Aging and arsenite loading control arsenic mobility from ferrihydrite-arsenite coprecipitates

Iron (oxyhydr)oxides may act as a sink and source of arsenic (As) in terrestrial and aquatic systems, therefore studying the mechanisms controlling Fe (oxyhydr)oxide transformation under changing environmental conditions is essential for elucidating the fate of As, in terms of release, mobility and speciation. We provide evidence that aging and As(III) loading control the mineralogical and structural evolution of As(III)-bearing Fe (oxyhydr)oxides, and thus influence the extraction of As with compounds which frequently occur in soil solution. During a one year timeframe, partial As(III) oxidation to As(V) and accumulation on poorly crystalline Fe (oxyhydr)oxides were promoted by increasing As(III) contents, which hindered the transformation of less into more crystalline materials. The changes in crystallinity degree and surface properties resulted in the release of different proportions of As(III) and As(V) from the aged compared to the fresh coprecipitates. The extraction of both As species through mechanisms of competition, disaggregation of Fe (oxyhydr)oxides, or both was assessed by employing several anions, whose efficiency decreased in the order: inorganic phosphate > citrate > organic phosphate > chloride ≥ silicate. The tested extractants are key components of environments such as aquifers, waterlogged soils or the rhizosphere of paddy soils, where aqueous As(III) is mobilized from the anoxic soil layers and sequestered by Fe (oxyhydr)oxides under more oxidizing environments.