Mechanism of p‑Substituted Phenol Oxidation at a Ti₄O₇ Reactive Electrochemical Membrane

This research investigated the removal mechanisms of p-nitrophenol, p-methoxyphenol, and p-benzoquinone at a porous Ti₄O₇ reactive electrochemical membrane (REM) under anodic polarization. Cross-flow filtration experiments and density functional theory (DFT) calculations indicated that p-benzoquinone removal was primarily due to reaction with electrochemically formed OH^•, while the dominant removal mechanism of p-nitrophenol and p-methoxyphenol was a function of the anodic potential. At low anodic potentials (1.7–1.8 V/SHE), p-nitrophenol and p-methoxyphenol were removed primarily by an electrochemical adsorption/polymerization mechanism on the REM. Increasing anodic potentials (1.9–3.2 V/SHE) resulted in the electroassisted adsorption mechanism contributing far less to p-methoxyphenol removal compared to p-nitrophenol. DFT calculations indicated that an increase in anodic potential resulted in a shift in p-methoxyphenol removal from a 1e^– direct electron transfer (DET) reaction that resulted in radical formation and significant adsorption/polymerization, to a 2e^– DET reaction that formed nonadsorbing products (i.e., p-benzoquinone). However, the anodic potentials were too low for the 2e^– DET reaction to be thermodynamically favorable for p-nitrophenol. The decreased COD adsorption for p-nitrophenol at higher anodic potentials was attributed to reaction of soluble/adsorbed organics with OH^•. These results provide the first mechanistic explanation for p-substituted phenolic compound removal during advanced electrochemical oxidation processes.