Reactive species conversion into ¹O₂ promotes substantial inhibition of chlorinated byproduct formation during electrooxidation of phenols in Cl⁻-laden wastewater

Highlights

• Addition of H₂O₂ during electrolysis enables reactive species conversion into ¹O₂.

• ¹O₂ stimulates phenol oxidation and circumvents chlorinated byproduct formation.

• Self-combination of O₂^•– is the major mechanism responsible for ¹O₂ generation.

• ¹O₂ reacts with phenol preferably via the cycloaddition pathway.

• Oxidation of phenols in coking wastewater is promoted in the ¹O₂-initiated system.

Abstract

The generation of chlorinated byproducts during the electrochemical oxidation (EO) of Cl⁻-laden wastewater is a significant concern. We aim to propose a concept of converting reactive species (e.g., reactive chlorines and HO^• resulting from electrolysis) into ¹O₂ via the addition of H₂O₂, which substantially alleviates chlorinated organic formation. When phenol was used as a model organic compound, the results showed that the H₂O₂-involving EO system outperformed the H₂O₂-absent system in terms of higher rate constants (5.95 × 10⁻² min⁻¹ vs. 2.97 × 10⁻² min⁻¹) and a much lower accumulation of total organic chlorinated products (1.42 mg L⁻¹ vs. 8.18 mg L⁻¹) during a 60 min operation. The rate constants of disappearance of a variety of phenolic compounds were positively correlated with the Hammett constants (σ), suggesting that the reactive species preferred oxidizing phenols with electron-rich groups. After the identification of ¹O₂ that was abundant in the bulk solution with the use of electron paramagnetic resonance and computational kinetic simulation, the routes of ¹O₂ generation were revealed. Despite the consensus as to the contribution of reaction between H₂O₂ and ClO⁻ to ¹O₂ formation, we conclude that the predominant pathway is through H₂O₂ reaction with electrogenerated HO^• or chlorine radicals (Cl^• and Cl₂^•−) to produce O₂^•−, followed by self-combination. Density functional theory calculations theoretically showed the difficulty in forming chlorinated byproducts for the ¹O₂-initiated phenol oxidation in the presence of Cl⁻, which, by contrast, easily occurred for the Cl^•-or HO^•-initiated phenol reaction. The experiments run with real coking wastewater containing high-concentration phenols further demonstrated the superiority of the H₂O₂-involving EO system. The findings imply that this unique method for treating Cl⁻-laden organic wastewater is expected to be widely adopted for generalizing EO technology for environmental applications.

Introduction

The electrochemical oxidation (EO) of organic pollutants in wastewater has received considerable attention in recent years because of its advantages, such as high efficiency, cleanness, flexibility, and sustainability (thanks to the development of sustainable energy as a power source) (Chaplin 2014; Jeon et al. 2018; Radjenovic and Sedlak 2015; Sirés et al. 2014). The EO process can proceed via i) direct electron transfer to the anode surface (Chaplin 2014; Panizza and Cerisola 2009; Radjenovic and Sedlak 2015) and ii) indirect oxidation by reactive oxygen species (ROS, e.g., HO^•; Eq. 1) (Brillas et al. 2009; Fabiańska et al. 2014; Jeon et al. 2018) or other oxidant agents (e.g., reactive chlorine species, RCS; (2), (3), (4), (5)) (Cho et al. 2014; Radjenovic and Sedlak 2015; Shen et al. 2019) produced from the electrolysis of water or ions (e.g., Cl^–) in the solution, respectively. The efficiency of surface-related EO (including direct and indirect HO^•-mediated pathways) is highly limited by the mass transfer of pollutants from the bulk to the anode surface or its surroundings (Díaz et al. 2011; Fabiańska et al. 2014; Liu et al. 2019; Radjenovic and Sedlak 2015). It has been reported that RCS (including chlorine radicals and free chlorine)-mediated EO plays a vital role in improving efficiency because the majority of RCS are available in the bulk solution, and the bulk chemical process avoids the diffusion of pollutants (Cho and Hoffmann 2014; Cho et al. 2014; Jasper et al. 2016; Yang et al. 2016). Nevertheless, this process has an undesirable drawback–––the formation of chlorinated byproducts that are even more toxic and recalcitrant than the parent molecules are (Cho and Hoffmann 2014; Jasper et al. 2016; Jasper et al. 2017; Panizza and Cerisola 2009; Radjenovic and Sedlak 2015; Yang et al. 2016). Inhibiting their generation and concurrently boosting EO efficiency are outstanding issues that have yet to be addressed.

