Chlorinated Byproduct Formation during the Electrochemical Advanced Oxidation Process at Magnéli Phase Ti₄O₇...

This research investigated chlorinated byproduct formation at Ti₄O₇ anodes. Resorcinol was used as a model organic compound representative of reactive phenolic groups in natural organic matter and industrial phenolic contaminants and was oxidized in the presence of NaCl (0-5 mM). Resorcinol mineralization was >68% in the presence and absence of NaCl at 3.1 V/SHE (residence time = 13 s). Results indicated that ∼4.3% of the initial chloride was converted to inorganic byproducts (free Cl₂, ClO₂^-, ClO₃^-) in the absence of resorcinol, and this value decreased to <0.8% in the presence of resorcinol. Perchlorate formation rates from chlorate oxidation were 115-371 mol m^-2 h^-1, approximately two orders of magnitude lower than reported values for boron-doped diamond anodes. Liquid chromatography-mass spectroscopy detected two chlorinated organic products. Multichlorinated alcohol compounds (C₃H₂Cl₄O and C₃H₄Cl₄O) at 2.5 V/SHE and a monochlorinated phenolic compound (C₈H₇O₄Cl) at 3.1 V/SHE were proposed as possible structures. Density functional theory calculations estimated that the proposed alcohol products were resistant to direct oxidation at 2.5 V/SHE, and the C₈H₇O₄Cl compound was likely a transient intermediate. Chlorinated byproducts should be carefully monitored during electrochemical advanced oxidation processes, and multibarrier treatment approaches are likely necessary to prevent halogenated byproducts in the treated water.