Dynamic Immobilization of Copper in Mining Soils Using Modified Biochars

This study investigated the dynamic passivation effects of pristine pine wood biochar (BC) and three modified biochars (BC-OH, BC-Mn, and BC-Mg) on copper (Cu)-contaminated soil from a mining area. Based on the Community Bureau of Reference (BCR) sequential extraction and Risk Assessment Code (RAC) analysis, the modified biochars significantly outperformed pristine BC, exhibiting a substantial reduction in the highly toxic weak acid-soluble fraction (F1) of Cu. Speciation evolution analysis revealed that different modifications drove distinct immobilization mechanisms, characterized by either a unidirectional (BC-Mg and BC-OH) or a two-step (BC-Mn) transformation process. The long-term passivation efficacy of the materials followed the order: BC-Mg > BC-Mn > BC-OH ≫ BC. Notably, the optimal material, BC-Mg, achieved a nearly 75% reduction in the F1 fraction after 70 days, demonstrating its potential as an ideal remediation material for the long-term mitigation of acute heavy metal toxicity.