Environmental aging enhanced the ability of biochar to remediate heavy metal-contaminated soil through combined adsorption and microbial community regulation: A mechanistic analysis

The remediation of heavy metal-contaminated soils via biochar involves both adsorption and microbial community regulation, yet the impact of environmental aging on this synergistic mechanism remains unclear. This study analyzed the aforementioned mechanisms through biochar-soil co-cultivation, batch adsorption experiments and 16S rRNA gene sequencing. The results revealed that the initial introduction of biochar did not significantly enhance heavy metal immobilization. After one year of natural aging, however, the efficacy of biochar in immobilizing heavy metals was closely associated with their initial residual fractions. When the inherent residual fraction was high e.g., Cu (> 94 %), biochar amendment induced negligible change in metal speciation. In contrast, in soils with low residual fractions, a 5 % biochar addition promoted the transformation of heavy metals into residual forms, resulting in maximum increases of 3.2 % for Pb and 7.2 % for Cd. Moreover, the stable fractions of heavy metals in biochar-remediated soils increased slightly with extended incubation time. After natural aging for one year, the content of mineral-related fractions increased significantly, substantially enhancing the adsorption (e.g., K d increase d to 499, 607 and 2114 mg/kg for Cu 2 + , Cd 2+ and Pb 2+ with 5 % biochar addition after 1-year incubation) of heavy metals on biochar-remediated soil. Additionally, after natural aging, biochar amendment enriched key metal-immobilizing microbial genera (e.g., Gemmatimonas , Sphingomonas , and Acidiferrimicrobium ), while reducing the abundance of metal-mobilizing Kouleothrix , which exhibited dose-dependent. This study demonstrated environmental aging enhanced the ability of biochar to remediate heavy metal-contaminated soil through combined adsorption and microbial community regulation. • The heavy metal (HMs) immobilization by biochar was related to their fraction content. • The stable fractions of HMs increased slightly with biochar dosage and incubation time. • Aging increased mineral-related fractions, substantially enhancing the adsorption of HMs. • The regulation of biochar on microbial communities remediating HMs is dose-dependent. • Aging enhanced biochar’s remediation of HMs-contaminated soil via adsorption and microbial regulation.