Quantitative analysis of free radical generation during nitrite degradation in antibiotic-containing aquaculture wastewater under long-wave ultraviolet radiation

This study developed a long-wave ultraviolet (UVA)/nitrite (NO 2 - ) system to degrade antibiotics in aquaculture wastewater, using NO 2 - as a treatment agent in line with the “waste-to-treat-waste” approach. It further advanced the understanding of reactive nitrogen species (RNSs)-mediated oxidation by quantifying radical dynamics, assessing transformation product ecotoxicity, and evaluating system performance under varying environmental conditions. The degradation of sulfamethoxazole (SMX), a model antibiotic, was primarily driven by RNSs, with nitric oxide radicals (NO•) emerging as the most impactful species. Steady-state analysis revealed that the RNSs concentration ([RNSs] SS = 1.79 × 10 −13 M) was significantly higher than that of hydroxyl radical (•OH) ([•OH] SS = 2.25 × 10 −14 M). Among RNSs, NO• was dominant ([NO•] SS = 1.51 × 10 −13 M), followed by nitrogen dioxide radicals (NO 2 •, [NO 2 •] SS = 2.76 × 10 −14 M) and peroxynitrite (ONOO⁻, [ONOO⁻] SS = 4.19 × 10 −18 M). RNSs contributed 55.97 % to SMX degradation, surpassing UVA (3.23 %) and •OH (40.80 %). Furthermore, ten transformation products were identified, showing relatively low toxicity and minimal ecological impact. Additionally, the UVA/NO 2 ⁻ system remained stable across varying temperatures and anion concentrations, making it promising for real-world wastewater treatment. This study establishes a quantified methodology for RNSs, providing critical insights into their kinetic behavior and role in antibiotic degradation.