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 (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>·</mo><msubsup><mi mathvariant="normal">O</mi><mn>2</mn><mo>-</mo></msubsup></mrow></semantics></math></inline-formula>) and carbonate radicals (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mrow><mi>CO</mi></mrow><mn>3</mn><mrow><mrow><mo>•</mo><mo>-</mo></mrow></mrow></msubsup></mrow></semantics></math></inline-formula>) 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.