Numerical analysis and experimental study for removal of copper and zinc from groundwater by dielectric barrier discharge

In recent years, the dielectric barrier discharge (DBD) technology, as a new advanced oxidation technology, has been widely studied and applied in the direction of water treatment due to its low energy consumption and environmental advantages (Van Nguyen et al. 2020; Shao et al. 2023). DBD has the advantages of high efficiency, rapidity, no chemical additives, and no secondary pollution in the treatment of industrial wastewater and domestic sewage. Coaxial DBD reactors have the advantage of stable and uniform discharge. However, the narrow discharge gap results in a more complex flow field within the discharge region. Effective gas–liquid mixing distribution in the discharge area enhances the removal efficiency of copper and zinc ions from water treated with DBD in high copper and zinc groundwater. Therefore, enhancing gas–liquid mixing distribution in the discharge area is crucial for utilizing the DBD reactor in treating high copper and zinc groundwater. In studies examining gas–liquid mixing in microchannel flow fields, Chekifi (2018) explored droplet generation in a conical coaxial flow structure microfluidic device under low Reynolds number conditions. Li et al. (2021) utilized Fluent software to simulate and analyze the cleaning efficacy of gas–liquid mixing jets from the nozzle to acquire the flow field distribution of the nozzle. Zheng et al. (2022) employed the volume of fluid (VOF) model to investigate the distribution pattern of the internal flow field of the water ring vacuum pump and achieved the assisted production of a new type of water ring vacuum pump. Based on the simulation outcomes, various types of pollutants can be accurately treated by adjusting the parameters and conditions of plasma discharge. In the research design of DBD reactor types, of note is the removal of tramadol analgesia from deionized water and final wastewater by Babalola et al. (2023) using a novel continuous flow DBD reactor. Zhang et al. (2018) designed a novel gas–liquid two-phase DBD reactor to treat phenol in water, using a tube covered with copper foil as a high-voltage electrode and a grounding electrode to increase the contact area between the plasma and the pollutant molecules, and the removal rate of phenol was 95.5%, which proved that hydroxyl radicals are the main active substances for removing phenol. In a process parameter study, Iervolino et al. (2019) investigated the effect of air flow rate, applied voltage, and type of process gas on the removal of several aqueous pollutants (methylene blue, phenol, paracetamol, caffeine, and ceftriaxone). In studies using DBD technology in combination with other techniques to treat target contaminants, Tang et al. (2012) combined DBD technology with integrated granular activated carbon (GAC) preconcentration to remove bisphenol A from water, and the experimental results showed that the adsorption capacity of DBD regenerated GAC could be maintained at a high level. The results of the study by Van Nguyen et al. (2020) show that a combination of DBD technology and other water treatment technologies can effectively treat groundwater to meet drinking water standards.

Groundwater is an essential source of drinking water in China, and the safety of water quality is a constant concern for the health of the residents (Chomba et al. 2022). According to the ‘Standards for Drinking Water Quality (GB5749-2022)’ in China (NHCC 2022), the standard limits for copper and zinc in groundwater are both 0.1 mg/L. However, in recent years, the groundwater in most cities in China has been seriously polluted, the water source has been decreasing, and the water quality is deteriorating (Chen 2020). Heavy metals are not only a serious environmental hazard but also pose several problems for human health. The problem of heavy metal pollution of water bodies is becoming more and more prominent and has received widespread attention (Badruddoza et al. 2013; Abtahi et al. 2017). There is increasing pollution of groundwater by copper and zinc heavy metals. The main causes of excessive copper and zinc levels in groundwater in China include industrial pollution, agricultural activities, landfills, geological conditions of groundwater, and improper sewage treatment. Long-term consumption of groundwater containing high levels of copper and zinc may cause acute or chronic poisoning of the human body, leading to digestive discomfort, organ damage, imbalance of trace element balance, and even affect the functioning of the nervous system and immune system (Ahmed et al. 2016; Bawuro et al. 2018; Abtahi et al. 2023). Various techniques have been used to remove Cu²⁺ and Zn²⁺ from water, such as adsorption (Yang et al. 2019), chemical precipitation (Yu et al. 2020), membrane filtration (Deng et al. 2020), and electrochemical deposition. Currently, the common methods are adsorption and precipitation. Adsorption of copper ions by anisotropic PVA wood-based hydrogels with 77.27% adsorption rate was demonstrated by Sun et al. (2024). Ferreira et al. (2019) used white beans (Phaseolus vulgaris L.) as a sorbent for the removal of zinc from rainwater with a removal rate of 63%. Precipitation, as a commonly used method for removing heavy metals from water, is effective but suffers from the disadvantages of generating slag that is difficult to dispose of, becoming affected by water quality, inconvenient for pH adjustment, expensive, and not allowing resource recovery. These methods prove to be costly and uneconomical, and are less effective in eliminating heavy metals.

