Iron (oxyhydr)oxides play an important role in controlling the mobility and toxicity of arsenic (As) in contaminated soils and groundwaters. Dynamic changes in subsurface geochemical conditions can impact As sequestration and remobilization since the fate of As is highly dependent on the dominant iron mineral phases present and, specifically, the pathways through which these form or transform. To assess the fate of arsenate [As(V)] in subsurface settings, we have investigated the Fe 2+ -induced

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... transformation of As(V)-bearing ferrihydrite (As(V)-FH) to more crystalline phases under environmentally relevant anoxic subsurface conditions. Specifically, we examined the influence of varying Fe 2+ (aq)/Fe(III)solid ratios (0.5, 1, 2) on the behavior and speciation of mineral-bound As species during the transformation of As(V)-FH to crystalline iron-bearing phases at circum-neutral pH conditions. At all Fe 2+ (aq)/Fe(III)solid ratios, goethite (GT), green rust sulfate (GRSO4) and lepidocrocite (LP) formed within the first 2 h of reaction. At low ratios (0.5 to 1), initially formed GRSO4 and/or LP dissolved as the reaction progressed, and only GT and some unreacted FH remained after 24 h. At Fe 2+ (aq)/Fe(III)solid ratio of 2, GRSO4 remained stable throughout the 24 h of reaction, alongside GT and unreacted As(V)-FH. Despite the fact that majority of the starting As(V)-FH transformed to other phases, the initially adsorbed As was not released into solution during the transformation reactions and ~99.9% of it remained mineral-bound. Nevertheless, the initial As(V) became partially reduced to As(III), most likely because of the surface-associated Fe 2+ -GT redox couple. The extent of As(V) reduction increased from ~34% to ~40%, as the Fe 2+ (aq)/Fe(III)solid ratio increased from 0.5 to 2. Overall, our results provide important insights into transformation pathways of iron (oxyhydr)oxide minerals in As contaminated, anoxic soils and sediments, and demonstrate the impact that such transformations can have on As mobility and also importantly oxidation state and, hence, toxicity in these environments.