on Bibliographic management will help you understand the different tools available.

The Library's  organising information activity will explain:

Smarter searching with library databases

Thursday, 30 March, 2023 - 19:30

Learn how to access library databases, take advantage of the functionality they offer, and devise a proper search technique.

literature search and bibliographic services

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Literature Search

The Prevention Services Clearinghouse periodically provides clarification on topics covered in the Handbook. To learn more, please visit the FAQ page.

Other Clearinghouses. The search begins by identifying citations from other evidence clearinghouses or repositories. A number of evidence clearinghouses overlap in content with the Prevention Services Clearinghouse (see Exhibit 3.1). Identifying studies that these other clearinghouses have reviewed is an efficient way of locating studies that may meet Prevention Services Clearinghouse eligibility criteria.

Exhibit 3.1. Clearinghouses Used to Identify Relevant Research

*Note: Additional clearinghouses may be used, depending on the program or service selected.

Bibliographic Databases. To ensure that searches are comprehensive, Prevention Services Clearinghouse staff also conduct searches of electronic bibliographic databases to identify additional potentially eligible studies not included on other clearinghouse sites. Trained staff use keywords to execute the searches. Content experts review these search terms for completeness, identify common synonyms, and suggest additional keywords. The following databases are included in all searches, with additional databases added as content experts recommend.

Exhibit 3.2 Bibliographic Databases Used to Identify Relevant Research

*Note: Additional databases may be used, depending on the program or service selected.

Grey Literature Scans. Finally, Prevention Services Clearinghouse staff scan the websites of federal, state, foundation, and private agencies who sponsor or conduct relevant research in order to identify any additional potentially eligible studies that may not be indexed in the standard electronic databases.


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Literature Searches

Make a Search Request / Request a Search Consultation (Opens Email)

Who Can Request Literature Searches?

All Northern Health staff, physicians, affiliated physicians and residents.

​ What Should You Include in Your Question*?

* Check out the Literature Search Subject Guide for tools and instructions to develop your search.

What Happens After You Submit a Request?

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Literature searches: what databases are available?

Posted on 6th April 2021 by Izabel de Oliveira


Many types of research require a search of the medical literature as part of the process of understanding the current evidence or knowledge base. This can be done using one or more biomedical bibliographic databases. [1]

Bibliographic databases make the information contained in the papers more visible to the scientific community and facilitate locating the desired literature.

This blog describes some of the main bibliographic databases which index medical journals.

PubMed was launched in 1996 and, since June 1997, provides free and unlimited access for all users through the internet. PubMed database contains more than 30 million references of biomedical literature from approximately 7,000 journals. The largest percentage of records in PubMed comes from MEDLINE (95%), which contains 25 million records from over 5,600 journals. Other records derive from other sources such as In-process citations, ‘Ahead of Print’ citations, NCBI Bookshelf, etc.

The second largest component of PubMed is PubMed Central (PMC) . Launched in 2000, PMC is a permanent collection of full-text life sciences and biomedical journal articles. PMC also includes articles deposited by journal publishers and author manuscripts, published articles that are submitted in compliance with the public access policies of the National Institutes of Health (NIH) and other research funding agencies. PMC contains approximately 4.5 million articles.

Some National Library of Medicine (NLM) resources associated with PubMed are the NLM Catalog and MedlinePlus. The NLM Catalog contains bibliographic records for over 1.4 million journals, books, audiovisuals, electronic resources, and other materials. It also includes detailed indexing information for journals in PubMed and other NCBI databases, although not all materials in the NLM Catalog are part of NLM’s collection. MedlinePlus is a consumer health website providing information on various health topics, drugs, dietary supplements, and health tools.

MeSH (Medical Subject Headings) is the NLM controlled vocabulary used for indexing articles in PubMed. It is used by indexers who analyze and maintain the PubMed database to reflect the subject content of journal articles as they are published. Indexers typically select 10–12 MeSH terms to describe every paper.

Embase is considered the second most popular database after MEDLINE. More than 32 million records from over 8,200 journals from more than 95 countries, and ‘grey literature’ from over 2.4 million conference abstracts, are estimated to be in the Embase content.

Embase contains subtopics in health care such as complementary and alternative medicine, prognostic studies, telemedicine, psychiatry, and health technology. Besides that, it is also widely used for research on drug-related topics as it offers better coverage than MEDLINE on pharmaceutics-related literature.

In 2010, Embase began to include all MEDLINE citations. MEDLINE records are delivered to Elsevier daily and are incorporated into Embase after de-duplication with records already indexed by Elsevier to produce ‘MEDLINE-unique’ records. These MEDLINE-unique records are not re-indexed by Elsevier. However, their indexing is mapped to Emtree terms used in Embase to ensure that Emtree terminology can be used to search all Embase records, including those originally derived from MEDLINE.

Since this coverage expansion—at least in theory and without taking into consideration the different indexing practices of the two databases—a search in Embase alone should cover every record in both Embase and MEDLINE, making Embase a possible “one-stop” search engine for medical research [1].

Emtree is a hierarchically structured, controlled vocabulary for biomedicine and the related life sciences. It includes a whole range of terms for drugs, diseases, medical devices, and essential life science concepts. Emtree is used to index all of the Embase content. This process includes full-text indexing of journal articles, which is done by experts.

The most important index of the technical-scientific literature in Latin America and the Caribbean, LILACS , was created in 1985 to record scientific and technical production in health. It has been maintained and updated by a network of more than 600 institutions of education, government, and health research and coordinated by Latin America and Caribbean Center on Health Sciences Information (BIREME), Pan American Health Organization (PAHO), and World Health Organization (WHO).

LILACS contains scientific and technical literature from over 908 journals from 26 countries in Latin America and the Caribbean, with free access. About 900,000 records from articles with peer review, theses and dissertations, government documents, conference proceedings, and books; more than 480,000 of them are available with the full-text link in open access.

The LILACS Methodology is a set of standards, manuals, guides, and applications in continuous development, intended for the collection, selection, description, indexing of documents, and generation of databases. This centralised methodology enables the cooperation between Latin American and Caribbean countries to create local and national databases, all feeding into the LILACS database.  Currently, the databases LILACS, BBO, BDENF, MEDCARIB, and national databases of the countries of Latin America are part of the LILACS System.

Health Sciences Descriptors (DeCS) is the multilingual and structured vocabulary created by BIREME to serve as a unique language in indexing articles from scientific journals, books, congress proceedings, technical reports, and other types of materials, and also for searching and retrieving subjects from scientific literature from information sources available on the Virtual Health Library (VHL) such as LILACS, MEDLINE, and others. It was developed from the MeSH with the purpose of permitting the use of common terminology for searching in multiple languages, and providing a consistent and unique environment for the retrieval of information. DeCS vocabulary is dynamic and totals 34,118 descriptors and qualifiers, of which 29,716 come from MeSH, and 4,402 are exclusive.

Cochrane CENTRAL

The Cochrane Central Register of Controlled Trials (CENTRAL) is a database of reports of randomized and quasi-randomized controlled trials. Most records are obtained from the bibliographic databases PubMed and Embase, with additional records from the published and unpublished sources of CINAHL, ClinicalTrials.gov, and the WHO’s International Clinical Trials Registry Platform.

Although CENTRAL first began publication in 1996, records are included irrespective of the date of publication, and the language of publication is also not a restriction to being included in the database.  You won’t find the full text to the article on CENTRAL but there is often a summary of the article, in addition to the standard details of author, source, and year.

Within CENTRAL, there are ‘Specialized Registers’ which are collected and maintained by Cochrane Review Groups (plus a few Cochrane Fields), which include reports of controlled trials relevant to their area of interest. Some Cochrane Centres search the general healthcare literature of their countries or regions in order to contribute records to CENTRAL.


ScienceDirect i s Elsevier’s most important peer-reviewed academic literature platform. It was launched in 1997 and contains 16 million records from over 2,500 journals, including over 250 Open Access publications, such as Cell Reports and The Lancet Global Health, as well as 39,000 eBooks.

ScienceDirect topics include:

Web of Science

Web of Science (previously Web of Knowledge) is an online scientific citation indexing service created in 1997 by the Institute for Scientific Information (ISI), and currently maintained by Clarivate Analytics.

Web of Science covers several fields of the sciences, social sciences, and arts and humanities. Its main resource is the Web of Science Core Collection which includes over 1 billion cited references dating back to 1900, indexed from 21,100 peer-reviewed journals, including Open Access journals, books and proceedings.

Web of Science also offers regional databases which cover:

Boolean operators

To make the search more precise, we can use boolean operators in databases between our keywords.

We use boolean operators to focus on a topic, particularly when this topic contains multiple search terms, and to connect various pieces of information in order to find exactly what we are looking for.

Boolean operators connect the search words to either narrow or broaden the set of results. The three basic boolean operators are: AND, OR, and NOT.

References (pdf)

You may also be interested in the following blogs for further reading:

Conducting a systematic literature search

Reviewing the evidence: what method should I use?

Cochrane Crowd for students: what’s in it for you?

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Grey literature and their sources

What is grey literature, when would you use it, what are its advantages and disadvantages, and how can you find it?

literature search and bibliographic services

This blog provides a step-by-step guide on how to conduct a systematic literature search.

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Grey Literature

1997- The Luxembourg Convention on Grey Literature;  expanded New York 2004 “...that which is produced on all levels of government, academics, business and industry in print and electronic formats, but which is not controlled by commercial publishers i.e. where publishing is not the main activity of the producing body”

 Report Conference Abstract Dissertation & Thesis Registered Trial Interview Patent Newsletter White paper Book Chapter

Reduce publication bias More access to global literature Often more current

Clinical trial registries Regulatory Information Conference proceedings Unpublished studies Studies reported in languages other than English

·          Campus guide on grey literature

·         NYAM grey literature report  

           Canada’s federal HTA agency

Overview of Literature Search

• To reduce bias, multiple databases need to be searched

• Suggested main databases to be searched in health care field       -Medline/PubMed       -Cochrane Library       -Embase       -Scopus

• Search specialized databases or websites for your topic additional to the suggested main databases

• Search scientific information packets, e.g., manufactures of products under review, for specific topics 

• Search cited references of main literature search results

 •Hand search in targeted journals

• Keep personal communications (e.g., principal investigators, colleagues, authors, experts in a discipline) to get subject experts' opinions on resources and search strategies

• Conduct Google/Google Scholar search to find additional documents

•Search strategy may be influenced by inclusion and exclusion criteria

•Search grey literature

Search Terms

Techniques for harvesting terms

One technique of collecting terms in PubMed

Methodology Filters

Methodology filters can help to get rid of undesirable study designs so as to reduce the size of a large retrieval. However systematic review attempts to maximize sensitivity in terms of retrieving all relevant documents. Therefore it is always preferable not to employ filters.

Sometimes retrieval sets are literally unmanageable. You‘d probably want to implement a methodology filter.

If you have to limit, use at all possible validated filters, which have been tested against gold-standard sets of bibliographic records. 

Iteration of Search

Signal of stopping search, other guides.

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Every day, around the world, students and scholars turn to Literature Online for the texts and information they need to advance their literary studies. Literature Online brings primary works, reference materials, and literary criticism together within an intuitive research environment that allows users to quickly find the information they need and make exciting new discoveries. Literature Online supports students and researchers studying English literature with the essential western literature canon and provides the contextual resources they need to understand them. For libraries, Literature Online reduces the work of managing journal subscriptions and saves budget on book renewals.

Literature Online is ever growing, and developed with the advice of leading scholars, making it an authoritative environment for research, teaching, and undergraduate study.

