Electrochemical reduction of Cr (VI) using a palladium/graphene modified stainless steel electrode

The discharge of wastewater is increasing with the accelerating process of industrialization. Industrial wastewater contains a variety of heavy metals, which cannot only pollute water bodies but also are harmful to human health. Therefore, it is necessary to give priority to treatment of industrial wastewater containing a large amount of heavy metals before discharged (Amanze et al. 2022). Chromium (Cr) is one of the important industrial raw materials, widely used in electricity plating, tanning, pigment manufacturing, metallurgy and mining, etc. (Wang et al. 2020), which results in the discharge of a large amount of Cr-containing wastewater. It was reported that 15% Cr(VI) in the metallurgical industry is difficult to be utilized, which enters into aquatic environment easily. In addition, a large amount of wastewater discharged from the tannery industry is also one of the main sources of Cr-containing wastewater. These wastewaters with heavy metal pollution characteristics currently lack a unified management program (Bhunia et al. 2022). The content of Cr ranks 24th in the earth's crust and can be represented from all valence states from −2 to +6 (Sinha et al. 2022). Among these forms, hexavalent chromium (Cr(VI)) and trivalent chromium (Cr(III)) can exist stably in water environment (Wang et al. 2020). Cr(VI) can cause respiratory damage, digestive damage and skin irritation in humans. Furthermore, Cr(VI) will affect the growth of plants and aquatic organisms (He et al. 2020). Therefore, 100 μg/L Cr(VI) was set as the concentration limit of industrial effluents discharged into groundwater by the United States Environmental Protection Agency (USEPA) (Monga et al. 2022). In contrast, Cr(III) is a trace element, which has a good effect on the prevention of diabetes and high blood pressure. And the toxicity of Cr(III) is approximately 100-fold less than Cr(VI) (Lyu et al. 2021). The most effective method for the removal of Cr(VI) contamination is reducing Cr(VI) to Cr(III).

The electrode material is crucial to the removal efficiency of an electrochemical process (Zhang et al. 2019). Conventional electrode materials are generally selected from carbon materials, stainless steel and other metals that are not very reactive (Stern et al. 2020; Villalobos-Neri et al. 2021). The metal electrodes have excellent electrical conductivity, which is beneficial for electron transfer in the reaction, but their small specific surface area limits their contact with Cr(VI). Carbon materials are often incorporated into the electrodes due to their large specific surface area, high conductivity and outstanding electrolyte accessibility (Wang et al. 2019). Graphite felt, carbon fiber felt and carbon aerogel have been used as work electrodes for electrochemical Cr(VI) reduction (Rana-Madaria et al. 2005; Huang et al. 2014; Chen et al. 2021). However, the strength of carbon materials is not high, which is not beneficial to the processing and repeated use of electrodes. At present, stainless steel/carbon composite electrodes are the development direction of new electrode materials.

Graphene is the name given to a single layer of carbon atoms densely packed into a benzene-ring structure. Individual graphene sheets have extraordinary electronic transport properties (Sun et al. 2022). With high aspect ratio (the ratio of lateral size to thickness), large surface, rich electronic states, and good mechanical properties, graphene is a suitable electrode candidate outperforming other carbon materials (Arvas et al. 2022). And graphene exhibits good adsorption (Ham et al. 2018) and catalytic (Wang et al. 2016) properties in wastewater treatment. For instance, compared with the stainless steel (SS) electrode, the graphene-modified stainless steel electrode had much lower charge transfer resistance and more oxygen-containing functional groups, which can promote the electrochemical reduction of Cr(VI) (Chen et al. 2022). Chemical vapour deposition (Zou et al. 2022) and chemical reduction are common methods for the preparation of graphene.

The electrochemical reduction of Cr(VI) is often accompanied by a hydrogen evolution reaction (HER) (Li et al. 2021). Pd, a kind of active metal, can store the H₂ produced by HER on the electrode surface (Granja-DelRío et al. 2021) and catalyze the stored H₂ to form H₂O₂ (Tian et al. 2020). H₂O₂ can reduce Cr(VI) in the aqueous solution under acidic conditions and in the presence of external power supply (Jialiang et al. 2021). However, agglomeration may occur if Pd nanoparticles are added to the aqueous solution alone (Hu et al. 2020), which make the catalytic sites of Pd cannot be fully exposed. Attaching Pd to graphene can effectively solve the above problems.

In this study, SS electrode was modified with palladium and graphene to fabricate a palladium/graphene-modified stainless steel (Pd/G-SS) electrode. With this electrode as the work electrode, the electrochemical Cr(VI) reduction under different conditions was investigated and the reaction mechanism for the electrochemical Cr(VI) reduction was explored via various trails.

