Global perspectives for biochar application in the remediation of heavy metal-contaminated soil: a bibliometric analysis over the past three decades

Abstract Herein, 7,308 relevant documents on biochar application for the remediation of heavy metal (HM)-contaminated soil (BARHMCS) from 1991 to 2020 were extracted from the Web of Science Core Collection and subjected to bibliometric and knowledge mapping analyses to provide a global perspective. The results showed that (1) the number of publications increased over time and could be divided into two subperiods, i.e., the slow growth period (SGP) and rapid growth period (RGP), according to whether the annual publication number was ≥300. (2) A total of 126 countries, 741 institutions, and 1,021 scholars have contributed to this field. (3) These studies are mainly published in Science of the Total Environment, Chemosphere, etc., and are mainly based on the categories of environmental science, soil science, and environmental engineering. (4) The top five keyword clusters for the SGP were biochar, biochar, sorption, charcoal, and HMs, and those for the RGP were adsorption, black carbon, nitrous oxide, cadmium, and pyrolysis. (5) The main knowledge domains and the most cited references during the SGP and RGP were discussed. (6) Future directions are related to biochar application for plant remediation, the mitigation of climate change through increased carbon sequestration, biochar modification, and biochar for HMs and multiple organic pollutants. NOVELTY STATEMENT Biochar application in the remediation of heavy metal-contaminated soil (BARHMCS) has become a popular research topic worldwide. Many excellent papers on this topic have been published, including some valuable reviews. However, there are no reviews including bibliometric and visual analyses. In the present study, bibliometric and visual analyses of relevant literature in the field of BARHMCS based on the Web of Science Core Collection were carried out to outline the development process of this field at a macro level, clarify the research hotspots, identify the knowledge domains that support this field, and explore future research directions. These efforts will no doubt help readers fully understand BARHMCS from a global perspective and provide a reference for future research. HIGHLIGHTS An overall global perspective of biochar remediation of heavy metal (HM)-contaminated soil was provided. The main popular research topics of each period were discussed. Knowledge domains were discussed. Five main future research directions were identified based on burst keyword analysis. Biochar modification and its effect on HMs and coexisting organic pollutants should be studied in the future for soil remediation purposes.


Introduction
Soil heavy metal (HM) pollution is a global environmental problem (Qin et al. 2019;Liu et al. 2020aLiu et al. , 2020b. According to previous reports, approximately 50,000 ha of forest, 55,000 ha of pasture, and 100,000 ha of farmland in Western Europe, the United States and China are polluted by HMs (Murtaza et al. 2017). The Soil Pollution Investigation Bulletin published in China showed that its total soil pollution rate was 16.1% (Ministry of Environmental Protection & the Ministry of Land and Resources 2014). The over-standard rates of cadmium (Cd), arsenic (As), copper (Cu), mercury (Hg), and lead (Pb) reached 7.0%, 2.7%, 2.1%, 1.6%, and 1.5%, respectively (Ministry of Environmental Protection & the Ministry of Land and Resources 2014). Soil HM pollution is a major hazard for the soil system, soil-plant system, and human health and has caused concern worldwide. In China, especially after the introduction of the Action Plan for Soil Pollution Prevention and Control in 2016, the overall prevention and control of HM pollution across the country have been strengthened, which means that China's efforts to remediate HM-contaminated soil have further increased (Chen et al. 2018); at the same time, research on repair methods and materials has become more in depth.
In recent years, biochar (biomass charcoal, black charcoal) has been favored by researchers because of its characteristics of improving soil physical and chemical properties and reducing the risk posed by soil HMs (Ghosh and Maiti 2020;Rathika et al. 2021;Conesa and Parraga-Aguado 2022;). It may become a novel alternative over conventional approaches due to its cost effectiveness, sustainability, and environmental friendliness (Chausali et al. 2021). Biochar is made from biomass such as straw, shell residue, wood, and domestic waste under anoxic or anaerobic conditions through pyrolysis at different temperatures (Chen et al. 2019b). It is a relatively stable solid substance characterized by a high specific surface area, high porosity, high cation exchange, and abundant surface functional groups, among other features, which are favorable in the remediation of HM-polluted soil (Chen et al. 2019b;Rathika et al. 2021;Conesa and Parraga-Aguado 2022;). Previous research has mainly focused on biochar preparation; the effect, process, and mechanism of biochar inactivation of soil HMs; and the effect of biochar on the transformation and content of HMs in the soil-plant system (Liu et al. 2015;Zhao et al. 2017;Liang et al. 2021;Conesa and Parraga-Aguado 2022;). Thousands of reports on biochar application in the remediation of HM-contaminated soil (BARHMCS) have been published, including some excellent reviews (Chen et al. 2019b;Lian et al. 2020;Chausali et al. 2021;Mounissamy et al. 2021). Although scholars have shown increasing interest in biochar in recent years, it has been difficult to perform traditional literature reviews in this field over a long time scale, given the thousands of available reports. Therefore, a systematic and in-depth analysis of these studies from a new perspective is urgent and necessary.
Bibliometric analysis is a scientific method used to quantitatively explore the development of a specific research field (Han et al. 2020;. Knowledge mapping is a tool for the quantitative analysis of literature using mathematical and statistical methods. It is a comprehensive system that integrates mathematics, philology, and statistics. Among the knowledge mapping analysis software packages, CiteSpace Java-based visualized software explored by Chen (PhD) is popular Guan et al. 2021;. With the help of bibliometric theory and knowledge graph analysis, the development context and cooperative relationships of a particular research field can be clearly presented, and its research hotspots, knowledge base, and future research trends can be explored (Guan et al. 2021;Niu et al. 2021;Wu et al. 2021). Scientific knowledge-based graph analysis is mainly used in research fields such as informatics, medicine, psychology, and management. In recent years, it has also quietly emerged in the fields of agriculture, forestry, ecology, and the environment (Han et al. 2020;Guan et al. 2021;).
In the present study, a bibliometric and visual analysis of the relevant literature in the field of BARHMCS based on the Web of Science Core Collection (WoSCC) was carried out to outline the development process of this field at a macro level, clarify the research hotspots, identify the knowledge domains that support this field, and explore future research directions. These efforts will no doubt help readers fully understand BARHMCS from a global perspective and provide a reference for future research.