DBD aims to address the shortcomings of traditional methods for removing zinc and copper in wastewater treatment, such as low removal efficiency, harsh operating conditions, the possibility of secondary pollution, and poor recovery rates. By improving the efficiency and effectiveness of zinc and copper removal, the technology hopes to promote environmental protection and resource recycling. In this paper, DBD generates low-temperature plasma to treat groundwater with high copper and zinc content; the gas–liquid distribution law of a gas–liquid hybrid DBD reactor and the mechanics of copper and zinc in groundwater was further analyzed to investigate in detail the effects of process parameters such as discharge power, initial pH value, gas flow rate, liquid flow rate, and dosage rate on the removal rate of copper and zinc ions in the hope of providing theoretical support for the application of low-temperature plasma technology in the treatment of groundwater.

Instruments: DBD reactor (Discharge length: 150 mm), CTP-2000K low-temperature plasma experimental power supply (Voltage: 0–30 kV; Discharge frequency: 5–20 kHz), two-stage 980 oil-free air compressor (Gas flow range: 0–80 L/min), FBⅠ/MN3 peristaltic pump (Flow: 50–570 mL/min), FM-6 gas flow meter (Measure range: 0.1–1 L/min), FM-6 liquid flow meter (Measure range: 100–500 mL/min), MSO5072 oscilloscope (Analog bandwidth: 70 MHz), B01 electronic (Balance accuracy: 0.001 g), flame atomic absorption spectrometer (FAAS), and portable pH meters (Accuracy: ±0.1pH).

Reagents: CuSO₄·5H₂O (Purity: 99%), ZnSO₄·7H₂O (Purity: 99%), and NaOH (Purity: 96%).

All experiments were conducted in 2023–2024 at laboratories of universities and companies in Xinjiang based on effluent data provided by Xinjiang De'an Environmental Protection Technology Company, with the following experimental setup:

• (1) Before the experiment, distilled water was used to clean the beaker and the fluid channel inside the dielectric blocking discharge reactor, which were dried for use. After drying, the low-temperature plasma power supply was turned on for 30 min before the experiment to warm up, and then the oscilloscope was turned on. The center frequency of the power supply needs to be adjusted to 10 kHz.

• (2) CuSO₄·5H₂O and ZnSO₄·7H₂O were dissolved and diluted in deionized water to simulate the high concentration of copper and zinc groundwater, then the simulated high zinc–copper groundwater was taken in a beaker for subsequent discharge treatment. The beaker was covered to prevent splashing of the liquid.

• (3) The discharge gas uses air supplied by the air compressor. Hence, the air compressor was turned on, the flow rate at the outlet was adjusted to the required flow rate, and then the gas flow meter was adjusted to stabilize the gas flow rate.

• (4) After the gas flow rate was stabilized, the peristaltic pump was opened and the liquid flow rate was adjusted to the required flow rate, following which the liquid flow meter was adjusted to stabilize the liquid flow rate.

• (5) After the gas flow and liquid flow were stabilized, the contact regulator was twisted to adjust the discharge voltage to the required voltage, and the discharge voltage and voltage were recorded with an oscilloscope for subsequent calculation of the discharge power.

• (6) The experimental procedure involved measuring and recording the changes in concentration at regular intervals, taking an appropriate amount of the treated sample, and filtering out the resulting precipitates, Cu(OH)₂ and Zn(OH)₂.

