A dual-purpose route for polluted water
Antibiotic residues in water are difficult to manage because treatment systems must remove persistent contaminants without adding major energy or chemical burdens. In Carbon Research , researchers led by Fan Li and Chun Hu describe a photocatalyst designed to address both issues at once: degrading emerging contaminants while producing H₂O₂ through oxygen reduction under visible light.
The material is a zinc single-atom catalyst anchored on graphitic carbon nitride, with asymmetrically coordinated Zn–N₂O₂ sites . According to the article, this coordination environment polarizes surface charge, broadens light absorption, and promotes directional electron transfer. The authors frame the approach as a way to couple water purification with value-added chemical generation, potentially lowering the overall energy demand of treatment.
Why asymmetric zinc sites matter
The team synthesized the catalyst by anchoring zinc single atoms onto graphitic carbon nitride and then characterized its structure and electronic properties using microscopy, X-ray photoelectron spectroscopy, extended X-ray absorption fine structure analysis, electrochemical measurements, and electron paramagnetic resonance. These results supported the presence of isolated zinc atoms coordinated by two nitrogen and two oxygen atoms.
That local structure was associated with stronger pollutant adsorption and improved charge behavior. Compared with undoped carbon nitride, the zinc-containing catalyst showed enhanced visible-light absorption, higher photocurrent, lower charge-transfer resistance, and reduced photoluminescence intensity, all consistent with more effective charge separation and transfer . The authors also report that ciprofloxacin adsorption on the modified catalyst was stronger than on pristine carbon nitride.
Pollutants help drive the chemistry
A central point of the paper is that the adsorbed pollutant is not only a treatment target but also part of the reaction pathway. The authors found that ciprofloxacin acted as a molecular modifier , further increasing charge polarization and extending the light absorption edge of the catalyst. Under illumination, electrons associated with the adsorbed pollutant were transferred to O₂, supporting H₂O₂ formation while accelerating pollutant breakdown.
In performance tests, the zinc single-atom catalyst achieved a ciprofloxacin degradation rate constant of 0.061 min⁻¹, compared with 0.028 min⁻¹ for the undoped material, and produced H₂O₂ at up to 5.8 mM h⁻¹ g⁻¹ in air. The system also removed more than 95% of several contaminants, including ciprofloxacin, tetracycline, rhodamine B, and norfloxacin, within 60 minutes under the reported conditions.
Mechanism, robustness, and environmental context
Mechanistic experiments indicated that direct electron transfer at the catalyst surface dominated ciprofloxacin degradation, rather than hydroxyl radical chemistry. Electron paramagnetic resonance and quenching tests supported a pathway in which oxygen is reduced through a two-step single-electron route to H₂O₂, while the adsorbed pollutant undergoes oxidative cleavage. Density functional theory calculations were consistent with stronger charge polarization and greater electron capture by adsorbed O₂ on the Zn–N₂O₂ catalyst than on comparison models.
The catalyst also retained activity over five cycles, with more than 95% ciprofloxacin degradation and little change in zinc loading or structure after reuse. Additional tests suggested tolerance to a range of pH conditions, common inorganic anions, humic acid, and different water matrices, including municipal wastewater and raw drinking water. The paper further includes a laboratory-scale life cycle assessment indicating a lower environmental impact than several reported heterogeneous Fenton-type systems and better environmental performance than a conventional homogeneous Fe/PMS system.
Limits and next steps
The authors note that the life cycle assessment was conducted using laboratory-scale data, and they indicate that future work should reduce catalyst consumption and the material inputs required for catalyst preparation. The article also states that data are available on request, and practical deployment will require further validation under scaled treatment conditions and broader contaminant mixtures.
Even so, the work presents a clear concept: carefully designed single-atom photocatalysis can use pollutant-catalyst interactions to improve both contaminant removal and chemical production. By linking asymmetric coordination, interfacial electron transfer, and visible-light utilization, the paper offers a route toward more resource-recovering wastewater treatment strategies.
Carbon Research
Experimental study
Not applicable
Direct charge transfer mediated by asymmetrically coordinated Zn–N₂O₂ sites in graphitic carbon nitride toward water purification with hydrogen peroxide photosynthesis
31-Jul-2026
All authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.