Hexavalent chromium, Cr(VI), is a groundwater contaminant and a Class-1 carcinogen in humans when ingested. To protect public health, in 2014 California set a Maximum Contamination Level (MCL) of 10 μg/L for Cr(VI) in drinking water. However, in 2017 this MCL was retracted due to a court ruling based on the economic infeasibility of using current remediation technologies to comply with a 10 μg/L MCL. Iron Electrocoagulation (Fe-EC) is a low-cost electrochemical process that uses an Fe(0) anode to produce <i>in-situ</i> Fe(II) ions that can reduce Cr(VI) to the insoluble Cr(III) species. Fe-EC batch reactors with two parallel plates have shown near 100% removal efficiencies for initial Cr(VI) concentrations as high as 20 mg/L. Nevertheless, these bench-scale reactors require high electric potentials to achieve low current densities, which can result in high energy demands if scaled-up, thus limiting the feasibility of using Fe-EC on larger scales. To achieve high Fe(II) dosage rates (mM/min) in a more energy-efficient manner, we implemented a novel spiral reactor design that significantly lowers ohmic resistance by decreasing the inter-electrode distance and increasing the surface area of the electrodes immersed in the electrolyte (Figure 1). Removal efficiencies under varying pH and operating conditions are investigated using synthetic California groundwater. This electrolyte contains an initial Cr(VI) concentration of 500 μg/L and relevant concentrations of co-occurring ions as well as species such as natural organic matter (NOM) known to inhibit Cr(VI) removal. Using ICP-MS, initial and final dissolved Cr(VI) concentrations are obtained. The final sludge is later studied with XRD and XANES for elemental composition and oxidation state, crystallography, and coordination geometry. Successful continuous flow treatment with high removal efficiencies, lower energy requirement, and a stable final sludge demonstrate the potential for this novel Fe-EC design to effectively remediate Cr(VI) contaminated groundwater on larger scales and at lower costs.

Rapid removal of hexavalent chromium in groundwater using a novel low-cost iron electrocoagulation reactor

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