Could Magnetite Be a Stable and Promising Iron Mineral Phase for Immobilizing Arsenic?.

Remediation approaches for Arsenic (As) contamination have been attracting scientists’ attentions for a long time, in part because successful strategies are difficult to develop and implement. One of the most commonly attempted remediation approaches involves stimulating reductive transformations that affect the concentration of As in solution; however iron (Fe) and/or As reduction also is commonly a principal cause of As problems. Recent studies have found that magnetite (Fe₃O₄) is capable of trapping a number of cations and also As(V) into its structure, making it a potential target mineral for the stabilization of As soil contamination.  And, magnetite is stable under typical Fe-reducing conditions where As is commonly mobilized and bioavailable, making it a promising long-term solution for As-affected areas. Despite magnetite’s potential to As remediation, its formation in soils and the factors that promote its production and incorporation of dissolved As are poorly established. To address this, we did a series of laboratory batch incubations, using aquifer solids and groundwaters from the Vineland Superfund site – a site with considerable inventories of bioavailable As. We explored to stimulate the formation of As-bearing magnetite, and evaluated its capability of maintaining As retention. Microcosms were amended with various combinations of nitrate, sulfate, Fe(II) (added as ferrous sulfate), and organic matter which was added to encourage reduction. All Vineland incubations were conducted at pH 6, similar to site conditions but much lower than is normally considered ideal for magnetite formation. Among these treatments, the one containing nitrate, Fe(II) and organic matter sequestered As on solid quickly, yielded considerable magnetite within 2 weeks (about 10% of total solid-phase Fe, suggested by Fe EXAFS spectroscopy), and maintained low aqueous As concentrations over the duration of the experiment (more than 5 weeks). Arsenic XANES spectroscopy suggested that the majority of the solid-bound As was As(V), and the presence of a spectral feature at 11,884 eV indicated some of this As(V) was substituted in the magnetite structure. In contrast, no other treatments were effective at producing magnetite or cleaning up the dissolved As, and many actually mobilized As.