It should also be noted that the total treatment time for Cu and Zn was 15 min and the sampling interval was 3 min. Samples were taken at regular intervals, and the precipitate was filtered and tested for the amount of copper and zinc ions remaining in the water. The amount of copper and zinc in solution was determined by flame atomic absorption spectrometry. Standard solutions of copper at 0, 1, 2, 3, 4, and 5 mg/L were prepared and the concentrations of the standard solutions were determined by flame atomic absorption spectrometry. The regression equation of the standard curve for copper and zinc was y = 0.1062x − 0.0668 with the correlation coefficient R² = 0.9535 and that of the standard curve for zinc was y = 0.1271x + 0.3671 with the correlation coefficient R² = 0.8694. From the correlation coefficients of the regression equations of the standard curves of copper and zinc, it can be seen that the concentration and absorbance of copper and zinc show a good linear relationship within a certain range.

The single-factor experiment was designed to study the effect of different process parameters on the removal of copper and zinc. The different discharge power and initial pH are examined to accommodate the complex environmental changes in actual production processes. The factor levels are presented in Table 1.

Ozone (O₃) is an oxidizing agent with strong oxidizing properties, which is produced in the discharge plasma reaction mainly through the process of ionization and recombination of oxygen molecules. Ozone can quickly diffuse in the plasma channel and diffuse into the water through the catalytic tube, part of the ozone molecules dissolve in the water and have a direct oxidation reaction with the organic matter, and part of the ozone molecules and water molecules combine to generate more oxidizing –OH. Ozone can react with metal ions in water by oxidizing them to higher valence forms such as hydroxides and oxides. These oxidation products usually have low water solubility and tend to form precipitates or solids that can be separated from water by precipitation, filtration, and so on. Ozone was detected by a portable ozone detector, and as can be seen in Figure 2(b), the amount of ozone in the water gradually increased as the discharge time increased.

By setting different gas–liquid boundary conditions to analyze the influence law of gas flow rate and liquid flow rate on the gas–liquid distribution in the reactor, we get the best gas–liquid mixing distribution effect. Both the air inlet and the water inlet are set up as velocity inlets and the flow outlet is set as the outflow boundary. The wall surface is set to a smooth wall surface. The gas is supplied by an air compressor, the rated exhaust pressure of the air compressor is 0.7 MPa, so the pressure at the inlet is set to 0.7 MPa, and the flow rate parameters of the inlet and the liquid inlet are shown in Table 2.

The effect of discharge power on the removal of Zn²⁺ from solution at an initial concentration of 5 mg/L, a gas flow rate of 30 L/min, a liquid flow rate of 200 mL/min, and a pH of 11 is shown in Figure 10(b). The maximum removal of Zn²⁺ was 95.67, 96.22, and 94.29% when the discharge power was 58, 94, and 129 W, respectively. As the discharge power increases, the rate of active particles produced in the reactor gradually increases and the removal of Zn²⁺ slightly increases. However, when the discharge power was too high, the removal of Zn²⁺ started to decrease slightly. To analyze the reason, when the voltage is greater than 120 V, the thermal effect of the dielectric blocking the discharge becomes more and more obvious, and the temperature of the solution increases, resulting in a decrease in the solubility of ozone. This is consistent with the results of Wang et al. (2022) who used multi-electrode dielectric blocking discharge plasma to treat printing and dyeing wastewater. As shown in Figure 10(d), after 3 min of DBD treatment, the energy yield reached a maximum of 1.59 g/(kW h) at a discharge power of 58 W. Therefore, the discharge power of 58 W can achieve better removal effects and economic benefits.

When the discharge power is 58 W, the concentration of Zn²⁺ is 5 mg/L, the liquid flow rate is 200 mL/min, and the pH is 11. The effect of the gas flow rate on the removal rate of Zn²⁺ is shown in Figure 11(b), it can be seen that the gas flow rate of 20 L/min has the best effect of 92.18% removal of Zn²⁺. However, the maximum removal of Zn²⁺ began to decrease when the gas flow rate continued to increase. The degradation efficiency started to decrease when the gas flow rate was gradually increased, which may be due to the fact that the excessive gas flow rate made the discharge region inside the reactor unstable, resulting in a decrease in the generation of reactive particulate matter, which reduced the chance of collision between the pollutants in the water and the reactive particulate matter, and as a result, the degradation effect was weakened. Therefore, the removal of Zn²⁺ was reduced. This is different from the phenomenon shown in Figure 7 as the gas flow rate has less influence on the removal of Zn²⁺ in the actual removal of Zn²⁺ and the control of the gas flow rate is less stable in the real situation.

