Removal of roxarsone from water by Fe electrocoagulation: Efficacy, mechanisms, and toxicity evaluation

Fe electrocoagulation technology was proposed for roxarsone (3-nitro-4-hydroxyphenyl-arsonic acid, ROX) removal from water. The effects of current density (CD) and initial pH on ROX removal and operating costs were evaluated using batch tests. Results showed a high total arsenic (TAs) removal efficiency (≥ 99.5 %) at 40 min (CD 1.75–8.77 A/m2, pH 4.6–8.6). Sludge treatment cost accounted for the highest proportion of total operating costs. High concentrations of HCO3− and humic acid had significant inhibition impacts on TAs removal. Hydroxyl radicals (•OH) were mainly responsible for ROX oxidation during Fe electrocoagulation. The E. coli growth tests showed the non-toxicity of final effluent. The TCLP tests indicated that the As-laden sludge was a non-hazardous waste. Possible mechanisms for ROX removal including flocculation, electrostatic attraction, inner-sphere complexation, and coprecipitation were proposed through XPS, XRD, FTIR, and zeta potential analyses combined with adsorption tests. Continuous-flow Fe electrocoagulation tests (165 h) showed the average TAs removal efficiency of above 99.9 %. Due to the aggregation and aging of flocs, its main crystalline phase was maghemite, which was different from that in the batch tests (Fe(OH)3 and lepidocrocite). This study provided a simple, environmentally safe, and cost-effective strategy to remove ROX using Fe electrocoagulation.