Method for removing sulfate radical in wastewater by bipolar membrane electrodialysis membrane stack

Abstract

The application belongs to the field of wastewater treatment and relates to a method for removing sulfate in wastewater by bipolar membrane electrodialysis membrane stack, which adopts a three-compartment structure bipolar membrane electrodialysis device, and the membrane stack is composed of two bipolar membranes and two anion exchange membranes to form an acid chamber, an intermediate chamber and a mixing chamber, and the electrode plate and the bipolar membrane on both sides of the membrane stack form an electrolyte chamber; each compartment is connected with a corresponding container through a pump; the steps for removing sulfate are as follows: the wastewater is introduced into the mixing chamber of the bipolar membrane electrodialysis device, the acid chamber and the intermediate chamber of the bipolar membrane electrodialysis device are filled with deionized water, the electrolyte in the electrolyte chamber is a 0.05-0.25 mol / L sulfate solution, the operating voltage is 9-15 V, the flow rate is 1.5 mL-5.5 mL / s, the peristaltic pump is opened and the flow rate value is set, the direct current power supply is turned on, the target voltage is set, and the bipolar membrane electrodialysis device is operated; during the process, the current value and the conductivity are monitored; when the current value does not change, the sulfate wastewater in the mixing chamber is replaced. Compared with the existing bipolar membrane electrodialysis membrane stack, the current efficiency is improved, the desalination rate is improved, and the energy consumption is reduced.

Description

Technical Field

[0001] This invention relates to a method for removing sulfate from wastewater using a bipolar membrane electrodialysis membrane stack, belonging to the field of wastewater treatment. Background Technology

[0002] With rapid industrialization, various industrial production processes, including petroleum, pharmaceuticals, seawater desalination, and chemicals, have rapidly generated large quantities of sulfate-rich wastewater. Sulfate wastewater pollutes vast areas, persists for extended periods, and is difficult to treat. Untreated high-sulfate wastewater acidifies surface and groundwater, damages soil structure, and ultimately pollutes the environment, leading to reduced crop yields. Currently, many countries require "zero discharge" of sulfate-rich wastewater because improper discharge threatens aquatic ecosystems. Sodium sulfate is currently being recovered from sulfate-rich wastewater to minimize its adverse environmental impact.

[0003] There are four main methods for treating industrial sulfate wastewater: chemical precipitation, physical adsorption, membrane separation, and biological treatment. In addition, bipolar membrane electrodialysis technology, as a novel treatment method, is more cost-effective than traditional physicochemical methods. Current sulfate-containing wastewater treatment projects are not performing well, highlighting the need for cost-effective and low-carbon sulfate wastewater treatment technologies. Developing sulfate-rich wastewater treatment technologies is becoming an increasingly important task worldwide, especially given the increasing water consumption.

[0004] Bipolar membrane electrodialysis (BMED), as a novel electrodialysis technology, is rapidly developing due to its high efficiency and environmental friendliness. A bipolar membrane is a special ion exchange membrane composed of three parts: an anion exchange layer, a cation exchange layer, and an intermediate catalyst layer. Compared to traditional electrodialysis, under the influence of a DC electric field, water diffuses into the intermediate layer and is dissociated into H+ and OH-. Anions X- pass through the anion exchange membrane into the acid chamber, reacting with H+ to form HX; cations M+ pass through the cation exchange membrane into the alkali chamber, reacting with OH- to form MOH. Here, MX is the electrolyte, and HX and MOH are the corresponding acids and bases formed. This not only removes salts from wastewater but also recovers acids and bases.