Database and search strategy
The relevant bibliographic records related to BARHMCS from 1991 to 2020 were retrieved from the WoSCC database, a comprehensive research database that is the most frequently used for knowledge mapping analysis Chen 2017;Guan et al. 2021; OR "Black carbon" OR "Biomass charcoal") AND (soil Ã )), "document type" ¼ "Article", and "language" ¼ "English". A total of 7,308 bibliographic records and 156,164 references were obtained. In the present study, the research time scale was artificially divided into two periods, i.e., the slow growth period (SGP) and the rapid growth period (RGP), according to whether the number of publications was !300.

Research framework and visualization software
The research framework (Figure 1) was modified according to our previous paper . The visualization software CiteSpace 5.7 R5 was used in the present study. First, the publication characteristics (including publication numbers; collaborations between countries/regions, institutions and authors; journals; and disciplines) of 7,308 bibliographic records were analyzed by CiteSpace software combined with WoSCC's analysis report. Then, the research hotspots in the SGP and RGP were discussed. After that, the knowledge domains in these two periods were analyzed based on the 156,164 references, and the top three most cited references in the SGP and RGP were examined in detail to determine the main knowledge domains, as well as the vital references supporting research on BARHMCS. Finally, all the burst keywords that continued to 2020 were classified and discussed to comprehensively analyse future directions.

Parameter setting and knowledge mapping
The parameter set was the same as that in our previous studies (Guan et al. 2021;. The time slice was set to 1 year. The top 50 keywords with the highest cooccurrence frequency (top 50) were extracted from each time slice. The network mapping for a specific object was generated by the g-index algorithm (k ¼ 25, LRF ¼ 3, LBY ¼ À1, and e ¼ 1.0). 1 K-core clustering was applied to generate clusters. Cluster labels were automatically extracted by the LLR and are shown in white in the mapping.
In the network mapping, one node represents an analysis object. The node size represents the frequency. A line connecting two nodes indicates that there is a relationship between these two nodes. The count frequency (CF) and betweenness centrality (BC) of these nodes were used to assess the importance of each node in the mapping. A node with a high BC is likely to be situated between two large communities or subnetworks; hence, the term betweenness, which is surrounded by a purple circle, has a relatively high BC value (!0.1), indicating its vital role in mapping and extensive connections to other nodes (Chen 2004(Chen , 2017Yang and Meng 2019). Burst strength (BS) was used to assess the strength of the burst keywords, i.e., where more attention was given in a short time (Chen 2004(Chen , 2017Guan et al. 2021;.

Publication numbers
The annual change in the number of publications can show the degree of academic attention given to a research field, which can indicate the development and evolution of the research field and reflect the overall trend in this field to a certain extent (Xiao et al. 2020;Guan et al. 2021;Wang et al. 2021b). Figure 2 shows that in the past 30 years, the number of publications in the BARHMCS research field has conformed to a cubic regression model 2 (y ¼ 0.008x 4 -0.2903x 3 þ 3.6925x 2 -16.449x þ 19.182, R 2 ¼ 0.9983), indicating that this field has received increasing attention, especially in recent years. The number of publications on BARHMCS increased from 1 in 1992 to 1,487 in 2020. The documents were distributed among the journals Science of the Total Environment, Chemosphere and Environmental Science and Pollution Research, among others, over the whole period.
The SGP . In the SGP, the percentage of publications was less than 20%, accounting for only 16.30% of all publications. The most cited reference was the paper titled Persistence of soil organic matter as an ecosystem published by Schmidt et al. (2011) in Nature in 2011, with 2,668 citations. This paper was also the most cited paper in the whole period from 1991 to 2020. The documents in this period were mainly published in the journals Environmental Science Technology, Chemosphere, Organic Geochemistry, Journal of Environmental Quality, and Geoderma.
The RGP (2014-2020). In the RGP, the percentage of publications was over 80%, accounting for 83.70% of all publications. The annual number of documents increased from 435 in 2014 to 1,487 in 2020, with an average value of 874 per year, which was 16.81 times that in the SGP, indicating that this field received a large amount of attention from the international academic community and developed rapidly in this period. Publications in this period were mainly published in the journals Science of the Total Environment, Environmental

