Zerovalent iron with high sulfur content enhances the formation of cadmium sulfide in reduced paddy soils

The objective of this study was to investigate the role of S impurity in zerovalent iron (ZVI) on the chemical speciation and solubility of Cd in the reduced soils. Two types of ZVI with contrastingly different S levels (0.009 and 1.08%) were used to find how the solubility and speciation of Cd would be influenced by the S impurity in ZVIs. Synchrotron-based X-ray absorption fine structure (XAFS) spectroscopy was used to determine the relative proportion of CdS and other species in the Cd-spiked soils amended with ZVIs with different S levels (hereafter low- and high-S ZVIs). Microscale distribution and speciation of Cd at the interface between ZVI and soil particles were investigated using micro-X-ray fluorescence (μ-XRF) and micro-XAFS (μ-XAFS) spectroscopy. The difference in S contents in ZVIs did not affect the soil solution Cd concentrations, but a significant decrease in exchangeable Cd was found in the soil with added high-S ZVI. Linear combination fitting (LCF) on Cd K-edge XAFS spectra of bulk soils determined up to 16% CdS in the reduced soils amended with ZVIs. The μ-XRF map of the soil amended with high-S ZVI showed that the spots with S accumulation were discretely distributed on the ZVI, and some S accumulating areas corresponded to Cd localization. The LCF on Cd K-edge μ-XAFS spectra for selected soil particles revealed that the proportion of CdS ranged from 20 to 87% (avg. 53 ± 22%) in the soil with low-S ZVI and from 64 to 98% (avg. 84 ± 14%) in the soil with high-S ZVI. A higher S content in ZVI, and probably in other amendments, decreases labile Cd fractions and enhances CdS formation in reduced soils.