Chemical and Physical Processes Governing Chromium(VI) Removal by Continuous Flow-Through Iron Electrocoagulation for Drinking Water Treatment

Abstract

Iron electrocoagulation (EC) is a promising approach for removing hexavalent chromium [Cr(VI)] from water. Understanding the chemical and physical processes that govern Cr(VI) removal under dynamic and environmentally relevant conditions can enable optimal designs of treatment systems. This study evaluated the rates, extents, and mechanisms of Cr(VI) removal in a flow-through EC reactor followed by a flocculation chamber and a settling basin. Using a short EC reactor residence time, low iron doses, and realistic water chemistries, the experiments revealed both advantages and challenges of EC. In water containing only Cr(VI) and NaCl, more than 98% of 500 μg/L Cr(VI) was removed using 2.5 mg/L Fe with an 11 s EC residence time. Cr(VI) was removed by reduction and formation of Cr(III)–Fe(III) precipitates. In the presence of dissolved silica and bicarbonate, Cr(VI) was chemically reduced, but the resulting Cr(III)–Fe(III) particles persisted at sizes that were difficult to separate by microfiltration (0.22 μm). Higher ionic strength and calcium mitigated these inhibitory effects by promoting particle aggregation that allowed for better particle removal. Near-complete Cr(VI) removal was achieved in a complex groundwater within the 11 s EC residence time at iron doses ≥5 mg/L. These results demonstrate that both chemical and physical processes need to be considered to accurately anticipate chromium behavior during treatment and to ensure the optimal design of treatment systems.