Accurate identification of radicals by in-situ electron paramagnetic resonance in ultraviolet-based homogenous advanced oxidation processes

Long Chen; Jun Duan; Penghui Du; Weiliang Sun; Bo Lai; Wen Liu

doi:10.1016/j.watres.2022.118747

Accurate identification of radicals by in-situ electron paramagnetic resonance in ultraviolet-based homogenous advanced oxidation processes

Water Res. 2022 Aug 1:221:118747. doi: 10.1016/j.watres.2022.118747. Epub 2022 Jun 15.

Authors

Long Chen¹, Jun Duan¹, Penghui Du², Weiliang Sun³, Bo Lai⁴, Wen Liu⁵

Affiliations

¹ The Key Laboratory of Water and Sediment Sciences, Ministry of Education, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, PR China.
² State Environmental Protection Key Laboratory of Integrated Surface Water-Groundwater Pollution Control, School of Environmental Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, PR China.
³ Department of Civil and Environmental Engineering, Louisiana State University, Baton Rouge 70803, LA, USA.
⁴ State Key Laboratory of Hydraulics and Mountain River Engineering, College of Architecture and Environment, Sichuan University, Chengdu 610065, PR China.
⁵ The Key Laboratory of Water and Sediment Sciences, Ministry of Education, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, PR China. Electronic address: wen.liu@pku.edu.cn.

PMID: 35728498
DOI: 10.1016/j.watres.2022.118747

Abstract

Accurate identification of radicals in advanced oxidation processes (AOPs) is important to study the mechanisms on radical production and subsequent oxidation-reduction reaction. The commonly applied radical quenching experiments cannot provide direct evidences on generation and evolution of radicals in AOPs, while electron paramagnetic resonance (EPR) is a cutting-edge technology to identify radicals based on spectral characteristics. However, the complexity of EPR spectrum brings uncertainty and inconsistency to radical identification and mechanism clarification. This work presented a comprehensive study on identification of radicals by in-situ EPR analysis in four typical UV-based homogenous AOPs, including UV/H₂O₂, UV/peroxodisulfate (and peroxymonosulfate), UV/peracetic acid and UV/IO₄^- systems. Radical formation mechanism was also clarified based on EPR results. A reliable EPR method using organic solvents was proposed to identify alkoxy and alkyl radicals (CH₃C(=O)OO·, CH₃C(=O)O· and ·CH₃) in UV/PAA system. Two activation pathways for radical production were proposed in UV/IO₄^- system, in which the produced IO₃·, IO₄·, ·OH and hydrated electron were precisely detected. It is interesting that addition of specific organic solvents can effectively identify oxygen-center and carbon-center radicals. A key parameter in EPR spectrum for 5,5-dimethyl-1-pyrroline N-oxide (DMPO) spin adduct, A_H, is ranked as: ·CH₃ (23 G) >·OH (15 G) >IO₃· (12.9 G) >O₂·^- (11 G) ≥·OOH (9-11 G) ≥IO₄· (9-10 G) ≥SO₄·^- (9-10 G) >CH₃C(=O)OO· (8.5 G) > CH₃C(=O)O· (7.5 G). This study will give a systematic method on identification of radicals in AOPs, and shed light on the insightful understanding of radical production mechanism.

Keywords: Advanced oxidation processes; Electron paramagnetic resonance; Homogeneous; Radical; Ultraviolet.

MeSH terms

Cyclic N-Oxides*
Electron Spin Resonance Spectroscopy
Free Radicals
Hydrogen Peroxide*
Oxidation-Reduction
Solvents

Substances

Cyclic N-Oxides
Free Radicals
Solvents
Hydrogen Peroxide