Biochar for multi-metal stabilization in smelting soils: Long-term performance and mechanisms under freeze-thaw and...

Despite extensive research, the long-term performance of biochar (BC) for stabilizing multiple heavy metals (HMs) under realistic environmental stresses remains unclear. This study investigated the effects of freeze-thaw and dry-wet cycles on the long-term stabilization behavior of HMs in smelting-contaminated soils amended with BCs under accelerated aging conditions. BC application increased the residual and carbonate fractions while reducing exchangeable fractions. However, freeze-thaw and dry-wet cycles induced contrasting effects on HM stability: Freeze-thaw cycling caused physical damage to BCs, resulting in a 3.4-fold increase in HM leaching and promoting the transformation of residual metals into exchangeable and carbonate fractions. In contrast, dry-wet cycling reduced HM mobility by driving metals from exchangeable to Fe/Mn oxide-bound fractions, indicating a shift toward a more stable chemical equilibrium. Multiscale characterization further revealed that the destabilization under freeze-thaw was primarily driven by structural breakdown of BC, whereas dry-wet cycles promoted the formation of phosphate and carbonate precipitates with HMs, thereby enhancing BC long-term stabilization capacity. These findings provide mechanistic insights into BC-mediated HMs stabilization under environmental stresses.