Combination of Precipitation-Adsorption-Bipolar Membrane Electrodialysis for Mine Water Treatment

Abstract

The process of mining produces a large amount of heavy metals and high-sulfate mine water, which is the main factor leading to environmental degradation in the mining area, and the removal of heavy metals and the recovery of sulfate from mine water is a difficult problem faced by mines today. Currently, sulfate is treated as a hazardous substance and is not recycled. In this paper, the precipitation–adsorption bipolar membrane electrodialysis (BMED) multi-technology-coupled recovery of sulfate method was used to treat mine water. The results showed that the coupling technology could remove heavy metals and sulfate in water better, while the sulfate desalination rate was about 96.8%, current efficiency was 54.2%, energy consumption was 0.823 kWh/kg, and the acid production concentration was 0.168 at an electrolyte concentration of 0.1 mol/L, an operating voltage of 12 V, an initial salt concentration of 30 g/L, and a flow rate of 3.5 mL/min. Mechanistic results showed that the precipitation–adsorption method could realize the removal of heavy metals from mine water. The BMED process realized the removal of sulfate and also the recovery of acid. The multi-technology coupling of precipitation–adsorption and bipolar membrane electrodialysis explored in this paper provides a direction for the in-depth treatment of mine water.

1. Introduction

With the development of human society, the demand for mineral resources is increasing, and in the process of exploiting and utilizing mineral resources, mine wastewater containing a large number of sulfate and heavy metal ions will be generated, causing very serious damage to the surrounding natural environment [1,2,3]. It mainly comes from surface infiltration water, underground drainage, rock pore water, pit water and mineral production and the processing of sewage produced during the mining process [4,5,6]. If untreated mine water is discharged directly, it will easily lead to an increase in the salinity of surface water and groundwater, affecting river life and the lives of the surrounding residents [7,8]. In China, every 1 ton of coal mining produces about 2.1 tons of mine water, that is, about 7 billion tons of mine water to be discharged annually, accounting for about 10% of the total industrial wastewater discharge in the country, but the treatment rate is only about 4.3% [9]; therefore, it is imperative to seek an efficient and environmentally friendly mine water treatment technology.

However, the current single treatment methods have certain limitations, and a combination of methods is needed for treatment [2,10]. Mining wastewater contains a variety of metal ions, organic matter, suspended solids and other pollutants, and certain treatment methods for the removal of specific pollutants are less efficient, cannot meet the requirements of wastewater treatment, and need to be combined with other methods of treatment. It is often difficult to remove all pollutants using a single treatment method. Precipitation is widely used in mine water treatment because of its cost-effectiveness, simple operation and rapid response [11]. Emma [12,13] removed >99% of aluminum, arsenic, cadmium, cobalt, copper, iron and manganese from AMD wastewater discharged in a copper–zinc mine using coke lime byproducts generated in the production of quick lime; the same amount of sludge was produced, and approximately 50–60% of the same amount of sulfate could be removed under the treatment of quick lime and hydrated lime. Chemical precipitation has a high removal efficiency for heavy metals in mine water, but due to the complex characteristics of mine water, it is not easy to deal with inorganic salts via conventional chemical precipitation, and heavy metals cannot be completely precipitated [14,15]. Adsorption, as an effective and cost-effective method, can efficiently remove the residual heavy metals from wastewater [16]. Wan [17] used cellulose nanofibrils (CNFs) as a functional scaffold, and significantly improved the mechanical strength and toughness of the membrane by forming an interlocking structure and assembling with Ti₃C₂Tx MXenes. The as-prepared MXene@CNF@FeOOH (MCF) membranes demonstrated stable and pH-independent static removal capacities for Sb species. Li [18] used an industrial by-product, namely red mud, to remove Mn ions from wastewater and found that the active groups in modified raw red mud were partially decomposed and transformed, which is not conducive to Mn adsorption. Bipolar membrane electrodialysis (BMED) is a new type of electrodialysis technology that can dissociate hydrolysis into H⁺ and OH⁻, and through the pressure difference and potential difference [19], it can convert inorganic salts into corresponding acids and bases without the introduction of other chemicals to realize the removal of inorganic salts in wastewater [20]. Secondly, the recovery of acids and bases from mine water has certain economic benefits and even alleviates the environmental impact of chemical production and the depletion of natural resources [21,22,23]. Liu [24] utilized the BMED technique to remove arsenic and other heavy metals from copper slag produced during the hydrometallurgical process of copper ores, and only a small amount of metals remained in the slag after treatment. Peng [16] treated carbocysteine wastewater with bipolar membrane electrodialysis (BMED), and investigated the migration and coexistence mechanisms of organic and inorganic ions in the wastewater. The results showed that the ideal migration of inorganic ions such as NH₄⁺ and Cl⁻ ions dominated in the feed chamber, and the removal of ammonium chloride reached 90%.

Therefore, a novel combination of precipitation + adsorption + bipolar membrane electrodialysis was investigated to treat heavy metals and sulfates in mine water. Compared with the traditional treatment methods, it has a wide range of application prospects, which can provide a more in-depth understanding of mine wastewater treatment technology and contribute to the environmental protection and sustainable development of China.