[0005] The core of a BMED (Built-in-the-Shelf) unit is the membrane stack, composed of bipolar membranes (BPM), anion exchange membranes (AEM), and cation exchange membranes (CEM) arranged in different configurations. The stack configuration significantly impacts performance. Typically, in BMED processes producing acids, the traditional three-membrane, two-compartment configuration is preferred due to its high current density and low energy consumption. However, BMED processes with a two-compartment configuration are affected by proton tunneling, which hinders water splitting, leading to reduced current efficiency in producing acids and bases from high-salinity wastewater. Due to proton tunneling, the current efficiency of a two-compartment membrane stack structure with both bipolar and anion exchange membranes is only 36%. This relatively low current efficiency prolongs BMED operating time, and the reverse diffusion of product molecules through the anion exchange membrane further reduces recovery rate and current efficiency. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention proposes a method for removing sulfate from wastewater using a bipolar membrane electrodialysis stack. To this end, this invention adds an AEM layer to the traditional BPM-AEM-BPM structure, resulting in a BPM-AEM-AEM-BPM membrane stack structure. This structure effectively prevents proton leakage and improves current efficiency while reducing energy consumption.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A method for removing sulfate from wastewater using a bipolar membrane electrodialysis stack is characterized by employing a three-compartment bipolar membrane electrodialysis device. The membrane stack of the bipolar membrane electrodialysis device consists of two bipolar membranes and two anion exchange membranes arranged in the order BP-AA-BP. Adjacent membranes are separated by partitions to form an acid chamber, an intermediate chamber, and a mixing chamber. The cathode chamber and anode chamber formed by the electrode plates and the bipolar membranes on both sides of the membrane stack constitute the electrolytic chamber. Each compartment is connected to a corresponding container via a pump. The steps for removing sulfate are as follows: (1) The wastewater is fed into the mixing chamber of the bipolar membrane electrodialysis device, and the inlet water of the acid chamber and intermediate chamber of the bipolar membrane electrodialysis device is deionized water. The electrode liquid in the electrode liquid chamber is 0.05~0.25mol / L sulfate solution. The operating voltage is 9~15V and the flow rate is 1.5mL~5.5mL / s. (2) Turn on the peristaltic pump and set the flow rate; turn on the DC power supply, set the target voltage, and run the bipolar membrane electrodialysis device; (3) During the process, monitor the current value and conductivity; (4) When the current value does not change, replace the sulfate wastewater in the mixing chamber.

[0008] Furthermore, a filtration process is added before the wastewater enters the bipolar membrane electrodialysis device.

[0009] Furthermore, the cathode chamber and the anode chamber are interconnected.

[0010] Furthermore, the polar liquid in the polar liquid chamber is 0.1 mol / L.

[0011] Furthermore, the operating voltage is 12V.

[0012] Furthermore, the initial salt concentration of the wastewater in the mixing chamber is 30 g / L.

[0013] Furthermore, the flow rate is 3.5 mL / s.

[0014] The positive effects of this invention are: (1) After treatment, the desalination rate of high-concentration sulfate wastewater reached over 96%, enabling the high-concentration sulfate wastewater to meet the discharge standards after treatment.

[0015] (2) No additional reagents are required during the process of removing sulfate, no solid waste is generated, and no pollution is caused to the environment.

[0016] (3) Compared with other membrane stacks, it can improve the sulfate removal rate and reduce energy consumption.

[0017] (4) It is easy to operate and has a good removal effect, and has good prospects for industrial application.

[0018] (5) Compared with existing bipolar membrane electrodialysis membrane stacks, it improves current efficiency, increases desalination rate and reduces energy consumption. Attached Figure Description

[0019] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is a schematic diagram of a bipolar membrane electrodialysis device for treating sulfate wastewater in a specific embodiment of the present invention.

[0021] Figure 2 This is a schematic diagram of the membrane stack configuration in a specific embodiment of the present invention.

[0022] Figure 3 The effect of membrane stack structure on sulfate removal rate and current efficiency.

[0023] Figure 4 The effect of electrolyte concentration in the polar liquid chamber on the performance of the BMED process.

[0024] Figure 5The effect of operating voltage on BMED process performance.

