Competitive adsorption, immobilization, and desorption risks of Cd, Ni, and Cu in saturated-unsaturated soils by biochar under combined aging

This study investigated saturated-unsaturated soils, which were closer to the actual field conditions than traditional batch and column experiments with large water-soil ratios. The competitive adsorption, immobilization, and desorption of Cd, Ni, and Cu in soils treated with original and KMnO4-modified biochars were investigated under combined aging. Moreover, the employment of a three-layer mesh method enabled the independent analysis of heavy metals on biochar and soil during aging. The results showed that the order of biochar adsorption capacities was Cd > Cu > Ni in tested soils, and competing with Ni and Cu enhanced the Cd adsorption on biochars. Cd desorbed most with the CaCl2 solution while Ni and Cu desorbed most with citric acid. Modified biochar had improved immobilization effects compared to original biochar, and maintained the most stable remediation effects. The maximum variations in the stable Cd fraction during aging were 7.21%, 13.26%, and 14.71% for modified biochar, original biochar, and CK, respectively. However, for Ni and Cu, the biochar application reduced the residual fraction and increased desorption by citric acid. However, the stable fractions of Ni and Cu remained dominant, accounting for 83.28-97.85% and 86.31-98.96%, respectively.