The electrocatalytic degradation of 1,4-dioxane by Co–Bi/GAC particle electrode

In recent years, organic pollutants in water have seriously threatened the environment and human health (Rathi et al. 2021). 1,4-dioxane (C₄H₈O₂) is considered to be an emerging pollutant in water, classified by international agencies as a possible human carcinogen (Group B), and is widely present in industrial wastewater and groundwater (Adamson et al. 2017; Lee et al. 2023), which is often used as a solvent and stabilizer in different industries (Ouyang et al. 2022). Landfills of agricultural chemical waste, pharmaceutical waste, plastic waste, everyday use of personal care and household cleaning products can lead to the production of harmful leachate containing high concentrations of 1,4-dioxane, which can negatively impact ecosystems (Inoue et al. 2021). Traditional wastewater treatment methods have limited effects on removing 1,4-dioxane, due to its stability and difficult degradation (Smarzewska & Morawska 2021). Previous studies have shown that adsorption techniques are not considered an effective way to remove 1,4-dioxane (Myers et al. 2018). The degradation of 1,4-dioxane by microorganisms is easily affected by environmental factors, which leads to the weakening of the removal effect of 1,4-dioxane by microorganisms (Zhao et al. 2018). To date, only aerobic degradation of 1,4-dioxane has been reported, and the lack of dissolved oxygen and nutrients limits the efficacy of microbial removal of 1,4-dioxane (Xiong et al. 2020). Therefore, it is necessary to develop an efficient, economical, and environmentally friendly wastewater treatment technology to solve this problem.

Electrochemical technology, as a potential wastewater treatment technology (Chen 2004), has received extensive attention in recent years. In most cases, both direct and indirect oxidation methods are used to treat organic wastewater (Li et al. 2023a, 2023b). The low current efficiency and electrode area of traditional two-dimensional (2D) electrochemical catalysis seriously affect its degradation efficiency (Qian et al. 2021). In a 3D electrochemical system, the particle electrode is polarized into a small electrolytic cell under the action of an electric field (Feng et al. 2017), which has a larger surface area and can provide more active sites via the corresponding catalytic structure, thus improving the efficiency and rate of the catalytic reaction (Li et al. 2021a, 2021b; Ma et al. 2021). In addition, the 3D electrochemical catalytic technology also has good current transmission characteristics, which can reduce the resistance between the electrodes and improve the overall efficiency of the electrochemical reaction (Qin et al. 2023). By introducing 3D electrochemical catalysis technology, we are able to overcome the limitations of 2D electrochemical catalysis and achieve a more efficient and reliable catalytic process (Zhang et al. 2013).

As a critical component of electrochemical treatment, the selection and performance of particle electrodes have an essential impact on the degradation efficiency (Zuo et al. 2023). Many materials are used as raw materials for particle electrodes, such as granular or powdered GAC (Xiao et al. 2023), auric oxide (Wang et al. 2022), kaolin (Zhang et al. 2019), etc. Columnar activated carbon has been widely used in particle electrodes due to its large specific surface area, developed micropore structure, and strong ability to generate ·OH (Zhang et al. 2013; Zhan et al. 2019). The particle electrode should have good conductivity and stability (Zhao et al. 2023). The electrocatalytic activity of the particle electrode is usually improved by loading a metal catalyst (Souza et al. 2014). Transition metals such as Fe, Cu, Co, and Mn are often used as metal catalyst supports, which are cheaper and more readily available than precious metals (Li et al. 2019; Li et al. 2023a, 2023b; Liu et al. 2023; Ren et al. 2023). Among them, metal Co is widely regarded as an efficient electrocatalyst for heterogeneous catalytic systems, promoting the generation of H* or ·OH free radicals in electrochemical systems (Appaturi et al. 2019). Li et al. synthesized two kinds of carbon-supported cobalt–ferrite spinel by a one-step solvothermal method. The results showed that both catalysts could effectively activate sulfites, rapidly reduce dechlorination, and oxidize the refractory antibiotic chloramphenicol (CAP) (Li et al. 2023a, 2023b). The oxygen evolution potential (1.46 V) of the Mn–Co/GAC particle electrode prepared by Ma et al. is much higher than that of GAC (1.1 V). The active surface area is 1.34 times that of GAC. The mass transfer resistance is much smaller than that of GAC, which provides favorable conditions for the degradation of pollutants (Ma et al. 2022). Therefore, the development of wastewater treatment technology based on 3D particle electrodes has excellent potential for the removal of organic pollutants (Ma et al. 2021).