[0025] Figure 6 The effect of initial salt concentration on the performance of the BMED process.

[0026] Figure 7 The effect of flow rate on BMED process performance. Detailed Implementation

[0027] The technical solutions of the present invention will be explained in more detail below with reference to specific embodiments. However, it should be understood that within the scope of the present invention, the various technical features of the present invention and the various technical features specifically described below (such as in the embodiments) can be combined with each other to form new or preferred technical solutions. Due to space limitations, they will not be described in detail here.

[0028] Taking Na2SO4 wastewater as an example, Example 1 like Figure 1 and Figure 2 As shown, a method for removing sulfate from wastewater using a bipolar membrane electrodialysis stack is described. The bipolar membrane electrodialysis device 6 comprises the following components: a pure titanium electrode as the cathode and an iridium-tantalum-titanium electrode as the anode; the bipolar membrane electrodialysis stack consists of two bipolar membranes and two anion exchange membranes arranged in a BP-AA-BP sequence, with a stack unit number N=1; adjacent membranes are separated by partitions, with inlets and outlets on the partitions ensuring water flow; the partitions are 0.8mm thick, forming a three-compartment structure including an acid chamber, an intermediate chamber, and a mixing chamber; the electrode plates and the bipolar membranes on both sides of the stack constitute the cathode chamber and the anode chamber, which are interconnected to form the electrolytic liquid chamber; the H2O generated by the bipolar membrane on the cathode side... + OH generated by the bipolar membrane on the anode side - Neutralization generates H2O, avoiding the generation of H+ by one side of the electrode. + / OH - This causes corrosion damage to the electrodes. Each compartment is connected to an external container for storing and circulating the solution: acid chamber container 4, intermediate chamber container 3, mixing chamber container 2, and electrode liquid chamber container 1. Each container is connected to the inlet and outlet of its respective compartment, and a peristaltic pump 5 is installed on the connecting pipeline. The peristaltic pump 5 drives the solution to enter from below the membrane stack and flow out from above, forming an independent closed-loop circulation. The device uses a regulated DC power supply and operates in constant voltage mode. The maximum output current of the power supply is 2A, and the maximum output voltage is 15V. After the membrane stack is energized, charged ions in the wastewater undergo selective migration. Cations combine with hydroxide ions dissociated from the bipolar membrane in the mixing chamber to form alkali, while anions migrate through the anion exchange membrane to the acid chamber and combine with hydrogen ions dissociated from the bipolar membrane to form acid. This not only removes cations and anions from the wastewater but also recovers valuable acids and alkalis. The OH- generated by hydrolysis enters the mixing chamber through the cathode-side bipolar membrane and reacts with Na+. +Combine to form NaOH and SO4 2- It enters the acid chamber through the anion exchange membrane and reacts with the H+ produced by hydrolysis. + H2SO4 is generated, while sodium sulfate remains in the mixing chamber.

[0029] The bipolar membrane electrodialysis technology uses local circulating water treatment. When the concentration in the mixing chamber reaches the preset treatment requirements, the wastewater treated in the mixing chamber is discharged and new sulfate wastewater is added.

[0030] The steps are as follows: (1) The mine wastewater is filtered through a microporous membrane to remove suspended solids. The filtered mine wastewater is then introduced into the mixing chamber of the bipolar membrane electrodialysis device 6. The acid chamber and intermediate chamber of the bipolar membrane electrodialysis device 6 are filled with deionized water, and the electrode liquid in the electrode liquid chamber is a 0.05~0.25mol / L Na2SO4 solution. The operating voltage is 9~15V and the flow rate is 1.5mL~5.5mL / s.

[0031] (2) Turn on the peristaltic pump 5 and set the flow rate; turn on the DC power supply and set the target voltage. At this time, run the bipolar membrane electrodialysis device 6.

[0032] (3) During the process, the current value is read by DC voltage every 10 minutes, and the conductivity is monitored online by a conductivity meter.

