Membrane-free electrodeionization using graphene composite electrode to purify copper-containing wastewater Open Access

Because of its excellent thermal and electrical conductivity, as well as high ductility, copper has been widely used in electroplating, metallurgy, machining, wire production, and other applications (Jiang et al. 2015; Kilany et al. 2020). These industrial processes produce a large amount of copper-containing wastewater, which has become a major industrial and environmental concern. Unlike organic matter, copper-containing pollutants do not degrade and can migrate into the biosphere through the food chain, eventually accumulating in the human body and causing irreversible damage (Shao et al. 2020; Liu et al. 2021). Copper can irritate the human central nervous system and cause cancer (Choi et al. 2020a). Nevertheless, copper is a precious resource. Therefore, the economical, effective treatment and reuse of copper-containing wastewater, along with copper recovery, are of great significance for preserving the safety of the environment, conserving natural resources, and protecting human health.

Many technologies have been adopted to remove copper from wastewater, including adsorption (Khan et al. 2021), chemical precipitation (Luna et al. 2020), membrane separation (Zhao et al. 2019; Li et al. 2021) and ion exchange (Ulloa et al. 2020; Yan et al. 2020). Adsorption is effective in treating low-copper effluents, however, the disposal of exhausted adsorbents is expensive (Hao et al. 2022). Chemical precipitation is a widely used method for treating wastewater that contains copper ion. The investment and operating costs of this method are low (Mark et al. 2020), but the treated effluent cannot meet discharge requirements, so further in-depth treatment is required to purify the water well enough to reuse it. Membrane separation is the most commonly used advanced-treatment technology; other examples include electrodeionization (Feng et al. 2007; Mahmoud & Hoadley 2012) and electrodialysis (Simona et al. 2015; Scarazzato et al. 2017). Membranes can remove almost all of the ions from wastewater, through either standalone or combined treatment. However, this method requires ion-exchange membranes, which are expensive and prone to membrane pollution, concentration polarization, and scaling (Ma et al. 2020). Ion-exchange technology has the advantages of good treatment effectiveness (Hamdaoui 2009; Choi et al. 2020b; Wołowicz & Hubicki 2020) and low investment cost, and the failed ion-exchange resin can be regenerated with strong acids and bases. However, the regeneration operation is complicated, and a large volume of acid and alkali liquid waste is produced, which results in secondary pollution. Therefore, green regeneration of ion-exchange resins is a very compelling area of study.

In this research, membrane-free electrodeionization (MFEDI) technology was applied to the treatment of copper-containing wastewater for the first time. This technology allows the production of pure water and regeneration of resin to be alternated in the same resin column (Shen & Chen 2019); that is, after resin-operation failure, high voltage is used to induce water electrolysis and ionization, producing H⁺ and OH⁻, and regenerating the resin. The regenerated ions are removed by water that flows from the top of the column to the bottom, and then the water is treated. This cycle repeats continuously. The entire process is green and pollution-free, since it does not require acid and alkali agents, or ion-exchange membranes.

However, the resin has a high copper adsorption capacity, which leads to low regeneration efficiency. In addition, hydroxide precipitation readily occurs during electric regeneration. To overcome these drawbacks, water electrolysis needs to be enhanced to produce more H⁺ for regeneration. Inspired by this, we developed a graphene composite electrode MFEDI for enhanced water electrolysis. Compared to a platinum-coated titanium electrode, a graphene composite electrode provides a larger specific surface area, with the possibility of decreasing the oxygen evolution potential, which means increased occurrence of the oxygen evolution reaction. The objectives of this research were to investigate the electrochemical properties of the graphene composite electrode we prepared, and examine its performance as an electrode used in a MFEDI system for purification of copper-containing wastewater.

A strong-acid cation resin (650C, Dow Chemical Company, Mitland, USA), weak-acid cation resin (D113, Zhejiang Zhengguang Industrial Co., Ltd, Hangzhou, China), strong-base anion resin (550A, Dow Chemical Company, Mitland, USA), and weak-base anion resin (D301, Zhejiang Zhengguang Industrial Co., Ltd, Hangzhou, China) were used in the experiments. The main properties of the resins are shown in Table 1. Both the anion and cation resins were chemically regenerated into R–OH and R–H types prior to use. The copper-containing wastewater, with a concentration of 5 mg/L, was prepared using chemically pure CuCl₂ (99%, Aladdin, Shanghai, China).