2. Materials and Methods

2.1. Test Reagents and Materials

Test reagents: anhydrous sodium sulfate (Na₂SO₄, 99.99%), Cd(NO₃)₂·4H₂O, manganese nitrate solution (Mn(NO₃)₂, 50 wt%), CuSO₄·5H₂O, BaCl₂, NaOH (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China)and methyl orange indicator (C₁₄H₁₄N₃NaO₃S, 0.04% aqueous solution). The laboratory produced all of the deionized water for the test. Methyl orange was purchased from Sinopsin Chemical Reagent Co., Ltd. (Shanghai, China), and other reagents were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China).

Configuration of simulated mine water: Cd(NO₃)₂·4H₂O, manganese nitrate solution, CuSO₄·5H₂O and anhydrous sodium sulfate were dissolved in deionized water to configure simulated mine water. The concentrations of Cd²⁺, Mn²⁺, Cu²⁺ and SO₄²⁻ were 10 mg/L, 10 mg/L, 10 mg/L and 10 g/L, respectively. Sulfate-simulated wastewater configuration: anhydrous sodium sulfate was dissolved in deionized water, in which the concentration of sodium sulfate was 10~50 g/L.

An inductively coupled plasma optical emission spectrometer (ICP-OES, HKYT, Beijing, China) was used to measure the concentrations of Cd²⁺, Cu²⁺ and Mn²⁺ in the solution before and after the test, and concentrations of SO₄²⁻ were measured by ion chromatography (ICS-3000, Dionex, Sunnyvale, CA, USA).

The actual mine water used in the test came from a mine in Lvliang City, Shanxi Province. The main components of the water are analyzed in

Table 1. Composition of the mine water.

Ingredient	SO42−	Cl−	NO3−	Na+	Ca2+	Mg2+	Fe	Pb	As	Cd	Cr	Cu	Mn
Concentration (mg/L)	1170	342	67	1020	27.13	10.54	16.47	0.14	0.0512	0.517	0.016	2.43	5.33

.

2.2. Test Method

2.2.1. Precipitation Experiment

The precipitation method was used to investigate the effect of different pH (7, 8, 9, 10, 11, 12), reaction temperature (15, 25, 35, 45, 55 °C), reaction time (10, 20, 30, 40, 50, 60, 90 min), stirring speed (200, 300, 400, 500, 600, 700 rpm) and precipitant concentration (0.1, 0.2, 0.3, 0.4, 0.5 g/L NaOH and 10, 20, 30, 40, 50 g/L BaCl₂) on the removal of heavy metals (Cd, Mn, Cu) and sulfate from mine water. We took a number of 200 mL conical flasks, and added 100 mL of simulated mine water, Cd²⁺, Cu²⁺, Mn²⁺ 10 mg/L and SO₄²⁻ at a concentration of 10 g/L to each. We put the conical flasks in a constant-temperature magnetic stirrer, and after a certain period of time removed the conical flasks, left them standing for 30 min, filtered the contents to obtain the supernatant, and determined the remaining Cd²⁺, Cu²⁺, Mn²⁺ and SO₄²⁻ concentration of the solution.

2.2.2. Adsorption Experiment

The adsorption method was used to investigate the effect of different adsorbents (CeO₂, humic acid and iron-based materials), pH (7, 8, 9, 10, 11 and 12), reaction temperatures (15, 25, 35, 45 and 55 °C), adsorbent dosages (0.1, 0.2, 0.3, 0.4 and 0.5 g/L), reaction times (10, 20, 30, 40, 50 and 60 min) and stirring speeds (200, 300, 400, 500, 600 and 700 rpm) on the removal of heavy metals (Cd, Mn, Cu) and sulfate from mine water. We took a number of 200 mL conical flasks and added 100 mL of simulated mine water, 10 mg/L Cd²⁺, Cu²⁺ and Mn²⁺ and SO₄²⁻ at a concentration of 10 g/L. We put the conical flasks in a constant temperature magnetic stirrer, and after 60 min removed the conical flasks amd let them stand for 30 min, and then filtered the contents to obtain the supernatant and determine the remaining Cd²⁺, Cu²⁺, Mn²⁺ and SO₄²⁻ concentration of the solution. The optimum process conditions for the removal of mine water by precipitation–adsorption were determined by comprehensive analysis. At the end of the process, the pretreated mine water was fed into the BMED system.

2.2.3. BMED Experiment

In this test, a self-made BP-A-A-BP three-compartment BMED device was used to treat mine wastewater. BP-A-A-BP is a membrane stack configuration in which bipolar membranes and anion-exchange membranes are arranged in an alternating pattern, and where AEM1 and AEM2 are two anion exchange membranes. The device structure’s schematic diagram and membrane stack configuration are shown in Figure 1a–c.