[0033] (4) When the current value does not change for 30 minutes, discharge the wastewater after the mixing chamber is treated and then add new sulfate wastewater.

[0034] The acid solution obtained by removing sulfate ions through bipolar membrane electrodialysis can be further separated into sulfuric acid by distillation.

[0035] The BPM-AEM-AEM-BPM membrane stack structure was adopted, with circulating water and a voltage of 10.5V. The initial acid chamber and intermediate chamber contained deionized water, while the mixing chamber contained sulfate wastewater. The flow rate in the acid chamber, intermediate chamber, and mixing chamber was 3.5 mL / min. The initial salt concentration of Na2SO4 was 30 g / L, and the concentration of Na2SO4 in the electrode solution was 0.1 mol / L. The results are shown in Table 1.

[0036] It should be noted that, for example Figure 3 As shown, using the traditional membrane stack structure method, the sulfate desalination rate is 84.2% and the current efficiency is 41.3%; after treatment according to the membrane stack structure method in this embodiment, the sulfate desalination rate is 96.8% and the current efficiency is 54.2%, both of which are significantly improved. Example

[0037] The BPM-AEM-AEM-BPM membrane stack structure was adopted, with circulating water and a voltage of 12V. The initial acid chamber and intermediate chamber contained deionized water, while the mixing chamber contained sulfate wastewater. The flow rate in the acid chamber, intermediate chamber, and mixing chamber was 3.5 mL / min. The initial salt concentration of Na2SO4 was 30 g / L, and the concentration of Na2SO4 in the electrode solution was 0.1 mol / L. The results are shown in Table 1. Example

[0038] The BPM-AEM-AEM-BPM membrane stack structure was adopted, with circulating water feed and a voltage of 13.5V. The initial acid chamber and intermediate chamber contained deionized water, while the mixing chamber contained sulfate wastewater. The flow rate in the acid chamber, intermediate chamber, and mixing chamber was 3.5 mL / min. The initial salt concentration of Na2SO4 was 30 g / L, and the concentration of Na2SO4 in the electrode solution was 0.1 mol / L. The results are shown in Table 1.

[0039] Table 1: Effect of electrolyte concentration in the polar chamber The experimental parameters were: initial Na₂SO₄ concentration in the mixing chamber of 30 g / L, operating voltage of 12 V, and flow rate in each chamber of the membrane stack of 3.5 mL / min. The effects of Na₂SO₄ concentrations of 0.05 mol / L, 0.1 mol / L, 0.15 mol / L, 0.20 mol / L, and 0.25 mol / L as the electrolyte solution in the polar chamber on the removal of sulfate by bipolar membrane electrodialysis were investigated. The experimental results are shown below. Figure 4 As shown in (a)-(c).

[0040] Depend on Figure 4 (a) Analysis shows that different electrolyte concentrations affect the membrane stack current. The current increases with reaction time, and stabilizes after 30 minutes. This is because in the early stage of desalination, the system resistance is high. Under the influence of the electric field, SO₄²⁻ in the mixing chamber passes through the anion exchange membrane into the acid chamber. The anions from the electrolyte consume the H₂ produced by the bipolar membrane in the catholyte chamber. + The electrolyte's cations consume the OH- produced by the bipolar membrane in the anolyte chamber. - The electrolyte, serving as both the cathode and anolyte, circulates in the system carrying current between the electrodes and the bipolar membrane, causing a decrease in system resistance. According to Ohm's law, this leads to an increase in current. In the later stages of desalination, the system remains stable, with minimal current variation, tending towards equilibrium. As the electrode solution concentration increases, the number of ions available for loading increases, and the increased concentration gradient enhances ion migration rates. Therefore, the degree of current change increases with increasing electrode solution concentration.