Publication contributions
Countries/regions. The number of publications is an important index for measuring the contribution and academic status. In the present study, the publication contributions as well as the top 10 countries/regions, institutions and associated countries, authors, journal sources, and disciplines were analyzed (Table 1). A total of 126 countries have published related research. The top 10 publishing countries are China, the United States, Germany, Australia, Spain, Pakistan, Canada, South Korea, Brazil, and Italy. Among them, China ranked first, with a total of 2,778 articles published during the whole research period, accounting for 38.01% of all articles, 1.81 times and 4.64 times the number from the United States and Germany, which ranked second and third, respectively. These results indicated that Chinese scholars pay close attention to this field and conduct much research, which reflects China's academic level and status in this research field. The BC value analysis showed that although the number of documents from the United States was lower than that from China, the United States had the largest BC value of 0.23, followed by Germany (BC ¼ 0.21) and France (BC ¼ 0.17), indicating that these countries have had an important academic influence in the field of BARHMCS. This conclusion is further supported by the country/region cooperation network map ( Figure S1). The United States is the largest node in the country/region network map, with an obvious purple outer circle, indicating that it had close cooperation with other countries in the field of BARHMCS and occupies an important position in this field.
Institutions. Analysis of institutional contributions can help readers understand which institutions are important in a certain field (Chen et al. 2020). A total of 741 institutions worldwide have contributed to the study of BARHMCS, and 11 institutions have published more than 100 papers. They are located in China (6), the United States (3), Spain (1), and Pakistan (1) ( Table 1). Among these institutions, the top three are all in China. The Chinese Academy of Sciences (Chinese Acad Sci) ranked first, with 605 papers, accounting for 8.28% of all papers. The second and third institutions were Zhejiang University (Zhejiang Univ, 212 papers, 2.90%) and University of Chinese Academy of Sciences (Univ Chinese Acad Sci, 200 papers, 2.74%). In addition, Nanjing Agricultural University (Nanjing Agr Univ), Northwest A&F University (Northwest A&F Univ), and Chinese Academy of Agricultural Sciences (Chinese Acad Agr Sci) each published more than 100 papers. As shown in Figure S2, the Chinese Academy of Sciences had the highest BC value of 0.34, indicating that it has close academic exchanges and cooperation with domestic and foreign research institutions, followed by Cornell University (BC ¼ 0.14).
Authors. Similarly, analysis of author contributions can help readers quickly understand who the major scholars in a certain research field are and what work they do and then promote academic exchanges and cooperation between scholars (Chen 2004;Guan et al. 2021). A total of 1,021 scholars worldwide have contributed to this field of research. The author co-collaboration network map is shown in Figure S3. Among the top 10 contributing scholars, Sik Ok Yong from Korea University ranked first, with 141 papers, accounting for more than 40% of the total publications in South Korea. This is 1.76 times the number for Johannes Lehmann (who ranked second) and 2.01 times that of Daniel C W Tsang (who ranked third). In addition, Hailong Wang (53) from Foshan University of Science and Technology and Genxing Pan (49) from Nanjing Agricultural University are prolific authors in China. All these authors have a strong global influence in the field of BARHMCS.

Publication journals
Analysing related journals can help readers understand the mainstream journals of a specific research field and quickly search for relevant literature. It can also help researchers identify target journals as they submit their own research manuscripts. The top 10 journals all had more than 100 articles and accounted for 32.12% of all articles ( Figure 2 and Table 1). The journal Science of the Total Environment ranked first, with 552 documents, accounting for 7.55% of all documents, followed by Chemosphere (334, 4.75%) and Environmental Science and Pollution Research (323, 4.42%).

Publication disciplines
The study of BARHMCS covered 100 disciplines over the past 30 years. The top 10 disciplines included environmental science, soil science, environmental engineering, agronomy, energy and fuel, botany, chemical engineering, water resources, earth science, biotechnology, and applied microbiology (Table 1), indicating that BARHMCS is a multidisciplinary research field. Among these fields, there were 3,769 papers on environmental science, accounting for 51.57% of all papers. Soil science and environmental engineering accounted for 18.42% and 11.33%, respectively.