The purpose of this study is as follows: (i) the degradation effects of 1,4-dioxane under different Co/Bi ratios, calcination temperatures, and calcination times will be compared; (ii) the range of process parameters (voltage, electrolyte concentration, dosage) were optimized by a single factor, and RSM confirmed the optimal experimental conditions; (iii) quenching experiments with specific free radical scavenger was carried out to identify the active substances in the Co–Bi/GAC particle electrode system. The results of this study will provide a necessary theoretical and experimental basis for the development of efficient and environmentally friendly wastewater treatment technology, which provides new ideas and methods for solving the problem of water pollution.

1,4-dioxane (C₄H₈O_2, 99.5% anhydrous solution), cobalt nitrate hexahydrate (CoH₁₂N₂O₁₂, 99.5%), dichloromethane (CH₂Cl₂, 99.5%), nitric acid (HNO₃), sodium hydroxide (NaOH), hydrochloric acid (HCl), sodium sulfate (Na₂SO₄), sodium chloride (NaCl), sodium carbonate (Na₂CO₃), sodium bicarbonate (NaHCO₃), sodium nitrite (NaNO₂), anhydrous methanol (MeOH), tert-butanol (TBA), p-benzoquinone(p-BQ) are all from Shanghai Sinopharm Group Chemical Reagents Co., LTD. Bismuth nitrate pentahydrate (BiH₁₀N₃O₁₄, 99%) is from Shanghai Aladdin Reagent Co., LTD., China. Deionized water was used to prepare solutions throughout the experiment.

First of all, the activated carbon particles were soaked in deionized water overnight, and the surface ash and water-soluble salt were removed. The activated carbon particle electrode was prepared by impregnation–calcination method. The specific process is as follows: the aqueous solution of Co (NO₃)₂·6H₂O and Bi (NO₃)₃·5H₂O was prepared as the impregnating solution, in which dilute nitric acid is added to the aqueous solution of Bi (NO₃)₃·5H₂O. The pretreated activated carbon was placed in a single or mixed impregnating solution, oscillated at 120 r/min at room temperature for 8.0 h, and filtered. The activated carbon was dried in an oven at 105 °C for 12.0 h, and then placed in a tube furnace and calcined at 750 °C for 2.0 h under an N₂ atmosphere to obtain particle electrodes loaded with different active components. It is recorded as Co/GAC, Bi/GAC, and Co-Bi/GAC.

The surface morphology and elemental composition of the particle electrode were characterized by scanning electron microscopy (SEM, Zeiss Sigma 500, Oberkochen, Germany) and energy-dispersive spectroscopy (EDS). X-ray photoelectron spectroscopy (XPS, Shimadzu AXIS SUPPA + , Milton Keynes, UK) is used to analyse the chemical states on the surface of a particle electrode. Fourier transform infrared spectroscopy (FTIR, China, iCAN8) was used to determine surface functional groups in the spectral range 400–4,000cm⁻¹. All electrochemical characterization experiments were performed at the electrochemical workstation (Koster, China). A 3D system was used for the determination. The prepared electrode (5 cm × 5 cm) was the working electrode, the platinum sheet (5cm × 5 cm) was the auxiliary electrode, and the Ag electrode (5 cm × 5 cm) was the reference electrode. Cyclic voltammetry (CV) was performed in 0.015 M Na₂SO₄ solution as an electrolyte at a scanning rate of 10–100 mV·s⁻¹. Electrochemical impedance spectroscopy (EIS) was performed in 0.015 M Na₂SO₄ electrolyte solution at a frequency of 0.1 Hz–1 MHz.

The extraction method of 1,4-dioxane refers to the literature (Mameda et al. 2018). The concentration of 1,4-dioxane was determined by a gas chromatograph with a flame ionization detector (Shimadzu, Japan GC-2010-FID) (Fedorov et al. 2023). Chemical oxygen demand (COD) was measured using the dichromate method described in the standard method (ASTM 2020).

The removal rate of 1,4-dioxane catalysed degradation by particle electrode at different calcination temperatures was studied, as shown in Figure 1((c), (d)). The experimental results show that different calcination temperatures significantly affect the removal of 1,4-dioxane by particle electrode. With the increase in temperature, the removal increases first and then decreases. When the calcination temperature was 750°C, the removal efficiency of 1,4-dioxane reached the highest 84.91% and the COD removal rate was 81.28% in 120 min. When the calcination temperature exceeded 750°C, the removal efficiency of 1,4-dioxane showed a decreasing trend. Previous studies have shown that lower calcination temperatures may lead to incomplete crystallization of the metal, resulting in reduced activity of the particle electrode (Ma et al. 2023). With the increase in calcination temperature, the supported elements and their metal oxides are entirely transformed into crystal forms conducive to electrocatalytic degradation (Sun et al. 2019), thus greatly improving the electrocatalytic efficiency of the particle electrode.