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Literature Online reduces the work of managing literary journal subscriptions and saves budget on maintaining print copies of fiction, plays, and poetry and renewing reference work.

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Bibliographic Analysis

Related terms:.

Locating scholarly papers of interest online

Maureen Henninger , in Social Media for Academics , 2012

Proprietary scholarly search services

Scopus, now officially SciVerse Scopus, is a product from Elsevier and was launched in November 2004 (coincidently within a month of the release of Google Scholar) to be a major competitor to Web of Science. At the time, the service was ‘covering 14,000 scientific titles plus 167 million scientific web pages, and delivering the largest collection of abstracts ever collected online in one place, going back forty years” ( Elsevier, 2004 , para. 3).

Content . Currently, Scopus is an enormous database and according to Elsevier has access to ‘over 18,500 titles from more than 5,000 international publishers … 14.4 million records’ ( Elsevier, 2011 , p. 4 ). It has its basis in the scientific fields using the citations from its own publications which include peer-reviewed journals, book series and conference proceedings. There is also the advantage of being able to include access to articles-in-press from Elsevier’s list of 2,199 journal titles. 10 Recently, Scopus began adding citations in the arts and humanities, thus moving towards a more multi-disciplinary coverage. Scopus, from the search interface, provides a list of all journals and other sources, categorized by journals, conference proceedings, trade publications, and book series.

We have already noted that the web-based scholarly search engines have added listings to their content by agreement and/or joint ventures with proprietary database vendors. In the case of Scopus, the content is extended by the additional harvested listing available on Scirus (see above) which it owns.

Ranking and displaying . By default, Scopus displays the results of a subject search by year in descending order; however, the user has the option of reversing this order, as well as sorting by number of citations, relevance, first author, and source title. There is a very nice feature for selecting items from the results’ list and printing a bibliography without having to first export the references into a bibliographic software package.

Bibliographic analysis . Scopus ranks journals according to the h-index, not the impact factor (the formula devised by Eugene Garfield) as done by the Web of Science; however, the data is available to calculate the impact factor. According to Elsevier, Scopus uses a ‘next generation context-based’ metrics, SNIP (source-normalized impact per paper) and SJR (SCImago Journal Rank). 11 Scopus has some excellent tools for analysis and visualization. For example, there is the usual citation tracker to find, check and track citations, an author identifier to automatically match an author’s published research including the h-index and a journal analyser which shows journal performance, which is helpful for publishing decisions ( Figure 4.10 gives some examples).

literature search and bibliographic services

Figure 4.10 . Scopus bibliometric visualizations: 1) h-index graph of 7 documents with self-citations removed, and 2) the SNIP (contextual citation impact) analysis of four journals 12

Social networking opportunities . Scopus provides no intrinsic social networking tools. However, Elsevier does have an add-on service, SciVal Experts, which is a directory of research expertise within an organization. It scans and analyses the Scopus publications to produce individual profiles which potentially can be used for collaboration within the organization or to identify external collaborators.

Web of Science

This is the oldest of the proprietary scholarly literature search services which are built around the concept of citation indexing. Its recent incarnation under Thomson Reuters, the Web of Science, was released in 1992, and while it was long regarded as a premier source for literature discovery, it has been the benchmark service for citation tracking and bibliometric analysis.

Content . The Web of Science consists of five databases covering scholarly literature from 1898 to the present: the three citation indexes of journal articles in science, social sciences and arts and humanities (this last one begins in 1975); and conference proceedings in both science and the social sciences beginning in 1990, covering international conferences, symposia, seminars, colloquia, workshops, and conventions. Web of Science, like Scopus, provides a list of journals included in the different databases; however, you are able to get journal title abbreviations within the search interface, the master list of journals is available at Thomson Reuters. 13 There is also an option within Web of Science to pass your search over to a beta service, Scientific WebPlus, to find web-based documents. This service is a collaborative one with Microsoft and various Thomson editors; 14 and the results come from repositories, news and blogs.

Ranking and displaying . By default, the results are listed by date (newest to oldest); however, the user can select several other sorts, for example times cited, first author, source title. You can also sort by relevance, which is a statistical ranking that considers how many of the search terms are found in each record. In addition, there are options to filter a subject search by almost any piece of metadata (see Figure 4.11 ).

literature search and bibliographic services

Figure 4.11 . Web of Science options for filtering search results

Bibliographic analysis . While both Web of Science and Scopus have excellent analytical tools, Web of Science provides the very powerful cited reference searching – the ability to track a specific author’s work to analyse the extent of the scholarly network in a particular field. This is particularly useful as a starting point in understanding the impact of a seminal piece of scholarship, invaluable for literature reviews for doctoral theses, for example (see Figure 4.3 on p. 64 to view the impact of the work of Henry Small in the discipline of bibliometrics). The service provides many other analytical reports such as citation reports (as shown in Figure 4.2 on p. 63 ).

Social networking opportunities . Web of Science does not have any built-in Web 2.0 social media tools. However, the beta service, Scientific WebPlus, provides a full range of tools – tagging, bookmarking, commenting, and voting (see Figure 4.12 ).

literature search and bibliographic services

Figure 4.12 . Results of Web of Science query passed through to Scientific WebPlus

A review on the practice of big data analysis in agriculture

Andreas Kamilaris , ... Francesc X. Prenafeta-Boldú , in Computers and Electronics in Agriculture , 2017

2 Methodology

The bibliographic analysis in the domain under study involved three steps: (a) collection of related work, (b) filtering of relevant work, and (c) detailed review and analysis of state of the art related work. In the first step, a keyword-based search for conference papers and articles was performed from the scientific databases IEEE Xplore and ScienceDirect, as well as from the web scientific indexing services Web of Science ( Thomson Reuters, 2017 ) and Google Scholar. As search keywords, we used the following query:

“Big Data” AND [“Precision Agriculture” OR “Smart Farming” OR “Agriculture“]

In this way, we filtered out papers referring to big data but not applied to the agricultural domain. Existing surveys ( Wolfert et al., 2017; Nandyala and Kim, 2016; Waga and Rabah, 2014 ; ( Chi et al., 2016; Wu et al., 2016 ) were also examined for related work. From this effort, 1330 papers were initially identified. Restricting the search for papers in English only, with at least two citations, the initial number of papers was reduced to 232. Number of citations was recorded based on Google Scholar. An exception was made for papers published in 2016–2017, where zero citations were acceptable.

In the second step, we checked these 232 papers whether they made actual use of big data analysis in some agricultural application. Use of big data analysis was quantified as satisfying some of its five “V” characteristics ( Chi et al., 2016 ) (see Section 3 ). We primarily targeted the first three “V”s (i.e. volume, velocity and variety), since dimensions V4 and V5 (i.e. veracity and valorization) were more difficult to quantify. From the 232 papers, only 34 qualified according to our constraints. We were forced to discard (also) a small number of interesting efforts which did not qualify in terms of the data analysis employed and the solutions provided. In the final step, the 34 papers selected from the previous step were analyzed one-by-one, considering the problem they addressed, solution proposed, impact achieved (if measurable), tools, systems and algorithms used, sources of data employed and which “V” dimensions of big data they satisfied.

Deep learning in agriculture: A survey

Andreas Kamilaris , Francesc X. Prenafeta-Boldú , in Computers and Electronics in Agriculture , 2018

The bibliographic analysis in the domain under study involved two steps: (a) collection of related work and (b) detailed review and analysis of this work. In the first step, a keyword-based search for conference papers or journal articles was performed from the scientific databases IEEE Xplore and ScienceDirect, and from the web scientific indexing services Web of Science and Google Scholar. As search keywords, we used the following query:

[ “deep learning”] AND [“agriculture” OR ”farming“]

In this way, we filtered out papers referring to DL but not applied to the agricultural domain. From this effort, 47 papers had been initially identified. Restricting the search for papers with appropriate application of the DL technique and meaningful findings 1 , the initial number of papers reduced to 40.

In the second step, the 40 papers selected from the previous step were analyzed one-by-one, considering the following research questions:

Which was the agricultural- or food-related problem they addressed?

Which was the general approach and type of DL-based models employed?

Which sources and types of data had been used?

Which were the classes and labels as modeled by the authors? Were there any variations among them, observed by the authors?

Any pre-processing of the data or data augmentation techniques used?

Which has been the overall performance depending on the metric adopted?

Did the authors test the performance of their models on different datasets?

Did the authors compare their approach with other techniques and, if yes, which was the difference in performance?

Our main findings are presented in Section 4 and the detailed information per paper is listed in Appendix B .

Mapping the relationships between work and sustainability and the opportunities for ergonomic action

Ivan Bolis , ... Laerte I. Sznelwar , in Applied Ergonomics , 2014

A map was drawn up of the relationships between work (in its multiple interpretations) and sustainability (sustainable development and corporate sustainability) based on a bibliographic analysis of articles that discuss these themes jointly in the current academic literature. The position of the discipline of ergonomics focused on work was identified from this map and, based on its specific academic literature, it was possible to identify where this discipline could contribute so that work and workers can be included in the discourse of sustainable development and considered in corporate sustainability policies. Ergonomics can be actively influential within the organization on issues relating to work improvements; it may boost integrated increases in the organization's performance and in workers' well-being; it can provide support for changes and new (environmental) sustainability-related work requirements to be considered; and it can contribute to the definition of the concept of work in a context of sustainable development.

Visual attractiveness in routing problems: A review

Diego Gabriel Rossit , ... Mariano Frutos , in Computers & Operations Research , 2019

5.1 Recommendations on the use of visual attractiveness measures

The correlation analysis performed on our experiment did not allow us to derive general relations between the various measures that permit to identify the most appropriate measures for each characteristic. However, based on the bibliographic analysis and the computational tests we can make some general recommendations:

Bending energy is a concept that is particularly useful for urban route planning. In these applications, the routes should be repeated frequently in a short period of time. That is why routes with numerous jagged turns can have a negative impact on the tires and brakes of the vehicle. Moreover, many turns in the path of a vehicle can lead to larger routing times considering the extra time needed to turn left at crossroads with traffic lights ( Lacomme et al., 2005 ). Conversely, in inter-city distribution plans where the nodes may represent a whole city and the details of actual paths are not specified, bending energy measures clearly makes no sense.

At least for the set of instances considered in this paper it does not seem worthwhile calculating the CLP measure because the number of average intra-route crossings ( I n t r a − C ) gives similar information and is faster and easier to compute (see Table 3 ).

It seems appropriate to use just one measure out of either convex hull overlaps ( CH ) or inter-route crossings ( I n t e r − C ) because those measures are both very time-consuming to be computed and provide similar information. However, from Table 3 we can see that at least for our implementation, I n t e r − C is faster to calculate. In addition, PROX c has a behavior similar to CH or I n t e r − C and, therefore, could be used as a proxy.

COMP b , COMP c and COMP d produce similar results. Because the computation of COMP b and COMP d requires the distances from the center of gravity, we recommend the use of COMP c .

PROX a and PROX b produce very similar information. However, we recommend the use of PROX b because its computation does not require the calculation of the distances from the center of gravity.

Researching for a literature review: Conducting a literature review search


Test Instruments

Beginning a literature review

Select a topic, scope, and purpose for the lit review and identify some preliminary search terms.

Nouns tend to work better than adjectives. For example, if I am researching the role storytelling plays within formal and informal mentoring situations, my first search string might be: the role storytelling plays within formal and informal mentoring situations

                        Based on my results, I may have to get more specific: storytelling, mentoring, higher education

Create a list of keywords, terms, and phrases related to your topic.

