The Vertical Co-Migration Behavior of Biochar and Cd in Farmland Near the Mining Area

Ultrapure water and toluene/methanol were used to extract the dissolved organic matter (DOM) from soils and the three-dimensional fluorescence spectroscopy and the parallel factor (EEM-PARAFAC) technology were employed to track litchi wood biochar within one year in the 0~100 cm soil profiles. The vertical co-migration of Cd in the farmland polluted by the Dabaoshan mine wastewater in Shaoguan, Guangdong, was analyzed. Biochar increased the content of dissolved organic carbon (DOC) in the soil layer of 0 to 60 cm and increased the pH of the topsoil alone. Based on the EEM-PARAFAC analysis, three components of soil DOM (one protein-like component and two humic-like components) were identified. The addition of biochar increased the humic-like content in 0~60 cm soil profiles. Moreover, the unique polycyclic aromatic hydrocarbon structures originated from high temperature pyrolysis of biochar were also detected in soil DOM extracted with toluene/methanol in the 0~60 cm soil profiles, indicating that the biochar underwent a significant vertical migration within one year. In addition, the available Cd decreased in the topsoil with biochar addition but was 148% higher than that in the control group in the 20~60 cm soil profiles. These findings suggest that more attention should be paid to the possible risk of biochar during the remediation of metal-contaminated soils.