Vertical redox zones of Fe–S–As coupled mineralogy in the sediments of Hetao Basin – Constraints for groundwater As contamination

The formation of iron-sulfur-arsenic (Fe–S–As) minerals during biogeochemical processes in As contaminated aquifers remains poorly understood despite their importance to understanding release and transport such systems. In this study, X-ray absorption Mössbauer spectroscopies complemented by electron microscopy, chemical extractions were used examine vertical changes As, Fe S speciation for the example sediments Hetao Basin. Reduction Fe(III), As(V) SO42- species shown co-occur aquifers. Iron oxides observed be predominantly goethite hematite (36 – 12%) appeared decrease abundance with depth. Furthermore, reduced (including arsenite sulfides) sulfur S(-II), S(-I) S0) increased from 16% 76% 13% 44%, respectively. major carrier sediments, lower groundwater concentration consists less desorbable reducible sediments. As-Fe sulfides (e.g., containing pyrite greigite) induced redox heterogeneities likely contribute localized concentrations. These results help further elucidate complex relationship between