Zerovalent Iron with High Sulfur Content Enhances the Formation of Cadmium Sulfide in Reduced Paddy Soils

Core Ideas Sulfur impurities in ZVI affect Cd chemical speciation and solubility in reduced soils. Exchangeable Cd decreased in the ZVI‐treated soil with greater S content. The addition of ZVI with high levels of S enhanced the formation of CdS. 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.