Influencing factors and chlorinated byproducts in electrochemical oxidation of bisphenol A with boron-doped diamond anodes

The influencing factors and formation of chlorinated byproducts in electrochemical oxidation of bisphenol A (BPA) with the boron-doped diamond (BDD) were systematically investigated using response surface methodology (RSM). The initial BPA concentration, chloride concentration, pH, and current density were studied as independent variables and the degradation rate of BPA (k), current efficiency (CE) and amount of Cl consumed (Delta Cl) were selected as responses. The results showed that current density was the most influential factor on both k and Delta Cl, and there were strong interactions between the influencing factors for all three responses, attributable to their effects on active chlorine, hydroxyl radicals and the state of BPA in this system. Based on the criteria of achieving the effective degradation of estrogenic BPA with relatively higher current efficiency and less generation of chlorinated byproducts, a final solution with 0.06 mM BPA and 40 mM NaCl (pH 8) and a current density of 15 mA cm(-2) optimized through RSM was proved to give a BPA degradation rate of 0.318 min(-1), a CE of 54.93% and Delta Cl of 3.55 mM. This showed great advantages compared with Pt anodes under the optimized conditions. The specific orientation of different forms of chlorine was also analyzed under different operation condition. Active chlorine, chloroform, and perchlorate showed similar variation trends with the operating factors. The amount of chlorine consumed in 240 min was 5.03 mM for BDD anode (while only 0.30 mM for Pt under similar conditions), indicating more varieties and amounts of chlorinated byproducts in the BDD system compared to Pt system due to its stronger oxidizing ability. This was further demonstrated through identification of different forms of chlorine and gas chromatographymass spectrometry analysis. Besides generation of more perchlorate, chlorinated intermediates such as 1,1,2,2-tetrachloro-ethane, 2,2-bis(chloromethyl)-1-propanol, 1,1,1-tris(chloromethyl)-ethane, 2,3,4,6-tetrachloro-phenol, and pentachloro-phenol were specifically detected in the BDD cell. Although the entire toxicity of the whole BPA solution decreased obviously, the formation of these byproducts should be cautiously concerned during the electrolysis. Hereby, electrochemical oxidation should be conducted under lower current density in weakly alkaline electrolyte for actual water treatment projects polluted with organics and inevitable chloride ions. (C) 2017 Elsevier Ltd. All rights reserved.