Arsenic is a toxic and carcinogenic metalloid, ubiquitous in the pedosphere and highly sensitive tofluctuations in soil redox conditions which dramatically influence both its toxicity and mobility. We reviewthe global biogeochemical cycle of arsenic in light of increasing resource usage and re-evaluate theimportance of anthropogenic and geogenic arsenic inputs to the exogenic cycle. Arsenic contamination isoften diffuse in European sedimentary basins. Despite this, concentrations in soil pore-water may be highduring periods of soil saturation caused by rising groundwater or surface flooding which is predicted toincrease due to climatic change. Standardless quantitative X-ray fluorescence spectrometry is used toanalyse for arsenic in moderately contaminated soils on the alluvial plain of the Saône River with samplepreparation protocols designed to optimize analytical precision and accuracy in-situ at trace arsenicconcentrations. Arsenic in these soils is shown to be associated with colloidal and clay sized iron andmanganese (hydr)oxides which undergo microbially mediated reductive dissolution during flooding, releasingsubstantial arsenic to the aqueous phase. If released arsenic is not subsequently removed with recedingflood water it is re-immobilized during soil oxidation and re-precipitation of metal oxides. We demonstratethrough a novel combination of wet chemistry, microbial ecology, spectroscopy and thermodynamic andkinetic modelling that sequential reduction-oxidation cycles result in aqueous arsenic attenuation duringreducing conditions due to increased co-precipitation and decreases in microbial activity due to depletion oflabile organic matter. Similar attenuation processes are observed in the absence of microbial activity for Crand As in pyrite-bearing clays when subjected to redox oscillations induced by addition of reduced humicsubstances. We demonstrate that the cumulative effects of successive redox cycling are therefore of greatimportance to contaminant mobility in a variety of environments.
