Selenium (Se) can be released into water systems by natural weathering processes and anthropogenic activities such as surface mining, oil refining, and agricultural irrigation. We investigated Se(VI) in the form of the oxyanion SeO<sub>4</sub><sup>2-</sup> which has a high solubility and limited affinity for adsorption. Iron electrocoagulation (EC) is a promising method for Se removal. With the release of Fe(II) from the elemental iron anode and further formation of iron-containing solids in suspension, Se can be removed in iron EC by adsorption, surface-mediated reduction, and precipitation. In a series of batch experiments, we explored the effects of pH (4, 5, 6, 7 and 8) and dissolved oxygen (oxic, partially oxic and anoxic) on Se removal. The highest extents of removal of dissolved Se under oxic and partially oxic conditions occurred at pH 4. The highest extent of removal of dissolved Se under anoxic conditions was achieved at pH 8, and the dissolved Se concentration decreased from 1000 µg/L to below 5 µg/L within 20 min. Overall extents of removal were greater at anoxic conditions and under these conditions Se was more rapidly removed at higher pH. Removal at oxic and partially oxic conditions was primarily the result of Se(VI) adsorption to ferrihydrite and lepidocrocite. In contrast, under anoxic conditions dissolved Se removal was the result of adsorption and chemical reduction by mixed valence iron-containing solids that formed at pH 6 and above. For comparison with EC results, we performed batch experiments using chemical coagulation (CC). In focusing on the conditions that had the greatest Se removal by EC, we found that Se removal was more effective by EC than CC at pH 4 under oxic conditions. At anoxic conditions, the rate and extent of Se removal at pH 7 were greater for EC than for CC, while at pH 8 the performances of EC and CC were similar.

Effect of water chemistry on selenium removal by iron electrocoagulation

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