The conductivity-examination device was the same as the MFEDI device, except that the length of the resin bed was 2.5 cm. A voltage-current experiment was conducted to investigate the resistivity of different resin forms.

Both the platinum-plated titanium and graphene composite electrodes were prepared by thermal decomposition (Chen & Chen 2005). The titanium mesh was cleaned ultrasonically for 10 min, two or three times. The titanium mesh was etched in boiling 37% HCl for 2 min to remove impurities on the surface of the electrode, then cleaned ultrasonically again for 10 min, two or three times; and then dried under ambient conditions. The precursor solution for the platinum-plated titanium electrode was prepared by dissolving 0.5 M H₂PtCl₆·H₂O (99%, Aladdin, Shanghai, China) in isopropanol solution, while the precursor solution for the graphene composite electrode was prepared by evenly mixing 1 g graphene and platinum-plated titanium electrode precursor solution. Next, the precursor solution was evenly brushed on the titanium-mesh substrate surface. The titanium-mesh matrix was then dried at 353 K for 3 min in an oven, transferred to a muffle furnace, and roasted at 673 K for 5 min, before being taken out and cooled under ambient conditions. Subsequently, the matrix was repeatedly brushed, dried, and roasted 8–10 times. Finally, this titanium-mesh substrate was placed in the muffle furnace and annealed for 1 h to complete electrode preparation.

The morphology of the electrode surface was characterized by SEM (SEM500, CIQTK, Hefei, China). The anode oxygen evolution behaviour was studied using linear sweep voltammetry (LSV) with a scan rate of 0.02 V/s using an electrochemical workstation (PARSTART MC2273, AMETEK, USA). LSV was carried out in a three electrode configuration with a saturated calomel electrode as reference electrode, a platinum electrode as counter electrode, and the prepared electrode as working electrode.

A high-voltage power supply (YT-DL, Nanjing Kenfan Electronic Technology Co., Ltd, Nanjing, China) was employed for electrical regeneration, operating with constant voltage. The pH and conductivity of various fractions were analyzed using a pH meter (PHS-2F, Rex, Shanghai, China) and a conductivity meter (IP67, Mettler Toledo, Zurich, Switzerland), respectively. The copper concentration was measured by flame atomic absorption spectrometer (TAS-990AFG, Persee, Beijing, China).

As shown in Figure 3, the regeneration solution (concentrate) was acidic, with a pH of approximately 3.8, due to more weak-base resins regenerated than weak-acid resins. The acidic concentrate can effectively prevent copper hydroxide precipitation.

The copper concentration in the regenerated solution was 47.3 mg/L at 2 min, then gradually decreased to 25.8 mg/L at 20 min. As regeneration progressed, the amount of copper adsorbed by the resin particles gradually decreased, and the copper concentration in the regeneration solution also decreased. However, the average copper concentration in the regeneration solution was only 35.0 mg/L; this may be due to the strong copper-adsorption capacity of the resin, making the resin difficult to regenerate. Furthermore, the regeneration current density was too low, which also leads to lower electric regeneration efficiency. The results above indicate low electric regeneration efficiency of the MFEDI column with the platinum-plated titanium electrode. The regeneration flow rate and time were used to calculate the total amount of regenerated copper, which was 82.8 mg.

Current density is an important factor for the electrical regeneration of the resins and energy consumption. Typically, electrical regeneration efficiency can be increased by increasing current density. However, current density was greatly influenced by the conductivity of the resin bed. As shown in Figure 3, the current density decreased gradually with the regeneration time, from 86 A/m² initially to 69 A/m² at 20 min. This is mainly due to the fact that after electric regeneration, a part of the cation exchange resins changed from R–Cu form to R–H form, and a part of the anion exchange resins changed from R–Cl form to R–OH form. The changes in the resin forms caused the resin-bed conductivity changes.

Obviously, the conductivity change of the packed resin has a significant influence on the current density. As shown in Figure 5, the conductivities were high for strong-acid resins and strong-base resins of all types, while those for weak-base resins and weak-acid resins were extremely low. As a result, the current density was determined by the weak-acid resins and weak-base resins. As the electrical regeneration proceeded, the conductivity of the weak-acid resins increased, while that for the weak-base resins decreased. Moreover, the composition of the acidic regeneration solution indicates more weak-base resins were regenerated than weak-acid resins. This explains why the current density decreased. To increase the current density, efforts should be focused on improving the regeneration efficiency of weak-acid resins.