Research hotspots
The visualization software CiteSpace was used to analyze keyword clusters in the SGP and the RGP, to explore the research hotspots in each period, and to understand the development context and evolutionary trend of BARHMCS.

Research hotspots in the RGP
The keyword cluster network map of the RGP is shown in Figure 4. There were 873 nodes and 7,175 lines. Similarly, the top five keyword clusters were identified. The top 25 keywords in each cluster are listed in Table S1. The largest cluster was #0 Adsorption, including 134 keywords, and the average year was 2016, including "soil" (CF ¼ 1,682), "adsorption" (CF ¼ 886), "sorption" (CF ¼ 771), "water" (CF ¼ 590), and "removal" (CF ¼ 531). This cluster mainly concerned the adsorption, desorption, application, and mechanism of biochar in relation to pollutants. The cluster "Black carbon" ranked second, with 131 keywords and an average year of 2015. The top five CF keywords were "black carbon" (CF ¼ 1,064), "organic matter" (CF ¼ 529), "microbial community" (CF ¼ 236), "mineralization" (CF ¼ 215), and "microbial bioma" (CF ¼ 173). The topics in this cluster were mainly related to the mineralization and decomposition of biochar in relation to soil organic matter and its priming effect. In fact, most pollution is a mixture of organic matter and HMs. Therefore, "organic matter" emerged in the RGP. To date, research on the compound pollution of HMs and organic matter has not been sufficiently advanced, and scholars worldwide have not put forward a clear concept to support clear classification. Complex pollution is different from single-source pollution; there are interactions between pollutants, and the environment where they are located is very likely to change; thus, they deserve in-depth study in the future.
The third largest cluster was #2 Nitrous oxide, with an average year of 2015, including 126 keywords, such as "charcoal" (CF ¼ 634), "impact" (CF ¼ 526), "nitrogen" (CF ¼ 516), "yield" (CF ¼ 395), and "manure" (CF ¼ 362). The effects of biochar application on soil fertility and crop yield were the main themes in this cluster. There were 117 keywords in cluster #3 Cadmium, with an average year of 2016. The main keywords were "biochar" (CF ¼ 3,355), "heavy metal" (CF ¼ 1,000), "amendment" (CF ¼ 769), "bioavailability" (CF ¼ 590), and "cadmium" (CF ¼ 483). The bioavailability of biochar to HMs, especially Cd, and the soil remediation efficiency of biochar have attracted much attention in this cluster. Cadmium is a nonessential element in organisms and has attracted extensive attention, especially in China. Our previous study showed that biochar amendment significantly (p < 0.05) decreased soil available Cd and plant Cd uptake by 23.2% and 40.0%, respectively (Liang et al. 2021). A meta-analysis performed by Nkoh et al. (2022) also showed that the biochar application rate varied from 1 to 10% and corresponded to a pH change of À0.30 to 3.44 units, a cation exchange capacity (CEC) change of À17.2 to 140 mmol kg À1 , and a pH buffering capacity (pHBC) change of 1.0 to 198.5 mmol kg À1 pH À1 , resulting in large HM reductions in plants, with values of 26.2% for Cd, 25.8% for Cu, 56.0% for Cr, 41.5% for As, 3.03% for Pb, 18.3% for Zn, 33.0% for N, and 22.8% for Mn. These results indicated that the addition of biochar to soil can enhance HM remediation efficiency and reduce plant uptake.

