| Heavy metal pollution is an international environmental problem,and biochar passivation remediation has become an effective means to solve this problem.In recent years,research on biochar passivation remediation of heavy metal Cd and Zn contaminated soils（BPRHM（Cd、Zn）CS）has gradually increased,while there is a lack of comprehensive and systematic analysis on the recent developments of this research theme.Therefore,the visualization analysis（Cite Space 5.8 R3 software）and Meta-analysis were combined to comprehensively and systematically study the development trends and hot topics in the field of BPRHM（Cd 、 Zn）CS.Firstly,the bibliometrics and visualization analysis methods were used to study the whole perspective and future research directions of HMBPR published from 2015 to 2021 based on the databases of China National Knowledge Infrastructure（CNKI）and Web of Science Core Collection（Wo SCC）.An amount of 156 Chinese literature and 1114 English literature were used to analyze the published characteristics,research hotspots,knowledge base,and future research trends on this topic.Then,using the method of Meta-analysis,110 Chinese publications and 135 English publications were screened out by setting the application of biochar alone as the only distinguishing variable between the control group and the experimental group.Based on these publications,the effective data were extracted according to the data standardization and statistical calculation methods.Finally,quantitative analysis and comprehensive evaluation were conducted on the effects of biochar on soil Cd and Zn available content and soil physicochemical properties（p H,OM,EC,CEC,etc.）under different experimental conditions,soil conditions,biochar characteristics,and application ratios.The main results of BPRHM（Cd、Zn）CS based on Cite Space visualization analysis are as follows:（1）The publication characteristics: In the past 7 years of development,a total of 139 institutions and 202 authors in Chinese literature have contributed to this topic,and a total of 20 journals have reported research on this topic,covering a total of 14 subject categories.A total of 71 countries/regions,280 institutions,and 340 authors contributed to the English literature,and a total of 186 journals reported relevant research,covering a total of 30 subject categories.（2）Research hotspots: The research hotspots of the Chinese literature mainly included #0 contaminated soil,#1 soil,#2 passivation remediation,#3 passivating agent,#4 rice,#5 soil remediation,and #6 biochar,etc.7 clusters.The research hotspots of English literature mainly included #0 Environmental Remediation,#1 Remediation,#2Mine Soil,#3 Grain Yield,#4 Modified Biochar,#5 Alkaline Soil,#6 Soil Pollution,#7Soil Remediation,#8 Soil Enzymes,and #9 Soil Science,etc.a total of 10 clusters.（3）Knowledge base: A total of 10 clusters including #0 Metal,#1 Endogenous Metals,and #2 Competitive Adsorption in the analysis of Co-citation references of English publications.In-depth analysis of 6 references that play a key supporting role in the research on this topic,these knowledge bases well support and expand the hot topics of the corresponding data.（4）Future research trends: Appropriate modification methods to improve the passivation effect of biochar,preparation and screening of engineered biochar,long-term effects of biochar application on soil,where biochar goes in the natural environment,the relationship between biochar and biochar,combined remediation of HMs-contaminated soils with other materials or technologies,passivation remediation of field soils with biochar under natural conditions,potential environmental risks and sustainable assessment of biochar applications,and effects of biochar on changes in the concentration of essential trace metal elements in crops,etc.deserved to be concerned in the future.The main results of Cd and Zn available content and soil physicochemical properties of BPRHM（Cd、Zn）CS based on Meta-analysis are as follows:（1）The application of biochar significantly（p<0.05）reduced the available content of Cd and Zn in the soil by 32.25% and 22.7% respectively.（2）The application of biochar in the field experiment was generally more beneficial than the pot experiment in reducing the available content of Cd and Zn in the soil,and the decreasing amplitudes were 38.14% and 60.84%,respectively.The application of biochar in the loamy soil had the largest decrease in the available content of Cd,reaching26.75%.The clay soil had the largest decrease in Zn,reaching 36.48%.When biochar was applied in acidic soil（p H≤5.5）,Cd and Zn available content decreased by 34.21%and 37.92%,respectively.（3）The feedstock,p H,pyrolysis temperature,pyrolysis time,and application ratios of biochar that had the greatest impact on Cd available content were livestock manure type,p H≤8,medium and high temperature（500 ℃10,medium temperature（400 ℃3.5h,and2 ～ 3%,and the decreasing amplitudes were 29.89%,27.23%,26.77%,25.81%,and30.38%,respectively.（4）The application of biochar also significantly（p<0.05）increased soil p H,OM,EC,CEC,and DOC,and the increasing amplitudes in these soil physicochemical properties were 8.44%,57.93%,39.61%,30.15%,and 27.47%,respectively.Among them,different characteristics and application ratios of biochar had different effects on soil pH and EC,but they all significantly（p<0.05）increased soil OM and CEC. |
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