The effective membrane area is 16 cm² (4 cm × 4 cm). The membrane pile consists of 2 bipolar membranes and 2 anion exchange membranes arranged in the order of BP-A-A-BP, and the number of membrane pile units is N = 2. A partition board separates the adjacent membranes. The water inlet and outlet on the partition board ensure the flow of water. The thickness of the partition board is 0.8 mm. The hydrolysis product OH⁻ enters the mixing chamber through the bipolar membrane on the cathode side and mixes with Na⁺ to make NaOH. The SO₄²⁻ enters the acid chamber through the anion exchange membrane and mixes with the hydrolysis product H⁺ to make H₂SO₄. Sodium sulfate remains in the mixing chamber.

2.3. Test Process

The process flow of BMED is shown in Figure 1c. The test device is composed of four compartments: polar liquid chamber, acid chamber, intermediate chamber, and mixing chamber. The solution is driven by peristaltic pump into and out of the bottom of the membrane pile to form an independent closed-circuit circulation. Each compartment links to an external beaker, which is used to store and circulate the solution. The two ends of the film stack are made of stainless-steel cathode plates and iridium tantalum titanium anode plates as electrode materials. In addition, the electrode plate, and the bipolar film on both sides of the film stack constitute the cathode chamber and the anode chamber, and the two polar liquid chambers are connected to each other, so the water in them can enter a beaker. The H⁺ generated by the bipolar film on the cathode side and the OH⁻ generated by the bipolar film on the anode side are neutralized and generate H₂O, to avoid the corrosion hazard of the electrode caused by H⁺/OH⁻ generated by the unilateral electrode. The device uses a regulated DC power supply; the maximum output current of the power supply is 2 A, and the maximum output voltage is 15 V.

The test was run in constant-pressure mode. The test steps were as follows:

(1) Before the test, 100 mL of a certain mass fraction of Na₂SO₄ salt solution was added to the salt chamber, 100 mL of deionized water was added to the acid chamber and the intermediate chamber, and 250 mL of a certain concentration of Na₂SO₄ solution was added to the polar chamber.

(2) The flow rate of each chamber in the test was the same, and the peristaltic pump was set to the specified value. The DC power supply was turned on, the target voltage was set, and the BMED device was run.

(3) During the test, the current value was read by the DC voltage every 10 min, and the conductivity was monitored online by the conductivity meter.

(4) The test is ended when the current value does not change in 30 min.

2.4. Analysis Methods and Evaluation Indexes

The concentration of acid produced by the BMED process was determined by neutralization titration with methyl orange as the indicator.

In this study, the removal rate f, current efficiency η, and energy consumption E were used as evaluation indexes, and the calculation methods were as follows:

$$f=\frac{C_0-C_i}{C_i}\times 100\%$$

(1)

where f is the sulfate removal rate (%), C₀ is the concentration of Na₂SO₄ at time 0, mg/L, and C_i is the concentration of Na₂SO₄ at the end of the test, mg/L.

$$\eta =\frac{z(C_t{V}_t-C_0{V}_0)F}{60NIt}\times 100\%$$

(2)

where η is the current efficiency (%); z is the number of charged ions; C_t and V_t refer to H⁺ concentration (mol/L) and and volume (L) generated at time t, C₀ and V₀ refers to H⁺ concentration (mol/L) and volume (L) generated at time 0, respectively. F is Faraday’s constant (96,485 C/mol); N is the logarithm of the membrane (=2); I is the average current in the bipolar membrane electrodialysis process (A); t is the bipolar membrane electrodialysis operation time (min).

$$E={\displaystyle \int }\frac{UI\mathrm{dt}}{(C_t{V}_t-C_0{V}_0)60M}$$

(3)

where E is the reaction energy consumption (kWh/kg), and acid concentration is the calculation basis, that is, the electricity consumed by the production of 1 kg H⁺; U is the operating voltage (V); I is the average current in the double stage membrane electrodialysis process (A); C_t and V_t refer to the concentration (mol/L) and volume (L) of H⁺ produced at time t, respectively; C₀ and V₀ refer to H⁺ concentration (mol/L) and volume (L) at time 0, respectively; and M is the molar mass of the acid, calculated in H₂SO₄ (98 g/mol).

The schematic diagram of this combined process is shown in Figure 2.

2.5. Statistical Analysis

Three sets of parallel experiments were carried out for each parameter to obtain the average values.

3. Results and Discussion

3.1. Experiments on Process Parameters of the Precipitation Method for the Treatment of Simulated Mine Water

Six 200 mL conical flasks were taken and 100 mL of simulated mine water was added to each, with Cd²⁺, Mn²⁺, Cu²⁺ of 10 mg/L, and SO₄² of 10 g/L. The conical flasks were placed on a thermostatic magnetic stirrer to investigate the effects of different influencing factors, and then the flasks were removed and left to stand for 30 min, and then filtered to obtain the supernatant, and then the Cd²⁺, Mn²⁺, Cu²⁺, and SO₄²⁻ concentrations of the remaining Cd²⁺, Mn²⁺, Cu²⁺, and SO₄²⁻ in the post-reaction solution were determined. The results of the influence of different factors on the removal rate of heavy metal ions and sulfate are shown in Figure 3.