Knowledge domains
The development of a specific field often needs to be based on the relevant research foundation of the previous period. Therefore, analyzing the co-citations of the documents can reveal the knowledge domains supporting the research topic ). In the present study, both the general knowledge domains and the top three cited references in the SGP and RGP were analyzed to determine which knowledge domains supported research on BARHMCS and which cited references played a vital supporting role.
Knowledge domains in the SGP General knowledge domains in the SGP. The co-citation map for BARHMCS in the SGP had 886 nodes and 5,366 lines and formed seven main knowledge domains, i.e., #0 Black carbon, #1 Sorption, #2 Biochar, #3 Black carbon, #4 Reuse, #5 Los Angeles, and #6 Organic matter. The node sizes were 189, 179, 164, 99, 68, 34, and 31, with average years of 2000, 2007, 1997, 2009, 1990, 1984, and 1991, respectively. Among them, the cluster labeled #1 Sorption was a relatively new knowledge domain, which was consistent with the keyword cluster with the same name, indicating that this domain knowledge well supported the study topic in the same period (Figures 3 and 5 and Tables S1 and S2). The cluster #0 biochar (1997) in the co-citation map was 14 years earlier than the keyword cluster (2000), indicating that the previous foundation and classic references about biochar provided strong support for the subsequent application of biochar in HM remediation.
Top three cited references in the SGP. Information on the top three cited references in the SGP is listed in Table 2. The most cited reference was a review paper by Schmidt and Noack (2000;CF ¼ 208) published in the journal Global Biogeochemical Cycles in 2000 entitled "Black carbon in soils and sediments: Analysis, distribution, implications, and current challenges." This paper pointed out that black carbon formed by the incomplete combustion of plants and fossil fuels is ubiquitous in nature, which is of great significance for understanding the cycle of biomass charcoal in biology, geochemistry and environmental processes, and the challenges for future research on biochar were also discussed. The second most cited reference was also a review paper. The paper by Glaser et al. (2002;CF ¼ 198) was published in the journal Biology and Fertility of Soils in 2002 and entitled "Ameliorating physical and chemical properties of highly weathered soils in the tropics with charcoal: A review." Considering that charcoal is the cause of the high organic matter content and soil fertility of anthropogenic soil (terra preta), this article reviews the relevant literature and finds that the addition of charcoal has a positive impact on soil fertility and crop yields, such as improving soil nutrient availability, soil nutrient retention, soil water maintenance, and soil structure stability. All these factors were attributed to the higher exchange capacity, larger surface area, and nutrient supplementation of biochar. Furthermore, within a certain temperature range, a higher carbonization temperature usually improves the exchange performance and surface area of biochar. These two review papers provided comprehensive knowledge on biochar and provided a theoretical basis for understanding and applying biochar in the remediation of HM-contaminated soil. The third most cited reference, entitled "Biochar sequestration in terrestrial ecosystems: A review," was published by Lehmann et al. (2006) in 2006 in the journal Mitigation and Adaptation Strategies for Global Change. This article proposed that biochar is a new type of soil carbon sink and is expected to become an important and long-term storage medium for atmospheric carbon dioxide in terrestrial ecosystems. In addition, consistent with the research findings of Glaser et al. (2002), which ranked second in terms of citation rate, the paper also  reported that the production of biochar and its application to soil will improve soil fertility and increase crop yields. This study provided solid theoretical support for comprehensively understanding the characteristics of biochar and its improvement of soil properties, increase in crop yields, effect on the carbon dioxide cycle and mechanism of action. It also provided solid theoretical support for the use of biochar in HM-contaminated soil remediation.
In this period, the largest cluster, #0 cadmium, was also the newest cluster, with an average year of 2013. It included 230 keywords. Similarly, the keyword cluster #3 cadmium was the most recently emerging cluster (2016) (Figure 4 and Table S1). These results indicated that the cluster #0 cadmium in the co-citation network map well supported the research hotspots related to biochar application to Cd-contaminated soils in recent years. In addition, cluster #7 mycorrhizal provided more support for studying the changes in soil microbiological properties with biochar supplementation in this period (Wang et al. 2018, 2;Gao et al. 2021;Shao et al. 2021) top three cited references in the RGP.
In this period, the most cited reference was by Lehmann et al. (2011), entitled "Biochar effects on soil biota: A review" (CF ¼ 1,078) and published in the journal Soil Biology & Biochemistry. This paper comprehensively expounds on the relationship between biochar and soil property improvement, the response of soil microorganisms to biochar supplementation, and the management and risks of soil biochar. This study provides highlights and theoretical support for biochar amendment of HM-contaminated soil. The second most cited reference was by Ahmad et al. (2014), entitled "Biochar as a sorbent for contaminant management in soil and water: A review" (CF ¼ 692) and published in the journal Chemosphere. This paper reviews the effects of pyrolysis conditions (including residence time, raw material type, temperature, and heat transfer rate) on biochar generation and biochar supplementation on remediation efficiency, and the possible mechanisms are discussed. It also provided a reference for parameter selection in subsequent biochar preparation, as well as an efficiency assessment of biochar use in HM-contaminated soil remediation. The third most cited reference was the same as the second most cited reference in the SGP (Table 2). But the CF value of this paper increased from 198 in the SGP to 596 in the RGP, indicating that this paper played a vital role in both of these periods.

