Recent advance on application of biochar in remediation of heavy metal contaminated soil: Emphasis on reaction factor, immobilization mechanism and functional modification

Soil contamination with heavy metals (HMs) poses a critical environmental challenge that demands immediate attention and resolution. Among the various remediation techniques, biochar emerges as an environmentally friendly option with obvious advantages. Biochar can be obtained by pyrolysis of various biomass and has significant effects in the remediation of heavy metal pollution contaminated soil. In this study, we examined 3489 articles on biochar-based remediation of soil heavy metal contamination published between May 2009 and October 2023, utilizing the Web of Science core collection database. Based on bibliometric methods and big data statistical analysis, CiteSpace visualization software is utilized to create a knowledge map of biochar research, allowing for an analysis of keyword clustering and a summary of the current research hotspots and development trends. Furthermore, this review emphasizes factors influencing the characteristics of biochar, including raw material types, pyrolysis temperature and pyrolysis method. At the same time, the optimal conditions for producing biochar are also presented. Additionally, the mechanisms of biochar remediation for heavy metal contaminated soil are introduced in detail, including electrostatic attraction, ion exchange, physical adsorption, surface complexation and precipitation. Meanwhile, the modification and combined effects of biochar are also reviewed. Finally, the advantages and potential risks of using biochar are explored. It is aims to serve as a reference for subsequent research and promote the application of environmental remediation technologies in polluted soils.