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

doi:10.1016/j.watres.2022.118747

Water Research

Volume 221, 1 August 2022, 118747

https://doi.org/10.1016/j.watres.2022.118747 Get rights and content

Highlights

•
A systematic EPR method to accurately identify radicals in UV-AOPs was presented.
•
EPR method on identification of SO₄·⁻ without the interference of ·OH was proposed.
•
Alkoxy and alkyl radicals produced in UV/peracetic acid system were identified.
•
Iodine-related radicals and hydrated electron in UV/IO₄⁻ system were identified.
•
Key characteristics on various DMPO-radical adducts in EPR spectrum were summarized.

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.

Graphical abstract

Introduction

Advanced oxidation processes (AOPs) have drawn intensive attention in water treatment area for organic pollutants degradation and pathogenic microorganisms’ inactivation (Miklos et al., 2018; Sun et al., 2016). Generally, AOPs involve production of radicals with high oxidization ability for degradation/transformation of pollutants (Ike et al., 2019; Ma et al., 2021). Particularly, ultraviolet (UV)-based AOPs, such as UV/H₂O₂, UV/peroxodisulfate (PDS, S₂O₈²⁻), UV/peroxomonosulfate (PMS, HSO₅⁻), UV/peracetic acid (PAA, CH₃C(=O)OOH), UV/IO₄⁻, UV/HClO, UV/NH₂Cl, etc. are most widely applied technologies due to efficient and rapid production of radicals (Ao et al., 2021; Lee et al., 2020; Zhang et al., 2021). The energy input of UV can directly break the specific bonds (such as peroxy bond in H₂O₂, PAA, PDS and PMS), and the reaction rate constant between radicals and pollutants is extremely high (∼10⁹ M⁻¹ s⁻¹), which is almost controlled by diffusion (Zhang et al., 2019b).

UV/H₂O₂ system is a classical UV-based AOP, in which hydroxyl radical (·OH), superoxide radical (O₂·⁻), and hydroperoxyl radical (·OOH) with strong oxidation capacity are produced (Sobańska et al., 2017). ·OH (E⁰ = 1.9–2.7 V vs. NHE), the most common reactive oxidation species (ROS), is a strong electrophilic radical and can attack organics through radical addition-elimination, electron transfer and hydrogen abstraction (Xu et al., 2017). O₂·⁻ (E⁰ = 0.94 V vs. SHE) has less reactivity due to its anionic form, which tends to attack organics through single electron transfer (Krumova and Cosa, 2016). However, the protonation radical (HOO·, E⁰ = 1.06 V vs. SHE) has higher reaction activity and can undergo hydrogen abstraction and radical addition reactions (Iuga et al., 2012; Marino et al., 2014). UV/PDS and UV/PMS systems are typical sulfate radical (SO₄·⁻)-based AOPs, and SO₄·⁻ (E⁰ = 2.6–3.1 V vs. NHE) has become a popular alternative of ·OH in recent years, due to its stronger oxidation ability and higher selectivity in broad pH ranges (Chen et al., 2021). UV/PAA system has drawn increasing interests in water treatment area because of the high efficiency, various radicals’ production, and low toxic by-products production (da Silva et al., 2020; Shah et al., 2015). Besides ·OH, electrophilic radicals including acetoxy radical (CH₃C(=O)O·) and acetylperoxyl radical (CH₃C(=O)OO·) also can be generated in this system (Ao et al., 2021). CH₃C(=O)OO· is considered to be the most concentrated active species in PAA system (∼10⁻¹⁰ M) (Wu et al., 2020), which has a longer life and tends to attack aromatic rings with high electron density (Wu et al., 2020). CH₃C(=O)O· is relatively unstable and would further undergo single molecule decay to form nucleophilic ·CH₃ (Ao et al., 2021; Kim et al., 2020). In UV/IO₄⁻ system, iodine-related radicals including iodate radical (IO₃·) and periodate radical (IO₄·) can be produced, accompanied by ·OH (Zhang et al., 2021). IO₃· and IO₄· are also active oxygen-center radicals containing halogen, which are electrophilic and can selectively react with organic substrates (Tian et al., 2017).

