Electrodialysis and Bipolar Membrane Electrodialysis (EDBM) for Efficient Conversion of Na2SO4 to H2SO4 and NaOH: A Pilot-Scale Study

Abstract:

This study demonstrates the performance of the YASA ET pilot-scale electrodialysis (ED) and bipolar membrane electrodialysis (EDBM) system for the conversion of sodium sulfate (Na₂SO₄) into sulfuric acid (H₂SO₄) and sodium hydroxide (NaOH). Using a 10-pair Saifu EDBM stack, the system operated under constant applied voltage (20 V) and controlled flow conditions. The process was characterized by real-time measurements of conductivity, pH, temperature, current, and energy consumption across the salt, acid, and base compartments. Results show efficient ion transport and water dissociation, with the acid and base streams reaching high concentrations of H₂SO₄ and NaOH, respectively. The system exhibited stable operation, validating the use of YASA ET’s DESALT® technology for large-scale acid-base production and zero-liquid-discharge (ZLD) applications. The findings highlight the scalability and reliability of the system for industrial applications in chemical resource recovery and desalination.

Keywords:   Electrodialysis, Bipolar Membrane Electrodialysis, Na₂SO₄ Conversion, H₂SO₄Production, NaOH Production, DESALT®, Zero-Liquid-Discharge (ZLD), Acid-Base Generation, Pilot-Scale Study, YASA ET.

YASA Environmental Technology Co., Ltd.

No. 588, Xinjinqiao Road, Pudong, Shanghai, China

Email: info@yasa.ltd

Website: www.yasa.ltd

1. Introduction of DESALT Pilot Equipment

2. 
   

The DESALT Pilot Equipment is a versatile platform designed for testing and developing electrodialysis (ED) and bipolar membrane electrodialysis (EDBM) processes. The system is constructed from corrosion-resistant materials to ensure high durability and extended operation, even in aggressive environments. The pilot skid integrates high-precision pumps for the circulation of feed solutions, acid, base, and electrodes. Real-time process monitoring is facilitated by flow meters and pressure gauges, while easy operation and control are ensured through the inclusion of sampling and ball valves.

A high-efficiency DC power supply powers the membrane stack, while a Siemens PLC-based control cabinet handles system operations. The pilot unit is equipped with pH and conductivity meters to accurately measure the process streams, with all sensors linked to a PLC and Modbus data recording software for automated data acquisition and analysis. Real-time monitoring, data visualization, and analysis are facilitated through Ethernet connectivity and data logging software.

For enhanced system safety and performance, the unit can be equipped with a heat exchanger and a ventilation module, ensuring stable operational conditions, even in high-temperature or gas-evolving environments. The system supports both ED and EDB membrane stacks, offering flexibility for desalination, acid-base generation, and ion separation research. Designed with reliable components, corrosion-resistant materials, and intelligent control, the YASA ET pilot equipment serves as an efficient and professional platform for membrane electrochemical experiments.

2. Bipolar Membrane Electrodialysis Test (Pilot Skid)

2.1. Feed Solutions

In this EDBM experiment, sodium hydroxide (NaOH) solutions were utilized as the working electrolyte across all process tanks. The electrode rinse solution was prepared by diluting 500 mL of 1 M NaOH with water to achieve a final volume of 1 L, ensuring sufficient ionic conductivity to maintain stable current flow and optimal electrode performance. The feed solution was prepared by dissolving 100 g/L of Na₂SO₄ (10% w/v) in 1 L of water, providing a balanced ionic matrix to enable efficient transport of Na⁺ and SO₄²⁻ ions during acid and base generation.

This solution configuration ensured stable osmotic conditions across the ion-exchange and bipolar membranes, minimizing unwanted back-diffusion and mitigating concentration polarization during the test. Both the acid and alkali loops were continuously recirculated to maintain uniform ionic distribution, steady temperature control, and consistent formation of H₂SO₄and NaOH.

2.2. Stack Configuration

The test was conducted using a Saifu EDBM stack, mounted on the YASA ET pilot skid system, which consisted of 10 cell pairs with a membrane size of 110 × 270 mm. Each cell pair comprised alternating Anion Exchange Membranes (AEM), Bipolar Membranes (BPM), and Cation Exchange Membranes (CEM), with the following specifications:

Table 1. Characteristics of the ion-exchange membranes (AEM, CEM) used in the EDBM stack.

