Biogeochemical processes of arsenic transformation and redistribution in contaminated soils: Combined effects of iron, sulfur, and organic matter

Microbially-mediated mobilization of soil arsenic (As) is greatly influenced by the soil properties. However, in soils with contrasting iron (Fe), sulfur (S), and organic matter (OM) contents, the biogeochemical pathways controlling As transformation and distribution remain unclear. Using sequential soil As extraction and X-ray absorption spectroscopy (XAS), we investigated the causal mechanisms of As reduction and redistribution in five soils during microbial incubation. Incubation of arsenate (As(V))-reducing bacteria resulted in a significant arsenite (As(III)) release (21.6–61.9% of total soil As (Astotal)). Thereafter, the re-immobilization of released As(III) was controlled by contrasting biogeochemical pathways, which were mainly dominated by soil Fe and S. For soil with high Fe content (191.1 g/kg), As immobilization is attributed to As(III)-readsorption by (neoformed) Fe-(oxyhydr)oxides, despite the presence of abundant S (10.3 g/kg); while in soils with relatively low Fe content (25.9–35.6 g/kg) and high S content (1.4–1.7 g/kg), As-sequestration depends largely on As-sulfide formation (5–47% of solid-phase As), including realgar and orpiment-like phases. In contrast, released As remains in solution in soils with relatively low Fe (27.5–52.4 g/kg) and S contents (0.6–1.0 g/kg). Arsenic-XAS results show that all soil As fractions, including residual As(V), can potentially be reduced (34–92% of Astotal), and solid-phase As(V) reduction was enhanced at higher OM content. Collectively, these results elucidate the dominant biogeochemical pathways controlling As fate in soils with different Fe, S, and OM contents.