Role of sulfur redox cycling on arsenic mobilization in aquifers of Datong Basin, northern China

Sulfur redox cycling potentially exerts important influences on arsenic (As) fate in shallow groundwater systems. Hydrochemical and sediment geochemical analysis combined with thermodynamic modeling study were conducted at Datong Basin to elaborate the effects of sulfur redox cycling on As speciation and mobilization under a strongly reducing environment. Dissolved As and sulfide concentration in 32 groundwater samples with depths of 19-40 m below the land surface varied from 8 to 2700 μg/L and from <5 to 490 μg/L, respectively, while dissolved Fe(II) was relatively low ranging from <20 to 280 μg/L. The apparent co-increase in dissolved sulfide and As concentration, especially for samples with As content larger than 500 μg/L, indicates that sulfidogenesis may significantly contribute to the mobilization of As via sulfide-induced reduction of both As-bearing Fe(III) oxide minerals and As(V). Thermodynamic calculations indicate that groundwater As might be also thiolated in the presence of high-level sulfide, particularly to a large extent for As(V) speciation, instead of sequestration by As-sulfide precipitates. Results of sequential extraction and scanning electron microscopy array on sediments indicate the presence of Fe(II) sulfide mineral phases and an appreciable amount of co-existent As in the sediments, suggesting the precipitation of Fe(II) sulfides can restrict the build-up of dissolved Fe(II) and sequester As from groundwater, but not strongly enough, thereby lowering down As to a moderate level of about 500 μg/L. Thus, redox processes involving As, S and Fe species under sulfidic conditions as observed in Datong Basin not only facilitate the enrichment of As(III) species and As(V) potentially existing as thiolated species, but also the depletion of Fe(II) concentration in groundwater due to Fe(II) sulfide formation.