In this study, we address these concerns by converting RCS into singlet oxygen (¹O₂), a reactive electrophile capable of oxidizing a variety of electron-rich organic molecules. The creation of ¹O₂ in the Cl^–-containing EO system is hypothesized to occur when H₂O₂ is added to the electrolytic cell, leveraging the homogenous reactions between HClO/ClO^– and H₂O₂ (Eq. 6) (Chen et al. 2019; Di Mascio et al. 2019; Guo and Liu 2020). Previous studies have shown that ¹O₂ enables the selective oxidative degradation of unsaturated organics, such as phenols (Mi et al. 2021; Yan et al. 2021a), olefins (Barrios et al. 2021; Di Mascio et al. 2019), and polycyclic aromatic compounds (Ji et al. 2021; Yan et al. 2021a), with the pathways including electrophilic addition or single electron transfer, highly depending on the molecular structures of contaminants. Moreover, ¹O₂ existing in the bulk solution is more resistant to being quenched by Cl^– and natural organic matter than HO^• (Barrios et al. 2021; Duan et al. 2020; Ji et al. 2021; Long et al. 2022; You et al. 2021); ¹O₂ has been demonstrated to show extraordinary oxidative capability for destroying organic pollutants under high-salinity conditions (Lee et al. 2009; Yi et al. 2019). As such, the conversion of RCS into ¹O₂ is expected to suppress the production of chlorinated compounds despite the presence of Cl^–. To date, to the best of our knowledge, few studies have demonstrated the evolution of ¹O₂ from RCS in an electrochemical system and explored its critical role in attenuating the formation of toxic chlorinated byproducts.

$${\mathrm{H}}_2\mathrm{O}\to \mathrm{H}{\mathrm{O}}^{•}\left(\text{ads}\right)+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}$$

$$\mathrm{C}{\mathrm{l}}^{-}\to \mathrm{C}{\mathrm{l}}^{•}\left(\text{ads}\right)+{\mathrm{e}}^{-}$$

$$\mathrm{C}{\mathrm{l}}^{-}+\mathrm{C}{\mathrm{l}}^{•}\left(\text{ads}\right)\to \mathrm{C}{\mathrm{l}}_2+{\mathrm{e}}^{-}$$

$$\mathrm{C}{\mathrm{l}}_2+{\mathrm{H}}_2\mathrm{O}\to \text{HClO}+\mathrm{C}{\mathrm{l}}^{-}+{\mathrm{H}}^{+}$$

$$\text{HClO}\rightleftharpoons \text{Cl}{\mathrm{O}}^{-}+{\mathrm{H}}^{+}$$

$$\text{Cl}{\mathrm{O}}^{-}+{\mathrm{H}}_2{\mathrm{O}}_2\to {}^1{\mathrm{O}}_2+\mathrm{C}{\mathrm{l}}^{-}+{\mathrm{H}}_2\mathrm{O}$$

In this study, phenolic compounds with different substituted groups were selected as the probe substances because phenolic wastewater has a wide range of sources, including pharmaceutical synthesis (Cheng et al. 2018; Gadipelly et al. 2014), pesticide production (Pliego et al. 2012), and plastic manufacturing (Chen et al. 2016; Ji et al. 2021), which use phenolic compounds as raw materials, as well as the coal and petroleum industry, which produces phenolic wastes (Wei et al. 2021; Zheng et al. 2020). The aims of the current research are to i) demonstrate the uniqueness of the H₂O₂-involving EO system for treating Cl^–-laden wastewater in terms of enhanced oxidation capacity and, more importantly, remarkable inhibition of chlorinated byproduct formation; ii) identify the generation of ¹O₂ in this system and elaborate the role of ¹O₂ in promoting the destruction of phenolic compounds; and iii) distinguish the pathways of phenol degradation as a consequence of reactive species conversion into ¹O₂. To achieve these goals, multiple lines of evidence from the experimental and theoretical aspects were provided. These include examination of the reaction rates of phenol degradation in different systems, determination of total organic chlorine (TOCl) and typical reaction intermediates, evaluation of the toxicity of the EO-treated solution with bioluminescent bacteria, recognition of reactive species by electron paramagnetic resonance (EPR) tests and kinetic modeling study, and density functional theory (DFT) calculations of the energy barrier of chlorinated byproduct formation. In addition, EO treatments with coking wastewater consisting of high-concentration phenolic compounds were performed to determine the role of reactive species conversion in promoting pollutant oxidation and mitigating solution toxicity.