Sulfate availability drives divergent evolution of arsenic speciation during microbially mediated reductive transformation of schwertmannite

The effect of SO₄^2– availability on the microbially mediated reductive transformation of As(V)-coprecipitated schwertmannite (Fe₈O₈(OH)_3.2(SO₄)_2.4(AsO₄)_0.004) was examined in long-term (up to 400 days) incubation experiments. Iron EXAFS spectroscopy showed siderite (FeCO₃) and mackinawite (FeS) were the dominant secondary Fe(II) minerals produced via reductive schwertmannite transformation. In addition, 25% to 65% of the initial schwertmannite was also transformed relatively rapidly to goethite (αFeOOH), with the extent of this transformation being dependent on SO₄^2– concentrations. More specifically, the presence of high SO₄^2– concentrations acted to stabilize schwertmannite, retarding its transformation to goethite and allowing its partial persistence over the 400 day experiment duration. Elevated SO₄^2– also decreased the extent of dissimilatory reduction of Fe(III) and As(V), instead favoring dissimilatory SO₄^2– reduction. In contrast, where SO₄^2– was less available, there was near-complete reduction of schwertmannite- and goethite-derived Fe(III) as well as solid-phase As(V). As a result, under low SO₄^2– conditions, almost no Fe(III) or As(V) remained toward the end of the experiment and arsenic solid-phase partitioning was controlled mainly by sorptive interactions between As(III) and mackinawite. These As(III)–mackinawite interactions led to the formation of an orpiment (As₂S₃)-like species. Interestingly, this orpiment-like arsenic species did not form under SO₄^2–-rich conditions, despite the prevalence of dissimilatory SO₄^2– reduction. The absence of an arsenic sulfide species under SO₄^2–-rich conditions appears to have been a consequence of schwertmannite persistence, combined with the preferential retention of arsenic oxyanions by schwertmannite. The results highlight the critical role that SO₄^2– availability can play in controlling solid-phase arsenic speciation, particularly arsenic–sulfur interactions, under reducing conditions in soils, sediments, and shallow groundwater systems.