The effects of different calcination times on the catalytic degradation of 1,4-dioxane by particle electrode were studied, as shown in Figure 1((e), (f)). The experimental results show that the removal rates of 1,4-dioxane and COD do not increase with the increase in calcination time. When calcined for 2 and 4 h, the removal rates of 1,4-dioxane and COD of Co–Bi/GAC particle electrode are 83.47 and 80.53%, 84.91 and 81.28%, respectively. When calcined for 2 h more, the removal rates of 1,4-dioxane and COD are only increased by 1.44 and 0.749%. With the extension of calcination time, the removal of 1,4-dioxane was not effectively improved. In summary, after calcination for 2 h, the supported elements can be almost completely converted into the corresponding metal oxides, with high electrocatalytic activity (Sun et al. 2019). Therefore, in this experiment, the roasting time of 2 h was selected as the best roasting condition for preparing particle electrodes.

By optimizing the three factors of Co/Bi mass ratio, calcination temperature, and calcination time, the preparation conditions with the best performance were obtained: Co/Bi = 1:0.25, calcined at 750 °C for 2 h, and this preparation condition was adopted in subsequent experiments.

As shown in Figure 7((c), (d)), with the increase of electrolyte concentration, the removal of 1,4-dioxane first increased and then decreased, and the EC showed a trend of first increasing, then decreasing, and then increasing. When the electrolyte concentration is 0.015 M, the removal rate of 1,4-dioxane is the highest, which is 84.87%, and the EC is 7.55 kWh/g 1,4-dioxane.

GAC as a carrier, the addition of Co–Bi/GAC particle electrode dose seriously affects the effectiveness of the entire process and operating costs. In order to determine the dosage of the particle electrode, 1,4-dioxane was degraded in a 3D system. As shown in Figure 7((e), (f)), with the increase in dosage, the removal rate of 1,4-dioxane gradually increased. When the dosage is 20 g, the removal rate of 1,4-dioxane is 84.87%, and the EC is 7.55 kWh/g 1,4-dioxane.

As can be seen from S3, P < 0.01 indicates that the regression of this model is good, and the R² of this model is 0.9390, so this model has good regression. The P value of the model's missing fitting item is 0.3259 (P > 0.05), indicating that the model's missing fitting item is not significant, that is, the regression model is significant. As can be seen from S3, F_A = 1.15, F_B = 0.0314, and F_C = 53.64. Therefore, the degree of influence of voltage, electrolyte, and dosage on the removal rate of 1,4-dioxane is as follows: dosage > voltage > electrolyte. According to the optimization of experimental data by the BDD response surface model, the optimal conditions for removing 1,4-dioxane are as follows: voltage 10.93 V, electrolyte concentration 2.07 g/L, dosage 18.88 g.

In summary, the one-step immersion calcination method prepared Co–Bi/GAC particle electrodes. The results of SEM, EDS, FTIR, XPS, CV, and EIS confirm that Co and Bi are uniformly distributed on GAC, and Co–Bi/GAC particle electrode has stable structure and excellent conductivity. RSM was used to optimize the process parameters such as voltage, electrolytes, and dosage. The results showed that the contaminant could be effectively degraded even in a high concentration of 1,4-dioxane solution of 500 mg/L. The highest removal rate was 84.87%, and the EC was 7.55 kWh/g of 1,4-dioxane. In addition, the particle electrodes showed good tolerance under all five acid–base conditions (3, 5, 7, 9, and 11). High-concentration solutions containing Cl⁻ and may participate in some competitive reactions and inhibit the degradation process. The stability test showed that after five cycles, the degradation effect of 1,4-dioxane remained at about 84%. In addition, an electrocatalytic degradation mechanism combining direct electrocatalysis and indirect electrocatalysis was proposed. The three-dimensional electrochemical system mainly removes 1,4-dioxane through the synergistic reaction of electrode adsorption, anodic oxidation and particle electrode oxidation. The abundant active sites and large specific surface area are the reasons for the catalytic activity of Co–Bi/GAC, in which ·OH is the main active free radical. This study elucidates the effect of bimetal-supported activated carbon particle electrodes on 1,4-dioxane electrocatalysis, which provides a new idea for designing more stable and efficient materials.

This study was supported by Fujian provincial industry–university–research collaborative innovation (2021Y4005), and Fujian Educational and Scientific Research Project for Young and Middle-aged Teachers (JAT200460).

R. W. conceptualized the whole article, developed the methodology, arranged the software, investigated the data, rendered support in formal analysis, and wrote the original draft. Z. D. developed the methodology, supervised the article, wrote the review and edited the article, and rendered support in funding acquisition. W. Z. wrote the review and edited the article. C. M. wrote the review and edited the article, and validated the data.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.