Arsenic mobility during flooding of contaminated soil: the effect of microbial sulfate reduction

In floodplain soils, As may be released during flooding-induced soil anoxia, with the degree of mobilization being affected by microbial redox processes such as the reduction of As(V), Fe(III), and SO₄^2–. Microbial SO₄^2– reduction may affect both Fe and As cycling, but the processes involved and their ultimate consequences on As mobility are not well understood. Here, we examine the effect of microbial SO₄² reduction on solution dynamics and solid-phase speciation of As during flooding of an As-contaminated soil. In the absence of significant levels of microbial SO₄^2– reduction, flooding caused increased Fe(II) and As(III) concentrations over a 10 week period, which is consistent with microbial Fe(III)- and As(V)-reduction. Microbial SO₄^2– reduction leads to lower concentrations of porewater Fe(II) as a result of FeS formation. Scanning electron microscopy with energy dispersive X-ray fluorescence spectroscopy revealed that the newly formed FeS sequestered substantial amounts of As. Bulk and microfocused As K-edge X-ray absorption near-edge structure spectroscopy confirmed that As(V) was reduced to As(III) and showed that in the presence of FeS, solid-phase As was retained partly via the formation of an As₂S₃-like species. High resolution transmission electron microscopy suggested that this was due to As retention as an As₂S₃-like complex associated with mackinawite (tetragonal FeS) rather than as a discrete As₂S₃ phase. This study shows that mackinawite formation in contaminated floodplain soil can help mitigate the extent of arsenic mobilization during prolonged flooding.