A comparative study on the efficient removal of hydroxychloroquine via UV/peroxymonosulfate and UV/peroxydisulfate systems triggered by reactive oxygen species: Process behaviors and the functional relevance of singlet oxygen

The efficient removal of hydroxychloroquine (HCQ) by UV/ peroxymonosulfate (UV/PMS) and UV/peroxydisulfate (UV/PDS) treatments triggered by reactive oxygen species (ROS) was comprehensively compared in terms of process behaviors and the functional relevance of singlet oxygen (¹O₂). Both UV/PMS and UV/PDS can remove HCQ quickly and the elimination rate followed the trend as UV/PMS > UV/PDS > UV/H₂O₂. The individual contribution of ROS was determined and the reaction rates of ROS with HCQ were ranked as SO₄•⁻ (1.44 × 10¹⁰ M⁻¹·s⁻¹) > HO• (4.03 × 10⁹ M⁻¹·s⁻¹) >¹O₂ (1.85 × 10⁸ M⁻¹·s⁻¹). The main operation factors influencing the HCQ degradation including UV intensity, PMS/PDS dosages, pH and water substrates were explored comparatively. UV/PMS showed strong resilience against interferences from natural organics and coexistent ions due to the action of ¹O₂. The radical and non-radical oxidation pathways of HCQ by ROS attack were postulated. The effects of pretreatments on disinfection by-products (DBPs) production obeyed the tendency as PMS/Cl₂ $\gg$ UV/PDS > UV/PMS. Although PMS/Cl₂ (for ¹O₂ scenario) induced massive DBPs over UV/PMS and UV/PDS, UV/PMS effectively regulated DBPs risk by cooperation of radical and non-radical routes. Comprehensive investigations concentrating on the functional relevance of ¹O₂ regarding selective oxidability, resistance to water substrates and especially DBPs enhancement were conducted for the first time. The findings can also provide valuable inspirations for the UV/PMS application as a promising ¹O₂-related technology for the HCQ elimination, taking into accounts of trade-offs among high efficiency, acceptable cost, insensitivity to environmental disturbance and reduced toxicity associated with DBPs.