Excitation of Triplet State Dissolved Organic Matter Sensitizes Formic Acid to Generate CO2•− and Its Reductive Degradation of Metronidazole
Surface waters contain numerous photoactive substances and low molecular weight carboxylic acids (LCAs). Hydroxyl radicals (HO•) can react with LCAs to generate the highly reducing carbon dioxide anion radical (CO2•−). Excited triplet state dissolved organic matter (3DOM*), a common oxidant in surface waters, may also oxidize LCAs to CO2•−, but this pathway remains unexplored. This study simulated sunlight-driven generation of CO2•− via 3DOM* using 4-benzoylbenzoic acid (CBBP) as a 3DOM* precursor and formate (HCOO−) as a model LCA. Metronidazole (MNZ) served as the target pollutant. Comparative degradation experiments in hν, hν/HCOO−, hν/CBBP, and hν/CBBP/HCOO− systems, combined with electron spin resonance spectroscopy and quenching tests, confirmed that CO2•− generated in the hν/CBBP/HCOO− system was the primary reactive species responsible for enhanced MNZ degradation, originating mainly from 3CBBP* oxidizing HCOO−. Under optimized conditions (8 mmol·L−1 HCOO−, 200 μmol·L−1 CBBP, 10 μmol·L−1 MNZ), 98.2% degradation was achieved within 30 min. Degradation efficiency increased with HCOO− concentration and was pH-independent. Cl−, NO3−, CO3^2−, and low concentrations of HCO3− inhibited degradation, while high HCO3− slightly promoted it. Humic acid (HA) inhibited degradation in a concentration-dependent manner. The system also performed well in real water matrices, suggesting potential for treating micropollutants via reductive pathways.