Mechanisms of S cooperating with Fe and Mn to regulate the conversion of Cd and Cu during soil redox process revealed by LDHs-DGT technology.

The activity changes of Cd and Cu in paddy field were strongly influenced by the transformation of S, Fe and Mn species. However, in the process of soil redox, how S cooperates with Fe/Mn to regulate the law and mechanism of Cd and Cu speciation transformation still needs to be studied. In this study, we used DGT technology based on layer double hydroxides (LDHs) combined with pore water sampling to investigate soil redox changes, rice growth, and the effects of different forms of sulfur (S0, SO42-) on soil Cd and Cu activities. The results showed that the concentrations of CDGT-Cd and Cu in the soil decreased rapidly in the anaerobic stage, but increased slowly in the oxidative stage. Multiple regression analysis showed that the changes of Cu and Cd concentrations mainly depended on the changes of Fe/Mn morphology. Sulfur treatment promoted the dissolution of Fe/Mn oxides in the short term (<48 h), and the activities of CDGT-Fe, Mn, and Cd increased simultaneously, but CDGT-Cu was not affected. However, after long-term anaerobic conditions (>10 d), sulfur addition reduced the activities of CDGT-Cd and Cu, and decreased the uptake of Cd and Cu by rice. During sulfate reduction, the sulfur addition treatment group resulted in a 24.5-50.2 % decrease in CDGT-Fe, indicating that sulfur addition may delay the release of Cd and Cu after rice planting by promoting the formation of FeS/FeS2. In addition, in the anaerobic stage, Cu formed sulfide before Cd and was fixed, and the higher thermodynamic stability of CuS would promote the dissolution of CdS in the oxidation stage. Overall, soil flooding with sulfur to enhance the generation of metal sulfides and secondary iron ores provides an opportunity to use sulfur as an environmentally friendly modifier to coordinate Fe, Mn to improve heavy metal-contaminated soils.