The effect of the gas flow rate on Zn²⁺ removal was analyzed when the discharge power was 58 W, the concentration of Zn²⁺ was 5 mg/L, the gas flow rate was 40 L/min, and the pH was 11. As shown in Figure 12(b), when increasing the liquid flow rate to 100 mL/min, the removal rate of Zn²⁺ had the best effect of 96.23%. With further increase in the flow rate, the removal efficiency of Zn²⁺ gradually decreased relative to the 100 mL/min flow rate. This is consistent with the phenomenon shown in Figure 9 that the gas–liquid mixing in the discharge region deteriorates when the liquid flow rate increases. The reason for this is that high liquid flow rates can lead to excessive dilution of reactive oxygen species (e.g. ozone) in the reaction chamber, reducing their concentration and thus their oxidizing capacity for heavy metals, and may also lead to excessive mixing effects in the reaction chamber, which can hinder the transport of reactive oxygen species and make it difficult for them to effectively come into contact with and react with the heavy metal ions, thus lowering the efficiency of the degradation. When the liquid flow rate is too high, the simulation results show that the gas–liquid mixing effect becomes worse, which is also reflected in the zinc removal rates.

When the discharge power is 58 W, the concentration of Zn²⁺ is 5 mg/L, and the gas flow rate is 40 L/min, the effect of the initial pH of the liquid flow rate of 100 mL/min solution on the removal of Zn²⁺ is as shown in Figure 13(b). The maximum removal of Zn²⁺ was 58.85, 75.84, 96.09, and 68.89% when the initial pH of the solution was 9, 10, 11, and 12, respectively. This is because the pH of the solution affects the formation of Zn(OH)₂ precipitates. A high pH increases the concentration of hydroxide ions (OH–) in the water, which in turn improves the formation of Zn(OH)₂ precipitates, but too high a pH can lead to the production of more free radicals in the reaction solution, which may lead to a decrease in the amount of free radicals (e.g. –OH) and thus a decrease in precipitation. After 3 min of DBD treatment at a pH value of 10–12, the Cu²⁺ content was reduced to less than 1 mg/L, meeting the drinking water standard (NHCC 2022). Therefore, it facilitates the removal of zinc from groundwater by adjusting the pH value of the groundwater between 10 and 12.

In this study, the feasibility of a gas–liquid hybrid DBD reactor for treating high concentrations of copper and zinc in groundwater was verified by simulation and experiment, and the optimal operating parameters were found to improve the removal efficiency. The copper and zinc removal rates (up to 98.68 and 96.28% in 12 min, respectively) reached a steady state under optimal operating conditions. This provides a theoretical basis for future low-temperature plasma technology in treating heavy metals in groundwater or developing a more efficient DBD reactor structure. The experimental results show the following:

• The mixed gas–liquid two-phase flow inside the DBD reactor is simulated using the VOF model to obtain the mixing of the flow field inside the reactor, which provides a good model for two-phase flow analysis. The gas–liquid mixing effect in the discharge reaction area is more affected by the liquid flow rate, and it is especially poor when the liquid flow rate exceeds 200 mL/min.

• In the practical application of the treatment process, the maximum removal of copper and zinc ions was achieved when the inlet flow rate was around 30 L/min and the inlet flow rate was around 200 mL/min.

• The maximum removal rate was 98.68 and 96.28% when the concentrations of Cu²⁺ and Zn²⁺ were 5 mg/L and the treated concentrations were 0.066 and 0.186 mg/L, respectively. These results were obtained using the gas–liquid mixing DBD emission method.

• The removal of copper and zinc depends on the oxidation of the low valent Cu²⁺ and Zn²⁺ to the high valent Cu(OH)₂ and Zn(OH)₂ precipitation compounds by the reactive species produced during the discharge process.

• The process parameters have an effect on the copper and zinc removal rates, especially the initial pH and discharge power, only when the initial pH is 11 and the discharge power is 56–58 W, then copper and zinc reach better removal effects and economic benefits.

This work was supported by the Silk Road Economic Belt Innovation Driven Development Pilot Zone and the Wuchangshi National Independent Innovation Demonstration Zone Science and Technology Development Plan [grant number 2022LQ04009].

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

The authors declare there is no conflict.