A combined biochar and biomineralization approach for enhanced soil Pb immobilization under accelerated aging

Long-term effectiveness remains a critical challenge for soil heavy metal immobilization. In this study, the combination of biochar and microbial induced carbonate precipitation (MICP), a biomineralization-based approach, referred to as BM treatment, was applied to a Pb-contaminated silty clay. A quantified accelerated aging experiment based on rainfall and wet-dry cycles was conducted to assess the stability of Pb across the simulated 100 years. BM treatment exhibited enhanced performance in Pb immobilization compared with single biochar and single MICP treatments under accelerated aging. Before aging, BM treatment was the only method capable of reducing soil Pb leachability to levels that meet the toxicity characteristic leaching procedure standard. After aging, the total Pb concentration in BM-treated soils showed the lowest reduction (10.6 %) after simulated 100 years, compared to reductions of 23.3 % and 18.1 % in biochar-treated and MICP-treated soils, respectively. BM treatment also significantly increased the fraction of generally stable Pb in the soil, maintaining a relatively high level (39.4 %-45.7 %) and enhanced calcium carbonate production (7.92 %) with minimal dissolution (2.14 % reduction) throughout the simulated 100 years. Biochar's porous structure facilitated treatment solution delivery, overcoming MICP limitations in fine-pored clay soils. This study sheds light on the potential large-scale application of BM treatment in long-term soil heavy metal immobilization.