The effect of pH on the removal of heavy metals and sulfate was large. After the pH of the solution reached 10, the removal of Cd²⁺ and Mn²⁺ basically did not change any more, and the maximum values reached 94.7% and 95.8%; the removal of Cu²⁺ and SO₄²⁻ was the highest at pH 7, reaching 94.9% and 84.6%. Temperature had little effect on heavy metal and sulfate removal. The removal rates of Cd²⁺, Mn²⁺, Cu²⁺, and SO₄² concentration were 94.4%, 94.6%, 94%, and 84% at a temperature of 25 °C, respectively. The reaction time had a greater effect on the removal of heavy metals and sulfate, and the precipitation of heavy metal ions was basically completed within 40 min, and the removal rates of Cd²⁺, Mn²⁺, and Cu²⁺ were 94.7%, 94.5%, and 94.9%, respectively, at 60 min; the removal of SO₄² reached the maximum at 60 min, which was 85.2%. The stirring speed had a greater effect on the removal of heavy metals and sulfate, and the removal rates of Cd²⁺, Mn²⁺, Cu²⁺, and SO₄² were 94.8%, 92.5%, 96.1%, and 85.7%, respectively, at a stirring speed of 600 rpm. The precipitant concentration had a greater effect on heavy metal and sulfate removal when the precipitant concentration was 0.4 g/L NaOH and 30 g/L BaCl₂. At this time, Cd²⁺, Mn²⁺, Cu²⁺, and SO₄²⁻ removal rates were 95.1%, 94.9%, 95%, and 86.3%, respectively. In summary, this is comparable to Makino’s result where the removal of Cd, Mn, and Cu was above 94% using NaOH to treat heavy metals in soil-washing solutions [25].

3.2. Experimental Study on the Treatment of Simulated Mine Water with the Precipitation Adsorption Method

In order to improve the treatment effect of simulated mine water, adsorbents were added in the process of wastewater treatment by the precipitation method. The effects of different adsorbents, different pH, different reaction temperatures, and different adsorbent dosages on the treatment effect were investigated. The results of the influence of different factors on the removal rate of heavy metal ions and sulfate are shown in Figure 4.

Humic acid had the best removal effect among the three adsorbents and was much better than CeO₂; for iron-based materials under the same conditions, the treatment effect was much better than that of CeO₂ and iron-based materials. When humic acid was used as the adsorbent, the optimal solution pH was 10, the optimal reaction temperature was 25 °C, the optimal humic acid dosage was 0.4 g/L, the optimal reaction time was 40 min, and the optimal stirring speed was 500 rpm. The removal rates of Cd²⁺, Mn²⁺, Cu²⁺, and SO₄²⁻ were 99.9%, 99.4%, 100%, and 88%, respectively, the removal rates were 99.9%, 99.4%, 100%, and 88%, respectively, and the heavy metals could meet the emission standards. The removal of heavy metals and sulfates was more significant after the adsorption process. The efficiency of removing heavy metals and inorganic salts using this method was comparable to Lu’s method, which involved the use of humic acid enhancers for treating inorganic salts, with an improvement of 43.5% [26].

3.3. Experimental Study on the Treatment of Simulated Sulfate Wastewater by Bipolar Membrane Electrodialysis

3.3.1. Comparative Test of the Membrane Pile Structure

BP-A-BP two compartment and BP-A-A-BP three compartment were selected as comparison treatments for high sulfate in simulated mine water. The test parameters were an initial Na₂SO₄ concentration of 30 g/L, an operating voltage of 12 V, an electrode chamber electrolyte Na₂SO₄ concentration of 0.1 mol/L, and a flow rate of 3.5 mL/min in each chamber of the membrane reactor. The treatment results of two different membrane reactor structures are shown in Figure 5.

As can be seen from the analysis of Figure 5, the current efficiency and sulfate desalinization rate of the BP-A-A-BP membrane reactor structure significantly improved compared to the BP-A-BP membrane reactor structure. The current efficiency and sulfate desalinization rate of the BP-A-A-BP membrane reactor structure were 54.2% and 96.8%, which was 11.9% and 12.6% higher than those of the BP-A-BP membrane reactor structure, respectively. This is because putting an anion exchange membrane between the bipolar membrane and the anion exchange membrane stops protons from leaking between the two membranes. Therefore, the BP-A-A-BP membrane pile structure is selected as the membrane pile structure to be tested in this study.

3.3.2. Effect of Electrolyte Concentration in Polar Chamber

The test parameters were the initial concentration of Na₂SO₄ in the mixing chamber, 30 g/L, the operating voltage, 12 V, and the flow rate of each chamber of the membrane reactor, 3.5 mL/min. The effects of 0.05 mol/L, 0.1 mol/L, 0.15 mol/L, 0.20 mol/L, and 0.25 mol/L Na₂SO₄ on sulfate removal by bipolar membrane electrodialysis were investigated, respectively. The test results are shown in Figure 6a–c.