Property	Value
Ion-exchange capacity	0.90 - 1.10 mmol/g
Wet thickness	70 - 80 µm
Water uptake	20 - 25 wt.%
Area resistance	4.5 - 5.5 Ω·cm²
Transport number	> 0.97
pH range	0 - 14
Temperature range	15 - 40 ˚C

The membranes used in the system exhibit superior chemical and thermal stability, with low area resistance and high transport number, ensuring efficient water splitting and acid-base production during both laboratory and pilot-scale EDBM applications.

Table 2. Characteristics of the Bipolar Membrane (YSEDBM-1) used in the EDBM stack.

Property	Value
Water Dissociation Efficiency	> 96%
Water Dissociation Voltage	1.0 -1.3 V
Thickness	200-220 µm
Applicable temperature	15 - 40 ˚C
IEC (ion exchange capacity)	1.7 – 1.9 mmol/g
Area electric resistance	0.7 – 0.9 Ω·cm²
Pressure	> 0.2 MPa
Selective permeability	> 95%
Application	EDBM for acid and base production

2.3. Electrolyte Type

The electrolyte solution in the electrode compartments consisted of 500 mL of 1 M NaOH diluted with 500 mL of deionized water, resulting in a final volume of 1 L. This concentration was selected to provide sufficiently high ionic conductivity, promoting stable and efficient current flow through the electrode chambers during the EDBM operation. The alkaline environment in the electrode loop was essential for facilitating smooth electron transfer reactions while preventing excessive gas evolution or voltage fluctuations.

The use of sodium hydroxide as the electrode electrolyte is particularly advantageous for bipolar membrane electrodialysis due to its compatibility with both cation- and anion-exchange membranes. Additionally, NaOH does not introduce multivalent ions or species that could negatively impact membrane performance. Furthermore, the alkaline solution minimizes the risk of electrode fouling and corrosion, enhancing the operational stability of the system during continuous acid and base production.

Overall, the chosen electrolyte composition supports efficient water splitting, protects the electrode surfaces, and contributes to the long-term performance and reliability of the EDBM system.

2.4. Volume and Voltage

The EDBM experiment was conducted under a constant applied voltage of 20 V, supplied by a regulated DC power source. Operating at a fixed voltage ensures a stable driving force for ion migration through the ion-exchange membranes and water dissociation at the bipolar membrane interface. This voltage level is sufficient to promote continuous water splitting into H⁺ and OH⁻ ions, enabling the formation of H₂SO₄ in the acid compartment and NaOH in the alkali compartment.

Each of the three circulating liquid streams (electrolyte, acid, and alkali) was maintained at a working volume of 1 L to ensure proper hydraulic circulation. This continuous recirculation helps sustain uniform ion distribution, reduces concentration gradients across the membranes, and mitigates concentration polarization, which can otherwise reduce current efficiency and increase electrical resistance. Adequate working volume is essential for buffering changes in ionic strength as Na₂SO₄ is progressively converted into acid and base.

The applied voltage of 20 V was chosen to balance efficient electrochemical performance with operational stability. Lower voltages would result in insufficient ion transport and water dissociation, while higher voltages could cause unwanted phenomena such as excessive Joule heating, increased membrane resistance, or gas formation at the electrodes, potentially damaging the system and reducing process efficiency.

2.5. Flow Rate Control

The circulation flow rates were carefully controlled during the experiment to ensure stable electrochemical performance:

·      Electrolyte compartment: 40 L/h

·      Acid and alkali compartments: 70 L/h each

Stable and balanced flow rates were critical for ensuring homogeneous mixing, reducing boundary layer effects, and preventing concentration polarization near the membrane surfaces. The system's peristaltic and magnetic drive pumps maintained constant hydraulic flow, ensuring steady operation throughout the test and supporting consistent ion transport across the membranes.

3. Procedure Overview

   
   

Before initiating the electrodialysis experiments, the EDBM system was thoroughly rinsed with tap water for 15–20 minutes to remove any residual contaminants from previous operations. After rinsing, all tanks were drained of tap water using the sampling valves.

The feed and electrolyte solutions were then prepared. The feed solution consisted of 1 L of water containing 100 g/L Na₂SO₄ (10% w/v). The electrode electrolyte solution was prepared by diluting 500 mL of 1 M NaOH with 500 mL of deionized water, resulting in 1 L of 0.5 M NaOH. Both solutions were transferred to their respective feed and electrode tanks.

