The enhanced stability of arsenic coprecipitated with magnetite during aging: An XAS investigation

The coprecipitation of magnetite (Fe3O4) with arsenic (As) is a potential remediation technique for As-contaminated groundwater that can be applied to meet increasingly stringent As drinking water limits. However, knowledge of the fate of As coprecipitated with magnetite during aging for extended periods is lacking, which is critical to predict the long-term efficiency of this As treatment strategy. In this work, I combined aqueous As measurements with solid-phase characterization by synchrotron-based Fe and As K-edge X-ray absorption spectroscopy (XAS) to track the transformation of magnetite and the speciation of coprecipitated As(V) or As(III) for up to a year in oxic or anoxic conditions. It was determined that the initial magnetite particle increased in crystallinity for all aging experiments, but some differences in solid-phase Fe speciation were detected depending on aging conditions. For the anoxic aging samples with initial As(V), a significant fraction (15% of the total Fe) of maghemite (a magnetic Fe oxide spinel with formula γ-Fe2O3) was identified, which was coupled to As(V) reduction [As(III) was ∼30% of the total sorbed As], suggesting electron transfer between magnetite and particle-bound As(V). In the oxic aging experiments, the initial particle crystallized, with a large fraction of Fe(III) (oxyhydr)oxides (i.e., maghemite and lepidocrocite, γ-FeOOH) in the final products. Despite increased crystallinity suggested by Fe XAS analysis, sorbed As was not released from the particles in any experiment (aqueous As never exceeded 1 μg/L). This remarkable stability of As coprecipitated with magnetite was revealed by As K-edge XAS to be largely due to the formation of distinct multinuclear As uptake modes [i.e., As(V) incorporation; hexanuclear 3C As(III) complexes]. These results demonstrate the unique potential of magnetite for long-term As sequestration.