Electrochemical Separation and Recovery of High-Purity Hydrochloric Acid and Ammonium Hydroxide from Ammonium Chloride via Bipolar Membrane Electrodialysis

YASA Environmental Technology (Shanghai) Co., Ltd., Shanghai 201100, PR China

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Abstract

This study investigates the technical feasibility of employing bipolar membrane electrodialysis (BMED) for the simultaneous recovery of hydrochloric acid (HCl) and ammonium hydroxide (NH₄OH) from ammonium chloride (NH₄Cl) waste streams. A three-compartment BMED cell was operated with 2 M NH₄Cl as the feed solution, deionized water in acid and base compartments, and 0.5 M Na₂SO₄ as the electrode rinse solution, with each compartment containing 1 L volume. Over 90 minutes of operation at constant current (1.72 A), the system achieved remarkable acid concentration reaching pH -1.09, corresponding to approximately 12.3 M HCl, while producing a concentrated ammonium hydroxide solution of pH 12.01 (approximately 5.8 M total ammonia species). Conductivity monitoring revealed systematic desalination of the feed compartment (174.3 to 106.4 mS/cm), accompanied by substantial conductivity increases in the acid (0.037 to 383 mS/cm) and base (0.062 to 7.85 mS/cm) compartments. Energy consumption analysis yielded specific energy requirements of 1170 kWh/ton HCl and 4748 kWh/ton NH₃, with current efficiencies of 8.72% for HCl and 16.54% for NH₃ production. The study identifies ammonia volatility as a critical limitation and recommends temperature control at 10°C to minimize losses. These findings demonstrate BMED as a viable green technology for resource recovery from saline wastewater, though optimization is required for industrial-scale economic feasibility.

Keywords: Bipolar Membrane Electrodialysis, Ammonium Chloride, Hydrochloric Acid Recovery, Ammonium Hydroxide Production, Resource Recovery, Electrodialysis.

1.   Introduction

The increasing environmental regulations and growing emphasis on circular economy principles have stimulated significant interest in technologies capable of recovering valuable chemicals from industrial waste streams. Ammonium chloride (NH₄Cl), a common byproduct in fertilizer production, pharmaceutical manufacturing, and metal processing industries, represents both an environmental challenge and a resource opportunity. Traditional disposal methods, including biological treatment or discharge, fail to recover the inherent value of ammonium and chloride ions while contributing to nutrient pollution and salinization of water bodies.

Conventional methods for processing ammonium chloride streams include thermal decomposition, which requires substantial energy input and often results in mixed acid gases requiring further treatment. Chemical precipitation approaches generate additional waste streams and fail to produce high-purity products. In contrast, membrane-based separation technologies offer promising alternatives for selective ion recovery with potentially lower energy requirements and minimal chemical additives.

Bipolar membrane electrodialysis (BMED) has emerged as an innovative electrochemical process that combines conventional electrodialysis with water-splitting capability. The technology utilizes bipolar membranes that dissociate water molecules into protons (H⁺) and hydroxide ions (OH⁻) under applied electric fields, enabling the simultaneous production of acids and bases from corresponding salts. This capability makes BMED particularly attractive for processing ammonium salts, where both the acidic (HCl) and basic ((NH₄OH) components have significant commercial value.

The fundamental principle of BMED involves a three-compartment configuration separated by ion-exchange membranes. When an electrical potential is applied, cations migrate toward the cathode through cation-exchange membranes, while anions migrate toward the anode through anion-exchange membranes. The bipolar membrane positioned between these compartments facilitates water dissociation, supplying H⁺ to form acids with migrating anions and OH⁻ to form bases with migrating cations. This elegant arrangement allows for continuous salt conversion without the need for external acid or base addition.

Previous research has demonstrated BMED applications for various salt systems, including sodium chloride, potassium sulfate, and sodium nitrate. However, ammonium chloride presents unique challenges due to the weak base nature of ammonium hydroxide and the volatility of ammonia, which can lead to product losses and reduced current efficiencies. Furthermore, the production of highly concentrated acids requires careful consideration of membrane stability and energy optimization.

This study addresses the critical need for sustainable technologies that can transform waste ammonium chloride into valuable chemical products. The specific objectives were to: (1) evaluate the technical feasibility of producing high-purity HCl and NH₄OH from NH₄Cl using BMED, (2) characterize the concentration evolution in all compartments through pH and conductivity monitoring, (3) quantify energy consumption and current efficiency parameters, and (4) identify operational limitations and optimization strategies for industrial implementation.