List both general and specific terms.

May use subject/specialized encyclopedias at this stage for overview and to identify other key phrases or terms. Thesauri or helpful sites such as Lexipedia may suggest alternate terms.

Begin to identify tools to use for searching. Subject databases within the field are appropriate; also consider tangentially related disciplines that may discuss your topic (for example, certain education topics are also covered extensively within the field of psychology).

Skim the information the library provides about what is in the database to help determine if it will contain topical info.

Example: JSTOR database: Though some titles are available through the most current issues, issues from the past 3-5 years are not available for most titles.

Note that Summon searches for books and articles, but doesn't have all the articles and books in it.

Armed with search terms and strategic search locations, start to cast the net by finding sources.

Finding multiple sources

Search for information using a variety of sources. For example, library catalogs may guide you to books and chapters of books, films, music, or other types of media.

Library databases will help you find articles within the database. Each database contains specialized, unique content (although there will be some overlap in the articles found in each database). Dissertations represent new research; the bibliographies may help identify key research you will also review.

Print indexes in the field may also be appropriate. Contact your subject librarian for assistance with searching in paper indexes.

Expand your search

After collecting all the relevant books, articles, dissertations and other materials, expand your search.

Helpful materials contain more than just the study or results presented; often the bibliographies and references of helpful materials help cast the research net wider.

Trace research backward by checking the bibliographies of the most helpful and also the most recent materials collected on a topic. Identify the sources cited that match with your own interest in the topic. Locate those sources. Continue branching backward.

Does your research contain any gaps? Start to address areas where coverage is light or non-existent by searching specifically for those areas--back to the top and identifying a new round of search terms and places to search.

How to know when you are done

Of course, you will feel that you are never done.

However, a point will arrive when searches in different locations no longer find new results related to your topic, and all the material found no longer cites material that has not been found or is not relevant.

Set up journal or search alerts if interested, and move to the next phase of your work.

Who is my librarian?

Each department, school, or program has a librarian assigned to work directly with its faculty and graduate students to provide in-depth research and subject specialization.

A listing of subject librarians .

Do you Interlibrary Loan (ILL)?

Interlibrary Loan (ILL) is the BGSU Libraries interlibrary loan (ILL) system.

Log in to your Interlibrary Loan (ILL) account in a separate window while searching for books and articles; this saves typing much of the citation information needed when requesting articles.

Interlibrary Loan (ILL) FAQ

Register for Interlibrary Loan (ILL) here . This account is separate from other accounts you may have at BGSU, including your library account. You will need to create a username and password to request and retrieve materials.

Created by the University of Minnesota Libraries, the Dissertation Calculator breaks down the process of writing a dissertation into discrete elements.

Many resources recommended to U of M students are also available to students and researchers at Bowling Green State University.


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Literature and Cognition

Cognitive science, with its guiding metaphor of the mind as computer, has made substantial progress toward and understanding of how people comprehend and produce discourse. The essays in this book apply these insights to problems in the interpretation of literature. The first two chapters present the outline of cognitive theory of discourse and use it to shed light on some classic issues in literary theory, including the roles of the author's intention and the reader's belief systems in the meaning of a literary work. The nest two chapters are more technical investigations of discourse coherence and metaphor. Then, the framework developed is used in the examination of two literary works, a sonnet by Milton and Gerard de Nerval's novella, "Sylvie." The final chapter draws parallels between conversation and literary discourse in an effort to understand the aesthetics of each.

Jerry R. Hobbs is senior computer scientist at SRI International.

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A bibliographic study reviewing the last decade of hydrochar in environmental application: history, status quo, and trending research paths

Biochar volume  5 , Article number:  12 ( 2023 ) Cite this article

171 Accesses

Metrics details

Hydrothermal carbonization (HTC) technology has increasingly been considered for biomass conversion applications because of its economic and environmental advantages. As an HTC conversion product, hydrochar has been widely used in the agricultural and environmental fields for decades. A CiteSpace-based system analysis was used for conducting a bibliometric study to understand the state of hydrochar environmental application research from 2011 to 2021. Researchers had a basic understanding of hydrochar between 2011 and 2016 when they discovered hydrochar could apply to agricultural and environmental improvement projects. Keyword clustering results of the literature published in 2017–2021 showed that soil quality and plant growth were the major research topics, followed by carbon capture and greenhouse gas emissions, organic pollutant removal, and heavy metal adsorption and its bioavailability. This review also pointed out the challenge and perspective for hydrochar research and application, namely: (1) the environmental effects of hydrochar on soils need to be clarified in terms of the scope and conditions; (2) the influence of soil microorganisms needs to be investigated to illustrate the impact of hydrochar on greenhouse gas emissions; (3) combined heavy metal and organic contaminant sorption experiments for hydrochar need to be conducted for large-scale applications; (4) more research needs to be conducted to reveal the economic benefits of hydrochar and the coupling of hydrochar with anaerobic digestion technology. This review suggested that it would be valuable to create a database that contains detailed information on how hydrochar got from different sources, and different preparation conditions can be applied in the environmental field.

Graphical Abstract

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The environmental risks of hydrochar on soils need to be clarified before mass engineering application.

The influence of soil microbes needs to be investigated to illustrate the impact of hydrochar on greenhouse gas emissions.

More research needs to be conducted to reveal the economic benefits of hydrochar and the coupling with anaerobic digestion.

Working on a manuscript?

1 introduction.

Hydrochar is defined as a solid carbon product with rich oxygen-containing functional groups (OFGs) produced by the hydrothermal decomposition of biomass (e.g., residues and wastes of plants and animals) in a reactor with water as the solvent and at moderate temperatures (Parshetti et al. 2013 ; Sevilla and Fuertes 2009a ). Hydrochar exhibits superior performance relative to raw biomass in terms of its mass and energy density, dehydration, and combustion performance. In recent years, hydrochar has drawn increasing attention for its economic and environmental benefits in various agricultural and environmental applications (Kambo and Dutta 2015 ; Khosravi et al. 2022 ; Wu et al. 2021a ).

The term “hydrochar” was coined by Spanish researchers Marta Sevilla and Antonio B. Fuertes in 2009 to describe a hydrothermal carbonization sample used as an intermediate in synthesizing highly crystalline graphitic carbon nano coils (Sevilla and Fuertes 2009a ). Further investigations have revealed that hydrochar is derived from dehydration condensation, polymerization, and aromatization reactions, including those of highly aromatic and reactive oxygen functional groups (Sevilla and Fuertes 2009b ). The first documented research on hydrochar for environmental applications appeared in 2011; it showed that the hydrothermal carbonization of biomass increases its carbon turnover time; thus, this technique is a potential way to sequester atmospheric carbon dioxide (CO 2 ) (Sevilla et al. 2011 ).

Hydrochar has recently attracted much attention in environmental applications because of its potential benefits and the scarcity of natural resources. The number of publications and review articles on the environmental applications of hydrochar has risen rapidly since 2011. For example, Goel et al. ( 2021 ) reviewed hydrochar adsorption to capture CO 2 . Luutu et al. ( 2021 ) used mate analysis to review the effects of hydrochar on plant growth when utilized as a soil conditioner. Lan et al. ( 2021 ) reviewed the progress of the application of hydrochar as a product to improve and remediate the soil environment. Zhang et al. ( 2020b ) reviewed the application of hydrochar as an adsorbent in water environment remediation. Even though most of these reviews looked at hydrochar in various environmental applications, only a few included bibliometric analyses.

It is worth noting that the review studies in the above examples are mainly reviews of a specific area of hydrochar, which are usually based on the author’s specific research area and background and are highly subjective. On the other hand, bibliometrics is based on literature and uses quantitative methods to analyze and process literature characteristics to obtain the necessary data, find out the patterns of change through data analysis, and predict future trends. Compared with traditional review methods, bibliometrics has significant macroscopic research advantages of objectivity, quantification, and modeling (Li et al. 2017 ). Bibliometric methodologies are knowledge-mapping tools for quantitative analyses of the structure, characteristics, and linkages within a research field. CiteSpace, the primary visualization tool for bibliometrics, is based on “co-occurrence clustering”. The literature is analyzed first by extracting information units (including references at the literature level, keywords at the topic level, authors, institutions, countries, and journals) and reconstructing them based on the type and strength of links between them to generate a network structure with different meanings (e.g., keyword co-occurrence, author collaboration, literature co-citation). Network nodes represent literature information units, whereas links represent connections between nodes (co-occurrences). Finally, through measurement, statistical analysis, and visualization of the network structure, implicit patterns and laws are discovered about the knowledge structure of specific disciplines and fields (Chen 2006 ). Bibliometric analysis can be presented straightforwardly and comprehensively with easy-to-understand graphical formats and precise representations of relationships between sources through software such as CiteSpace. In this way, researchers can uncover topics likely to be the focus of future research. For example, Wu et al. ( 2019 , 2020 , 2021b ) conducted a bibliometric analysis of biochar research using CiteSpace to reveal its history, current status, and future trends. Based on this, the goals of this study are to explore recent popular research topics and trends in the environmental applications of hydrochar using CiteSpace and to anticipate future trends by conducting visual scientometric analyses.

The objectives of this study are to perform (1) a systematic assessment of the level of scientific development of environmental applications of hydrochar, (2) an analysis of the present state of environmental applications of hydrochar, and (3) an assessment of potential shortcomings of these applications, which will aid scholars in anticipating future research directions in this field. This study provided a concise scientometric analysis of the environmental applications of hydrochar and provided insight into the field’s developmental history and possible future trends of hydrochar applications in different sectors.

2 Data sources and research methods

2.1 data collection and processing.

Data collection for this study was based on the Web of Science (WoS) core collection database. The search formula [TI = (“hydrochar” or “hydro-char”) or TI = (“biochar” and “hydrothermal carbonization”) or TI = (“biochar” and “hydrothermal liquefaction”)] were used in the data collection process with a search period from January 1, 2011 through December 31, 2021. The retrieved literature types, subject categories, journals, highly cited literature, countries, institutions, authors, and keywords were analyzed statistically. CiteSpace was used for data visualization and analysis, and synonyms were created based on the collected data.

2.2 Scientometrics analysis methods

The JAVA-based quantitative bibliometric visualization tool CiteSpace (version: 6.1. R3), developed by Prof. Chaomei Chen, was used for analyzing the data in this study, and the results were highly objective (Chen 2006 ). CiteSpace provided the authors, co-keyword, and cluster analysis functions for understanding the relationships between authors and keywords and identifying emerging trends, hot spots, and gaps in hydrochar research. The size of the node in the network mapping graph indicated the frequency or number of occurrences of each entry (e.g., keyword or author). When there were two connected nodes, a correlation was found between them.