The H⁺ and OH⁻ ions generated by the electrolytic and ionization reactions exchanged with the Cu²⁺ and Cl⁻ ions adsorbed by the cation and anion resins, respectively, thus regenerating the anion and cation resins. The higher the concentration of H⁺ and OH⁻, the weaker the adsorption capacity of the resin for salt ions, and the higher the electric regeneration efficiency. Relevant studies (Shen & Chen 2019) have shown that the layered filling of anion and cation resins used in this study can effectively promote the water ionization reaction (3). Therefore, to further enhance the electric regeneration efficiency, it is important to promote the electrolytic reaction (1). Optimising the electrode performance is the most effective way to achieve this.

The treatment and regeneration performance data of the MFEDI device with the platinum-plated titanium electrode demonstrated that MFEDI technology can be used for Cu²⁺-containing wastewater treatment, but it is necessary to develop a new electrode to improve the electric regeneration efficiency of weak-acid resin. Graphene has excellent physical and electrochemical properties, with a large specific surface area and abundant surface functional groups. After modification and reduction, a better-dispersed phase at the nanoscale forms on the original electrode surface, which significantly improves the electro-chemical performance of the electrode. Therefore, the development of a graphene composite electrode may greatly improve the performance of the MFEDI system.

The copper concentration in the regeneration solution reduced slowly from 55.4 mg/L to 38.0 mg/L, owing to the decrease of copper ions in the resins with the regeneration. The average copper concentration after mixing the liquid from all of the regeneration cycles was 50.4 mg/L, which was 1.4 times that of the MFEDI device with platinum-plated titanium electrodes. This provided further evidence that the electrode performance was optimised after graphene loading, and the electric regeneration efficiency improved accordingly. The regeneration flow rate and time were used to calculate the total amount of regenerated copper, which was 119.3 mg. The concentrated regeneration solution could be fed to a pretreatment unit, such as reverse osmosis, so that the copper can be recovered after this concentrating process.

The current density decreased gradually, from 100 to 79 A/m². which was higher than that of the MFEDI system with platinum-plated titanium electrodes. This can be explained by the acidic regeneration solution and the improved regeneration efficiency of the weak-acid resin. On the one hand, more weak-base resins were regenerated than weak-acid resins, and the conductivity of the resin bed reduced, leading to the decrease of the current density. On the other hand, water electrolysis was enhanced after loading graphene, and more H⁺ was produced in the anode to regenerate weak-acid resin. Consequently, the regeneration efficiency of the weak-acid resin improved, resulting in higher current density compared to MFEDI system with platinum-plated titanium electrodes.

This work presented an improved MFEDI system capable of high electric regeneration efficiency and excellent purification performance in the treatment of copper-containing wastewater. The advantages of this system were attributed to the unique graphene composite electrode, which enabled a lower oxygen evolution potential and improved water electrolysis, due to the larger specific surface area of the electrode. The conductivity of the treated effluent was less than 1.0 μs/cm, indicating satisfactory treatment effectiveness. Compared to that of an MFEDI system with a platinum-coated titanium electrode, the average copper concentration in this system increased by 1.4 times to 50.4 mg/L, which demonstrates that the regeneration efficiency noticeably improved. The pH of the regeneration solution remained in the range of 3.9–4.1, and no hydroxide precipitation occurred during the entire process. The energy consumption of the system decreased from 1.55 to 1.48 kWh/m³, and the water recovery rate increased from 85 to 90%.

The MFEDI system with a graphene composite electrode produced a relatively low amount of harmful waste, and exhibited a high rate of copper and pure-water recovery, which are significant environmental and economic benefits. In addition, the system showed high electric regeneration efficiency, which reduced energy consumption and wastewater discharge, and avoided problems associated with ion-exchange membranes. These results confirmed the capabilities of the graphene composite electrode MFEDI system for achieving highly efficient copper removal and recovery from wastewater, which should increase the application of MFEDI for the treatment of copper-containing wastewater.

This work is supported by Zhejiang Provincial Natural Science Foundation of China (LQ20B070004), and the Young Doctoral Innovation Fund of Ningbo Polytechnic (10600300300504). Additional support is provided by Open Project of Zhejiang Collaborative Innovation Center for High Value Utilization of Byproducts from Ethylene Project (NZXT2018309).

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

The authors declare there is no conflict.