Electrogenerated oxychlorides induced overlooked negative effects on electro-oxidation wastewater treatment in terms of over-evaluated COD removal efficiency and biotoxicity.

The high-efficiency and environmentally-friendly electro-oxidation (EO) would lose its competitive edge because of the production of oxychloride by-products (ClO), which has not yet drawn significant attention in academic and engineering communities. In this study, the negative effects of the electrogenerated ClO were compared among four commonly used anode materials (BDD, TiO, PbO and Ru-IrO) in terms of ClO interference on the evaluation of electrochemical COD removal performance and biotoxicity. Apparently, the COD removal performance of various EO systems were highly enhanced with increasing current density in the presence of Cl, e.g., the amounts of COD removed by various EO systems from the phenol solution with an initial COD content of 280 mg L at 40 mA cm within 120 min decreased in the order: TiO of 265 mg L > BDD of 257 mg L > PbO of 202 mg L > Ru-IrO of 118 mg L, which was different from the case with the absence of Cl (BDD of 200 mg L > TiO of 112 mg L > PbO of 08 mg L > Ru-IrO of 80 mg L) and the results after removing ClO by anoxic sulfite-based method (BDD of 205 mg L > TiO of 160 mg L > PbO of 153 mg L > Ru-IrO of 99 mg L). These results can be ascribed to the ClO interference on COD evaluation, the extent of which decreased in the order: ClO > ClO (where ClO cannot impact COD test). The highest overrated electrochemical COD removal performance of TiO may be associated with its relatively high production of ClO and the low mineralization extent. The chlorella inhibition ratio of ClO decreased in the order: ClO > ClO >> ClO, which accounted for the biotoxicity increasement of the treated water (PbO 68%, TiO 56% BDD 53%, Ru-IrO 25%). Generally, the inevitable problems of overrated electrochemical COD removal performance and biotoxicity increasement induced by ClO should deserve significant attention and effective countermeasures should be also developed when employing EO process for wastewater treatment.