[0041] Depend on Figure 4(b) Analysis shows that electrolyte concentration is the main factor affecting the product acid concentration and sulfate removal rate. The sulfate removal rate and acid production concentration first increase and then decrease with the increase of electrode solution concentration. When the electrolyte concentration is 0.10 mol / L, the sulfate removal rate and acid production concentration are the highest, and the H2SO4 concentration obtained at this time is 0.167 mol / L, with a sulfate removal rate of 96.1%. After 0.15 mol / L, the sulfate removal rate and acid production concentration remain basically unchanged.

[0042] picture Figure 4 (c) Analysis reveals the relationship between electrolyte concentration and system current efficiency and energy consumption. As electrolyte concentration increases, energy consumption and current efficiency first increase and then decrease. When the Na₂SO₄ solution concentration in the electrode chamber reaches 0.05 mol / L, the electrolyte content is too low, and the electrode chamber resistance is too high, leading to sluggish ion kinetics and a significantly reduced reaction rate. Increasing the Na₂SO₄ solution concentration increases the electrolyte content in the electrode chamber, reduces the resistance of the electrode chamber, lowers the overall resistance of the membrane stack, and accelerates the reaction. When the electrolyte concentration increases from 0.15 mol / L to 0.25 mol / L, the high electrolyte concentration leads to uneven ion concentration at the membrane interface, increasing the Joule heat required to overcome the resistance to ion migration. This is because the electrical energy consumed to overcome ion migration resistance accounts for a large portion of the energy, which cannot be effectively used for bipolar membrane water dissociation and the migration of desired ions, thus resulting in a decrease in current efficiency.

[0043] The above experimental results show that the conductivity of a system is defined as the transfer of charge under the influence of an electric field. Since ions are the charges in an electrolyte, the conductivity of the electrolyte depends on the number of ions present in the system and their mobility. The anions of the electrolyte consume the H₂ produced by the bipolar membrane in the cathodic chamber. + The electrolyte's cations consume the OH- produced by the bipolar membrane in the anolyte chamber. - The electrolyte, serving as both the cathode and anode electrolytes, circulates in the system carrying current between the electrodes and the bipolar membrane. Therefore, the number and mobility of ions determine the rate of the chemical reaction, and the results indicate that a Na₂SO₄ electrolyte concentration of 0.1 mol / L in the bipolar chamber was selected as the optimal concentration for the experiment.

[0044] Effect of operating voltage Operating voltage is a crucial parameter in bipolar membrane electrodialysis, directly impacting water dissociation, ion migration in the solution, and energy consumption. Excessive operating voltage results in a significant portion of the total electrical energy being converted into Joule heat, increasing system energy consumption; the current is primarily used for water dissociation, leading to poorer separation efficiency. Conversely, insufficient operating voltage weakens the driving force for ion migration in the solution, also resulting in poorer separation. Therefore, the operating voltage range selected in this experiment was 9–15 V.

[32] The test results are shown in Figure 5 As shown in (a)-(c).

[0045] Depend on Figure 5 (a) Analysis shows that different operating voltages are the main factors affecting the membrane stack current. Under different operating voltages, the membrane stack current first rises rapidly and then stabilizes. This is because, in the initial stage, the acid chamber and intermediate chamber contain deionized water, which has very poor conductivity, resulting in a relatively high overall membrane stack resistance in bipolar electrodialysis. The membrane stack current is affected by both the operating voltage and the membrane stack resistance. The high initial system resistance reduces the electrochemical activity during the forward transport of charged ions, thus resulting in a low initial current. As desalination proceeds, under the influence of the electric field, H+ generated in the intermediate layer of the bipolar membrane... + SO₄²⁻ from the mixing chamber migrates to the acid chamber through the anion exchange membrane and the positive bipolar membrane, respectively, and combines to form H₂SO₄. This increases the number of ions available for current loading, leading to a higher operating voltage and a gradual increase in the membrane stack current. In the later stages of desalination, the decomposition rate of Na₂SO₄ is comparable to the acid-base synthesis rate, resulting in minimal change in membrane stack resistance and a relatively constant current. According to the second Wien effect, voltage can accelerate the water dissociation rate in the intermediate layer of the bipolar membrane; therefore, the higher the operating voltage, the shorter the time required for the system to complete desalination. The five control experiments used the same membrane stack with identical resistance. According to Ohm's law (U=IR), the current is directly proportional to the voltage applied across the membrane stack.