Future directions
If a keyword is identified as a burst keyword, it means that this topic was extensively studied in a short period of time and research on this topic might be continued in the following years . In general, the burst keywords in the last 5 years are analyzed to explore the future research directions of a specific field ). In the present study, we analyzed burst keywords in the most recent 7 years, i.e., the time scale of the RGP, but only the burst keywords that continued to 2020 were discussed in terms of future directions because these topics are more likely to continue to be studied in the future. The burst test revealed a total of 123 burst keywords. Among them, 24 keywords continued to burst until 2020, and these keywords were further artificially analyzed and classified into five categories (Table 3).
In the category Biochar, there were five keywords, i.e., "activated carbon" (BS ¼ 4.82), "thermal treatment" (BS ¼ 3.53), "torrefaction" (BS ¼ 3.38), "food waste" (BS ¼ 2.21), and "sorbent" (BS ¼ 2.32). These keywords indicated that research on biochar mainly considered the thermal conditions for biochar preparation and the sorbent properties of biochar, and food waste was the most studied raw biochar material. Our previous meta-analysis of the effects of biochar on soil available Cd and Cd uptake by plants also supported these results. Domestic residue-derived biochar was regarded as the most suitable type of biochar and significantly decreased (p < 0.05) the soil available Cd concentration and plant Cd uptake by 36.04% and 53.17%, respectively (Liang et al. 2021). Therefore, further searching for low-cost, high-efficiency raw biochar materials, exploring their appropriate preparation process parameters, and applying them to the remediation of HM-contaminated soil are scientific issues that still need to be explored in the field of BARHMCS.
(BS ¼ 2.78), and "biodiversity" (BS ¼ 2.89). These keywords indicated that biochar supplementation of soil had positive effects on soil nutrient cycling and microbial diversity, and acidic paddy soil was the most studied soil. Similarly, the meta-analysis showed that the application of biochar significantly (p < 0.05) decreased the concentration of bioavailable Cd in loamy and clayey soils by 33.06% and 17.00%, respectively, while the effects were not significant (p > 0.05) in sandy soils (Liang et al. 2021). In soil systems, microorganisms play a positive role in the physical and chemical properties of the soil. Therefore, the effects of biochar on soil microbial community structure, especially the impact of HM-tolerant bacteria, and the response of soil microbes to biochar supplementation in relation to soil nutrient improvement, as well as the effects on the soil-plant-rhizosphere microbial system, warrant further study. Combining biochar application with other remediation methods can further improve the efficiency and reduce the risks of HM-contaminated soil remediation. For example, compared to municipal solid waste compost alone, the combination of municipal solid waste þ10% pigeon pea biochar significantly (p 0.05) reduced the HM content in spinach leaves and roots by 20.62%-41.88% (Mounissamy et al. 2021). The long-term effect of maize straw-derived biochar (15 t ha À1 ) was to decrease Cd bioavailability in slightly Cd-polluted paddy soil and moderately Cd-polluted paddy soil by 20.0% and 34.5%, respectively . The category Combined remediation included the keywords "silicon" (BS ¼ 4.79), "nanocomposite" (BS ¼ 2.98), "nitrification inhibitor" (BS ¼ 2.89), and "earthworm" (BS ¼ 2.49). In the process of combined remediation, biochar combined with physics, chemistry, and biology still deserves further study. Silicon and nanomaterials are regarded as suitable soil additives because of their relatively stable performance, large specific surface area, and nutrient supplementation of soil (Zhu et al. 2019;Chausali et al. 2021), and they can be combined with biochar for combined soil remediation . For example, biochar-Fe 3 O 4 nanocomposites (BFNCs) promoted the transport of Cd by 2.5 times in soils compared to biochar alone. The adsorption of BFNCs was greater in red soil than in paddy soil, and the BFNCs derived from wheat straw showed more potential for Cd adsorption than those derived from wood chips (Chen et al. 2019a). Therefore, the potential HM adsorption potential of nanocomposites varied with soil type and raw biochar material.
Among the animal-based combined remediation methods, those with earthworms were the most studied, while methods involving plants were the most studied bioremediation methods. The addition of passivators to soil to reduce available HMs, improve plant biomass, and alleviate hazards is a research hotspot in the field of phytoremediation (Yu et al. 2020;Zand et al. 2020;Mehta et al. 2021;Liu et al. 2021b). These methods may be combined with biochar application to remediate HM-contaminated soils. Moreover, biochar can also be combined with activators to improve HM phytoextraction efficiency. For example, both biochar and ethylene diamine tetra-acetic acid (EDTA) can enhance the phytoextraction of Pb from artificially polluted soil by Brassica juncea, while the combined use of biochar and EDTA was a more advantageous option than individual amendments for the treatment of Pb-contaminated soil (Rathika et al. 2021). Phytoremediation-biochar synergy can effectively remediate areas contaminated with HMs (Ghosh and Maiti 2020) and undoubtedly will be another valid method in the field of phytoremediation due to its cost effectiveness, sustainability, and environmental friendliness Gonzaga et al. 2022).
In the category Pollutant type, "Cd" (BS ¼ 3.53) was the most studied HM element. Cadmium is considered to be a priority due to its high mobility, high bioaccumulation, and toxicity (Liang et al. 2021). In China, joint reports from the Ministry of Environmental Protection (MEP) and the Ministry of Land and Resources (MLR) showed that among HMs and metalloids, Cd ranks first in terms of pollution, with Cd pollution detected in approximately 7% of soil samples (MEP and MLR 2019). Therefore, the application of biochar in Cd-contaminated soil remediation, especially the soil Cd remediation of agricultural land in China, is still an important topic for Chinese scholars and governments. Moreover, under natural conditions, soil pollution often involves the interaction of multiple pollutants, so the joint remediation of multiple-metal pollution, HM pollution, and organic compound pollution in the soil is a problem that needs attention in the future. Triticum aestivum L. and Zea mays L. were the most studied species in the category of Plant. These two crops are ranked first and third in the world, respectively. Biochar application in low-medium HM-contaminated soils, especially in agricultural land, will promote soil remediation and is being tested as a suitable method in practice. For example, the long-term effect of maize straw-derived biochar (15 t ha À1 ) was a decrease in Cd bioavailability in slightly Cdpolluted paddy soil and moderately Cd-polluted paddy soil of 20.0% and 34.5%, respectively . Furthermore, wheat straw can be used as a raw biochar material and can be sustainably produced (Singh et al. 2020). Chen et al. (2019a) showed that the BFNCs derived from wheat straw showed more potential for Cd adsorption than those derived from wood chips, likely stemming from a greater content of minerals such as CaCO 3 . Therefore, these keywords were research hotspots in the RGP. Both have burst since 2018 and continued to do so until 2020, and they are expected to continue to be research hotspots in the future.
