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.