3.1 Characteristics of publication type and outputs

A total of 2403 papers on hydrochar were included in the WoS core collection from January 1, 2011 to December 31, 2021. The main types of literature were papers, conference proceedings, review papers, online publications, conference abstracts, and letters. Among these types, the proportion of dissertation articles was the largest. An analysis of the wide range of the composition of publication types indicated the importance of hydrochar research. Generally, the number of publications suggests the level of attention given to a field by scientists and, in some ways, it reflects the pace and course of development of the field. Thus, the number of publications in hydrochar research is a good indicator of the growth trend. Hydrochar publications were expected to continue to receive academic attention for some time, as shown in Fig.  1 , based on the increasing number of annual hydrochar publications and their cumulative numbers. A literature analysis provided evidence that researchers were increasingly paying attention to hydrochar research due to its importance in producing bioenergy, fertilizer, and waste biomass. There have been two distinct periods of research on hydrochar in environmental applications: a “logarithmic development” period with an average annual growth rate of 67.49% (2011–2016) and a “steady development” period with an average annual growth rate of 35.77% (2017–2021).

figure 1

The number of published documents on hydrochar in environmental applications each year (the inserted figure is the cumulative number of publications from 2011 to 2021). Hydrochar has been published in environmental applications in 2403 papers between 2011 and 2021. The number of publications has steadily increased over time. As research progresses, it is gaining increasing attention in environmental applications. Research on hydrochar in environmental applications has been divided into two distinct periods: a “logarithmic development” period (2011–2016) and a “steady development” period (2017–2021)

3.2 Subject categories and journal distribution

The hydrochar research involved 72 subjects (Additional file 1 : Fig. S1), of which 10 were selected for analysis, as shown in Table 1 . The most popular subjects were “Energy Fuels”, followed by “Environmental sciences”, “Engineering chemical”, “Engineering environmental”, “Biotechnology applied microbiology”, “Agricultural engineering”, “Green sustainable science technology”, “Chemistry multidisciplinary”, “Chemistry physical”, and “Materials science multidisciplinary”. It is generally recognized that hydrochar research is a multidisciplinary field that has received attention from various perspectives. The research on hydrochar was more often centered on environmental categories, suggesting that hydrochar is a popular topic for environmental studies and has attracted widespread interest. Additionally, applications of hydrochar in green and sustainable science and technology have become a popular research topic.

It was possible to identify the major journals that strongly influence the field by counting the number of related publications published by specific journals (Additional file 1 : Fig. S2). This finding helps researchers to select key journals for submissions and reading based on their subject categories. In this study, the publication outputs of 401 journals in the field of hydrochar were determined, and the 10 journals with the most publications were selected for analysis (Table 1 ). Bioresource Technology had the highest number of publications, followed by Science of the Total Environment. In terms of journals, most of them have published a relatively high number of manuscripts on the environmental applications of hydrochar, thus strongly influencing this field.

Double-map overlay analysis reveals the internal relationship between disciplines by evaluating the subject distribution, citation track and research focus of the paper. A double-map overlay is produced by superimposing and simplifying disciplines using a Z-score. According to the color-coded lines, there is an association between a citing journal and a different field; the thickness of the lines indicates how similar the fields are. Fig.  2 shows a double-map overlay analysis for hydrochar research. On the left is the subject distribution of the research literature. This section of the figure shows that the research on hydrochar is mainly applied to zoology, physics, chemistry, and materials science. On the right is the subject distribution of cited research literature. This section shows that hydrochar researchers are more involved in environmental, toxicology, nutrition, chemistry, physics, and materials science, indicating that the research on hydrochar is multidisciplinary. The thickness of a connection indicates how closely disciplines are related, while the color indicates the path of their relationship. For example, a rose line connects physics to environmental science; a yellow line connects zoology to environmental science. Researchers from other disciplines would explore the diverse characteristics of this field.

figure 2

A double-map overlay analysis for hydrochar research. On the left is the subject distribution of research literature. It can be seen that the research on hydrochar is mainly applied to zoology, physics, chemistry, and materials science. On the right is the subject distribution of cited research literature. Research on hydrochar is more involved in environmental, toxicology, nutrition, chemistry, physics, and materials science, indicating that the research on hydrochar is multidisciplinary

3.3 Analyses of countries and issuing institutions

Visual representations of the overall strength of scientific research and the influence of the specific countries in a given research field were based on the statistics of the author’s countries of related articles. Overall, hydrochar research was conducted in 90 countries worldwide. Hydrochar research varied from country to country. For readers to have more direct information, we summarized the main countries (top 10) in hydrochar research (top 3). Researchers can obtain a more thorough understanding of research dynamics in a particular field by analyzing the geographical distribution of research activities and the cooperation between institutions. According to the regional heatmap for hydrochar research (Fig.  3 and Additional file 1 : Table S1), China has the most publications (1009, 41.99%, mainly in the fields of energy fuels, soils, and water treatment), followed by the United States (319, 13.28%, energy fuels, soils, and water treatment), Germany (253, 10.53%, soils, energy fuels, and catalysts), Spain (141, 5.87%, energy fuels, soils, and electrochemistry), Italy (120, 4.99%, energy fuels, soils, and catalysts), England (101, 4.20%, energy fuels, soils, and water treatment), Australia (100, 4.16%, energy fuels, and water treatment), Korea (99, 4.12%, energy fuels, soils, and water treatment), India (94, 3.91%, soils, and water treatment), and Canada (92, 3.83%, energy fuels, soil science, and water treatment). Lines between countries and regions indicate their cooperation in Fig.  3 , indicating the bilateral cooperative relationship between various countries in hydrochar research. Accordingly, China has the highest productivity and the most nodes compared to other nations and regions. Despite this, it is crucial to strengthen collaborations with other high-yield countries in hydrochar research. Generally, the number of articles published by an institution in a field represents its level of influence and research (Additional file 1 : Fig. S3). China was shown to be the high-yield hydrochar research institution, as seven of the ten institutions with the most publications were based in China (Additional file 1 : Table S2). The Chinese Academy of Sciences posted the most articles (116 publications) and the University of Hohenheim (Germany) occupied the second highest (87 publications) ranking. The University of Trento (Italy) was ranked eighth (35 publications) and the University of Florida (United States) was ranked tenth (32 publications).

figure 3

Contributions of various countries worldwide in the published documents for hydrochar in environmental applications. There has been a worldwide push to study hydrochar by governments, as indicated. Comparatively, China is the most productive country in the world. The line indicates a high degree of cooperation between countries and regions

3.4 Analyses of authors and highly cited publications

Table 1 summarizes the top 10 authors with the highest number of publications and co-citations in the field based on the author publication statistics in the WoS core collection database (Fig.  4 ) and CiteSpace-based cited author analysis (Additional file 1 : Fig. S4). Generally, the number of publications of an author reflects their ability to conduct research in a particular field. In contrast, co-citations reflect the recognition of the author’s knowledge and expertise by peers. Therefore, the number of publications and co-citations is generally regarded as indicators of a scholar’s academic contribution and influence in a particular research field. Author analyses allow readers to learn about the researchers in the field and the researchers’ topics of interest to help readers communicate appropriately in academia according to their research needs; these analyses reflect the research capabilities of the researchers’ countries. From 2011 to 2021, a total of 6649 authors contributed to hydrochar research from their respective fields. According to Fig.  4 , many hydrochar application research groups have formed, with close cooperation among the researchers within the groups among several core authors. As shown in Fig.  4 and Table 1 , we found that Prof. Kruse Andrea from Germany had the most publications, and his research encompasses several disciplines, as evidenced by his articles. Meanwhile, the author is highly interested in the environmental applications of hydrochar, particularly in improving soils and growing plants. Spanish scholar Prof. Sevilla Marta had the highest co-citation frequency, as shown in Additional file 1 : Fig. S4 and Table 1 , and proposed the name “hydrochar” in early 2009 as a new method of CO 2 sequestration, laying a foundation for subsequent research. In China, Prof. Shicheng Zhang of Fudan University had the highest number of publications, ranking second in total, indicating that this author had conducted in-depth research on hydrochar for environmental remediation applications. Among Chinese scientists, Prof. Zhengang Liu ranked fourth in both publications and total citations, indicating a thorough research background in hydrochar. The total cited frequency of literature provides insight into influential authors and essential topics over time in a field, as high total cited pieces of literature are usually of high quality. The top 10 citations for environmental applications of hydrochar are shown in Fig.  5 and Additional file 1 : Table S3. According to Kambo et al. (University of Guelph, Canada), the most frequently cited article compared the production, physicochemical properties, and environmental applications of hydrochar and concluded that the reaction temperature controls these properties.

figure 4

A scheme of the author and their cooperation relationship contributed to hydrochar in environmental applications. Collaborating with researchers can help research progress. Researchers collaborated on hydrochar in environmental applications between 2011 and 2021

figure 5

A scheme of highly cited literature on hydrochar in environmental applications

3.5 Popular research topics

An analysis of keywords in a research field can accurately pinpoint popular research topics and possible future research directions based on their correlations with research themes in the literature. Keyword view analysis (Additional file 1 : Fig. S5 and Fig.  6 ), a keyword clustering of cited authors (Fig.  7 a), and timeline view analysis (Fig.  7 b) were performed using CiteSpace. Keyword views provide a good visualization of present and past popular research topics in a field. In keyword views, nodes represent the analyzed objects, with larger nodes indicating more frequent appearances or citations. The color and thickness of the inner circles of the nodes represent the frequencies of occurrences or citations in different periods. Lines between nodes indicate co-occurrences or co-citations, and the thicknesses of the lines represent the strength of the co-occurrences. Cited authors keyword clustering views are based on cited authors, and they display the main research focus of the most cited authors; the automatic clustering label is generated by the spectral clustering algorithm, which then extracts the labeled words from the relevant cited literature. The clustering timeline view is implemented on the keyword clustering analysis of cited authors, focusing mainly on outlining the relationships between the clusters and the historical span of literature in a particular cluster and relationships between the clusters and a segment of popular topic outbreak time.

figure 6

The keyword network map of hydrochar in environmental applications during 2017–2021. The red circles represent hydrochar in environmental applications, and the black circles represent hydrochar in other applications

figure 7

a Cluster view of the total cited author keywords of hydrochar in environmental applications; b cluster timeline view of seven major clusters of co-citation of hydrochar in environmental applications in recent years through fisheye view to show more details toward 2021

To more intuitively reflect the popular trends and development history of environmental applications of hydrochar, a keyword view analysis from two time periods, 2011–2016 and 2017–2021, was adopted in this study. Hydrothermal carbonization was the subject of 202 papers published between 2011 and 2016. The keywords associated with the raw materials or products of hydrochar were “biomass”, “activated carbon”, “carbon”, “black carbon”, and “biochar”. According to the literature analysis, this observation reflected the focus on hydrochar preparation during this period. However, the keywords “soil” and “adsorption” during this period indicated that researchers began to pay attention to the roles of hydrochar as a soil additive and an adsorbent. The keywords included “carbon sequestration”, “greenhouse gas emission reduction”, “soil quality improvement”, and “remediation of organic pollutants”; however, their relevance was limited, and the exploration of these topics was in its early stages.

A significant increase in the literature on hydrochar was observed between 2017 and 2021 (Fig.  1 ). As displayed in Fig.  6 , the hydrochar research in 2017–2021 showed a more significant diversity and development trend than that in 2011–2016, with a significant increase in keywords, nodes, and intersecting lines. According to the visualization and statistical analyses, the research on the environmental applications of hydrochar from 2017 to 2021 could be divided into “soil quality and plant growth”, “carbon capture and greenhouse gas emission”, “organic pollutant removal”, “heavy metal adsorption and its bioavailability” and “anaerobic digestion”.

