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

The effect of SO42– 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 SO42– concentrations. More specifically, the presence of high SO42– concentrations acted to stabilize schwertmannite, retarding its transformation to goethite and allowing its partial persistence over the 400 day experiment duration. Elevated SO42– also decreased the extent of dissimilatory reduction of Fe(III) and As(V), instead favoring dissimilatory SO42– reduction. In contrast, where SO42– 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 SO42– 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 SO42–-rich conditions, despite the prevalence of dissimilatory SO42– reduction. The absence of an arsenic sulfide species under SO42–-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 SO42– availability can play in controlling solid-phase arsenic speciation, particularly arsenic–sulfur interactions, under reducing conditions in soils, sediments, and shallow groundwater systems