In AOPs systems, clarification of the production-evolution mechanism, reaction activity and selectivity of radicals are important to clearly understand the subsequent reactions with target pollutants (Yang et al., 2020). Therefore, identification of radicals is a prerequisite procedure. Electron paramagnetic resonance (EPR) is a cutting-edge technology to directly determine radicals by means of spectrum characteristics recognition (Gerson and Huber, 2003). Trapping agent, such as 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), is used to convert short-lived radical into stable/metastable radical addition product, and then the radical-adduct is identified based on the hyperfine splitting from N and H nuclei on characteristic spectrum, due to the difference of polarity and electronegativity of radical groups (Brustolon and Giamello, 2009; Davies, 2016). However, there are still inconsistency in the recognition of EPR spectrum in previous studies, which causes a misleading to accurately identify the signals of radicals: (1) incorrect information recognizing on basic parameters of EPR spectrum, such as unit and plotting; (2) incorrect applied determination conditions, such as selected solvent, solution pH and trapping agent; (3) incorrect identification of radicals’ spectra due to interference by impurities signals; (4) insufficient description of radicals’ spectra with only peak shape, number or intensity ratio. Therefore, establishment of accurate measurement and identification method for radicals by using EPR is very important.

The objective of this work was to provide a comprehensive study to identify radicals in typical UV-based AOPs through in-situ EPR method, and meanwhile, clearly illustrate the possible erroneous zone in EPR test for specific AOP system. DMPO was used as a trapping agent to capture radicals in conventional and emerging UV-based AOPs, including UV/H₂O₂, UV/PDS (and PMS), UV/PAA, and UV/IO₄⁻. EPR experiments and EasySpin simulation in MATLAB software were used to describe the characteristics of radicals’ spectra. Moreover, the mechanisms of radicals’ formation and evolution in the UV-AOPs were also proposed. This study can provide an overall and deep insights into identification of radicals in UV-based and even universal AOP systems through EPR.

Section snippets

Radicals production in various UV-based AOPs

Chemicals used in this study are presented in Text S1 of Supplementary Data (SD). In this study, an in-situ and integrated reaction and detection instrument was established on an EMXplus-6/1 EPR spectrometer (Bruker, Germany) (Fig. 1), and the schematic diagram of this in-situ EPR experimental set-up was also presented in Fig. S1. The radicals were generated in a quartz standard sampling tube under UV light irradiation. A 254 nm low-pressure mercury lamp (CEL-LPH120-254, Beijing Zhongjiao

Radical signals recognition in EPR spectrum

As a radical trapping agent, DMPO can capture short-lived radicals and convert them to stable DMPO radical addition products (Spulber and Schlick, 2010). The radical (labeled as R·) prefers to attack the C=N bond of DMPO molecule (structure shown in Fig. S3) and add on α-C, and then the free electron will transfer to N–O group to form DMPO adduct radical (DMPO-R·) (Villamena et al., 2005).

The spectral signal of DMPO-R· detected by EPR is isotropic and has an average g factor around

Conclusion

This study presented an overall summary on identification of radicals by EPR in typical UV-based homogenous AOP systems, and carefully discussed the challenges in this area. The obtained findings can provide a reference for identification of radicals in such UV-AOPs, and even in other AOPs with similar radicals’ production. The primary and important findings are:

(1)
In UV/H₂O₂ system, ·OOH and O₂·⁻ are also produced and the roles cannot be ignored. Accurate identification of these two radicals can

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors gratefully acknowledge financial supports from National Key Research and Development Program of China (No. 2021YFA1202500), National Natural Science Foundation of China (NSFC) (No. 21906001 and 51721006), China Postdoctoral Science Foundation (No. 2021M690208 and 2021M700213), and the Beijing Nova Program (Z191100001119054).

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

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Radicals production in various UV-based AOPs

Radical signals recognition in EPR spectrum

Conclusion

Declaration of Competing Interest

Acknowledgments

Water Res.

Free Radic. Biol. Med.

Chem. Eng. J.

Water Res.

Methods

J. Photochem. Photobiol. A

Water Res.

Water Res.

Sci. Total Environ.

Appl. Catal. B

Water Res.

Free Radical Bio. Med.

Appl. Catal. B

Chemosphere

Water Res.

Water Res.

J. Hazard. Mater.

Chem. Eng. J.

Sens. Actuators B

Org. Lett.

Chem. Eng. J.

Appl. Catal. B: Environ.

Water Res.

Water Res.

Water Res.

Sci. Total Environ.

Chem. Eng. J.

Water Res.

Appl. Catal. B

The solvation of nitroxide radicals in ionic liquids studied by high-field EPR spectroscopy

Phys. Chem. Chem. Phys.

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