The adverse impacts of produced ClOx- on assessing electrochemical oxidation performance for COD removal and biotoxicity

The effects of oxychlorides (ClOx-) produced by electrochemical oxidation of real wastewater on evaluation of COD removal performance and biotoxicity are rarely reported. In this study, an electrolytic reaction system with Ti/RuO2-IrO2-SnO2-Sb2O5 as anode was constructed to treat the air floatation produced water and effluent water. The experimental results showed that increasing the current density promoted the production of ClOx- and exacerbated the overestimation of the COD removal efficiency in electrochemical oxidation when treating the two types of water, mainly due to the produced ClO3- acted as a mask for the determination of COD by the dichromate method. When treating the air floatation produced water, ammonia nitrogen and organic matter in the water consumed HClO/ClO- and inhibited the production of ClOx- by blocking the conversion of HClO/ClO- to ClO3-. As a result, the degree of over-evaluation of COD removal was relatively low in this case. Further analysis by cyclic voltammetry revealed that the indirect oxidation of ammonia nitrogen could rapidly capture and react with active chlorine, thus inhibiting the formation of ClOx-. The Chlorella bioassay experiments showed that toxic by-products were formed during electrolysis of chlorinated wastewater, and ClOx- was the main substance for the increase of biotoxicity.