Interaction of As with organic matter and Fe (III)-organic matter associations

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Arsenic (As) contaminations in periodically water-logged soils such as Fluvisols are reported throughout the world. Generally, Fe(lll) (hydr)oxides (FHOs) control the soil solution concentrations of As and hence its bioavailability. Redox oscillations, however, can cause coprecipitation of FHOs with dissolved organic matter (QM). Besides of Sorption of As to pure FHOs, binding of As to OM via multivalent metal bridges (ternary As complexes) has been suggested as an important sorption mechanism of As. Therefore, FHO-OM coprecipitates are complex adsorbent mixtures whose capability to immobilize As may significantly differ from that of pure FHOs commonly used in laboratory studies. These coprecipitates might be altered upon redox oscillations with respect to both Fe mineralogy and OM. The cumulative effects of redox oscillations on As immobilization are unknown. Likewise, the mobilization potential of As in FHO-OM coprecipitates is unclear. The objectives of the project are to elucidate (i) the binding of As to OM via metal bridges, (ii) changes in coprecipitate composition upon redox oscillations and its effect on As sorption, and (iii) the mobilization of As from FHO-OM coprecipitates. To achieve these goals, batch and bioreactor experiments as well as diffusion in thin gradients (DGT) experiments will be conducted. By using analytical tools such as 57Fe-Mössbauer and synchrotron X-ray absorption spectroscopy, this project will help to gain a fundamental understanding of the abiotic controls of As mobility in organic-rich soils subject to redox oscillations.