Quantitative structure-activity relationships for reactivities of sulfate and hydroxyl radicals with aromatic contaminants through single-electron transfer pathway.

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 (ΔGSET∘) 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 ΔGSET∘ and subsequent SET reactions. To test the hypothesis, a quantitative structure-activity relationship (QSAR) model was developed for predicting ΔGSET∘ using the highest occupied molecular orbital energies (EHOMO), a measure of electron distribution and donating ability. The standardized QSAR models are reported to be ΔG°SET=-0.97×EHOMO - 181 and ΔG°SET=-0.97×EHOMO - 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.