Electrochemical activation of periodate with graphite electrodes for water decontamination: Excellent applicability and selective oxidation mechanism

Highlights

• Electrochemical activation using graphite electrodes can successfulliy activate periodate (PI) for water decontamination.

• ¹O₂ as the primary reactive species is produced in the E-GP/PI system through the combination of O₂^•−.

• A dual descriptor model is proposed to reveal the selective oxidation behavior of ¹O₂.

• The E-GP/PI system shows strong pH tolerance, long-lasting performance, and high resistance to aqueous matrices.

• BPA degradation mechanism is proposed based on experimental results and DFT calculations.

Abstract

Advanced oxidation technologies based on periodate (PI, IO₄⁻) have garnered significant attention in water decontamination. In this work, we found that electrochemical activation using graphite electrodes (E-GP) can significantly accelerate the degradation of micropollutants by PI. The E-GP/PI system achieved almost complete removal of bisphenol A (BPA) within 15 min, exhibited unprecedented pH tolerance ranging from pH 3.0 to 9.0, and showed more than 90% BPA depletion after 20 h of continuous operation. Additionally, the E-GP/PI system can realize the stoichiometric transformation of PI into iodate, dramatically decreasing the formation of iodinated disinfection by-products. Mechanistic studies confirmed that singlet oxygen (¹O₂) is the primary reactive oxygen species in the E-GP/PI system. A comprehensive evaluation of the oxidation kinetics of ¹O₂ with 15 phenolic compounds revealed a dual descriptor model based on quantitative structure−activity relationship (QSAR) analysis. The model corroborates that pollutants exhibiting strong electron-donating capabilities and high pK_a values are more susceptible to attack by ¹O₂ through a proton transfer mechanism. The unique selectivity induced by ¹O₂ in the E-GP/PI system allows it to exhibit strong resistance to aqueous matrices. Thus, this study demonstrates a green system for the sustainable and effective elimination of pollutants, while providing mechanistic insights into the selective oxidation behaviour of ¹O₂.

Introduction

Frequent detection of emerging organic contaminants (EOCs) in water calls for the exploration of high-efficiency and long-lasting water treatment technologies, as they will pose undesirable effects to human health and ecological environment. Advanced oxidation processes (AOPs) are considered as one of the most promising pathways to control EOCs, where highly reactive oxygen species (ROS) are formed to transform and mineralize EOCs. Widely studied AOPs, like the Fenton and Fenton-like reactions, primarily utilize the generation of hydroxy radicals (^•OH) and sulfate radical (SO₄^•−) from hydrogen peroxide (H₂O₂), persulfate, and ozone, which are capable of oxidizing almost all the organic pollutants without discrimination (Ren et al., 2020; Zhang et al., 2020). However, the high oxidation ability of these radicals results in the disadvantage associated with their low resistance to naturally occurring substances such as humic acid (HA), halide ions, and bicarbonate (Li et al., 2016; Zhu et al., 2020). The critical obstacle impedes the actual application of radical-based AOPs. Consequently, there has been significant interest in the field regarding the strategic manipulation of the chemical reactivity of oxidizing species to achieve selective oxidation of pollutants, particularly in the development of non-radical oxidation systems.

