Household-scale treatment units for reductive removal of hexavalent chromium from groundwater

Abstract

Legacy chromium-laden hazardous industrial solid waste has been a major reason for severe groundwater pollution in low-to-middle income countries, often impacting marginalized communities who lack access to centralized water treatment solutions for their potable needs. This study aimed to develop low-cost, simple-to-adopt, and robust decentralized treatment units for total Cr removal using methods that do not generate toxic sludge post-treatment. The research involved a systematic comparison of the extents and rates of Cr (VI) reduction in groundwaters using ferrous iron (Fe (II)) for chemical coagulation (CC) with Fe (II) sulphate and for electrocoagulation (EC) with iron electrodes in batch experiments over a range of Cr (VI) (1.5–13 mg L−1) and Fe(II) (3–42 mg L−1) concentrations. While complete Cr removal was achieved by CC within 5 min, the extent of Fe(II)-Cr(VI) reaction depended on initial Fe(II) dosage. In contrast, the reaction kinetics of EC was controlled by initial Cr(VI) concentration and Fe(II) dosing rate, with the extent of reaction determined by the duration of current applied. Time-dependent Cr(VI) and Fe(II) concentrations in groundwater were modelled using rate equations and constants derived from past studies performed on simpler matrices. Subsequently, treatment units that combined either CC- or EC-based Cr(VI) reduction with sand filtration, to remove generated Fe(III)-Cr(III)-oxyhydroxide colloids, were built to obtain potable groundwater. Efficiencies of Cr(VI) removal from spiked and real-contaminated groundwaters by CC- and EC-based treatment units were ~98 % and >99 %, respectively. Although EC-based unit was more efficient than CC-based unit for groundwaters with initial Cr(VI) >4 mg L−1, it exhibited longer treatment time. The costs of treated water for EC and CC were estimated to be USD 0.05 and USD 0.13 per 100 L, respectively. Results indicated that point-of-use decentralized Cr(VI) treatment units could be a promising solution for marginalized communities.