Quantitative structure-activity relationships for reactivities of sulfate and hydroxyl radicals with aromatic...

Sulfate radical anion (SO₄^•-) and hydroxyl radical (OH) based advanced oxidation technologies has been extensively used for removal of aromatic contaminants (ACs) in waters. In this study, we investigated the Gibbs free energy (ΔG_SET^∘) of the single electron transfer (SET) reactions for 76 ACs with SO₄^•- and OH, respectively. The result reveals that SO₄^•- possesses greater propensity to react with ACs through the SET channel than OH. We hypothesized that the electron distribution within the molecule plays an essential role in determining the ΔG_SET^∘ and subsequent SET reactions. To test the hypothesis, a quantitative structure-activity relationship (QSAR) model was developed for predicting ΔG_SET^∘ using the highest occupied molecular orbital energies (E_HOMO), a measure of electron distribution and donating ability. The standardized QSAR models are reported to be ΔG^°_SET=-0.97×E_HOMO - 181 and ΔG^°_SET=-0.97×E_HOMO - 164 for SO₄^•- and OH, respectively. The models were internally and externally validated to ensure robustness and predictability, and the application domain and limitations were discussed. The single-descriptor based models account for 95% of the variability for SO₄^•- and OH. These results provide the mechanistic insight into the SET reaction pathway of radical and non-radical bimolecular reactions, and have important applications for radical based oxidation technologies to remove target ACs in different waters.