Spatial and temporal variations of Cu and Cd mobility and their controlling factors in pore water of contaminated paddy soil under acid mine drainage: A laboratory column study

Abstract

Acid mine drainage (AMD) poses a potential threat to human health worldwide, due to its high content of inorganic contaminants including heavy metals. Nevertheless, AMD is commonly used for irrigation of paddy soils. To determine the extent to which AMD affects contaminant levels in such practices, the effect of continuous AMD flooding on pH, redox potential Eh and the migration of Cu and Cd in contaminated paddy soil was studied in column experiments. By means of simulated AMD, dynamic changes of Cu and Cd concentrations in pore water were measured and the controlling factors pH, Eh and presence of Fe, dissolved organic carbon and sulfate were determined over a period of 60 days. Minerals in the soil were assessed by means of an Eh-pH diagram and solid-phase mineral detection. During continuous flooding with AMD-simulated water the soil pH increased, while Eh decreased over time. After 60 days the soil pH stabilized. Cu and Cd concentrations in the pore water negatively correlated with pH and with sulfate concentrations. Five-step sequential extraction illustrated that the fraction of exchangeable Cu increased significantly during AMD flooding. The overall content of Cu increased from initially 0.29 mg/g to 0.41 mg/g, while the content of Cd decreased from 9.2 mg/g to approximately 7.2 mg/g. Mobility factors were calculated and these conformed that Cd mobility significantly increased in contaminated soils during continuous AMD flooding. Our findings indicate that the release of Cu and Cd under AMD flooding can increase potential environmental risks, even though they lead to formation of metal sulfide deposits under anaerobic conditions. The presented data improves our understanding of the impact of overlying water conditions on the mobility of toxic metals in contaminated paddy soils.