Plants will produce a large amount of oxygen-free radicals and other substances under HM stress, which will cause lipid peroxidation in cells and damage plant cells (Emamverdian et al. 2020). Meanwhile, plants employ their own antioxidant systems, including antioxidant enzymes (Yu et al. 2013;Ahmad et al. 2020;Sharifi et al. 2021;Wang et al. 2021a), nonantioxidant enzymes and osmotica, to resist this kind of stress and alleviate hazardous HMs (Yang et al. 2016(Yang et al. , 2018Huang et al. 2019;Emamverdian et al. 2020;Sharifi et al. 2021). "Lipid peroxidation" (BS ¼ 2.89) is an important indicator for measuring the stress caused by HMs in plants. Therefore, the effects of biochar on plant HM toxicity and the activation of antioxidant systems and nonenzymatic systems in the field of BARHMCS warrant further study.
The category of Others included three keywords, i.e., "climate change mitigation" (BS ¼ 3.53), "greenhouse" (BS ¼ 3.21), and "health risk assessment" (BS ¼ 2.89). This meant that the effects of biochar under climate change and carbon cycles were studied extensively in the last 3 years and might continue to be studied in the future.
The data from Intergovernmental Panel on Climate Change (ICPP) showed that human activities are estimated to have caused approximately 1.0 C of global warming (IPCC 2018) above pre-industrial levels, resulting in a threat to global ecological sustainability, and terrestrial ecosystems may be more affected, causing more hazards to humans, as they are close to farming activities (IPCC 2019). Global warming is likely to reach 1.5 C between 2030 and 2050 if it continues to increase at the current rate (IPCC, 2018). Therefore, how to effectively mitigate this environmental event has become a problem that scientists and governments all over the world need to consider. Interestingly, in addition to the novel characteristics of improving soil properties and pollution remediation, biochar is also an important material for increasing soil carbon sinks, participating in the global carbon cycle and becoming a suitable strategy to mitigate climate change through increased C sequestration (Schmidt and Noack 2000;Glaser et al. 2002;Khadem et al. 2021;Moreno et al. 2022;Nkoh et al. 2022).
For example, the results from a mesocosm experiment showed that straw-derived biochar decreased the global warming potential by 375.6 g CO 2 -eq m À2 season À1 , primarily due to a decrease in CO 2 uptake (Cao et al. 2021). The results from a 6-year field experiment in a double rice cropping system showed that straw-derived biochar application had a higher potential for soil carbon sequestration than straw incorporation, and long-term (>3 year) straw incorporation enhanced nitrogen use efficiency (Liu et al. 2021a). How much of an impact will the addition of biochar have on the global and regional carbon cycles? What feedback will occur as biochar is added to HM-contaminated soil under global climate change? These interesting and considerable scientific problems are worthy of in-depth and systematic research in the future.
On the other hand, biochar application to HM-contaminated soil may also cause other negative effects, such as pH increases. How does this kind of increase affect organisms in the soil-plant system? Moreover, for a specific soil, HM type, HM concentration, soil-plant system, region and climatic zone, how much biochar is appropriate? Does biochar application to soil cause new pollution? Additionally, the effectiveness of biochar in mitigating pollution may decrease with time due to aging factors, such as leaching of alkaline biochar, while related research is limited (O'Connor et al. 2018). What are the effects of the long-term and in situ application of biochar at the field scale? All these issues are related to health risk assessment in the field of BARHMCS and are worthy of in-depth study in the future.
In addition to the future directions based on the CiteSpace analysis results, we believe biochar modification and the combination of HMs and organic matter should receive more attention in the future.
Biochar, an eco-friendly material, has attracted increasing attention due to its low cost, high surface area, and abundant functional groups (Liu and Zhang 2022). Improving the adsorption efficiency of biochar through modification is becoming an important scientific and technological question and has been studied in recent years. Biochar modification is the activation of the feedstock and/or original biochar through physical and chemical methods to achieve the desired purpose ( The modification methods mainly include chemical oxidation (KMnO 4 , Fe 2 O 3 , AlCl 3, FeCl 3 , MgCl 2 , and Fe 2 (NO 3 ) 3 , etc.), chemical reduction (NaOH, KOH, NH 4 OH, etc.), metal impregnation (Fe, Mn, Ag, Zn, etc.), low-temperature plasma (Wu et al. 2012), organic matter grafting , ozone oxidation (Jimenez-Cordero et al. 2015;, and carbon nanotubes or graphene (Inyang et al. 2014;Ghaffar and Younis 2014). Modified feedstock/biochar has very high adsorption and immobilization capacities for pollutants as a result of an increase in the number of sorption sites, high porosity, more oxygenated functional groups, and larger surface area compared to those of unmodified biochar Nkoh et al. 2022). The aim of the optimization process is to obtain the maximum adsorption capacity and efficiency of the biochar used in the extraction of HMs, and process parameters include the heating temperature, heating time, and heating rate as the three most important factors in the preparation of biochar, which should receive increased attention.
Under normal conditions, HMs and organic pollutants coexist in soil, and these two types of pollutants may interact in the environment, increasing the ecological risk. Biochar can potentially be used to reduce the bioavailability and leachability of HMs and organic pollutants in soil simultaneously through adsorption and other physicochemical reactions (Zhang et al. 2013). Changes in the bioavailability and leachability of pollutants are more complex than expected. For example, a 60-day field exposure study conducted by Beesley et al. (2010) showed that Cu and As concentrations in soil pore water increased more than 30-fold with biochar amendment, which was associated with significant increases in dissolved organic carbon and pH, whereas Zn and Cd decreased significantly.

