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 As release and transport in such systems. In this study, X-ray absorption and Mössbauer spectroscopies complemented by electron microscopy, and chemical extractions were used to examine vertical changes of As, Fe and S speciation for the example of sediments in the Hetao Basin. Reduction of Fe(III), As(V) and SO 4 2- species were shown to co-occur in the aquifers. Iron oxides were observed to be predominantly goethite and hematite (36 – 12%) and appeared to decrease in abundance with depth. Furthermore, reduced As (including arsenite and As sulfides) and sulfur species (including S(-II), S(-I) and S0) increased from 16% to 76% and from 13% to 44%, respectively. Iron oxides were the major As carrier in the sediments, and the lower groundwater As concentration consists with less desorbable and reducible As in the sediments. The formation of As-Fe sulfides (e.g., As containing pyrite and greigite) induced by redox heterogeneities likely contribute to localized lower groundwater As concentrations. These results help to further elucidate the complex relationship between biogeochemical processes and minerals formation in As contaminated aquifers. ga1 • Speciation of As, S and Fe is mainly characterized by XAS. • Fe(III) oxides mainly include goethite and hematite, and the content decrease with depth. • The reduced As (As(III) species and As sulfides) species generally increases with depth. • Heterogenous Fe sulfides formation contributes to lower groundwater As concentration. [ABSTRACT FROM AUTHOR]