[0046] Depend on Figure 5 (b) Analysis shows the changes in product acid concentration and sulfate removal rate under different operating voltages. Under the action of a DC electric field, SO₄²⁻ in the mixing chamber and H₂O in the electrode chamber... + The ions migrate to the acid chamber through both the anion exchange membrane and the positive bipolar membrane to form H₂SO₄. As desalination proceeds, the concentration of SO₄²⁻ in the mixing chamber gradually decreases, and the OH⁻... - The concentration gradually increased. At different operating voltages, the sulfate removal rate was above 91% at the end of the experiment, reaching 96.2% at an operating voltage of 15V. After the reaction, the H2SO4 concentration in the acid chamber ranged from 0.151 to 0.163 mol / L under various operating conditions. The decrease in H2SO4 concentration was mainly due to H... + This is caused by leakage. Within the operating voltage range of 9–12V, the concentration of H₂SO₄ in the acid chamber increases with increasing operating voltage. This is because increasing the operating voltage shortens the reaction running time, allowing H₂SO₄ to react more readily. + The effect of leakage on H2SO4 concentration decreased, but when the operating voltage was further increased to 15V, the H2SO4 concentration in the acid chamber decreased slightly (0.157 mol / L). This is because the increased ion migration rate leads to H2SO4 concentration... + The leak worsened.

[0047] Figure 5(c) shows the current efficiency and energy consumption of the system under different operating voltages. When the input voltage is as low as 9V, the electric field is too weak to counteract the membrane's resistance to ion migration, resulting in low current efficiency. As the operating voltage gradually increases from 12V to 15V, the current efficiency gradually decreases. This is because the increase in operating voltage enhances water decomposition and ion migration, and increases the amount of H+ passing through the anion exchange membrane. + More severe leakage leads to a decrease in current efficiency during bipolar membrane electrodialysis. Furthermore, energy consumption gradually increases with increasing operating voltage, reaching a maximum of 0.981 kWh / kg H₂SO₄ at an operating voltage of 15V. The voltage applied to the membrane stack serves three main purposes: water dissociation under the influence of the electric field and the bipolar membrane, SO₄²⁻ migration, and Joule heating due to membrane stack resistance. Higher operating voltages result in greater current, faster water decomposition, and more heat generation on the membrane stack, leading to an increase in total electrical energy.

[0048] The above experimental results show that the high temperatures generated during long-term desalination can damage the equipment and ion exchange membranes. Excessive operating voltage results in low current efficiency, which is detrimental to practical applications. Conversely, excessively low operating voltage leads to low energy consumption, long operating times, and low acid concentrations, which is also undesirable. Therefore, selecting a suitable operating voltage that achieves short desalination time, high desalination rate, and high acid concentration is crucial. Research results indicate that 12V is the optimal operating voltage.

[0049] Effect of initial salt concentration The experimental parameters were as follows: electrolyte concentration in the electrode chamber (Na₂SO₄ concentration 0.1 mol / L), operating voltage 12 V, and flow rate in each chamber of the membrane stack 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 mixed chamber on the removal of sulfate by bipolar membrane electrodialysis were investigated. The experimental results are shown in [Figure number missing]. Figure 6 As shown in (a)-(c).