The analysis of Figure 6a demonstrates that variations in electrolyte concentration have an impact on the variation of the membrane pile current, with the current exhibiting a progressive increase as the reaction time extends. When the reaction time reaches 30 min, the current tends to stabilize. The reason is that in the early stages of desalination, the resistance of the system is large. Under the action of electric field force, SO₄²⁻ in the mixing chamber enters the acid chamber through the anion exchange membrane. The bipolar membrane in the cathodic liquid chamber produces H⁺, which the anion of the electrolyte consumes, while the bipolar membrane in the anodic liquid chamber produces OH⁻, which the cation of the electrolyte consumes. The electrolyte, which acts as the cathode and anode electrolyte, circulates through the system by carrying the current between the electrode and the bipolar film, causing the system resistance to decrease, and according to Ohm’s law, the current will then rise. In the later stage of desalting, the system remained stable, and the current changed little and tended to be stable. When the concentration of the electrode liquid increased, the available ions increased, and the increase in concentration difference increased the migration rate of ions. Therefore, with the increase in electrode liquid concentration, the degree of current change will be greater.

It can be seen from the analysis of Figure 6b that electrolyte concentration is the main factor affecting the acid concentration and sulfate removal rate of the product. With the increase in electrode liquid concentration, the sulfate removal rate and acid production concentration first increased and then decreased. When the electrolyte concentration was 0.10 mol/L, the sulfate removal rate and acid production concentration were the largest, and the H₂SO₄ concentration was 0.167 mol/L and the sulfate removal rate was 96.1%. After 0.15 mol/L, the removal rate of the sulfate and the concentration of acid production were basically unchanged.

The analysis of Figure 6c shows the current efficiency and energy consumption of the electrolyte concentration system. With the increase in electrolyte concentration, energy consumption and current efficiency first increased and then decreased. When the concentration of Na₂SO₄ solution in the electrode chamber reaches 0.05 mol/L, the electrolyte content is too low, the electrode resistance is too high, the kinetic activity of ions becomes dull, and the reaction rate is greatly reduced. The concentration of Na₂SO₄ solution directly affects the electrolyte content in the electrode chamber. As the concentration increases, the resistance of the electrode chamber lowers, resulting in a drop in the overall resistance of the membrane pile. This decrease in resistance accelerates the reaction. When the electrolyte concentration increases from 0.15 mol/L to 0.25 mol/L, the high concentration of the electrolyte leads to uneven ion concentration at the membrane interface, which increases the Joule heat consumed to overcome the carrier ion migration resistance because the electrical energy consumed to overcome the ion migration resistance accounts for most of the energy. The inefficiency of using these energies for the hydrolysis of bipolar membranes and the movement of necessary ions leads to a reduction in current efficiency [27].

The above test results show that the electrical conductivity of a system is defined as the transfer of charge under the action of an electric field. Since ions are the charges in the electrolyte, the conductivity of the electrolyte depends on the number of ions present in the system and the mobility of the ions. The bipolar membrane in the cathodic liquid chamber makes H⁺, which the anions of the electrolyte consume. In the anodic liquid chamber, the cations of the electrolyte consume OH⁻, which the bipolar membrane makes. The electrolyte, which acts as the cathode and anode electrolyte, circulates through the system by carrying a current between the electrode and the bipolar film. Therefore, the number and mobility of ions determine the rate of chemical reaction, and the study results determine the selection of 0.1 mol/L Na₂SO₄ electrode chamber electrolyte concentration as the optimal concentration for the test.

3.3.3. Effect of Operating Voltage

Operating voltage is an important parameter in the bipolar membrane electrodialysis process, and directly affects the dissociation of water, the migration of ions in solution, and energy consumption. If the operating voltage is too large, a large part of the total electric energy will be converted into joule heat, which will lead to the increase in system energy consumption. The current is mainly used in the dissociation of water, resulting in a poor separation effect. If the operating voltage is too small, the driving force of ion migration in the solution will be too small, and the separation effect will be poor. Therefore, the selection range of operating voltage for this test is 9~15 V, and the test results are shown in Figure 7a–c.

It can be seen from the analysis of Figure 7a that different operating voltages are the main factors affecting the current of the film pile. Under different operating voltages, the current of the membrane pile rises rapidly at first and then stabilizes. The reason is that in the initial stage, the acid chamber and the intermediate chamber contain deionized water and the conductivity is poor, resulting in greater resistance to bipolar membrane electrodialysis. The operating voltage and the resistance of the membrane pile have an impact on the current of the membrane pile. Too large a resistance in the initial system reduces the electrochemical activity during the forward transport of the charged ions, so the initial current is low. With the progress of desalination, under the action of an electric field, H⁺ produced in the middle layer of the bipolar membrane and SO₄²⁻ in the mixing chamber migrate to the acid chamber through the anion exchange membrane and the positive bipolar membrane, respectively, to form H₂SO₄. The ions available for loading current increase, and the operating voltage increases, so the current of the membrane pile gradually increases. At the late stage of desalting, the decomposition degree of H₂SO₄ is like the rate of acid–base synthesis, resulting in little change in the resistance of the film pile, with the current basically remaining unchanged. According to the second two-dimensional effect, the voltage can accelerate the hydrolysis separation rate in the middle layer of the bipolar film, so the higher the operating voltage, the shorter the time for the system to complete the desalination. The five controlled trials used the same membrane stack with the same resistance [28]. According to Ohm’s law (U = IR), the current is proportional to the voltage applied at both ends of the membrane stack.