The preparation of the chemicals does not require additional purification. The natural graphite powder (C, ∼99.5%) was obtained from Jinrilai Graphite Company, Qingdao. The palladium chloride (PdCl₂, >59% for Pd) powder, N-methyl pyrrolidone (C₅H₉NO, >99.5%), acetone (C₃H₆O, >99.5%), and ethanol (C₂H₆O, >95%), potassium dichromate (K₂Cr₂O₇, >99.8%), sulfuric acid (H₂SO₄, 95% ∼ 98%), sodium sulfate (Na₂SO₄, >99.0%) and other chemicals (analytically pure) were purchased from Sinopharm Chemical Reagent Co., Ltd. Polyvinylidene fluoride ((C₂F₂)_n, PVDF) was supplied by Shanghai Macklin Biochemical Co., Ltd. The 304 SS mesh (60 mesh) was provided by Wuxin Company, Changzhou.

Pd powder was obtained by reducing divalent Pd (Dai et al. 2014). After graphene oxide was prepared by oxidizing natural graphite powder according to the improved Hummers method (Zomorodkia et al. 2021), reduced graphene oxide (termed as graphene in this text) was synthesized via hydrothermal reduction (Serrapede et al. 2020). The palladium/graphene (Pd/G) slurries with various Pd content proportions were prepared as follows: (1) Pd powder with various weight (0, 1 mg, 3 mg, 5 mg or 7 mg) and graphene with fixed weight (100 mg) were sequentially added into 5 mL N-methyl pyrrolidone, followed by dispersing evenly through magnetic stirring; (2) 100 mg PVDF was dispersed evenly into 5 mL N-methyl pyrrolidone; (3) the above two solutions were mixed to obtain five Pd/G slurries with different Pd content proportions (0, 1, 3, 5 and 7%). The Pd/G-SS electrodes were prepared by adhering Pd/G slurries to the SS meshes through adhesive PVDF. The preparation process is as follows: (1) the 304 SS mesh (60 mesh) was cut into 20 mm × 45 mm, then rinsed with acetone, ethanol and DI water in an ultra-sonication, and finally dried in the nitrogen gas; (2) SS was soaked in the Pd/G slurries for 30 s, then dried at 80 °C for 2 h; (3) by repeating the second step three times, electrodes with a fixed graphene loading (1.50 mg/cm²) and varied Pd content proportions (0, 1, 3, 5 and 7%) can be fabricated. Among these electrodes, the Pd/G-SS electrode with a Pd content proportion of 0 was termed as the graphene-modified stainless steel (G-SS) electrode in this text.

Synthetic Cr(VI)-containing wastewater was prepared by dissolving K₂Cr₂O₇ in DI water. The pH was adjusted by adding H₂SO₄ and Na₂SO₄ solution was used as the electrolyte. Unless otherwise specified, the test device was run with the following parameters: Cr(VI) concentration of 10.0 mg/L, pH 2, Na₂SO₄ concentration of 10 g/L, electrode potential of −0.6 V and Pd/G-SS electrode with Pd content proportion of 3%. The system was in turn run at different Pd content proportions (0, 1, 3, 5 and 7%), different electrode potentials (−0.6 V, −0.7 V, −0.8 V, −0.9 V and −1.0 V), different pH (1, 1.5, 2, 2.5 and 3), and different electrolyte concentrations (2 g/L, 6 g/L, 10 g/L and 14 g/L). Under each condition, the test was repeated at least in triplicate and the Cr(VI) concentration was measured every 20 min.

Under the optimum working conditions, adsorption tests without external power supply and electrochemical Cr(VI) reduction tests were successively carried out to analyze the reaction mechanism. The Cr(VI) adsorption percent was examined by comparing the residual concentration after the reaction with the initial concentration. Fourier transform infrared radiation (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to examine the change of functional groups and the valence states of Cr. The electrochemical performance of Pd/G-SS electrode was tested by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests.

Cr(VI) concentration was measured by the diphenylcarbazide spectrophotometry. H₂O₂ concentration was measured by the Titanium Potassium Oxalate Spectrophotometry. The functional groups were analyzed with a FTIR spectrometer (VERTEX 70, Bruker, Germany, 400–4,000 cm⁻¹). The micromorphology of graphene and Pd/G slurry were observed by a transmission electron microscopy (TEM) (Tecnai G220, FEI Company, Netherlands). XPS (AXIS-ULTRADLD-600W, Kratos Company, UK) was performed with an Al X-ray monochromatic source. CV test (2.5 mV·s⁻¹, negative scan first and then positive scan) and EIS test (frequency range: 0.1–10 kHz, amplitude: 5 mV) were performed using a potentiostat mentioned above.