Once the solutions were prepared, the DC power supply was connected to the anode and cathode terminals of the EDBM membrane stack, and the system was powered on to begin the electrodialysis operation. A Modbus-based data acquisition system was connected to the control interface, enabling continuous logging of electrical parameters throughout the experiment.

pH and conductivity probes were placed in the acid, base, and electrolyte tanks to monitor the evolution of pH and electrical conductivity during the acid and alkali formation. Measurements were recorded at 5-minute intervals throughout the experiment, with all data subsequently exported for detailed analysis and interpretation.

4. Results and Discussion

   
   

4.1. Electrical Conductivity graph

The conductivity profiles of the salt, acid, and base compartments reveal the characteristic progression of ion transport and water-splitting reactions during the electrodialytic conversion of Na₂SO₄ into H₂SO₄ and NaOH. Initially, the salt-stream conductivity increased slightly from 25 mS/cm at 0 min to 30 mS/cm at 5 min, reflecting early stabilization and membrane hydration. However, as Na⁺ and SO₄²⁻ ions were removed from the feed, the salt conductivity declined steadily, reaching 18 mS/cm at 40 min.

In contrast, the acid compartment displayed a significant rise in conductivity as H₂SO₄ was formed. Acid conductivity increased sharply from 1 mS/cm at 0 min to 55–56 mS/cm between 35 and 40 min. Similarly, the base compartment exhibited a more moderate rise in conductivity, reaching 31–32 mS/cm by 40 min.

These conductivity trends confirmed the efficient splitting of Na₂SO₄, with the progressive depletion of the feed salt and corresponding increases in acid and base conductivity.

Figure 1: The conductivity profiles of the salt, acid, and base compartments during the electrodialysis process, showing the characteristic decrease in salt conductivity and the increase in acid and base conductivities over time as Na₂SO₄ is converted into H₂SO₄and NaOH.

4.2. pH vs Time graph

The pH profiles of the salt, acid, and base compartments clearly illustrate the evolution of ion transport and water-splitting reactions. In the acid compartment, the pH decreased sharply from 2.9 at 0 min to ~1.5 between 25 and 40 min, reflecting the sustained formation of sulfuric acid. The salt compartment also underwent gradual acidification, reaching a pH of ~2.0 by 40 min. Meanwhile, the base compartment remained highly alkaline, fluctuating between pH 12.1 and 12.3 throughout the experiment.

These pH trends confirm efficient water dissociation and acid-base production, with the acid compartment becoming strongly acidic and the base compartment maintaining a stable high pH. As given below

Figure 2: The pH variations in the salt, acid, and base compartments throughout the electrodialysis process, reflecting the acidification of the salt compartment and the sustained alkalinity in the base compartment as acid and base products are generated.

4.3. Temperature Profiles

The temperature profiles of the salt, acid, and base compartments show a consistent rise in temperature throughout the EDBM operation, primarily driven by Joule heating and exothermic water-splitting reactions occurring at the bipolar membrane interface. In both the salt and acid compartments, the temperature started at approximately 22.0˚C at 0 min and increased to 30.0 – 30.2˚C by 35 – 40 min.

The base compartment consistently exhibited higher temperatures than the other two, starting at 23.5˚C and reaching 31.5 – 32.0˚C by 40 min. This elevated temperature is typical of the base compartment due to the higher resistance and viscosity of the NaOH solution. As you can see in the figure given below.

Figure 3: The temperature increase over time in the salt, acid, and base compartments, driven by Joule heating and exothermic water dissociation reactions at the bipolar membrane interface.

4.4. Current Vs Time

The current-time profile reveals a characteristic rise-plateau-decline behavior. Initially, the current increased sharply from 0.30 A at 0 min to 1.10 A at 5 min, reflecting the high initial conductivity of the Na₂SO₄ feed solution. The current continued to rise as ion migration and water splitting stabilized, peaking at approximately 2.20 A at 25 – 30 min. Afterward, the current gradually declined due to ion depletion in the salt compartment.

This progression of current confirms the expected transport behavior, from a rapid initial conductivity-driven rise to a plateau during optimal acid/base generation, and a decline as ionic strength decreases.

Overall, the current evolution from 0.30 A → 2.20 A → 0.50 A—confirms the expected progression of ion transport: rapid initial conductivity-driven rise, a plateau during maximized acid/base generation, and a final decline governed by Na₂SO₄ depletion and increasing membrane resistance. As shown in the figure below

Figure 4: The current-time profile during the electrodialysis operation, demonstrating the rise, plateau, and eventual decline in current as ion transport stabilizes and the feed solution is depleted.