Periodate (PI, IO₄⁻) with a high oxidation potential (E⁰ = + 1.60 V) has recently attracted significant interest in the field of water decontamination owing to its inherent advantages in terms of ease of storage and transportation (Chen et al., 2022; Guo et al., 2022; He et al., 2022). Although the application of PI is largely hampered by its potential to release iodine species, it can be solved by regulating the decomposition pathway of PI to minimize the production of harmful iodine species (Zong et al., 2021). Yet, alone PI-mediated oxidation process fails to oxidize EOCs effectively (Lee and Yoon 2004). Many strategies have been proposed to intensify the oxidative ability of PI via stimulating the formation of various ROS (e.g., ^•OH, iodate radical (^•IO₃), superoxide radical (O₂^•−), and singlet oxygen (¹O₂)) (Eqs. (1)−(5)) or inducing direct electron-transfer activation of PI (Bokare and Choi 2015; Choi et al., 2018; Lee et al., 2014; Liu et al., 2022; Long et al., 2021). These strategies mainly include ultrasound (Lee et al., 2016), transition metal oxides (Chatraei and Zare 2013), carbonaceous materials (Long et al., 2022), and photoactivation (Liu et al., 2022). Although the above methods succeed in improving the oxidation capacity of PI, they also lead to the formation of secondary pollutant and increase energy consumption. Moreover, these methods still face challenges in activating PI over a wide pH range. For instance, metal-involved catalysts such as MnO₂ (Du et al., 2020), Co single-atomic catalyst (Long et al., 2021), Ru/Co₂O₃ (Zhang et al., 2023), and S-ZVI⁰ (Zong et al., 2022) are viable candidates for accelerating the degradation of micropollutants by PI, but their pH effectiveness range is limited to acidic conditions. As such, it is necessary to design PI-based AOPs with strong resistance to reaction pH in order to extend their applicability.

IO₄⁻ + energy → IO₃^• + O^•−

O^•− + H⁺ → ^•OH

3IO₄⁻ + 2OH⁻ → 2O₂^•− + 3IO₃⁻ + H₂O

2O₂^•− + 2H₂O → H₂O₂ + ¹O₂ + 2OH⁻

IO₄⁻ + 2O₂^•− + H₂O → IO₃⁻ + 2OH⁻ + 2¹O₂

Over the past few decades, electrochemical processes have been introduced in AOPs for efficient activation, as they are environmentally friendly, energy-efficient, use-friendly, and suitable for automation (Son et al., 2021; Song et al., 2021). During electro-activation, the electric field can provide electrons as co-catalysts to activate the oxidant, thereby inducing the formation of ROS and successfully avoiding the risks associated with exogenous catalysts (Long et al., 2023). For instance, Yao et al. proposed that the electrochemically activated peroxymonosulfate (PMS) system exhibits strong reactivity in degrading contaminants and performs effectively over a wide pH range (3.0−11.0) (Yao et al., 2021). Thus, it is expected that PI can also be activated by the electrochemical method over a broad pH range; however, knowledge in this field is still limited. As far as we know, only one recent research has demonstrated the feasibility of establishing an electro-activated PI oxidation process, in which nano-confined Fe₂O₃ was used as a working electrode to accelerate the removal of bisphenol A (BPA) by PI (Guo et al., 2022). However, the performance of the electro/Fe₂O₃CNT/PI system reduces under alkaline conditions. Considering that metal-based catalysts generally suffer limitations in pH-dependent catalytic activity (Chen et al., 2021; Son et al., 2021), using a metal-based working electrode to activate PI may be not the optimal choice. Graphite (GP) is a cheap and easily accessible non-metallic electrode material that has attracted much attention in electrochemical AOPs due to its low oxygen evolution potential and high hydrogen evolution potential. Song et al. reported that electrochemical activation of PMS using GP electrodes can completely degrade sulfamethoxazole (SMX) within 30 min (Song et al., 2020). Therefore, it is reasonable to hypothesize that using GP working electrodes for electrochemical activation of PI (E-GP/PI) can significantly enhance the removal of micropollutants. However, the performance of the E-GP/PI system has not been fully demonstrated, and there is a lack of knowledge on its pH tolerance and application potentiality.

Thus, the objective of the study is to investigate the oxidation activity and reaction mechanism of the E-GP/PI system. To our delight, the E-GP/PI system, characterized by the dominance of non-radical ROS, not only can effectively and selectively degrade contaminants but also shows outstanding pH adaptation, transcending other PI-based AOPs currently reported. The electro-activation mechanism was fully discussed by demonstrating the main ROS and the transformation pathway of PI. By revealing the activity differences of the system in degrading 15 phenolic compounds with different substituent groups, a new quantitative structure−activity relationship (QSAR) model was proposed to reveal the selective activity of the primary ROS, thus shedding light on the selective oxidation mechanism. Finally, the application potential of the E-GP/PI system and the degradation pathways of BPA were discussed.