A Straightforward Approach to Establish the High-Purity Single Sulfate Radical System: Determination of Organic Contaminants’ Second-Order Reaction Rates and QSAR Modeling

Accurate determination of sulfate radical (SO4•–) reaction kinetics is critical for advancing contaminant degradation in water treatment processes. While SO4•–-based advanced oxidation processes exhibit high reactivity, higher than 4 order of magnitude variations in reported second-order rate constants kSO4·− for organic contaminants compromise the reliability of existing kinetic models, particularly for emerging contaminants. A high-purity SO4•– system based on the UV/persulfate system (>99.8%, with a steady-state concentration of 3.66 × 10–13 M) was developed through optimized oxygenation (8.0 mg/L DO) and radical quenching (400 mM tert-butyl alcohol) at pH 7.0. Experimental determination revealed distinct kSO4·− ranges: (2.8–9.2) × 109 M–1·s–1 for phenols, (3.0–6.5) × 109 M–1·s–1 for alkoxybenzenes, (5.3–17.5) × 109 M–1·s–1 for anilines, and (0.29–1.08) × 108 M–1·s–1 for alcohol quenchers, demonstrating 30–60-fold differences between contaminants’ categories. Subsequently, quantitative structure activity relationship (QSAR) models incorporating frontier molecular orbital parameters achieved cross-validated R2 = 0.77–0.96, with nucleophilicity-index-based models showing peak performance for anilines (R2 = 0.96). This methodological framework enables precise prediction of emerging contaminant degradation kinetics based on their structures, providing essential data for optimizing radical-based water treatment systems to meet potential needs for targeted removal technologies for organic emerging contaminants for water environmental protection in the future.