Enhanced degradation of pharmaceutical contaminants by medium-pressure UV irradiation in the presence of nitrate: Reactive species, toxicity and DBPs formation

This study comparatively investigated the degradation kinetics and mechanisms, changes in toxicity, and formation of disinfection byproducts (DBPs) for two pharmaceutical contaminants (PCs), emtricitabine (FTC) and trimethoprim (TMP), using the medium-pressure UV/nitrate (MPUV/NO₃⁻) process. Alkaline conditions markedly enhanced direct MPUV photolysis of both FTC and TMP, with FTC being more susceptible than TMP. The presence of 2 mM NO₃⁻ enhanced degradation rate (k_obs) by 1.2–2.3-fold for FTC and 1.8–10.8-fold for TMP under MPUV across different pH conditions, attributed to •OH and reactive nitrogen species (RNS) generation. Under the MPUV/NO₃⁻ process, both FTC and TMP showed a decrease in k_obs with increasing pH. Their k_obs initially increased with NO₃⁻ concentration but plateaued at higher levels. Structural differences led to distinct reactive species dominating FTC and TMP degradation. Anions and humic acid inhibited FTC and TMP degradation to varying degrees. Based on LC/TOF/MS analysis and DFT calculations, the cytosine moiety in FTC and the pyrimidine ring in TMP were identified as vulnerable sites, leading to the proposed degradation pathways. Most FTC transformation products showed increased developmental toxicity but decreased mutagenicity. Nitrated TMP products exhibited notably higher developmental toxicity. The Vibrio fischeri inhibition assay showed that the MPUV/NO₃⁻ process had the ability to control toxicity. Pretreatment with MPUV/NO₃⁻ enhanced the formation of trichloronitromethane during post-chlorination. Additionally, chloramination outperforms chlorination in controlling DBPs formation and their associated toxicity. Overall, this study provides critical insights into the transformation pathways and ecological risk assessment of PCs in nitrate-rich wastewater under polychromatic UV irradiation.