Key Takeaways & Executive Findings
- •• • CO2•− generated via 3CBBP* oxidizing HCOO− achieved 98.2% degradation of 10 μmol·L−1 metronidazole within 30 min under 8 mmol·L−1 HCOO− and 200 μmol·L−1 CBBP, demonstrating a highly efficient reductive degradation route for recalcitrant pollutants. • • The degradation efficiency increased with HCOO− concentration (up to 8 mmol·L−1) and remained unaffected across pH variations, indicating robustness for applications in diverse water chemistries without pH adjustment. • • Common anions (Cl−, NO3−, CO3^2−, low HCO3−) inhibited MNZ degradation, whereas high HCO3− (≥? mmol·L−1) slightly enhanced it; humic acid (HA) suppressed degradation proportionally to its concentration, highlighting matrix-specific interference that must be managed in real water treatment. • • The hν/CBBP/HCOO− system exhibited effective MNZ degradation in real water samples, underscoring its practical viability for treating micropollutants in natural and engineered aquatic systems.
Abstract
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.
1. Introduction
Conventional advanced oxidation processes (AOPs) rely on hydroxyl radicals (HO•) to degrade organic pollutants, but their non-selective reactivity often leads to incomplete mineralization and the formation of toxic byproducts, especially for electron-deficient compounds like nitroimidazoles and perfluorinated substances. Reductive pathways, particularly those involving carbon dioxide anion radicals (CO2•−), offer a promising alternative due to their high reduction potential and selectivity toward halogenated and nitroaromatic contaminants. However, the generation of CO2•− typically requires expensive reagents or complex photochemical systems, limiting its practical application.
This study addresses this bottleneck by exploiting the ubiquitous triplet states of dissolved organic matter (3DOM*) in surface waters to oxidize low molecular weight carboxylic acids (LCAs) such as formate, thereby producing CO2•− under solar irradiation. Using 4-benzoylbenzoic acid (CBBP) as a model 3DOM* precursor, the authors demonstrate a simple, cost-effective system that achieves rapid and efficient degradation of metronidazole, a common antibiotic pollutant. The findings not only elucidate a novel natural pathway for CO2•− formation but also provide a framework for developing sustainable water treatment technologies that harness solar energy and naturally occurring organic matter.
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WANG Jin, HOU Jie, LIU Huaying, ZHAO Qilin, SUN Donggou, LI Yingjie (2026). Excitation of Triplet State Dissolved Organic Matter Sensitizes Formic Acid to Generate CO2•− and Its Reductive Degradation of Metronidazole. Environmental Chemistry. https://doi.org/10.7524/j.issn.0254-6108.2024120701
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Frequently Asked Questions
What is the maximum degradation efficiency and optimal conditions for metronidazole removal in the hν/CBBP/HCOO− system?
Under optimal conditions of 8 mmol·L−1 HCOO−, 200 μmol·L−1 CBBP, and 10 μmol·L−1 metronidazole, the system achieved 98.2% degradation within 30 minutes under simulated sunlight.
How do common water constituents (anions and humic acid) affect the degradation process?
Chloride, nitrate, carbonate, and low concentrations of bicarbonate inhibited degradation, while high bicarbonate slightly promoted it. Humic acid inhibited degradation in a concentration-dependent manner, likely by scavenging reactive species or absorbing light.
Is the CO2•− generation pathway dependent on pH?
No, the degradation efficiency was found to be independent of initial pH, suggesting the system operates effectively across a wide pH range without adjustment.
What is the primary source of CO2•− in the system?
Electron spin resonance and quenching experiments confirmed that CO2•− originates mainly from the oxidation of formate by the triplet state of CBBP (3CBBP*), not from direct photolysis or other pathways.
Does the system perform effectively in real water matrices?
Yes, the hν/CBBP/HCOO− system exhibited good degradation performance for metronidazole in real water samples, indicating its potential for practical application in natural waters.
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