The Cr(VI) removal efficiency within 60 min increased when the electrode potential was decreased from −0.6 V to −0.9 V, but it decreased as the electrode potential was further decreased to −1.0 V (Figure 4(b)). The decrease of electrode potential from −0.6 V to −0.9 V promoted the HER and further promoted the generation of H₂O₂, resulting in the increase of Cr(VI) removal efficiency (Kim et al. 2015). The decrease of Cr(VI) removal efficiency when the electrode potential being dropped to −1.0 V can be attributed to the violent HER, which was similar to the case of high Pd content proportion (5 and 7%). Since the Cr(VI) removal efficiency within 60 min reached the highest level of 99.70 ± 0.30% at the electrode potential of −0.9 V, this electrode potential was used in subsequent experiments.

The Cr(VI) removal efficiency within 60 min increased obviously when the initial pH was decreased from 3.0 to 2.0, but it decreased as the initial pH was further decreased to 1.5 and 1.0 (Figure 4(c)). Low pH was beneficial to reduce the reaction overpotential of electrochemical Cr(VI) reduction, enhancing the removal of Cr(VI). Besides, both the formation of H₂O₂ and the reduction of Cr(VI) depended on strong acid conditions (Jin et al. 2016a; Zhang et al. 2022). Therefore, the Cr(VI) removal efficiency was extremely low at the initial pH of 3 (2.00 ± 0.30%) due to the electrochemical reduction being hindered. After the initial pH was decreased below 2.0, the reduction between Cr(VI) and H₂O₂ was limited by H₂O₂ formation due to limited catalytic sites of Pd. And the ability of Cr(VI) to obtain electrons directly from the electrode was weakened due to HER exacerbating electron consumption at low pH. As a result, the highest Cr(VI) removal efficiency of 99.70 ± 0.30% was obtained at the initial pH of 2.0, and this initial pH was used in subsequent experiments.

The Cr(VI) removal efficiency within 60 min increased when the electrolyte (Na₂SO₄) concentration was increased from 2 g/L to 6 g/L, but it decreased as the electrolyte concentration was further increased to 10 g/L and 14 g/L (Figure 4(d)). Increasing the electrolyte concentration can improve the electrochemical Cr(VI) reduction via reducing the solution resistance and promote the HER (Hou et al. 2012). Although the HER and the electrochemical Cr(VI) reduction competed for electrons, the electrochemical Cr(VI) reduction was improved due to the H₂O₂ formation in the presence of Pd on the electrode (Kim et al. 2015). However, more electrolyte cations would be attracted to the cathode with the increase of the electrolyte concentration, resulting in a difficulty for Cr(VI) species to reach the cathode (Chen et al. 2022). When the electrolyte concentration was set at 6 g/L, the Cr(VI) removal efficiency was up to 99.70 ± 0.00% and the residual Cr(VI) concentration was 0.03 ± 0.00 mg/L, and this electrolyte concentration was used in subsequent experiments.

After the electrochemical reduction, the infrared peak at 3,440 cm⁻¹ (corresponding to O-H of carboxyl group (Tadjenant et al. 2020)) was further decreased as compared with the peak after adsorption (Figure 6(c)). Cr(VI) absorbed by -COOH²⁺ will be reduced to Cr(III) after the electrochemical reduction and -COOH²⁺ will lose 2 H⁺ to form -COO⁻ to absorb Cr(III) (Liu et al. 2019), resulting in the destruction of O-H of carboxyl groups. XPS spectra of the Pd/G-SS electrode showed that only a characteristic peak at 576.6 eV (representing Cr(III) (Hou et al. 2012)) was detected after the electrochemical reduction (Figure 6(d)). This implied that all absorbed Cr(VI) was electrochemically reduced. Besides, the Cr(VI) removal efficiency was up to 99.70 ± 0.0% in the electrochemical reduction test (section 3.2). These results implied that Cr(VI) was mainly removed via the electrochemical reduction.

Electrochemical reduction method based on the Pd/G-SS electrode can effectively remove Cr(VI). Under the optimal condition with Pd content proportion of 3%, electrode potential of −0.90 V, initial pH = 2, and electrolyte concentration of 6 g/L, the Cr(VI) removal efficiency reached 99.70 ± 0.00% within 60 min and the residual Cr(VI) concentration was 0.03 ± 0.00 mg/L. The removal of Cr(VI) involved four pathways: (1) direct electrochemical reduction by accepting electrons, (2) indirect electrochemical reduction by H₂O₂ that was generated from H₂ in the presence of Pd, (3) adsorption through hydrogen bond, and (4) chemical reduction through alkoxy groups donating electrons. The indirect electrochemical reduction considerably promoted the Cr(VI) removal while a small amount of Cr(VI) was removed via adsorption and chemical reduction. Electrochemical Cr(VI) reduction could be used as a pretreatment technology to solve the problem of excessive Cr(VI) concentration.

This work was supported by the National Natural Science Foundation of China under Grant number 21577108.

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

The authors declare there is no conflict.