Effect of Chloride Ions on the Degradation of Phenol and the Formation of Chlorinated Byproducts in BDD and RuO2-IrO2/Ti Anode Systems

Electrochemical oxidation (EO) effectively degrades phenolic compounds in phenol-containing wastewater, but the chlorinated byproducts generated during the process raised concerns. In this study, degradation efficiency of phenol and formation rules of chlorinated byproducts in the presence of chlorine (Cl − )were systematically and comparatively studied in two EO systems with boron-doped diamond (BDD) and ruthenium-iridium-coated titanium (RuO 2 -IrO 2 /Ti) as anodes. The results showed that Cl − significantly enhanced phenol degradation efficiency of the two anode systems. At the same Cl − concentration, degradation efficiency of BDD-EO was superior to that of RuO 2 -IrO 2 /Ti-EO, it had a higher apparent removal rate for phenolic compounds, including phenol, 4-NP, 4-CP, BPA, and RES, and was less affected by coexisting ions. However, in the BDD-EO system, the total amount of produced chlorinated byproducts (3.593   mM Cl) was approximately 50 times greater than that produced in the RuO 2 -IrO 2 /Ti-EO system (0.0723   mM Cl). These chlorinated byproducts included inorganic chloro-byproducts (ClO 3 − and ClO 4 − ) and organic chloro-byproducts (such as chloroform, trichloroacetic acid, and trichloroacetaldehyde). The results confirmed that HOCl generated by the oxidation of Cl − on BDD and RuO 2 -IrO 2 /Ti anodes through the • OH/direct electron transfer was the key active species for enhancing the phenol degradation and driving the formation of chlorinated byproducts. The toxicity assessment shows that the additive toxicity of inorganic chloro-byproducts in the RuO 2 -IrO 2 /Ti-EO system is approximately 1/57 that in the BDD-EO system, and the toxicity of organic chloro-byproducts is approximately 1/18. Selecting the appropriate anode is therefore critical to balance treatment efficiency with the need to minimize toxic chlorinated byproducts.