Leaching behavior and mechanism of Cu and Cd in acid paddy soil under acid mine drainage

Heavy metal pollution seriously affects soil quality, especially in paddy soils that have been polluted for a long time by acid mine drainage (AMD). However, the underlying mechanisms of the complicated AMD flooding processes in paddy soil remain poorly understood. In this study, the leaching behaviors and speciation distribution mechanisms of Cu and Cd were investigated by batch and in-situ leaching experiments under simulated AMD irrigation conditions. The results suggested that continuous AMD flooding for 180 days caused a decrease in pH and Eh until these values stabilized, at day 84 and day 70, respectively. Between pH 4.5 and 6.5, more Cu than Cd was adsorbed which was consistent with the findings of the surface complexation model (SCM). The maximum adsorption capacities of the soil was 264.84 mg/kg towards Cu and 125.96 mg/kg towards Cd. Tessier's five-step extraction showed that most adsorbed Cd was present in exchangeable fraction (F1) and carbonate-bound fraction (F2) at different depths and time points, while Cu was mainly found in Fe/Mn-bound fraction (F3) and organic-bound fraction (F4). Correlation analysis showed that migration of Cu and Cd was most susceptible to the influence of pH, Eh, and Fe. In all assessed layers of the column, Cu was retained to a greater extent than Cd. The leaching of AMD significantly increased the mobility of soil Cd. The presented data improves our understanding of the impact of long-term exposure and leaching on the mobility of heavy metals in contaminated paddy soils and assist in developing appropriate management strategies.