Optimization of electrochemical sequential reduction-oxidation of chlorophene with CoNi alloy anchored ionic liquid-graphene cathode: Comparison, mechanism and toxicity study

Electrochemical sequential reduction-oxidation is process shows great promise for decomposition of organic compounds in aqueous solution. We performed comparative experiments to investigate the effects of ionic liquid on catalytic cathode for chlorophene removal. Electrochemical sequential reduction-oxidation processes with CoNi nanoparticles anchored on the ionic liquid functionalized graphene (CoNi/IL-rGO) cathode showed better performance in terms of removal efficiency, mineralization, and dechlorination rate of chlorophene compared with the performance of a similar cathode without the ionic liquid (CoNi/rGO). The results suggest that the combination of graphene and ionic liquid promoted dispersion of CoNi nanoparticles and formation of small particle sizes, contributed to higher electrocatalytic activity. We investigated the factors affecting chlorophene degradation in the CoNi/IL-rGO cathode system in detail. Under the optimal conditions the removal rates of chlorophene reached 99.6%, 99.7%, and 95.1% in cathodic compartment 1, cathodic compartment 2, and the anodic compartment, respectively, at pH 5.3 after 120 min with 0.05 mol L-1 at a current of 68 mA cm(-2). The intrinsic reaction kinetics showed that removal of chlorophene followed pseudo first-order reaction kinetics and kinetic constants were calculated. Several reaction products were identified by liquid chromatography/mass spectrometry (LC-MS/MS) and ion chromatography (IC) and detailed a reaction pathway is proposed. In the cathodic compartment, reductive dechlorination occurred during bubbling of hydrogen to form dechlorination products. These intermediates were prone to further oxidization by center dot OH generated from H2O2 on the CoNi/ILrGO electrode under air. We also evaluated the toxicity of products by a luminescent bacteria assay, which confirmed that the toxicity of chlorophene was effectively eliminated during the electrochemical sequential reduction-oxidation process.