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

Hydroxyl radical ((OH)-O-center dot) and sulfate radical anion (SO4 center dot-) 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)-O-center dot and SO4 center dot- at pH 3 in UV/H2O2 and UV/persulfate systems, respectively. The second-order rate constants (k) of IBU with (OH)-O-center dot and SO4 center dot- were determined to be 3.43 +/- 0.06 x 10(9) and 1.66 +/- 0.12 x 10(9) M-1 s(-1), respectively. We also theoretically calculated the thermodynamic and kinetic behaviors for reactions of IBU with (OH)-O-center dot and SO4 center dot- 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)-O-center dot and SO4 center dot-, but due to the steric hindrance SO4 center dot- exhibits significantly higher energy barriers than (OH)-O-center dot. The theoretical calculations corroborate our experimental observation that SO4 center dot- has a smaller k value than (OH)-O-center dot 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. (C) 2017 Elsevier B.V. All rights reserved.