Removal of 2-Methyl-4-Isothiazolin-3-One by VUV/UV/Persulfate for Sustainable Wastewater Reclamation: Effects of Inorganic Anions on the Concentrations and Contributions of Hydroxyl Radicals and Sulfate Radicals

The non-oxidizing antimicrobial 2-Methyl-4-Isothiazolin-3-one (MIT) poses a significant environmental risk given its frequent detection in municipal wastewater. This study showed that the combination of Vacuum UV/UV (VUV/UV) and persulfate (PDS) efficiently achieved the rapid transformation and removal of 10 μM MIT within 90 s, which is much faster than UV, UV/PDS, and VUV/UV. Increasing the PDS dosage improved MIT degradation, whereas changes in pH between 4 and 10 had little effect. Radical quenching experiments showed that 93% of the MIT oxidation was attributable to the hydroxyl radical (•OH) and the sulfate radical (SO4•−). SO4•− and •OH at concentrations of 8.6 × 10−12 M and 1.5 × 10−12 M accounted for 32% and 61% of the MIT degradation, respectively, and the greater contribution of •OH was attributed to its higher reaction rate constant with MIT compared to SO4•−. Sulfate had a negligible impact on the radical concentrations. Chloride (1 mM) reduced the SO4•− and •OH concentrations by 61% and 27%, respectively. And the SO4•− contribution to MIT degradation fell to 19%. Nitrate (5 mM) readily quenched •OH but minimally affected SO4•−. The •OH concentration decreased by 79%, reducing its contribution to 27%. Bicarbonate/carbonate (5 mM) simultaneously reduced the SO4•− and •OH by 26–30% and had little effect on their contributions. Because of the quenching effect of organic matter and inorganic anions on radicals, secondary effluent inhibited the degradation of MIT. After a 120 s treatment, the total organic carbon, UV254, and fluorescence regional integration were reduced by 5%, 8%, and 17–24%, respectively. This study provides a quantitative analysis of how inorganic ions alter the concentrations and contributions of •OH and SO4•−, elucidating the MIT removal mechanisms in VUV/UV/PDS for sustainable wastewater reclamation.