It can be seen from the analysis of Figure 7b that the acid concentration and sulfate removal rate of products change under different operating voltages. Under the action of a direct-current electric field, SO₄²⁻ in the mixing chamber and H⁺ in the electrode chamber migrate to the acid chamber to form H₂SO₄ through the anion exchange membrane and the positive bipolar membrane, respectively. With progress in desalting, the concentration of SO₄²⁻ in the mixing chamber gradually decreased, and the concentration of OH⁻ gradually increased. At different operating voltages, the sulfate removal rate was higher than 91% at the end of the test, and at 15 V, the sulfate removal rate reached 96.2%. After the reaction, the concentration of H₂SO₄ in the acid chamber ranged from 0.151 to 0.163 mol/L. The decrease in H₂SO₄ concentration was mainly caused by H⁺ leakage. In the operating voltage range of 9~12 V, the concentration of H₂SO₄ in the acid chamber increased with the increase in the operating voltage, which is because with the increase in the operating voltage, the reaction running time is shortened, and the influence of H⁺ leakage on the concentration of H₂SO₄ is reduced. When the operating voltage is further increased to 15 V, the concentration of H₂SO₄ in the acid chamber is slightly decreased (0.157 mol/L). This is because the acceleration of ion migration rate leads to increased H⁺ leakage.

Figure 7c shows the current efficiency and energy consumption of the system at different operating voltages. When the input current is low to 9 V, the electric field force is too weak to offset the film’s resistance to ion migration, so the current efficiency is low. As the operating voltage gradually increases from 12 V to 15 V, the current efficiency gradually decreases. The reason is that the increase in operating voltage enhances water decomposition and ion migration, and H+ leakage through the anion exchange membrane is more serious, resulting in a higher current efficiency of the bipolar membrane electrodialysis process. In addition, with the increase in operating voltage, the energy consumption gradually increases. When the operating voltage is 15 V, the energy consumption reaches its maximum value of 0.981 kWh/kg H₂SO₄. The voltage applied to the membrane stack is used for three main purposes: the dissociation of water under the action of electric field forces and bipolar membranes, the migration of SO₄²⁻, and Joule heat due to the resistance of the membrane stack. The larger operating voltage produces a larger current, faster water decomposition, and more thermal energy generated on the membrane pile, resulting in an increase in total electrical energy.

The test results demonstrate that the high temperatures produced by heat energy will harm the machinery and ion exchange membrane during the long-term desalination process. The operating voltage is too large, and the current efficiency is low, which is not conducive to practical applications. The operating voltage is too low, the energy consumption is low, the working time is long, and the acid concentration produced is low, which is also not desirable. Therefore, it is important to select a certain operating voltage so that the desalting time is short, the desalting rate is high, and the acid concentration is high. The research results confirm that the operating voltage of 12 V is the best operating voltage.

3.3.4. Effect of Initial Salt Concentration

The test parameters were an electrolyte concentration of the electrode chamber of Na₂SO₄ 0.1 mol/L, an operating voltage of 12 V, and a flow rate of each chamber of membrane reactor of 3.5 mL/min. The effects of initial salt concentrations of 10 g/L, 20 g/L, 30 g/L, 40 g/L, and 50 g/L Na₂SO₄ in the mixing chamber on sulfate removal by bipolar membrane electrodialysis were investigated, and the test results are shown in Figure 8a–c.

It can be seen from the analysis of Figure 8a that the initial salt concentration was the main factor affecting the current change of the film pile. At the start of desalination, the current increased with the increase in reaction time. When the reaction time reached 50 min, the current with the initial salt concentration of 30 g/L reached 996 mA and tended to be stable. The reason is that at the beginning of the desalination reaction, the acid chamber contained deionized water, and there were few charged ions and the resistance of the membrane pile was high, resulting in a small initial current. With the increase in desalting time, the concentration of charged ions in the acid chamber increased gradually, the conductivity of the acid chamber increased, the resistance decreased, and the current of the film pile increased gradually under constant voltage.

It can be seen from the analysis of Figure 8b that the acid concentration and sulfate removal rate of the product change under the initial salt concentration. When the concentration of H₂SO₄ increases from 10 g/L to 50 g/L, the sulfate removal rate increases from 94.2% to 96.8%. With an increase in the initial salt concentration, the higher the concentration of Na⁺ and SO₄²⁻ in the solution, the higher the concentration of recovered acid. The reason is that under the action of direct current electric field, more SO₄²⁻ enters the acid chamber through the anion exchange membrane, and the H⁺ concentration generated by bipolar membrane hydrolysis increases correspondingly. When the concentration of Na₂SO₄ is 50 g/L, the H₂SO₄ concentration can reach 0.243 mol/L, which is 4.76 times the H₂SO₄ concentration (0.051 mol/L) when the concentration of Na₂SO₄ is 10 g/L.

