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

Zhihui Yang; Rongkui Su; Shuang Luo; Richard Spinney; Meiqiang Cai; Ruiyang Xiao; Zongsu Wei

doi:10.1016/j.scitotenv.2017.03.039

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

Sci Total Environ. 2017 Jul 15:590-591:751-760. doi: 10.1016/j.scitotenv.2017.03.039. Epub 2017 Mar 13.

Authors

Zhihui Yang¹, Rongkui Su¹, Shuang Luo¹, Richard Spinney², Meiqiang Cai³, Ruiyang Xiao⁴, Zongsu Wei⁵

Affiliations

¹ Institute of Environmental Engineering, School of Metallurgy and Environment, Central South University, Changsha 410083, China; Chinese National Engineering Research Center for Control & Treatment of Heavy Metal Pollution, Changsha 410083, China.
² Department of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio 43210, USA.
³ School of Environmental Science and Engineering, Zhejiang Gongshang University, Hangzhou 310018, China.
⁴ Institute of Environmental Engineering, School of Metallurgy and Environment, Central South University, Changsha 410083, China; Chinese National Engineering Research Center for Control & Treatment of Heavy Metal Pollution, Changsha 410083, China. Electronic address: xiao.53@csu.edu.cn.
⁵ Grand Water Research Institute - Rabin Desalination Laboratory, The Wolfson Faculty of Chemical Engineering, Technion - Israel Institute of Technology, Technion City, Haifa 32000, Israel. Electronic address: zongsuw@technion.ac.il.

PMID: 28302308
DOI: 10.1016/j.scitotenv.2017.03.039

Abstract

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.

Keywords: DFT; Hydroxyl radical; Ibuprofen; Sulfate radical anion; Trace organic contaminants.