Conclusion
In the present study, bibliometric and visual analyses of the field of BARHMCS was performed using the WoSCC database, and the main conclusions are as follows: 1. The number of publications in the field of BARHMCS over recent decades fit a cubic regression model (R 2 ¼ 0.9983). The whole research period could be divided into two subperiods, the SGP and RGP, accounting for 16.30% and 83.70% of all publications, respectively. 2. A total of 126 countries, 741 institutions, and 1,021 scholars have contributed to this field. Among them, the countries of China, the United States, and Germany contributed the most; the institutions of the Chinese Academy of Sciences, Zhejiang University, and the University of the Chinese Academy of Sciences were the top three institutions; and Ok Yong Sik, Johannes Lehmann, and Daniel CW. Tsang were the top three authors in terms of publications. These papers were mainly published in the journals Science of the Total Environment, Chemosphere, and Environmental Science and Pollution Research, mainly in the disciplines environmental science, soil science, and environmental engineering. 3. The research hotspots in the SGP mainly involved the preparation conditions for biochar and its influence on air pollution, the processes and mechanisms of adsorption and desorption of pollutants with biochar, and the effects of biochar on soil organic matter and HM remediation. The research hotspots in the RGP mainly involved pollutant adsorption and desorption processes and their associated mechanisms with biochar addition, the mineralization and decomposition effects of biochar on soil organic matter and their initiation effects, the impacts of biochar on soil fertility and crop yield, the bioavailability of HMs, especially Cd, with biochar supplementation, and remediation efficiency. 4. Future research directions worth focusing on include raw material selection; preparation parameter exploration; soil property changes, especially changes in soil microbiological characteristics with biochar supplementation, the combination of biochar and other methods to remediate HM-contaminated soil and HM-organic compound-contaminated soil; biochar application in the remediation of HM-contaminated agricultural land, especially land used for wheat and corn cultivation; the response of soil carbon in the biogeochemical cycle; biochar modification; and biochar remediation of HMs and organic pollutants that coexist in soil.

Notes
1. The values were automatically determined by the software CiteSpace (5.7 R3). 2. The equation for the annual publication curve was obtained using Excel, and the fitting result was evaluated by the R 2 value; the closer the value is to 1, the better the fitting effect. In general, R 2 values >0.95 are considered reasonable.

Authors contributions
Prof. Kehui Liu contributed to the experimental design and original writing of the manuscript. Mrs Jiayi Liang Ningning Zhang, Guangluan Li, and Jieyi Xue extracted the data and drew the knowledge maps.
Dr Yi Li provided constructive discussion and improved the manuscript.
Prof. Fangming Yu contributed to the paper organization and manuscript revision and provided funding support.

Disclosure statement
We declare that we have no financial or personal relationships with other people or organizations that could have inappropriately influenced our work; there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, this manuscript.

Funding
This project was supported by the National Natural Science Foundation of China (42267005), Key Project of Guangxi Natural Science Foundation (2022GXNSFDA080008) and the Guangxi Key Laboratory of Landscape Resources Conservation and Sustainable Utilization in Lijiang River Basin (LRCSU21Z0212).