[0050] Depend on Figure 6 (a) Analysis shows that the initial salt concentration is the main factor affecting the change in membrane stack current. At the beginning of desalination, the current increases with increasing reaction time. When the reaction time reaches 50 min, the current at an initial salt concentration of 30 g / L reaches 996 mA and tends to stabilize. This is because at the beginning of the desalination reaction, the acid chamber contains deionized water with very few charged ions, resulting in high membrane stack resistance and a small initial current. As the desalination time increases, the concentration of charged ions in the acid chamber gradually increases, the conductivity of the acid chamber increases, the resistance decreases, and the membrane stack current gradually rises under constant voltage.

[0051] Depend on Figure 6(b) Analysis shows the changes in product acid concentration and sulfate removal rate under the initial salt concentration. It can be seen that when the Na₂SO₄ concentration increases from 10 g / L to 50 g / L, the sulfate removal rate increases from 94.2% to 96.8%. With the increase of the initial salt concentration, the Na₂SO₄ concentration in the solution... + The higher the SO2-4 concentration, the higher the recovered acid concentration. This is because, under the influence of a DC electric field, more SO2-4 passes through the anion exchange membrane into the acid chamber, and the bipolar membrane hydrolysis produces H+. + The concentration increases accordingly. When the concentration of Na2SO4 is 50 g / L, the obtained H2SO4 concentration can reach 0.243 mol / L, which is 4.76 times the H2SO4 concentration obtained when the concentration of Na2SO4 is 10 g / L (0.051 mol / L). Depend on Figure 6 (c) Analysis shows that the initial salt concentration affects both current efficiency and energy consumption. With increasing initial salt concentration, current efficiency first increases and then decreases, while energy consumption first decreases and then increases. The BMED process exhibits the highest energy consumption and lowest current efficiency when the initial Na₂SO₄ concentration is 10 g / L. This is because a lower concentration in the mixing chamber results in a smaller total number of mobile ions, leading to higher overall membrane voltage and resistance, and consequently, higher energy consumption. Increasing the Na₂SO₄ concentration from 10 g / L to 30 g / L increases the number of ions in the system, accelerating the reaction and resulting in higher current efficiency and lower energy consumption. However, increasing the Na₂SO₄ concentration from 30 g / L to 50 g / L is problematic due to higher osmotic pressure and higher salt content in the feed solution, which hinders water entry into the bipolar membrane. The prolonged ion leakage during this process negatively impacts current efficiency. As the Na₂SO₄ concentration increases from 10 g / L to 30 g / L, the energy consumption of the BMED system gradually decreases. This is because the increased Na₂SO₄ concentration leads to a decrease in membrane stack resistance, making ion migration energy the dominant process. However, when the Na₂SO₄ concentration increases from 30 g / L to 50 g / L, the BMED system consumes significantly more energy. This is because the desalination time increases substantially, resulting in greater energy consumption during the desalination process.

[0052] In summary, while a low initial salt concentration offers higher current efficiency and lower energy consumption, it significantly limits its application in treating high-concentration mine wastewater and results in lower recovered acid concentrations. Conversely, a high initial salt concentration reduces current efficiency and increases system energy consumption. Considering all factors, appropriately increasing the initial salt concentration is beneficial for acid recovery; therefore, an initial salt concentration of 30 g / L was set as the optimal concentration for this experiment.

[0053] Effect of flow rate The experimental parameters were: electrolyte concentration in the electrode chamber 0.1 mol / L, initial Na₂SO₄ concentration in the mixing chamber 30 g / L, and operating voltage 12 V. The effects of flow rates 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. The experimental results are shown in […]. Figure 7 As shown in (a)-(c).

[0054] Depend on Figure 7 (a) Analysis shows that the flow rate affects the current variation of the membrane stack. The membrane stack current is the largest when the flow rate is 3.5 mL / min. This is because as the flow rate increases, the turbulence in different compartments intensifies, the diffusion boundary layer is compressed, leading to a decrease in membrane stack resistance and a gradual increase in membrane stack current.

