Elucidation of pH-Dependent Pathways for Benzophenone 1 Transformation by Heat-Activated Peroxydisulfate: A Combined Mechanistic and Environmental Assessment

Benzophenone 1 (BP1), a transformation product (TP) of UV filter benzophenone 3, was investigated for degradation by heat-activated peroxydisulfate (S2O82–). The study focused on degradation kinetics, radical speciation, TPs, ecotoxicity, and matrix effects. Complete BP1 degradation was achieved within 60 min at 50 °C, while kinetic experiments conducted at 40 °C revealed a strong pH dependence of the reaction rate. Radical-identification experiments showed sulfate radicals (SO4·–) as the dominant oxidant across tested conditions, with hydroxyl radicals (HO·) contributing marginally. Transformation pathways were governed by solution chemistry: acidic conditions favored carbonyl cleavage forming benzoic acid, whereas slightly alkaline conditions (pH = 8) promoted opening of the dihydroxy-substituted aromatic ring, yielding propenyl ketone derivates, supported by density functional theory (DFT) calculations. Predictive ecotoxicity assessment using the U.S. EPA Ecological Structure–Activity Relationships model (ECOSAR) indicated a higher aquatic toxicity for some TPs. However, experimental bioassays with Lemna minor revealed no significant adverse effect of either BP1 or treated solutions. Matrix components further modulated transformation, with nitrite inhibiting degradation, bicarbonate enhancing reactivity, and real water matrices exhibiting complex combined effects. These findings highlight the critical role of solution chemistry and temperature in controlling the S2O82– -based oxidation performance and TP-related risks.