Effects of Aging on The Bioavailability of Cu and Cd Immobilized by Biochars with Varying Endogenous Heavy Metal Concentrations

Abstract

Biochar is widely used because of its good passivation effect on soil heavy metals; however, the biochar itself may contain heavy metals due to the difference in raw materials, and whether the passivation effect of biochar with high endogenous heavy metals is altered after aging, and whether there is a potential environmental risk should be a concern. In this study, three types of biochar (BB, MB, and HB) with varying endogenous Cu and Cd levels, derived from Pennisetum giganteum grown in background, moderately contaminated, and heavily polluted areas, were used as the study objects. The effects of aging on the bioavailability of Cu and Cd in contaminated soil, when subjected to alternating dry-wet and freeze-thaw conditions, were analyzed using the CaCl₂ extraction and the thin-film diffusive gradient (DGT) extraction method. We examined the effects on the bioefficacy of Cu and Cd in contaminated soil by using the BCR sequential extraction method. To analyze the changes in the morphological distribution of soil Cu and Cd. The results showed that the three types of biochar were alkaline, and the contents of Cu in HB and Cd in MB were the highest, being 3.72 and 3.29 times higher, respectively, than those of BB. After 30 days of pre-cultivation, a dosage of 2% to 10% biochar increased the soil pH by 0.32 to 4.50, respectively. All of them significantly reduced soil Cu and Cd bioefficacy, among which a 10% dosage of the three kinds of biochar reduced soil CaCl₂ extraction of Cu and Cd by 98.2%~99.1% and 86.9%~97.0%, respectively, and DGT extraction of Cu and Cd by 80.0%~89.9% and 81.7%~99.0%, respectively. Compared with the control treatment, the addition of biochar significantly reduced the soil acid-soluble Cu content and promoted its transformation to oxidizable, reducible and residual Cu; however, the 5% and 10% MB treatments significantly increased the soil acid-soluble Cd content, which has a substantial potential environmental risk, and thus high endogenous pollutant biochar needs to be paid extra attention to when applied in large quantities. In addition, alternating dry-wet and freeze-thaw aging decreased the CaCl₂- and DGT-extracted soil Cu and Cd contents in each treatment group, which further promoted the transformation of soil Cu and Cd from active to inactive states. Overall, the stabilizing effects of the three types of biochar at the exact dosage were HB > MB > BB for soil Cu, and BB > HB > MB for soil Cd. The results of this study are of great significance for the development of biochar application standards and for evaluating the stability of biochar in long-term passivation and remediation.