Enhanced Removal of Photosensitive Antibiotics in Water Using CO2: A Beneficial Exploration of CO2 Resource Utilization

The utilization of carbon dioxide (CO2) offers an effective approach for alleviating the carbon-reduction pressures associated with fossil energy consumption. However, studies on the use of CO2 as an auxiliary agent in water treatment to enhance the removal of emerging contaminants are limited. In this study, the photodegradation of ciprofloxacin (CIP) was investigated using ultraviolet (UV) irradiation combined with CO2 dosing (UV/CO2). The results demonstrated that the UV/CO2 system effectively degraded CIP, with CO2 concentration and solution pH exerting a critical influence. Inorganic anions and metal cations had negligible effects on CIP degradation efficiency, whereas natural organic matter (NOM) had a pronounced inhibitory effect. Mechanistic analysis revealed that superoxide radicals (·O2-) and carbonate radicals (CO3•-) were the primary oxidizing species, whereas the excited triplet state of CIP (3CIP*) and singlet oxygen played crucial roles in initiating radical generation. LC–MS analysis and density functional theory calculations indicated that the main degradation routes involved defluorination, decarboxylation, and epoxidation of the piperazine ring. Toxicity assessment indicated that the transformation products generated by UV/CO2 were less toxic than the parent compound. Furthermore, the UV/CO2 process demonstrated high energy efficiency, with a low electrical energy per order (EEO) value of 0.4193 kWh·m−3·order−1. These findings suggest that the UV/CO2 system is a promising alternative for the treatment of photosensitive organic pollutants and provides a beneficial pathway for CO2 utilization.