Effect of Fe3+ as an electron-transfer mediator on WO3-induced activation of peroxymonosulfate under visible light

doi:10.1016/j.cej.2021.128529

Chemical Engineering Journal

Volume 411, 1 May 2021, 128529

https://doi.org/10.1016/j.cej.2021.128529 Get rights and content

Highlights

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Presence of Fe³⁺ enhances WO₃-mediated PMS activation under visible light.
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Fe³⁺ (electron-transfer mediator) facilitates electron transfer from WO₃ to PMS.
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SO₄⁻ acts as a primary oxidant for pollutant degradation in the Fe³⁺/PMS/WO₃ system.
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Positive Fe³⁺ effect was observed in the degradation of various organic pollutants.
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Fe³⁺/PMS/WO₃ system can be used repeatedly with regular injection of PMS only.

Abstract

The effect of ferric ions (Fe³⁺) on the degradation of organic pollutants through the photocatalytic activation of peroxymonosulfate (PMS) was investigated. In the presence of Fe³⁺, the degradation of 4-chlorophenol (4-CP) was significantly enhanced in the PMS/tungsten oxide (WO₃) system under visible light irradiation. The enhanced degradation efficiency by Fe³⁺ is primarily ascribed to the role of Fe³⁺ as an electron-transfer mediator, as it induces a cascadal electron transfer from WO₃ to Fe³⁺ to PMS. The production of the sulfate radical (SO₄ radical dot ⁻) and its significant contribution to the degradation of 4-CP in the Fe³⁺/PMS/WO₃ system were verified via electron paramagnetic resonance (EPR) spectroscopy and degradation experiments using radical scavengers (methanol and tert-butyl alcohol), respectively. The addition of Fe³⁺ to the PMS/WO₃ system also significantly increased the degradation rate of other organic pollutants, such as phenol, 2,4-dimethylphenol, bisphenol A, sulfamethoxazole, sulfanilamide, and propranolol. The degradation efficiency of the Fe³⁺/PMS/WO₃ system was largely maintained in multiple degradation cycles by the regular addition of PMS only. In addition, the Fe³⁺/PMS/WO₃ system exhibited a higher degradation efficiency than the conventional cobaltous ion (Co²⁺)/PMS system at equal transition metal ion concentrations. The addition of Fe³⁺ for the purpose of enhancing the degradation efficiency is not restricted to the PMS activation system. This method can be applied to the activation of other oxyanions, such as peroxydisulfate (S₂O₈²⁻) and bromate (BrO₃⁻), for wastewater treatment.

Graphical abstract

Introduction

Peroxymonosulfate (PMS, HSO₅⁻)-based water treatment processes have received a significant amount of attention as they are cost efficient and environmentally benign [1], [2]. Although PMS itself can be used as an oxidant in the water treatment process, the application of PMS alone is restricted to specific pollutants [3], [4]. To extend the utilization of PMS to a broad range of pollutants, PMS is routinely activated to more reactive radical species, such as hydroxyl ( radical dot OH) and sulfate radicals (SO₄⁻), with the use of various techniques; transition metal ions (e.g., Fe²⁺ and Co²⁺) [5], [6], catalysts (e.g., biochar, carbon nanotube, and ruthenium oxide (RuO₂) nanosheet) [7], [8], [9], [10], heat [11], ultrasound [12], and UV-C (λ = 254 nm) [13], [14].

Recently, semiconductor photocatalysts, capable of utilizing solar radiation as an energy source, have been extensively studied as viable PMS activators. Activation of PMS by photocatalysis is based on photo-induced electron transfer from the valence band (VB) to the conduction band (CB) (i.e., CB electron (e_cb⁻)-VB hole (h_vb⁺) pair generation) (reaction (1)) and the subsequent reduction of PMS to SO₄ radical dot ⁻ (reaction (2)). Titanium dioxide (TiO₂) with UV-A (λ = 365–370 nm) [15], carbon nitride (g-C₃N₄) [16], zinc ferrite (ZnFe₂O₄) [17], and cobalt oxide (Co₃O₄) [18] with visible light (λ > 420 nm) have been utilized as photocatalytic PMS activation systems for water treatment.photocatalyst + visible light → e_cb⁻ + h_vb⁺e_cb⁻ + HSO₅⁻ → SO₄ radical dot ⁻ + OH⁻

Tungsten oxide (WO₃) has been theoretically considered an efficient photocatalyst because it is highly visible-light-responsive (band gap = 2.4–2.8 eV), powerful (oxidation potential = +3.1 V_NHE), stable, and inexpensive [19], [20]. However, the more positive CB position of WO₃ (+0.5 V_NHE) in comparison to the reduction potential of dioxygen (electron acceptor, −0.28 ~ −0.05 V_NHE) significantly reduces the photocatalytic activity of WO₃, as one-electron transfer from CB to dioxygen is inhibited and rapid charge recombination between CB electrons and VB holes occurs readily [21]. Therefore, various surface modification techniques, such as metal deposition [22], [23], semiconductor coupling [24], [25], [26], and carbon material conjunction [27], [28], have been employed to facilitate electron transfer from CB to dioxygen via multi-electron transfer processes. In addition, pure WO₃ and surface-modified WO₃ (e.g., WO₃/MoS₂/Ag, Co(OH)₂/WO₃, and Bi₂WO₆/WO₃) have been coupled with PMS, which plays a dual role as an electron acceptor and SO₄ radical dot ⁻ precursor, and applied to water treatment [29], [30], [31], [32]. However, the PMS/pure WO₃ system exhibited low efficiency for the degradation of organic pollutants under visible light [29]. Although the WO₃-mediated photocatalytic degradation in the presence of PMS was enhanced by surface modification [30], [31], [32], the high cost of surface modification and toxic metal ion leaching from the WO₃ surface into water [31] limit the practical application of the PMS/surface-modified WO₃ system.

