A new insight into Cd reduction by flooding in paddy soil: The different dominant roles of Fe and S on Cd immobilization under fluctuant pe plus pH conditions

The unsteady comprehensive system of pe + pH strongly affects the late of Cd in paddy soils. However, the specific pe + pH threshold determining Cd bioavailability is largely unknown especially considering the roles of Fe and S reduction. The experiment set different water managements to obtain paddy soil samples with unstable pe + pH, and chemical analysis, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) characterization were applied to reveal the dynamic process and mechanism about how Fe and S controlled Cd mobilization. The results showed that low pe + pH was favorable to soil Cd immobilization. Compared with high and medium pe + pH, the exchangeable Cd content decreased by 67.57 % and 64.71 %at low pe + pH, respectively. The XPS results showed that the contents of Fe(H) and S(-II) increased to 65.1 %and 75.2 %at low pe + pH condition, which was higher than that in other treatments. In the process of flooding for reducing Cd mobility, first it was attributed to the formation of amorphous iron oxides that can provide amount of adsorption sites for Cd. After then, S2- began to play a dominant role to combine with Cd-2(+) to form CdS with continuous decreased pe + pH. Therefore, Fe and S played the different dominant roles on Cd immobilization in paddy soil, and soil pe + pH value could work as a threshold.

Automated summary

This study found that low pe + pH is conducive to the immobilization of Cd in paddy soils, with Fe and S playing different dominant roles in Cd immobilization, and soil pe + pH value can work as a threshold.