Overlooked role of CO3·- reactivity with different dissociation forms of organic micropollutants in degradation kinetics modeling: A case study of fluoxetine degradation in a UV/peroxymonosulfate system.

Abstract

Selective oxidizing agent carbonate radical (CO₃^•-) is an important secondary radical in radical-based advanced oxidation technology for wastewater treatment. However, the role of CO₃^•- in removing ionizable organic micropollutants (OMs) under environmentally relevant conditions remains unclear. Herein we investigated CO₃^•- effect on degradation kinetics of fluoxetine in UV/peroxymonosulfate (PMS) system based on a built radical model considering CO₃^•- reactivity differences with its different dissociation forms. Results revealed that the model, which incorporated CO₃^•- selective reactivity (with determined second-order rate constants, k_src,CO3·-, of 7.33 ×10⁶ and 2.56 ×10⁸ M^-1s^-1 for cationic and neutral fluoxetine, respectively) provided significantly more accurate predictions of fluoxetine degradation rates (k). A good linear correlation was observed between k_src,CO3·- from experiments and literatures for 24 ionizable OMs and their molecular orbital energy gaps and oxidation potentials, suggesting the possible electron transfer reaction mechanism. Cl^- slightly reduced the degradation rates of fluoxetine owing to rapid transformation of Cl^• with HCO₃^- into CO₃^•-, which partially compensated for the quenching effects of Cl^- on HO^• and SO₄^•-. Dissolved organic matter significantly quenched reactive radicals. The constructed kinetic model successfully predicted fluoxetine degradation rates in real waters, with CO₃^•- being the dominant contributor (∼90 %) to this degradation process.