Comparison of the reactivity of ibuprofen with sulfate and hydroxyl radicals: An experimental and theoretical study.

Hydroxyl radical (•OH) and sulfate radical anion (SO4•-) based advanced oxidation technologies (AOTs) are effective methods to treat trace organic contaminants (TrOCs) in engineered waters. Although both technologies result in the same overall removal of TrOCs, the mechanistic differences between these two radicals involved in the oxidation of TrOCs remain unclear. In this study, we experimentally examined the degradation kinetics of neutral ibuprofen (IBU), a representative TrOC, by •OH and SO4•- at pH3 in UV/H2O2 and UV/persulfate systems, respectively. The second-order rate constants (k) of IBU with •OH and SO4•- were determined to be 3.43±0.06×109 and 1.66±0.12×109M-1s-1, respectively. We also theoretically calculated the thermodynamic and kinetic behaviors for reactions of IBU with •OH and SO4•- using the density functional theory (DFT) M06-2X method with 6-311++G** basis set. The results revealed that H-atom abstraction is the most favorable pathway for both •OH and SO4•-, but due to the steric hindrance SO4•- exhibits significantly higher energy barriers than •OH. The theoretical calculations corroborate our experimental observation that SO4•- has a smaller k value than •OH in reacting with IBU. These comparative results are of fundamental and practical importance in understanding the electrophilic interactions between radicals and IBU molecules, and to help select preferred radical oxidation processes for optimal TrOCs removal in engineered waters.