[0055] Depend on Figure 7 (b) Analysis shows that flow rate affects the product acid concentration and sulfate removal rate. At a flow rate of 3.5 mL / min, the sulfate removal rate reaches a maximum of 96.2%, and the acid concentration produced reaches 0.168 mol / L. Flow rate has almost no effect on the concentration of the produced acid.

[0056] Depend on Figure 7 (c) Analysis shows that flow rate is one of the factors affecting current efficiency and energy consumption. In the BMED treatment of simulated mine water, as the flow rate increased from 1.5 mL / min to 4.5 mL / min, the current efficiency increased from 51.2% to 57.8%. When the flow rate increased from 4.5 mL / min to 5.5 mL / min, the current efficiency decreased while energy consumption increased. This is because the turbulence in each chamber was too high, leading to self-circulation of the solution in each chamber, increased shear stress of the liquid during longitudinal flow, and a short residence time for Na₂SO₄. There was insufficient time for ions to migrate through the anion exchange membrane and bipolar membrane, and some ions could not complete the mass transfer process, thus reducing current efficiency and increasing total energy consumption. High flow rates lead to greater head loss, further exacerbating energy consumption and shortening membrane lifespan. Experiments show that maintaining the flow rate within a reasonable range is crucial in the BMED process.

[0057] The above experiments show that too low a flow rate leads to reduced current efficiency and increased energy consumption, while too high a flow rate results in greater pressure on the ion exchange membrane, shortening its lifespan. Therefore, maintaining the flow rate within a reasonable range is crucial during the BMED process. Taking all factors into consideration, a flow rate of 3.5 mL / min was selected as the optimal flow rate for this experiment.

[0058] The technical content and features of the present invention have been disclosed above. However, those skilled in the art may still make various substitutions and modifications that do not depart from the spirit of the present invention based on the teachings and disclosures of the present invention. Therefore, the scope of protection of the present invention should not be limited to the content disclosed in the embodiments, but should include various substitutions and modifications that do not depart from the present invention, and should be covered by the claims of this patent application.

Claims

1. A method for removing sulfate ions from wastewater using a bipolar membrane electrodialysis membrane stack, characterized in that, A bipolar membrane electrodialysis device with a three-compartment structure is adopted. The membrane stack of the bipolar membrane electrodialysis device consists of two bipolar membranes and two anion exchange membranes arranged in the order of BP-AA-BP. Adjacent membranes are separated by partitions to form an acid chamber, an intermediate chamber, and a mixing chamber. The cathode chamber and anode chamber formed by the electrode plates and the bipolar membranes on both sides of the membrane stack constitute the electrolytic chamber. Each compartment is connected to a corresponding container through a pump. The steps for removing sulfate are as follows: (1) Wastewater is introduced into the mixing chamber of the bipolar membrane electrodialysis device, and the acid chamber and intermediate chamber of the bipolar membrane electrodialysis device are both filled with deionized water. The electrode liquid in the electrode liquid chamber is a 0.1 mol / L sulfate solution. The operating voltage is 12 V and the flow rate is 3.5 ml / min. (2) Turn on the peristaltic pump and set the flow rate; turn on the DC power supply, set the target voltage, and run the bipolar membrane electrodialysis device; (3) During the process, monitor the current value and conductivity; (4) When the current value does not change for 30 minutes, replace the sulfate wastewater in the mixing chamber.

2. The method for removing sulfate from wastewater using a bipolar membrane electrodialysis membrane stack according to claim 1, characterized in that, A filtration process is added to the wastewater before it enters the bipolar membrane electrodialysis device.

3. The method for removing sulfate from wastewater using a bipolar membrane electrodialysis membrane stack according to claim 1, characterized in that, The cathode chamber and the anode chamber are interconnected.

4. The method for removing sulfate from wastewater using a bipolar membrane electrodialysis membrane stack according to claim 1, characterized in that, The initial salt concentration of the wastewater in the mixing chamber is 30 g / L.

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