The Additional file 1 : Table S4 contains clustering information derived from the keyword clustering analysis of cited authors in Fig.  7 a and a clustering timeline view in Fig.  7 b. Since CiteSpace clusters closely related keywords, the keyword with the largest value within each cluster was selected as the representative of that category and assigned a tag. A good clustering result was characterized by two values, Q and S. A clustering module value of Q > 0.3 indicated a significant clustering structure; a clustering average profile value of S > 0.5 indicated a reasonable clustering class, and a clustering average profile value of S > 0.7 indicated a convincing clustering. Therefore, the clustering Q = 0.84 and S = 0.95 in this study demonstrated convincing clustering results. We can quickly grasp the hotspots of research on hydrochar using Additional file 1 : Table S4 and Fig.  7 , as well as the history of their development and the research directions of the disciplines involved. The largest cluster is #0 “nanoparticle”, and the clustering timeline is from 2011 to the present. Based on the keywords included in #0 in Additional file 1 : Table S4, we could roughly conclude that the most prominent research area regarding hydrochar involves the creation of nanomaterials by carbonization technology and studying their physicochemical properties. Next is cluster #1 “porous carbon”; the cluster timeline is from 2015 to the present. Based on the keywords included in #1 in Additional file 1 : Table S4, we could approximate that most researchers use biomass wastes as raw materials and modify them by nitrogen (N) doping or potassium hydroxide (KOH) activation to increase the specific surface area and pore size of hydrochar in order to create porous carbon for energy production/storage, capacitor/fuel cell preparation, and CO 2 absorption/capture for environmental use. Cluster #2 is “temperature” and the clustering timeline is 2011 to the present. Based on the keywords included in #2 in Additional file 1 : Table S4, we found that the research hotspot of this cluster should be about hydrochar preparation and properties. We speculated that temperature might be the most critical factor affecting hydrochar properties based on the cluster tag words. Based on other clustering keywords, we identified different categories of raw materials used to prepare hydrochar, including but not limited to “sewage sludge”, “agricultural waste”, “food waste”, “plant lignin”, “algae”, “poultry”, and “livestock manure”. Cluster #3 is “adsorption” and clustering timeline is from 2013 to 2021. Based on the keywords included in #3 in Additional file 1 : Table S4, we could roughly conclude that researchers usually use agricultural waste and livestock manure as raw materials to prepare hydrochar for environmental applications such as soil improvement. Because hydrochar research is multidisciplinary, one cluster contains multiple research directions, and a single research direction occurs in multiple clusters. According to the cluster keywords, hydrochar as an adsorbent covers clusters #4 “methylene blue”, #8 “hexavalent chromium”, #12 “carbon sequestration”, and #13 “adsorption mechanism”. Hydrochar is a product of the efficient conversion of biomass resources with economic and environmentally friendly applications. Therefore, it has a wide range of applications in removing heavy metals and organic pollutants from soil/water environments and carbon fixation. Several tag words, including cluster #5 “hydrothermal treatment”, cluster #6 “reactivity”, cluster #7 “conversion”, cluster #9 “hydrothermal liquefaction”, cluster #10 “hydrothermal carbonization”, cluster #11 “wet torrefaction” and cluster #14 “anaerobic digestion”, appear to describe hydrochar research, mainly pertaining to the preparation and characterization of hydrochar for energy and fuel applications.

4 Discussion

As described in Sect.  3.2 , research on hydrochar is a multifaceted and multidisciplinary field that has received a wide variety of attention. Taking the keyword clustering discussion in Sect.  3.5 as an example, researchers have typically used biomass waste as a raw material and modified it by N doping or KOH activation to increase the specific surface area of hydrochar in terms of pore size, forming porous carbon for applications in energy production/storage, preparation of capacitors/fuel cells, and CO 2 adsorption/capture for environmental application. The same research hotspot is involved in multiple disciplinary areas. Moreover, there are some other applications such as hydrochar preparation, energy production, and others. However, according to the distribution of publishing journals and subject categories, it can be seen that the research on hydrochar is more focused on environmental disciplines. Therefore, this study was mainly intended to describe hydrochar in environmental applications. According to Fig.  6 , the popular hydrochar research topics in environmental application fields were “soil quality and plant growth”, “carbon capture and greenhouse gas emissions”, “organic pollutant removal”, “heavy metal adsorption and its bioavailability”, and “anaerobic digestion”. Table 2 summarizes the main influencing results, influencing factors, and mechanisms of hydrochar in different environmental application fields.

4.1 Effects of hydrochar on soil properties and plant growth

Hydrochar showed great potential in improving the structure, nutrient cycle, and water retention abilities of soil due to its developed pore structure, abundant OFGs, rich organic matter content, high aromaticity, and large O/C and H/C ratios (Zhang et al. 2020c ). Therefore, hydrochar has been studied extensively as a cost-effective method for soil improvement in both pot experiments and field experiments (Adjuik et al. 2020 ; Malghani et al. 2014 ). Soil aggregation was positively correlated with O/C and H/C ratios. A study by George et al. ( 2012 ) showed that the addition of hydrochar has a positive effect on soil aggregation, which is presumably related to the organic matter in hydrochar. Additionally, the formation of cation bridges between soil and hydrochar surface active functional groups promoted soil agglomerate stability (Heikkinen et al. 2019 ). Studies have shown that applying poultry litter hydrochar to sandy soils can increase the soil ion exchangeability, water retention capacity, and soil porosity while reducing soil bulk density, and these positive effects may be controlled by the hydrophilic surface and well-developed pore structure of poultry fecal hydrochar (Mau et al. 2020 ). Furthermore, hydrochar contains high levels of dissolved organic carbon (DOC), which can be degraded and utilized by microorganisms and used as a slow-release organic fertilizer (Song et al. 2020 ). The application of hydrochar leads to an increase in the conversion rate of nitrate (NO 3 − ), which reduces the rate of soil nitrification and facilitates denitrification and mineralization. This phenomenon is particularly evident when hydrochar with high C/N ratios, high DOC, and low mineral N content is applied. Plants do not use the fertilizer-based N, but they mineralize NH 4 + from hydrochar to meet their N needs (Bargmann et al. 2013b ; Egamberdieva et al. 2016 ; Thuille et al. 2015 ). The application of hydrochar to soils can promote microorganisms to fix mineral N and reduce N leaching, especially for the case of NO 3 − , which positively impacts soil N utilization and soil ammonia volatilization (Chu et al. 2020 ). Thus, it is possible to apply hydrochar instead of N fertilizer, which affects N partitioning between soil microorganisms.

Moreover, the addition of hydrochar can change microbial activity and microbial community composition in soil, thereby improving soil characteristics. Studies have shown that when hydrochar is applied to soil, it has a positive effect on the root colonization of arbuscular mycorrhizal fungi, possibly due to changes in pH that occur after hydrochar addition to soil and because the porous structure of hydrochar can protect fungal hyphae from herbivores (Salem et al. 2013 ). The composition of the archaeal community of soils changes significantly after the application of hydrochar, possibly because the dissolved organic matter (DOM) released from the hydrochar provides an additional matrix for archaeal growth, resulting in its enrichment (Ji et al. 2020 ). Sun et al. ( 2020 ) found that hydrochar plays a negative and positive role in bacterial and fungal richness and diversity, respectively, because hydrochar is acidic and promotes fungal growth, whereas bacteria prefer neutral conditions. A positive correlation exists between soil rhizobia and soil sulfur (S), and a negative correlation was found between soil rhizobia and soil N. The study of Scheifele et al. ( 2017 ) compared the effects of hydrochar and pyrochar on soil rhizobia and found that the effect of hydrochar on rhizobia was more pronounced because hydrochar provided more usable S than pyrochar, and hydrochar led to a decrease in the availability of N in soil solutions, which had a positive effect on soil rhizobia. In addition, hydrochar contains higher contents of nutrients (P, K, Ca, and Mg) than those present in agricultural soils, making it a beneficial agricultural fertilizer for improving soil nutrient balance and enhancing crop growth (Fei et al. 2019 ; Melo et al. 2016 ). For example, sludge hydrochar provides plant-available P over time, primarily as Al-associated and Ca-associated P (Shi et al. 2019 ).

However, excessive application of hydrochar was reported to negatively affect plant growth. Hydrochar has been shown to have initial phytotoxicity (Hitzl et al. 2018 ), which may be attributed to its low pH value, salinity, organic pollutant contents (Mumme et al. 2018 ), polyphenol contents, volatile fatty acid contents (Puccini et al. 2018 ), polycyclic aromatic hydrocarbons (PAHs) contents (Lang et al. 2019b ), furan contents (Celletti et al. 2021 ), and to the presence of other toxic compounds. Moreover, hydrochar could enrich heavy metals and volatile organic compounds (Wang et al. 2016 ). As hydrochar decomposes, heavy metals and organic contaminants are released and adsorbed by components of soil matrices, such as carbonates, iron oxides, and clays (Lang et al. 2019a ). It is concerning that the hydrochar application ratio has a direct effect on plant growth, especially during the seed germination and seedling root development stages (Vozhdayev et al. 2015 ). Low application ratios promote plant growth (de Jager and Giani 2021 ). In contrast, high application ratios inhibit plant growth (Bargmann et al. 2013a ). Additionally, the high water retention capacity of hydrochar results in poor soil aeration, and the production of CO 2 can cause root hypoxia, adversely impacting plant growth (Fornes et al. 2017 ).

Soil improvement is intended to increase crop yields; therefore, the long-term effects of hydrochar should be considered for soil improvement initiatives to reduce the toxic effects of harmful substances on crops and decrease risks for soil contamination. In some studies, modifications to hydrochar have been shown to reduce its biotoxicity and alleviate its inhibitory effects on plant growth. For example, nitric acid-modified hydrochar did not exhibit phytotoxic properties and stimulated seedling growth; this phenomenon occurred because dilute nitric acid treatment reduced the biotoxicity, increased the N content, and stimulated the seedling growth of hydrochar (Fornes and Belda 2017 ). Calcium oxide (CaO) modification to hydrochar also reduced the biotoxicity of hydrochar and enhanced crop germination. This result may occur because CaO modification increases the pH of hydrochar and reduces the amount of PAHs precursors in hydrochar; additionally, CaO adsorbs metals and organic compounds that inhibit crop growth (Lang et al. 2019b ; Mumme et al. 2018 ). Furthermore, heat treatment of hydrochar can reduce phytotoxicity associated with hydrochar because the aromaticity and thermal stability of hydrochar can be improved, and the phytotoxic and genotoxic substances present in it can be biodegraded (Bahcivanji et al. 2020 ; Hitzl et al. 2018 ).

4.2 Effects of hydrochar on carbon capture and greenhouse gas emissions

Global population growth and greenhouse gas (GHG) emissions have led to global warming, which has caused scholars to focus their environmental research on global change and the realization of “peak carbon dioxide emissions” and “carbon neutrality”. Early published studies have shown the positive impact of hydrochar on carbon sequestration and GHG emission reduction (Malghani et al. 2014 ; Mestre et al. 2014 ; Sevilla et al. 2011 ). The high specific surface area and rich mesoporosity of hydrochar make it an effective technology for capturing CO 2 and reducing CO 2 emissions (Balahmar et al. 2015 ; Coromina et al. 2016 ; Sangchoom and Mokaya 2015 ; Sevilla et al. 2016 ). Therefore, hydrochar modified with alkali activation and magnetization has improved adsorption and capture capacity for CO 2 (Hao et al. 2017 ; Liu et al. 2018 ). A porous activated carbon obtained by activating hydrochar with potassium salts and glucosamine hydrochloric acid has the highest CO 2 adsorption capacity among the reviewed literature reports (Durán et al. 2018 ; Gallucci et al. 2020 ; Kishibayev et al. 2021 ; Lu et al. 2021 ; Shi et al. 2021b ), attaining 26.24 mmol/g at 0 °C and 20 bar (Cui et al. 2021 ). The addition of N-containing functional groups to hydrochar can enhance its porous properties, and acid–base interactions can occur between acidic CO 2 and basic N-containing functional groups (Cui et al. 2021 ; Jiang et al. 2020 ; Shi et al. 2021a ). The metal oxide nanoparticle load can promote the physical adsorption of CO 2 to hydrochar at metal sites surrounded by N atoms and hydroxyl groups (Vieillard et al. 2018 , 2019 ). Therefore, metal (hydrogen) oxide nanoparticle-loaded hydrochar materials are used as inexpensive large-scale CO 2 trapping materials to reduce GHG emissions and mitigate global warming (Jaberi et al. 2020 ; Vieillard et al. 2019 ). It is necessary to understand the available active sites and their nature as they are affected by the activation temperature as adsorption proceeds.

