Enhanced Immobilization of Arsenic from Acid Mine Drainage by Detrital Clay Minerals

Arsenic (As) is a potent carcinogen and the most common metal(loid) contaminant in drinking water sources globally. In acidic, Fe-rich systems, nanocrystalline Fe(III) precipitates (Fe(III)(NP)) are the main scavengers of As. However, the redox cycling of Fe(III)(NP) highly enhances As mobility and bioavailability. Notably, the irreversible release of As in runoff resulting from reductive dissolution of Fe(III)(NP) makes the effective remediation of As an ongoing environmental challenge. Here, we show for the first time that detrital clay minerals originating from the partial weathering of coal mining waste substantially increased total As uptake by acid mine drainage (AMD) sediments. The As immobilization mechanisms by the AMD sediments were investigated by the combined use of microbial community structure characterization (16S rRNA), chemical extractions, and synchrotron-based X-ray fluorescence (XRF), diffraction (XRD), and absorption (XANES). The use of an X-ray spot size as small as one micrometer allowed a detailed examination of the heterogeneous AMD sediments. Our results suggest that during sustained redox cycling of iron in Fe(III)(NP)-clay mixed-mineral systems, the clays controlled As mobility by (1) enhancing heterogeneous precipitation of Fe(III)(NP) under oxic conditions, which then adsorbed or incorporated As; and (2) facilitating the transfer of As from Fe(III)(NP) to clay during microbially mediated reduction of Fe(III)(NP) coatings under anoxic conditions. Designing remediation strategies that incorporate clay could become a promising low-cost strategy for As remediation in mining-impacted areas.