Biochar amendment gradually immobilized soil As and Sb over 2 years

Biochar is a promising candidate for the sustainable remediation of soils, especially those contaminated with cationic heavy metals, because of its liming effect and tunable surface functionality. Despite its potential, prior studies have highlighted biochar's limitations in immobilizing soil oxyanions, such as arsenic (As) and antimony (Sb), particularly in the short term. This shotcoming is primarily attributed to an increase of soil pH following biochar amendment, and factors like competition with phosphate. In this study, biochar amendments were applied to three soils with varying levels of oxyanions including As and Sb, and cations including cadmium (Cd) and lead (Pb). These treatments generally resulted in short-term failure of oxyanion immobilization. However, a noteworthy phenomenon unfolded over a 2-year period, where biochars gradually transitioned from initial mobilization or poor immobilization to eventual successful immobilization of oxyanions (up to 87.0% for As and 100% for Sb). Temporal changes in Cd and Pb differed from As and Sb, exhibiting no improvement in immobilization rates over time. Potential mechanisms driving this process were investigated, suggesting a decline in soil pH, progressive oxidation of soil carbon fractions, and direct adsorption effects as contributing factors. This study sheds light on the temporal shift in biochar's immobilization performance, highlighting a gradual increase in the efficacy in oxyanion immobilization. The findings offer valuable insights into the dynamic nature of biochar's remediation capabilities.