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Abstract

A novel method generates images of porewaterarsenic speciationto assess fine-scale redox cycling in contaminated soil and sediment,which is crucial to understanding arsenic mobility in the environment.
We present a newapproach combining diffusive equilibrium in thin-filmgels and spectrophotometric methods to determine the spatial distributionof arsenite, arsenate, and phosphate at submillimeter resolution.The method relies on the simultaneous deployment of three gel probes.Each retrieved gel is exposed to malachite green reagent gels differingin acidity and oxidant addition, leading to green coloration dependenton analyte speciation and concentration. Hyperspectral images of thegels enable mapping the three analytes in the 2.5-20 mu Mrange. This method was applied in a contaminated brook in the Harzmountains, Germany, together with established mapping of dissolvediron. The use of two-dimensional (2D) gel probes was compared to traditionalporewater extraction. The gels revealed banded porewater patternson a mm-scale, which were undetectable using traditional methods.Small-scale correlation analyses of arsenic and iron microstructuresin the gels suggested active iron-driven local redox cycling of arsenic.Overall, the results indicate continuous net release of arsenic fromcontaminant particles and deepen our understanding of arsenate transformationunder anaerobic conditions. This study is the first fine-scale 2Dcharacterization of arsenic speciation in porewater and representsa crucial step toward understanding the transfer and redox cyclingof arsenic in contaminated sediment/soil ecosystems.