Self-Promoting Mechanism of Oxalic Acid in Ce–CN-Assisted Photocatalytic Ozonation for Water Treatment

Small molecular carboxylic acids (SCAs) are common in natural waters, but their impact on advanced oxidation processes (AOPs) is understudied. This study investigates the effects of various carboxylic acids, such as oxalic acid (OA), formic acid (FA), acetic acid (AA), succinic acid (SA), and benzoic acid (BA), on a Ce single-atom-doped graphitic carbon nitride (Ce–CN) photocatalytic ozonation (PCO) system. Among the tested SCAs, only OA significantly promoted O3 activation and pollutant degradation. When degrading atrazine (ATZ), Ce–CN performed worse than CN, but Ce–CN exhibited superior activity when OA was the target. The OA removal was 98% in the PCO/Ce–CN system, while it was 82% in the PCO/CN system in 30 min. The introduction of OA also enhanced ATZ removal from 55 to 86% within 5 min in the PCO/Ce–CN system. Moreover, OA’s yield during macromolecular organic compound degradation and its proportion among SACs affected PCO/Ce–CN activity. Mechanistic studies showed that OA could bind with Ce–CN, forming a Ce–OA complex that facilitated ligand-to-metal electron transfer (LMET), reducing the redox potential of the Ce(III)/Ce(IV) cycle, enhancing the ATZ removal efficiency, and boosting ozone activation. O3 decomposed into *O and •OH on Ce–CN, which were the primary reactive oxygen species for pollutant degradation. These findings highlighted the unique role of OA in Ce-based PCO systems, offering insights for enhancing AOP performance.