The impact of hydrochar on GHG emissions has also attracted much attention. In some studies, the application of hydrochar to soils has been shown to increase CO 2 emissions (Andert and Mumme 2015 ; Yue et al. 2016 ), possibly due to the abundant DOM present in hydrochar, which results in an excitation effect on microbial activity and stimulates soil decomposition and mineralization of natural organic matter (Yue et al. 2016 ). It is also possible that the labile carbon (C) in hydrochar can provide an additional matrix for soil microbes, such as actinomycetes, fungi, and methanogens (Ji et al. 2020 ). A study by Breulmann et al. ( 2017 ) found that the hydrothermal carbonization temperature was associated with hydrochar stability, affecting CO 2 emissions. Hydrochar produced at high temperatures contains lower quantities of labile C and higher amounts of aromatic C and may release less CO 2 than hydrochar produced at low temperatures (Liu et al. 2017 ). Hydrochar can affect soil enzymes and microbial activity through the presence of toxic substances, which reduce the mineralization in the soil and reduce CO 2 emissions (Niu et al. 2021 ). Furthermore, results have shown that hydrochar can significantly reduce CO 2 emissions when applied to soils containing low water contents, and moisture variability can affect the soil’s rate of microbial C degradation (Adjuik et al. 2020 ). According to existing studies, differences in conditional parameters and application environments for hydrochar may affect CO 2 emissions.

For the effect of hydrochar on methane (CH 4 ) emissions, the research results are not consistent; both inhibitory effects (Chen et al. 2021 ; Li et al. 2021a ; Wu et al. 2021c ) and promoting effects (Ji et al. 2020 ; Zhou et al. 2018 ) have been observed. A study by Li et al. ( 2021a ) has shown that clay-hydrochar introduction can increase the porosity and aeration of paddy soils, which is beneficial for inhibiting the activity of methanotrophs. Simultaneously, methanotrophs can serve as aerobic microbes that utilize CH 4 and oxygen (O 2 ) for nutrition, which is conducive to CH 4 consumption and mitigating CH 4 emissions. However, it is worth noting that high applications suppress the emission ratio of CH 4 less than low applications (Wu et al. 2021c ; Zhou et al. 2018 ). Though a high amount of hydrochar application increases the inhibition of methanotrophs, at the same time, large amounts of DOM released from hydrochar cause an increase in CH 4 emission (Cheng et al. 2021 ; Sun et al. 2020 ), which is because the decomposition of DOM by microorganisms in anaerobic and partially aerobic environments can produce a large amount of CH 4 . A study also found that removing a part of the DOM from hydrochar by water-washing can reduce CH 4 emissions (Chen et al. 2021 ). Compared with hydrochar, biochar reduces CH 4 emissions by 37.5% (Kammann et al. 2012 ). Furthermore, a study by Cervera-Mata et al. ( 2021 ) showed that hydrochar at different hydrothermal carbonization temperatures has different effects on CH 4 emissions, which is a result of the fact that an increase in temperature can reduce the amount of easily decomposed DOM present, and CH 4 emissions caused by DOM decomposition are further reduced (Ji et al. 2020 ). Overall, the DOM in hydrochar may be an essential factor in determining soil CH 4 emissions and shows a positive correlation. Therefore, the application level and preparation temperature of hydrochar should be considered when applying hydrochar to soil environment.

Soil nitrous oxide (N 2 O) emissions from hydrochar have been documented in several studies (Chen et al. 2021 ; Li et al. 2021a ; Xu et al. 2020 ). The acidic surface functional mass of hydrochar helps reduce soil pH and improve the adsorption capacity of hydrochar for NH 4 + , and N 2 O reductase is negatively correlated with pH so that N 2 O emissions can be reduced (Hou et al. 2020 ; Li et al. 2021a ). Hou et al. ( 2020 ) found that the effects of the application of sawdust hydrochar (pH = 3.71) and aging hydrochar (pH = 7.04) on N 2 O in low-fertility soils were consistent, and it was speculated that there might be a soil matrix effect. Xu et al. ( 2020 ) found that although hydrochar can increase the abundance of denitrifying bacteria, it can still reduce N 2 O emissions. Zhou et al. ( 2018 ) found that an essential factor that regulates soil N 2 O emissions is the availability of soil labile organic C (referred to as DOC) and inorganic N, which serve as the C source and energy for heterotrophic denitrifiers. The high content of DOC in hydrochar could favor the reduction of N 2 O to N 2 , resulting in a reduction in N 2 O emissions (Hou et al. 2020 ). Therefore, it can be assumed that the substrate (C/N) supply of C and N is the main factor that affects N 2 O emissions. Hydrochar application can increase the C content of soils, and while a higher C/N ratio in hydrochar correlates with a higher fixation of organic N in soils, thereby reducing mineralization and nitrification, this can lead to inhibition of denitrification in hydrochar-treated soils (Adjuik et al. 2020 ). At the same time, hydrochar contains humus (containing an amount of DOC), which can improve soil porosity, increase oxygen flux, and reduce N 2 O emissions (Li et al. 2021a ).

In general, hydrochar has shown substantial promise in reducing GHG emissions, and future research should focus on the effects of hydrochar on GHG emissions. More research is needed to elucidate the mechanism of interaction between hydrochar and GHG in the future to clarify how the relationships affect GHG emissions and to optimize the stability of hydrochar.

4.3 Effects of hydrochar on organic pollutant removal

Currently, hydrochar is mainly used in environmental sciences, especially for environmental remediation applications, due to the low material cost and desirable properties such as specific pore structure, abundant OFGs, and N-containing functional groups. Hydrochar is effective in adsorbing organic pollutants, such as pesticides (Eibisch et al. 2015 ; Liu et al. 2019 ), antibiotics (He et al. 2016 ; Nogueira et al. 2018 ), dyes (Islam et al. 2017b ; Vozhdayev et al. 2015 ), PAHs (de Jager and Giani 2021 ; Li et al. 2021b ), and estrogen (Bargmann et al. 2013a ; Yu et al. 2019 ). Due to its limited porosity and specific surface area, hydrochar is usually activated (Qian et al. 2016 ), acidized (Jiang et al. 2019 ; Nguyen et al. 2019 ), or magnetized after alkali activation to enhance adsorption (Liu et al. 2014 ; Zhu et al. 2014a , b , c ). Thermal activation of hydrochar can enhance the sorption of methylene blue (MB) (Buapeth et al. 2019 ) and bisphenol A (Yu et al. 2020 ) because of the increased specific surface area and porosity of hydrochar. The nitric acid modification process enhances OFGs and N-containing functional groups and unsaturated bonds on the surface of hydrochar, improving their adsorption capacity for MB (Nguyen et al. 2019 ). Alkali activation can increase the specific surface area and porosity of hydrochar and its adsorption performance (Islam et al. 2017a , b ). The alkali activation of a magnetic hydrochar composite with iron oxide enhances the adsorption capacity of hydrochar for triclosan, tetracycline, and malachite green (Liu et al. 2014 ; Zhu et al. 2014a , b , c ). It is common to use iron oxide or zero-valent iron loadings to modify hydrochar to improve its removal of organic pollutants (Kermani et al. 2019 ; Rahmi et al. 2019 ; Ye et al. 2020a ). Moreover, urea oxidation of hydrochar is an effective method for doping heteroatoms to form N-containing functional groups; the increased numbers of pyridinic-N, pyridonic-N, and graphitic-N are believed to help anchor organic molecules via a coupling effect between electron configuration and binding energy to improve adsorption capacity (Hou et al. 2021 ; Xiao et al. 2020 ).

Organic pollutants can be removed by adsorption to hydrochar through various mechanisms, including hydrophobic interactions, electrostatic interactions, partitioning interactions, π-π interactions, pore filling, and hydrogen bonding as shown in Fig.  8 (Ning et al. 2017 ; Tian et al. 2018 , 2019 ; Wei et al. 2020 ). Eibisch et al. ( 2015 ) studied the removal of isoproturon in agricultural soils using hydrochar and attributed adsorption to hydrogen bonding due to the low surface acidity and the abundant OFGs of hydrochar. A study by Liu et al. ( 2019 ) pointed out that the partitioning of atrazine to hydrochar may be the primary adsorption mechanism due to the rich C–OR bonds in hydrochar. Rattanachueskul et al. ( 2017 ) investigated the adsorption of tetracycline by magnetized hydrochar with iron loading and found that the adsorption was mainly based on hydrogen bonding interactions between the OFGs on the modified hydrochar and the –OH, C=O, and –NH 2 groups on the tetracycline molecule; in addition, the aromatic structure on hydrochar induced π-π electron donor-receptor (EDA) interactions, or cation-π bonding, and could be conjugated to the ring of the TC molecule. Hydrochar has a large specific surface area and a good microporous structure, which can also increase the hydrochar adsorption capacity for tetracycline (Chen et al. 2017b ). Leng et al. ( 2015 ) found that the adsorption capacity of rice husk hydrochar prepared with ethanol as solution for the cationic dye malachite green was significantly increased compared to the conventional method (water as solution) because the richer OFGs in hydrochar prepared with ethanol as solvent induce the stronger electrostatic interaction and hydrogen bonding between the negatively charged carboxylic acid anion and the positively charged malachite green. Tran et al. ( 2017b ) investigated the adsorption of the cationic dye methylene green on hydrochar; hydrochar formed a hydrogen bonding and n-π interactions with the N and O atoms of methylene green, and acrylic-modified hydrochar improved the surface OFGs of hydrochar and reduced the adsorption capacity; therefore, it was speculated that π-π interactions between the aromatic structure of hydrochar and the benzene ring of methylene green and pore filling dominated the adsorption. Li et al. ( 2018 ) investigated the anionic dye methyl orange adsorption on hydrochar modified with protonated amines; electrostatic interactions were the adsorption mechanism for methyl orange because protonated amines-modified hydrochar was positively charged. Surface hydrophobicity and the carbonyl groups in hydrochar are conducive to removing aromatic pollutants such as 2-naphthol through hydrophobic interactions and hydrogen bonding (Li et al. 2021b ). In addition, the high aromacity, hydrophobicity, porosity and low surface OFGs of hydrochar result in higher adsorption capacities for 1-butanol (Han et al. 2017 ).

figure 8

A schematic diagram of possible adsorption and degradation mechanisms of organic pollutants by hydrochar

Researchers have studied technology combinations to enhance the organic pollutant degradation capabilities of hydrochar as shown in Fig.  8 . It is mainly combined with advanced oxidation processes such as photocatalytic technology, Fenton-like technology, electrochemical catalytic technology, sonocatalytic technology, and persulfate (PS)/peroxymonosulfate (PMS) oxidation technology to increase the catalytic degradation capacity for organic pollutants of hydrochar.