A variety of photocatalytic PMS activation systems have been developed and implemented for the degradation of aqueous organic pollutants. Nevertheless, more efficient, economical, and eco-friendly methods need to be developed for practical applications in water treatment. In this study, we developed a convenient method for enhancing the degradation efficiency of the WO₃-mediated photocatalytic PMS activation system (the PMS/WO₃ system under visible light). The addition of ferric ions (Fe³⁺) to the PMS/WO₃ system under visible light significantly enhanced its activity for the degradation of organic pollutants through PMS activation. The degradation efficiency of the Fe³⁺/PMS/WO₃ system (ternary system) under visible light was compared with that of binary systems under visible light. The degradation kinetics in the Fe³⁺/PMS/WO₃ system under visible light was measured as a function of various experimental parameters, such as Fe³⁺ concentration, PMS concentration, pH, organic pollutant type, and number of uses. Furthermore, the oxidant species primarily involved in the degradation process was identified and its generation mechanism was investigated.

Section snippets

Materials and chemicals

Materials and chemicals used in this study were as follows: tungsten oxide (WO₃), potassium peroxymonosulfate (KHSO₅·0.5KHSO₄·0.5K₂SO₄), potassium persulfate (K₂S₂O₈), potassium periodate (KIO₄), potassium bromate (KBrO₃), iron(III) perchlorate hydrate (Fe(ClO₄)₃·xH₂O), iron(II) perchlorate hydrate (Fe(ClO₄)₂·xH₂O), cobalt(II) perchlorate hexahydrate (Co(ClO₄)₂·6H₂O), methanol (CH₃OH), tert-butyl alcohol ((CH₃)₃COH), perchloric acid (HClO₄), sodium hydroxide (NaOH), 4-chlorophenol (ClC₆H₄OH),

Effect of Fe³⁺ on 4-CP degradation in the PMS/WO₃ system under visible light

The effect of Fe³⁺ on the degradation of organic pollutants in the photocatalytic PMS activation system was investigated (Fig. 1). WO₃ and 4-chlorophenol (4-CP) were selected as the photocatalyst and model organic pollutant, respectively. In the absence of Fe³⁺, the concentration of 4-CP gradually decreased in the PMS/WO₃ system under visible light, with 13% of 4-CP remaining after 60 min. The electron transfer from WO₃ CB (+0.5 V_NHE) [21] to PMS (E⁰(HSO₅⁻/SO₄ radical dot ⁻) = +1.19 V_NHE) [37] is

Conclusions

We investigated the effect of Fe³⁺ on WO₃-mediated photocatalytic PMS activation. The addition of Fe³⁺ significantly enhanced the degradation of organic pollutants in the PMS/WO₃ system under visible light. The acceleration of electron transfer from WO₃ CB to PMS can enhance PMS activation (i.e., the production of SO₄ radical dot ⁻) and the degradation efficiency. In this respect, Fe³⁺ acts as an electron-transfer mediator, which facilitates electron transfer from WO₃ CB to PMS.

Fe³⁺ is highly abundant,

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.

Acknowledgements

This research was supported by Young Research Program (NRF-2019R1C1C1004640) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT.

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Effect of Fe3+ as an electron-transfer mediator on WO3-induced activation of peroxymonosulfate under visible light

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials and chemicals

Effect of Fe3+ on 4-CP degradation in the PMS/WO3 system under visible light

Conclusions

Declaration of Competing Interest

Acknowledgements

Chem. Eng. J.

Chem. Eng. J.

Appl. Catal. B: Environ.

J. Hazard. Mater.

Chem. Eng. J.

Chem. Eng. J.

Appl. Catal. B: Environ.

Ultrason. Sonochem.

Chemosphere

Chem. Eng. J.

J. Environ. Chem. Eng.

Appl. Surf. Sci.

Appl. Catal. A: Gen.

Appl. Surf. Sci.

Appl. Catal. B: Environ.

J. Photochem. Photobiol. A: Chem.

J. Alloy. Compd.

Appl. Catal. B: Environ.

Chemosphere

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Electrochem. Commun.

Appl. Geochem.

Sep. Purif. Technol.

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Effect of Fe³⁺ as an electron-transfer mediator on WO₃-induced activation of peroxymonosulfate under visible light

Effect of Fe³⁺ on 4-CP degradation in the PMS/WO₃ system under visible light