Chemical Speciation of Cadmium and Sulfur K-Edge XANES Spectroscopy in Flooded Paddy Soils Amended with Zerovalent Iron

A microcosm experiment using cadmium (Cd) spiked soils with contrastingly different sulfur (S) contents (low-S and high-S soils, pH 6) were conducted to determine how zerovalent iron (ZVI) alters Cd solubility and speciation in relation to soil S and redox processes. Synchrotron-based X-ray absorption near edge structure (XANES) spectroscopy was used to determine both Cd and S speciation, particularly focusing on the relative proportion of CdS and other species in oxidized surface and reduced subsurface paddy soils. Along with rapid soil reduction (approximately -200 mV in 5 d), additions of ZVI decreased Cd in the soil solution and soil's exchangeable fractions. Compared to low-S soil, high-S soil with three times greater amount of available S showed more rapid removal of Cd from the soil solution and a smaller proportion of Cd in exchangeable fractions. Linear combination fitting on Cd K-edge XANES spectra of soil found that (i) at least 55% of Cd was present in association with soil colloids including kaolinite, ferrihydrite and humus, rather than precipitates with hydroxide, carbonate, and sulfate, and (ii) CdS occurred more in the subsurface than surface soil profiles and more in high-S soil (45%) than low-S soil (32%) in the presence of ZVI. Sulfur K-edge XANES analysis indicated the reduction of intermediate S species with increasing depth in the soil microcosm. Overall, ZVI enhanced CdS formations and reduced Cd in exchangeable fraction and in soil solution, and such effects appeared notably more in high-S than low-S soils.