The surface OFGs (O–C=O, C–O) of hydrochar can be used as a photosensitizer. Molecular oxygen can be activated by hydrochar under sunlight irradiation to generate a large number of reactive oxygen species (ROS), including superoxide anions (·O 2 − ) and hydroxyl radicals (·OH), to enhance the degradation of pollutants (Chen et al. 2017a ). Metal oxides (Ti, Zn, and Ag) modified hydrochar owns the higher light response, faster electron transfer, photogenerated carriers, and electron-hole pairs, thus enhancing the degradation of organic pollutants by generating ROS (holes, ·OH, and ·O 2 − ) (Leichtweis et al. 2021 ; Zhou et al. 2019 ). Ye et al. ( 2020b ) prepared a novel multiphase photocatalytic composite by combining hydrochar and FeAl layered double hydroxide (FeAl-LDH). The composites as electron-hole pairs, Fe in the LDH layer, and surface OFGs of hydrochar were found to facilitate the electron transfer process and generate more ·O 2 − , hydrogen peroxide (H 2 O 2 ), and ·OH. On the other hand, surface OFGs of hydrochar and hydrochar-derived DOM may undergo energy transfer to produce singlet oxygen ( 1 O 2 ) and form electron-transferred ·OH.

Fe oxide-modified hydrochar can improve the performance of Fenton-like oxidation technology. Liang et al. ( 2017 ) synthesized oxidized Fe-Zn hydrochar as a nonhomogeneous photo-Fenton catalyst. They found that oxidized Fe-Zn hydrochar can generate electrons and holes under visible light irradiation. H 2 O 2 traps electrons to produce ·OH; at the same time, Fe(II) and H 2 O 2 can also generate ·OH. As a result of forming iron (hydr)oxides on the surface of hydrochar ([email protected]), Liang et al. ( 2018 ) found that H 2 O 2 predominantly reacted with photo-generated holes in [email protected] rather than Fe(II)/(III) and photo-generated electrons to produce ·O 2 − .

Hydrochar can be combined with electrochemical oxidation to improve the ability to degrade 2,4-dichlorophenol. Electrochemical anodes can produce ·O 2 − and ·OH, and the surface OFGs of hydrochar can form persistent free radicals (PFRs), acting as electron donors, to produce ROS to remove pollutants (Cao et al. 2020 ). Khataee et al. ( 2021 ) pointed out that the prominent role of Cu oxide-modified hydrochar is to produce electron-hole pairs to facilitate the sonocatalytic dyes process, and oxidation of adsorbed water molecules or surface hydroxyl groups can produce ·OH and ·O 2 − . The increase in the specific surface area of Cu oxide-modified hydrochar can indirectly affect the sonocatalytic degradation capacity.

Hydrochar can activate PS/PMS and H 2 O 2 , generating ·SO 4 − and ·OH, improving the degradability of pollutants. The PFRs of Hydrochar are primarily responsible for the activation of PS/PMS, and its defective structure and graphite structure also promote ROS production (Wei et al. 2020 ). Hydrochar not only plays as a carrier for the dispersion of Co 3 O 4 nanoparticles but also acts as an electron shuttle between Co 3 O 4 and PMS/sulfamethazine molecules (Tian et al. 2020 ). Moreover, hydrochar can generate 1 O 2 through non-radical PMS activation because of its highly graphitized C domain with low polarity and enriched ketonic (C=O) functionality (Liu et al. 2021a ). Graphite-N in N-doped hydrochar can enhance the PFRs of hydrochar, as it not only promotes PMS activation in nonradiative pathways by showing strong electron transfer but it also promotes the formation of oxygen-centered PFRs by structural defects, increasing the ability of the product to degrade contaminants through free radical oxidation (Yu et al. 2020 ).

4.4 Effects of hydrochar on heavy metal adsorption and metal bioavailability

Heavy metal pollution has been the focus of attention in multiple areas, including environmental applications. Research on the applications of hydrochar for heavy metal immobilization increased dramatically between 2011 and 2021. Early articles focused mainly on the adsorption of heavy metals (Pb, Cu, Cd, Sb) by hydrochar or modified hydrochar in soil and water (Chen et al. 2015 ; Elaigwu et al. 2014 ; Sun et al. 2015 ; Xue et al. 2012 ). The heavy metal adsorption mechanisms by hydrochar can be broadly classified into the following four categories as shown in Fig.  9 : (1) electrostatic interactions between heavy metal cations and negatively charged minerals on hydrochar; (2) cation exchange between metal ions and mineral ions (e.g., K + , Na + , Ca 2+ , and Mg 2+ ) on hydrochar; (3) complexation of metals with functional groups (e.g., –OH, –COOH, –CHO, –C–) on the surface of hydrochar; and (4) complexation of heavy metals and mineral ions (e.g., SiO 3 2− , PO 4 3− , and CO 3 2− ) on hydrochar to induce precipitation.

figure 9

A schematic diagram of possible adsorption mechanisms of heavy metal by hydrochar

Several studies have aimed to increase the heavy metal removal capability of hydrochar by enhancing interactions between heavy metals and hydrochar through hybridization, acid and alkali treatments, or other modifications. For example, Tang et al. ( 2016 ) used Ni/Fe hybridized hydrochar to remove Pb(II), and adsorptive precipitation dominated Pb(II) removal by Ni/Fe hybridized hydrochar. Researchers have found that Pb adsorption to phosphoric acid-modified hydrochar is a synergistic process involving surface complexation, cation exchange, and partial precipitation because phosphoric acid-modified hydrochar is rich in surface acidic OFGs (–COOH) that can complex with Pb(II), while hydrochar also contains mineral ions that can be ion exchanged (K + , Na + , Ca 2+ , and Mg 2+ ) and can precipitate (CO 3 2− ) with Pb(II) (Zhou et al. 2017 ). The adsorption capacity of Cd on nitric acid-modified hydrochar increases 1.9–9.9 times relative to that on unmodified hydrochar, and the adsorption process involves electrostatic interactions, oxygen-containing complexation, ion exchange, and π-π interactions (Zheng et al. 2021 ). KOH-modified hydrochar increases Pb removal by a factor of 5 compared with unmodified hydrochar, demonstrating that the adsorption mechanism is mainly ion exchange followed by surface complexation (Petrovic et al. 2016 ). As a result of modifications with anaerobic fermentation, the adsorption capacity of Cd is enhanced by approximately 3.1–3.4 times, and the adsorption mechanisms include electrostatic attraction, ion exchange, and functional group complexation (Fu et al. 2021 ). A cyclodextrin-functionalized hydrochar results in 47.28 mg g −1 of Cd adsorption, with surface complexation and electrostatic interactions being the main mechanisms (Qu et al. 2021 ). In a study by Shi et al. ( 2018 ) the heavy metals Cr and Ni were removed from water using polyethyleneimine-modified hydrochar; the results showed that N-containing groups were the main adsorption sites.

Recent studies have investigated the co-adsorption of heavy metals and organic pollutants by hydrochar. For example, Guo et al. ( 2018 ) activated rapeseed shell hydrochar with KOH to remove MB and Cr from water and found that the modified hydrochar showed excellent pH tolerance. Li et al. ( 2019 ) investigated the adsorption of MB and Cu from water using polyamine carboxylated-hydrochar and found the adsorption process to have rapid kinetics (5 min) and large capacities (1239 and 141 mg g −1 , respectively). Surface complexation contributes to the adsorption of Cu(II) by amino and carboxylate groups, while π-π interactions, hydrogen bonding, and electrostatic attractions dominate the adsorption of MB.

Many researchers have studied the effects of hydrochar on the bioavailability of heavy metals in contaminated soils when used as a soil amendment since heavy metals cannot be easily degraded (Han et al. 2019 ). The bioavailabilities of heavy metals are essential indicators for evaluating their mobility and ecological impact, i.e., whether they can cause toxic effects on living organisms or are absorbed by them. To date, several studies have examined the effects of hydrochar on heavy metal mobility and bioavailability (Wagner and Kaupenjohann 2014 ; Wang et al. 2016 ; Watson et al. 2021 ). For example, heavy metals adsorbed to hydrochar are more stable than those in sewage sludge or manure feedstock, which reduces the bioavailability and environmental risks of heavy metals (Lang et al. 2018 ; Zhang et al. 2018 ). However, in addition to reducing the bioavailability of heavy metals through the hydrothermal carbonization process, hydrochar can directly adsorb heavy metals after it is applied to soils, or it can indirectly affect the bioavailability of heavy metals in soil by influencing soil properties.

Hydrochar reacts with soil heavy metals through adsorption, precipitation, complexation, and ion exchange after application. Heavy metals in soils bind to soil components in different manners, such as adsorption-desorption, dissolution-precipitation, complexation-dissociation, and oxidation-reduction, resulting in spatial migration, morphological changes, and varying bioavailabilities (Khan et al. 2021 ; Ren et al. 2018 ). Teng et al. ( 2020 ) found that soils amended with iron-modified rice husk hydrochar decreased mobility and bioavailability of the heavy metals Pb and Sb due to cation exchange, complexation, and precipitation. In contrast, the mechanisms controlling Sb immobilization in soils may be complexation, reduction, and electrostatic interactions. Moreover, iron-modified hydrochar can oxidize Sb(III) to Sb(V) and reduce the overall toxicity of Sb. Alkali-modified hydrochar is more effective than pristine hydrochar at reducing heavy metal bioavailability because it increases soil pH and forms precipitate with heavy metals (Xia et al. 2019 ).

As mentioned earlier, hydrochar can indirectly enhance the immobilization and adsorption of heavy metals in soils through soil properties. Adding hydrochar to soils reduces heavy metal bioavailability due to changes in soil properties, as Fei et al. ( 2019 ) showed. Soil pH and cation exchange capacity (CEC) affect the adsorption and transport of heavy metals; low pH values usually indicate that heavy metals are more easily leached, and high pH values induce heavy metal precipitation in the soil, thereby reducing the migration of heavy metal ions; in soils with high CEC, heavy metals adsorb more readily on cation exchange sites (Fei et al. 2019 ; Xia et al. 2020 ). It has been reported that amino-functionalized hydrochar derived from pinewood sawdust at 200 °C increased soil CEC by 8% and soil organic matter by 59.6%, while also lowering the bioavailability of heavy metals in plants by 45.9–52.5% (Xia et al. 2020 ). Therefore, hydrochar can reduce soil heavy metals bioavailability and relieve soil stress.

4.5 Effect of hydrochar on anaerobic digestion

Note that the effect of hydrochar on anaerobic digestion is a popular topic and is gaining increasing attention (Fig.  7 b). Anaerobic digestion is widely used and effectively adds value to high-moisture wastes, such as food, organic residues, and sewage sludge since it can convert these products into energy. However, the limitations of anaerobic digestion usually stem from recalcitrant substrates when utilizing wastes. A significant problem is the quality of produced digestion residues (digestate) since digestion residues are often used as soil conditioners (Codignole Luz et al. 2018 ). There has been growing interest in using hydrochar in waste anaerobic digestion since it increases the value of anaerobic digestion and improves environmental and agronomic benefits. Many studies systematically assessed the advantages of hydrochar for the anaerobic digestion of wastes, including its effect on accelerated organic matter solubilization and hydrolysis (Ren et al. 2020 ; Wang et al. 2017 ), altering microbial diversity (Usman et al. 2020 , 2021 ), reduced N loss (Zhang et al. 2020a ), and reduced phytotoxicity (Celletti et al. 2021 ; Usman et al. 2020 ; Xu et al. 2018 ).