It can be seen from the analysis of Figure 8c that the initial salt concentration affects the current efficiency and energy consumption. With an increase in the initial salt concentration, the current efficiency first increases and then decreases, and the energy consumption first decreases and then increases. When the initial concentration of Na₂SO₄ is 10 g/L, the energy consumption of the BMED process is the highest, and the current efficiency is the lowest. The reason is that when the concentration of the mixing chamber is low, the total amount of mobile ions is lower, the overall membrane reactor voltage and resistance are higher, and the reaction energy consumption is also higher. When the concentration of Na₂SO₄ increases from 10 g/L to 30 g/L, the number of ions in the system increases, the reaction is accelerated, the current efficiency is increased, and the energy consumption is reduced. When the concentration of Na₂SO₄ increases from 30 g/L to 50 g/L, it is due to high osmotic pressure and a high salt content in the feed solution, which are not conducive to water entering the bipolar film. The ion leakage that occurs during this process has a longer duration, which has a negative impact on the current efficiency [29]. When the concentration of Na₂SO₄ increases from 10 g/L to 30 g/L, the energy consumption of the BMED system gradually decreases because the resistance of the membrane pile decreases when the concentration of Na₂SO₄ increases, and the energy consumption of ion migration is dominant. When the concentration of Na₂SO₄ increases from 30 g/L to 50 g/L, the BMED system consumes more energy. The reason is that the desalting time is greatly increased, and more energy consumption is generated during the desalting process.

To summarize, while a low initial salt concentration provides improved current efficiency and lower energy consumption, it poses significant limits in treating high-concentration mine drainage and results in a lower recovered acid content. Nevertheless, if the initial salt concentration is excessively high, the current efficiency will decline while the energy consumption of the system will increase. Considering that increasing the initial salt concentration correctly promotes acid recovery, the ideal concentration for this test is established at 30 g/L.

3.3.5. Flow Rate Effect

The test parameters were 0.1 mol/L electrolyte concentration in the electrode chamber, 30 g/L initial Na₂SO₄ concentration in the mixing chamber, and 12 V operating voltage. The effects of flow rates in each chamber of 1.5 mL/min, 2.5 mL/min, 3.5 mL/min, 4.5 mL/min, and 5.5 mL/min on sulfate removal by bipolar membrane electrodialysis were investigated, and the test results are shown in Figure 9a–c.

It can be seen from the analysis of Figure 8a that the flow rate affects the current change in the membrane reactor. When the flow rate is 3.5 mL/min, the maximum current of the membrane pile can be obtained. The reason is that with the increase in flow velocity, the turbulence in different compartments is enhanced, and the diffusion boundary layer is compressed, resulting in a decrease in the membrane stack resistance and a gradual increase in membrane stack current.

According to the analysis in Figure 9b, the acid concentration and sulfate removal rate of the product are affected by the flow rate. When the flow rate was 3.5 mL/min, the highest sulfate removal rate was 96.2% and the acid production concentration was 0.168 mol/L. The flow rate has little effect on the acid concentration produced.

As can be seen from the analysis of Figure 9c, flow rate is one of the factors affecting current efficiency and energy consumption. In the process of the BMED treatment of simulated mine water, the current efficiency increased from 51.2% to 57.8% as the flow rate increased from 1.5 mL/min to 4.5 mL/min. When the flow rate increased from 4.5 mL/min to 5.5 mL/min, the current efficiency decreased and the energy consumption increased. The reason is that the degree of turbulence in each chamber was too large, which led to the self-circulation of the solution in each chamber; the shear stress of the liquid increased during the longitudinal flow of the solution [30], and the residence time of Na₂SO₄ was too short. There was not enough time for ions to migrate through the anion exchange membrane and the bipolar membrane, and some ions could not complete the mass transfer process, which reduces the current efficiency and increases the total energy consumption. High flow rates lead to more head loss, further increasing energy consumption and thus shortening the service life of the film. The test shows that it is very important to keep the flow rate within a reasonable range during BMED.

The above tests show that too small a flow rate will lead to lower current efficiency and increased energy consumption. An excessive flow rate will lead to greater pressure on the ion exchange membrane and will shorten the service life of the membrane. Therefore, it is important to keep the flow rate within a reasonable range during BMED. Under comprehensive consideration, 3.5 mL/min was selected as the optimal flow rate for this test.

