Kinetics, contributions, and pathways of the degradation of artificial sweeteners by primary and secondary radicals during UV/persulfate

UV/persulfate (UV/PS) is considered an effective process for the degradation of emerging micropollutants in aquatic media. However, under the influence of complex water matrices such as wastewaters, radicals created during UV/PS will be reduced and transformed, so the chemical process of effectively obtaining the radicals in the system is very important to improving degradation efficiency. Thus, in the study, neotame (NEO, an artificial sweetener), as an emerging contaminant, was selected as the target compound to investigate in terms of its degradation and the role of free radicals in a range of water matrices during the UV/PS process. Based on the low concentration probe method (probe concentration ≤ 0.2 μm, more than 3-fold improvement in radical detection accuracy), kinetic modeling was developed to determine the role of primary (•OH and SO₄•⁻) and secondary (e.g. Cl•, Cl₂⁻•, CO₃•⁻, and NO₂•) radicals. Results indicated that UV/PS was effective in decomposing NEO (>93.7 %) within 7 min and was mainly attributed to •OH and SO₄•⁻. Acidic environments promote NEO degradation with a greater contribution from SO₄•⁻. Natural organic matter inhibited NEO degradation by quenching radicals (especially •OH). The k_obs of NEO degradation in the presence of Cl⁻ remained almost unchanged due to the production of Cl• and Cl₂⁻• compensating the depletion of SO₄•⁻. The presence of HCO₃⁻ quenched a part of primary radicals, which led to a decrease in k_obs of NEO degradation, but CO₃•⁻ began to play a partial degradation role. In the presence of NO₃⁻, UV-activated production of •OH and NO₂• promoted NEO degradation. Based on 39 transformation products obtained, 3 degradation pathways and 7 radical attack ways were proposed for NEO degradation by primary and secondary radicals in the UV/PS system. This study provides meaningful insight into the role of primary and secondary radicals in NEO degradation using UV/PS systems.