Organic matter control on the reactivity of Fe(III)-oxyhydroxides and associated As in wetland soils: A kinetic modeling study

Iron (Fe) reactivity and arsenic (As) reactivity in wetland soils were studied by applying a generalized dissolution rate law to data recovered from reductive dissolution experiments using As-bearing-Fe(III)-oxyhydroxides (ferrihydrite and lepidocrocite). Although As does not correspond to a separate mineral, the kinetic law can be successfully used to investigate the dynamics of As. This was possible as As was coprecipitated in all the tested Fe(III)-oxyhydroxides. The generalized rate law was also applied to available published and here produced datasets of reduction experiments of Fe(III)-oxyhydroxides (with reducing agent: ascorbate, Shewanella putrefaciens, purified soil Fe(III)-reducing bacteria and no purified autochthonous wetland soil bacteria). A comparison of the calculated kinetic parameters and modeling demonstrates that Fe reactivity is strongly increased in the wetland soil as compared to simple bacterial reduction experiments. Dissolved organic matter appears to be a key factor in the control of the Fe(III)-oxyhydroxide dissolution rate. More specifically, organic matter by strongly binding Fe(II) prevents Fe(II) readsorption and subsequent Fe secondary mineral formation, both of which are known to strongly decrease Fe(III)-oxyhydroxide dissolution rates. Arsenic solubilization is driven by Fe dissolution with the extent of the reduction pathway and therefore indirectly by the occurrence of dissolved organic matter. In this type of organic environment, where the formation of Fe secondary minerals is reduced or inhibited, As is not taken up and is thereby strongly solubilized. Therefore, wetlands appear to be favorable areas for the active transfer of As from the soil to both surface- and ground-waters.