3.4. Mine Water Test

The sulfate concentration in wastewater from a mine in Shanxi was as high as 1170 mg/L, and Cd²⁺, Mn²⁺, and Cu²⁺ were 0.517 mg/L, 5.33 mg/L, and 2.43 mg/L. According to Integrated wastewater discharge Standard [31], the total Mn discharge limit is 2 mg/L, the total Cu discharge limit is 1 mg/L, the total Cd discharge limit is 0.1 mg/L, and the total sulphate discharge limit is 400 mg/L. Based on preliminary experiments, we took 100 mL of wastewater in a conical flask, adjusted the pH to 10, added 0.4 g/L humic acid, 0.4 g/L NaOH, and 30 g/L BaCl₂, and placed it on a thermostatic magnetic stirrer for 60 min, with the temperature set at 25 °C and the stirring speed at 600 rpm. Then, 30 min later, the filtered solution was taken and the concentrations of heavy metals and sulfate were detected. The filtered solution was added into the mixing chamber of the bipolar membrane electrodialysis system, and deionized water was added into the acid chamber and the intermediate chamber. The operating conditions were 0.10 mol/L electrode solution concentration, 12 V operating voltage, and 3.5 mL/min flow rate, and the concentrations of heavy metals and sulfate in the mixing chamber were measured after the test was completed. The treatment process is shown in Figure 10. The test results are shown in

Table 2. Data on mine water stages.

Methodology	Pharmaceuticals	RE(Cd2+)/%	RE(Mn2+)/%	RE(Cu2+)/%	RE(SO42−)/%	Current Efficiency
Precipitation	NaOH, BaCl2	94.26%	92.21%	96.78%	75.23%
Adsorption	Humic acid	99.93%	98.04%	99.91%	79.36%
BMED					94.19%	51.26%
Total removal rate		99.93%	98.04%	99.91%	94.19%	51.26%
Standard deviation		0.05	1.19	0.92	0.43	0.49

.

The actual mine wastewater was treated with combination precipitation–adsorption–bipolar membrane electrodialysis. Cu²⁺, Mn²⁺, Cd²⁺, and SO₄²⁻ can be removed by the following reaction equation. The concentrations of Cu²⁺, Mn²⁺, Cd²⁺, and SO₄²⁻ after the end of the treatment were 0.0022 mg/L, 0.1041 mg/L, 0.0004 mg/L, and 67.977 mg/L, which meets the industrial wastewater discharge standard. It was shown that the treatment method had a short reaction time and low energy consumption (94% removal of heavy metals and sulfates). The heavy metal removal of >98% and inorganic salt removal of >94% by this technique is similar to the treatment results reported in the literature [32,33]. This result demonstrates the effectiveness of combined treatment in removing heavy metals and inorganic salts from mine wastewater.

$$\begin{gathered} C{u}^{2+}+2O{H}^{-}\to Cu{\left(OH\right)}_2\downarrow \\ M{n}^{2+}+2O{H}^{-}\to Mn{\left(OH\right)}_2\downarrow \\ C{d}^{2+}+2O{H}^{-}\to Cd{\left(OH\right)}_2\downarrow \\ S{O}_4^{2-}+B{a}^{2+}\to BaS{O}_4\downarrow \end{gathered}$$

(4)

4. Conclusions

In this paper, precipitation–adsorption–bipolar membrane electrodialysis (BMED) technology was used to treat mine wastewater, and a novel BMED membrane stack for treating sulfate wastewater was proposed to validate the feasibility of this coupled technology for removing heavy metals and sulfates from mine wastewater. The operation conditions were optimized, and the main research conclusions are as follows.

(1) In the precipitation method for the treatment of simulated mine water, the optimal operating conditions were determined as follows: solution pH 10, reaction temperature 25 °C, reaction time 60 min, stirring speed 600 rpm, and precipitant concentration 0.4 g/L NaOH and 30 g/L BaCl₂. At this time, the removal rates of Cd²⁺, Mn²⁺, Cu²⁺, and SO₄²⁻ were 95.1%, 94.9%, 95%, and 86%, respectively, and 94.9%, 95% and 86.3%, respectively.

(2) Humic acid had the best removal effect among the three adsorbents. When humic acid was used as the adsorbent, the optimum solution pH was 10, the optimum reaction temperature was 25 °C, the optimum humic acid dosage was 0.4 g/L, the optimal reaction time was 40 min, and the optimal stirring speed was 500 rpm The removal rates of Cd²⁺, Mn²⁺, Cu²⁺, and SO₄²⁻ were 99.9%, 99.4%, 100%, and 88%, respectively, and the emission standards of the heavy metals could be met.

(3) In the BMED method for the treatment of simulated sulfate water, the optimal operating conditions were an electrode liquid concentration of 0.1 mol /L, operating voltage of 12 V, initial salt concentration of 30 g/L, flow rate of 3.5 mL/min, acid concentration of 0.168 mol/L, desalting rate up to 96.8%, current efficiency up to 58.2%, and an energy consumption of up to 0.823 kW·h/kg.

(4) In the actual mining wastewater treatment process, the combined process has a good removal rate of >98% for heavy metals and >94% for sulfate, and can recover acid from the wastewater.

In summary, the combined precipitation–adsorption–bipolar membrane electrodialysis treatment process investigated in this paper provides a new direction. Although the proposed technology can effectively treat mine wastewater, there are multiple challenges that need to be addressed in field-scale applications. Future research should focus on reducing the energy consumption of BMED systems.