4.5. Electrical Energy Consumption Vs Time

Energy consumption increases steadily throughout the EDBM operation, reflecting the gradual rise in electrical resistance as Na₂SO₄ is depleted from the feed compartment and as H₂SO₄ and NaOH concentrations increase in their respective product loops. At the start of the experiment, the energy consumption is 7.33 kWh at 0 min, rising to 7.35 kWh at 5 min as current flow stabilizes and initial water dissociation begins. As ion transport intensifies, energy consumption continues to increase, reaching 7.38 kWh at 10 min, followed by a further rise to 7.40 kWh at 15 min. This upward trend reflects the gradual reduction in available Na⁺ and SO₄²⁻ ions in the diluate, which increases the electrical resistance of the system.

Between 15 and 20 min, energy consumption reaches approximately 7.42 kWh, and it continues to climb to 7.44 kWh at 25 min, corresponding to the accumulation of H⁺, OH⁻, and ionic species in the acid and base streams, which adds additional resistive load. By 30 min, the energy consumption reaches 7.46 kWh, marking a cumulative increase of roughly 0.13 kWh over the course of the experiment. The overall monotonic rise in energy consumption from 7.33 kWh to 7.46 kWh indicates that as ion availability diminishes and product concentrations increase, the system requires more electrical energy to sustain the applied voltage and drive continued water-splitting and ion migration. As shown in the figure given below.

Figure 5: The cumulative energy consumption throughout the electrodialysis process, indicating a steady increase in energy consumption as ionic strength decreases and the concentrations of H₂SO₄ and NaOH rise in the respective product streams.

4.6. Energy Consumption Vs Current

The relationship between current and energy consumption during the EDBM process reveals how electrical performance shifts as Na₂SO₄ is depleted from the diluate and as H₂SO₄and NaOH accumulate in the product streams. At the beginning of the operation, the system exhibits the highest energy consumption values, 7.45 – 7.46 kW,h while the current is approaching zero (0.0 – 0.05 A). This behavior is characteristic of a transport-limited condition toward the end of an EDBM cycle, where ion availability is low, and resistance is high, causing energy consumption to remain elevated even though current becomes minimal.

As the current increases, indicating improved ion transport, energy consumption decreases slightly. When the current reaches about 0.2 A, the energy consumption falls to 7.40 kWh, reflecting reduced resistance in the membrane stack as more Na⁺ and SO₄²⁻ ions participate in charge transport. At a higher current of 0.7 A, the energy consumption further decreases to approximately 7.38 kWh, demonstrating the system’s most electrically efficient operating point, where ion availability and membrane conductivity are optimal.

However, when the current continues to increase to 1.1 A, the energy consumption rises again to 7.34 kWh. Although this value is lower than those observed during low-current operation, the upward trend indicates the onset of additional resistive losses from evolving acid and base concentrations, which increase solution viscosity and local resistance. This produces a characteristic U-shaped profile, where energy consumption is highest at very low currents (ion depletion phase), decreases during moderate currents (optimal ion transport), and then begins to rise again as current increases further (developing concentration gradients and resistive heating).

Overall, the graph reveals that energy consumption is inversely related to current at early stages (0.0 – 0.7 A) but becomes directly related to current at higher currents (0.7 – 1.1 A). This reflects transitions between transport-limited, optimal-transport, and resistance-governed regimes typical of EDBM systems converting Na₂SO₄ into H₂SO₄ and NaOH. As shown in the figure below.

Figure 6: The relationship between energy consumption and current, illustrating the U-shaped profile as energy consumption decreases during optimal ion transport and rises again due to resistive losses at higher currents.

Conclusion

This study successfully demonstrated the efficient conversion of Na₂SO₄ into H₂SO₄ and NaOH using the YASA ET pilot-scale bipolar membrane electrodialysis (EDBM) system equipped with a 10-pair Saifu EDBM stack. The system operated stably under a constant applied voltage of 20 V and controlled flow conditions, showing predictable electrochemical behavior and efficient acid-base generation.

The conductivity, pH, temperature, current, and energy consumption profiles confirm that the system performed as expected, with efficient ion transport and water dissociation. The results also highlighted the system's ability to produce high-quality acid and base products, demonstrating the feasibility of scaling up for industrial applications, such as zero-liquid-discharge (ZLD) strategies and large-scale acid-alkali recovery processes.

These findings validate the use of DESALT® and Saifu membrane configurations for future industrial-scale applications, underscoring their suitability for effective acid-base production and broader chemical resource reclamation.