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/m(2), 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 (center dot 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.