New Mechanistic Insights into the Transformation of Reactive Oxidizing Species in an Ultraviolet/Sulfite System under Aerobic Conditions: Modeling and the Impact of Mn(II)

Sulfite [S­(IV)] photolyzed with 254 nm ultraviolet (UV) light was used as a surrogate for S­(IV)-based advanced oxidation processes (AOPs) to elaborate the S­(IV) chain transformation process, which plays a pivotal role in S­(IV)-based AOPs but is poorly understood. The level of degradation of ibuprofen in the UV/S­(IV) system increased with an increase in the initiation rate of S­(IV) chain transformation, and SO4 •– was the major oxidant. Trace Mn­(II) in the UV/S­(IV) process promoted ibuprofen degradation, S­(IV) transformation, and peroxymonosulfate production at pH 6.0–8.0 but inhibited these processes at pH 9.0. A mathematic model (model 1) was constructed for the UV/S­(IV) system, and it revealed that 40% of dosed S­(IV) was transformed to SO4 •– but ∼95% of the generated SO4 •– was reduced by S­(IV), resulting in the low oxidation capacity of the UV/S­(IV) system. Upon incorporation of the reactions related to Mn­(II)/Mn­(III) transformation into model 1, model 2 was built to simulate the transformation kinetics of S­(IV), ibuprofen, and peroxymonosulfate in the UV/Mn­(II)/S­(IV) system, which demonstrated that Mn­(III) tended to oxidize S­(IV) and worked as a chain carrier at pH 6.0–8.0 but was apt to disproportionate and acted as a chain terminator at pH 9.0. These results will guide the efforts to optimize the performance of S­(IV)-based AOPs.