The humic acid in hydrochar accelerates the solubilization and hydrolysis of organic matter during anaerobic digestion. Humic acid functions as a surfactant, facilitating the dissolution of organic matter and hydrolysis, and competes for electrons in the reaction system with methanogenic bacteria, inhibiting their activity (Wang et al. 2017 ). The surface OFGs of hydrochar can mediate direct interspecific electron transfer and promote anaerobic digestion (Ren et al. 2020 ), and alter microbial diversity and abundance (Geobacter) (Usman et al. 2021 ). Additionally, hydrochar can degrade aromatic and phenolic compounds, contributing to the enrichment of methanogenic bacteria in anaerobic digestion (Usman et al. 2020 ). As a result, hydrochar can adsorb ammonium (NH 4 + ) during anaerobic digestion and lower the inhibitory effect of NH 4 + on anaerobic digestion; also, NH 3 emissions from anaerobic digestion can be reduced, thereby reducing atmospheric pollution (Zhang et al. 2020a ). Appropriate acidification and saline leaching treatment of hydrochar will reduce the potential phytotoxic effects of combined alkalinity and high electrical conductivity (Celletti et al. 2021 ). As a result, hydrochar is an ideal biological substrate for reducing the time required for the anaerobic digestion process and for increasing its value.

4.6 Comparison of hydrochar and pyrochar in environmental applications

Biochar is a solid carbon-rich product produced from biomass using various thermochemical methods, e.g., pyrolysis, dry torrefaction, gasification, and hydrothermal carbonization (Liu et al. 2021b ). Generally, pyrochar refers to pyrolytic carbonization products (300–700 °C), and hydrochar refers to hydrothermal carbonization products (180–260 °C). There are numerous applications for both types of biochar, including soil amendment and plant growth, CO 2 capture and GHGs emissions, organic pollutant removal, heavy metal adsorption, and reducing heavy metal bioavailability. Pyrochar and hydrochar differ significantly in terms of physicochemical properties as a result of different preparation methods. Compared with pyrochar produced at the typical temperature range, hydrothermal carbonization has lower dehydration and ash to liquid phase process, resulting in a lower C, ash, and pH content in hydrochar (Fang et al. 2018 ). A higher production temperature results in a higher specific surface area and larger pore size in pyrochar (Liu et al. 2021b ), whereas pyrochar is richer in heavy metals and alkaline earth metals than hydrochar, while its H/C ratio and O/C ratio are lower (Kumar et al. 2020 ; Shen et al. 2019 ). As a result, the performance of hydrochar and pyrochar in environmental applications differs.

As a soil amendment, pyrochar can be used on acidic soils to enhance pH improvement, while hydrochar can be used on alkaline soils to reduce pH (Zhang et al. 2019 ). Additionally, hydrochar outperformed pyrochar in improving soil aggregates since soil aggregates correlated positively with the H/C and O/C ratios (George et al. 2012 ). Contrary to pyrochar, hydrochar was not as effective as pyrochar in improving soil water holding capacity because hydrochar is unstable and biodegradable, and fungal fixations create hydrophobicity (Fang et al. 2018 ). The decomposition of char leaching into the soil can negatively affect plant growth when hydrochar or pyrochar is applied (Lang et al. 2019a ). Pyrochar has a higher heavy metal content than hydrochar, so that it may be more phytotoxic than hydrochar because of its high heavy metal content (Shen et al. 2019 ). In contrast, pyrochar has a larger specific surface area and is more stable than hydrochar, and contains alkaline functional groups (Liu et al. 2021b ); Therefore, pyrochar can better sequester carbon and capture CO 2 than hydrochar in soil. In addition, hydrochar provides fertile substrates for soil microorganisms (such as methanobacteria and actinomycetes ), which results in enhanced microbial activity, which may contribute to GHG emissions due to its instability and ease of decomposition (Ji et al. 2020 ). Consequently, washing treatment of hydrochar (Chen et al. 2021 ) or alkali modification (Cui et al. 2021 ; Jiang et al. 2020 ; Shi et al. 2021a ) is a good way to improve the CO 2 capture capacity of hydrochar and reduce GHG emissions.

Organic pollutants and heavy metal pollutants can be significantly reduced when used as adsorbents by hydrochar and pyrochar. The functional units of action and adsorption mechanisms of hydrochar and pyrochar are interchangeable in removing organic and heavy metal pollutants from the environment (Wu et al. 2019 , 2020 , 2021b ; Zhang et al. 2019 ). Despite this, the main adsorption mechanisms or their applicability to different organic and heavy metal pollutants differ due to the different content of functional units in the two types of biochar. In the case of toluene, an organic pollutant that is removed by surface adsorption and pore filling, pyrochar has a higher specific surface area, porosity, and aromatization, so it is more efficient at removing toluene than hydrochar (Liu et al. 2021b ). In the case of PAH organic pollutants, pyrochar significantly removes them by pore filling and interacting with N-heterocyclic structure (Liu et al. 2021b ). Hydrochar, on the other hand, has a predominant interaction on PAH organic pollutant removal via hydrophobic interaction (Li et al. 2021b ). In the case of the positively charged heterocyclic organic pollutants, hydrochar has a much higher adsorption capacity than pyrochar because of its rich surface OFGs. Hydrochar not only produces electrostatic effects on organic pollutants but also contains hydrogen bonding effects (Kambo and Dutta 2015 ). In the case of heavy metals, pyrochar has a large specific surface area and porous structure, and is generally alkaline, which make pyrochar a good material to remove heavy metals by electrostatic interaction, ion exchange, and precipitation; hydrochar has an advantage over pyrochar when it comes to heavy metal pollutant adsorption due to its abundant OFGs, which can easily interact electrostatically and complexity with heavy metals (Liu et al. 2021b ).

5 Conclusion

This study provided a comprehensive scientometric review of the trends in research on environmental applications of hydrochar from 2011 to 2021. An overview of the present state of research in hydrochar was provided, along with some significant conclusions. First, keyword clustering analysis revealed that the main research areas for the environmental applications of hydrochar were “soil quality and plant growth”, “carbon capture and greenhouse gas emissions”, “organic pollutant removal” and “heavy metal adsorption and its bioavailability” and that research on hydrochar has diversified over time. Second, the emergence of the term anaerobic digestion was noted; in addition to the major popular topics, hydrochar and anaerobic digestion have received increasing attention. The development of these research topics will facilitate the synthesis of hydrochar-based functional materials. Finally, although hydrochar is widely used in environmental applications, the increasing number of hydrochar-related publications each year indicates that hydrochar is still a major research topic that needs further development. Because mass engineering and production of hydrochar are approaching quickly, more attention should be given to the following aspects.

6 Possible further trends

The potential environmental risks of hydrochar should be concerned during its application processes. The biological toxicity and its effects on crop growth are not addressed, especially the methods involved in eliminating or mitigating the toxic effects of hydrochar by modifying and applying it with an appropriate application ratio. In addition, the increased reactivity of hydrochar may affect their risk concerns. A careful evaluation of the scope/conditions of hydrochar applications and the environmental effects of hydrochar applications on soils in different regions under various climate conditions is a fundamental requirement before their wide and environmentally friendly applications.

The effects of hydrochar on different GHG emissions require further comprehensive study. Hydrochar should also be considered in terms of its relationship with soil microorganisms to clarify the impacts of this relationship on GHG emissions. Moreover, the stability of hydrochar should be considered in GHG emissions. A better understanding of the available active sites and the properties of hydrochar is needed to enhance CO 2 capture capacity of hydrochar. Hydrochar helps promote anaerobic digestion by accelerating the solubilization and hydrolysis of organic materials and reducing N loss, GHG emissions, and organic pollutants. Future research will focus on optimizing the physicochemical properties of hydrochar, enhancing its economic benefits, and coupling hydrochar with anaerobic digestion technology to treat toxic and hazardous pollutants.

The physicochemical properties of hydrochar should be determined before its application because the application performances of hydrochar are diverse due to the different raw materials, processes, and modifications. More research needs to be conducted to select the optimal conditions and to expand the application range of hydrochar to improve its performance. The establishment of a database that records in detail the environmental applications of hydrochar from different sources and under various preparation conditions would provide a valuable theoretical basis for the development and application of hydrochar in the environmental field.

Data availability

All data generated or analyzed during this study are included in this published article and its Additional files.

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We thank Ms. Mengting Ge for assisting with the graphs revision.

This work was funded by the National Natural Science Foundation of China (Nos. 42107398, 42277332 and 42207453), Natural Science Foundation of Jiangsu Province (BK20210358 and BK20221428), Ecological Environment Research Project of Jiangsu Province (Policy Guidance 2021006), and China Postdoctoral Science Foundation (2020M68618). Y. F. Feng thanks the support of “333” High-level Talents Training Project of Jiangsu Province (2022-3-23-083).

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Junxia Huang and Yanfang Feng contributed equally to this paper

Authors and Affiliations

Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, School of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing, 210094, China

Junxia Huang, Huifang Xie, Bingyu Wang & Qiang Zhang

Key Laboratory of Agro-Environment in Downstream of Yangtze Plain, Ministry of Agriculture and Rural Affairs, Institute of Agricultural Resources and Environment, Jiangsu Academy of Agricultural Sciences, Nanjing, 210014, China

Yanfang Feng

Centre for Agricultural Resources Research, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Shijiazhuang, 050021, China

Faculty of Civil Engineering and Mechanics, Kunming University of Science and Technology, Kunming, 650500, China

Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3), Department of Environmental Science and Engineering, Fudan University, Shanghai, 200433, China

Shicheng Zhang

Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China

Zhengang Liu

University of Chinese Academy of Sciences, Beijing, 100049, China

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Data collection and analysis were performed by Junxia Huang and Yanfang Feng. Bingyu Wang and MinLi Wang as the corresponding author contributed to the conception, supervision and funding of the study. The first draft of the manuscript was written by Junxia Huang and Bingyu Wang. All authors read and approved the final manuscript.

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Correspondence to Minli Wang or Bingyu Wang .

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Handling Editor: Jun Meng.

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Additional file 1: figure s1..

A scheme to illustrate the subject categories of hydrochar in environmental applications. Figure S2. A scheme to illustrate the journal categories of hydrochar in environmental applications. Figure S3. A scheme to illustrate the institutional and their cooperation relationships of hydrochar in environmental applications. The use of co-authored articles as part of academic collaboration is an important method of collaborating on research projects. Hence, it is crucial to know how academics communicate, as well as their institutions. Institutions collaborated on hydrochar in environmental applications between 2011 and 2021. Figure S4. A scheme of the most cited author contributed to hydrochar in environmental applications. A high co-citation frequency shows that peers recognize the author’s expertise and knowledge. Figure S5. The keywords network map of hydrochar in environmental applications during 2011–2016. Table S1. Top 10 countries with publications of hydrochar in environmental applications during 2011–2021. Table S2. Top 10 institutions with publications of hydrochar in environmental applications during 2011–2021. Table S3 . Top 10 cited literature of hydrochar in environmental applications during 2011–2021. Table S4. Keyword clustering information of hydrochar in environmental applications during 2011–2021.

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Huang, J., Feng, Y., Xie, H. et al. A bibliographic study reviewing the last decade of hydrochar in environmental application: history, status quo, and trending research paths. Biochar 5 , 12 (2023). https://doi.org/10.1007/s42773-023-00210-4

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Received : 31 October 2022

Revised : 02 February 2023

Accepted : 07 February 2023

Published : 13 March 2023

DOI : https://doi.org/10.1007/s42773-023-00210-4

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  23. A bibliographic study reviewing the last decade of hydrochar in

    2.1 Data collection and processing. Data collection for this study was based on the Web of Science (WoS) core collection database. The search formula [TI = ("hydrochar" or "hydro-char") or TI = ("biochar" and "hydrothermal carbonization") or TI = ("biochar" and "hydrothermal liquefaction")] were used in the data collection process with a search period from January 1 ...