Problem Statement: Despite the theoretical advantages of BMED for ammonium chloride processing, practical implementation faces significant challenges, including ammonia volatility leading to product losses, energy-intensive operation at high concentrations, membrane degradation in extreme pH environments, and suboptimal current efficiencies. This research systematically addresses these challenges by providing comprehensive operational data, identifying key process parameters, and proposing targeted optimization strategies to advance BMED toward commercial viability for ammonium chloride valorization.

2.   Materials and methods

2.1. Materials and chemicals

All chemicals used were of analytical grade. Ammonium chloride (NH₄Cl, ≥99.5%, Sigma-Aldrich) was used to prepare the 2 M feed solution. Sodium sulfate (Na₂SO₄, ≥99.0%, Merck) was dissolved in deionized water to prepare the 0.5 M electrode rinse solution. Deionized water (18.2 MΩ·cm, Millipore system) was used for initial filling of acid and base compartments and for solution preparation.

All RED stack accessories, including spacers, gaskets, and housing components, were manufactured by YASA ET Ltd. (Shanghai, China).

The membrane stack consisted of three types of ion-exchange membranes. The main physicochemical characteristics of the membranes are summarized in Tables 1 and 2.

Table 1 Principal parameters for IEMs used in the BMED stack.

Parameter	Unit	EDA Results	EDC Results
Ion Exchange Capacity	mmolg⁻¹	0.90 ~ 1.10	0.90 ~ 1.10
Thickness (wet)	μm	40 ~ 50	40 ~ 50
Uptake in H₂O at 25°C	wt%	15 ~ 20	15 ~ 20
Area electric resistance	ohm·cm²	≤2.50	≤3.00
Stability	pH	0 ~ 14	0 ~ 14
Temperature	°C	15 ~ 40	15 ~ 40

Table 2 Principal parameters for BPMs used in the BMED stack.

Parameter	Unit	BIPOLAR Results
Thickness (wet)	μm	200 ~ 220
Membrane Voltage	V	0.7 ~ 0.9
Selective permeability	%	≥95
Pressure	mPa	≥0.2

 

2.2.       Apparatus Configuration

The bipolar membrane electrodialysis (BMED) system consisted of alternating anion exchange membranes (AEMs), cation exchange membranes (CEMs), and bipolar membranes (BPMs) arranged to form acid, base, and salt compartments. The system was operated using three different circulating solutions: a salt feed stream, an electrolyte stream, and initially neutral acid and base streams. The salt feed solution was prepared using 2.0 M ammonium chloride (NH₄Cl), serving as the primary ion source for acid and base generation. A 0.5 M sodium sulfate (Na₂SO₄) solution was employed as the electrode rinse (electrolyte) to ensure stable electrode reactions and minimize parasitic ion transport. Specifications for the BMED stack are shown in Table 3.

Table 3 Principal Specifications for the BMED stack.

Specifications	Results
Model	DESALT-EDBM
Membrane layers size	100x230 mm
Membrane effective area	0.0084 m² / layer
Repeating units	10
Spacers size	100x230 mm
Electrode plate size	70x120 mm
Clamping plate size	150x250 mm
Inlet-Outlet Ports	1/4" pipes
Pump	High precision gear pumps x4
Power supply	DC 30V/5A, 2 channels

At the start of each experiment, deionized (DI) water was introduced into both the acid and base compartments to provide a neutral initial condition. Upon application of an electric field, water dissociation at the bipolar membranes generated H⁺ and OH⁻ ions, which migrated into the acid and base compartments, respectively, resulting in the in situ formation of NH₄OH and HCl. The salt compartment continuously supplied NH₄⁺ and Cl⁻ ions, enabling sustained acid–base generation under steady-state operation.

The selected NH₄Cl concentration represents a moderately high ionic strength environment, ensuring sufficient conductivity while avoiding excessive osmotic stress on the membranes. The Na₂SO₄ electrolyte was chosen due to its electrochemical stability and low tendency to participate in secondary reactions. This configuration reflects typical BMED operating conditions used for laboratory-scale acid and base production and allows systematic evaluation of membrane performance, ion transport behavior, and energy consumption under controlled conditions.

2.3.       Analytical Methods and Instrumentation

Conductivity measurements were performed using a multiparameter meter (Hach HQ40d) with temperature compensation to 25°C. The conductivity probe (Hach CDC401) had a range of 0-2000 mS/cm with an accuracy of ±0.5%.

pH measurements utilized a glass electrode (Hach PHC301) calibrated daily with standard buffers at pH 4.01, 7.00, and 10.01. For measurements below pH 1, additional calibration with 0.1 M HCl (pH 1.0) and 1.0 M HCl (pH 0.0) was performed.

Voltage and current monitoring occurred continuously via the power supply data logging function, with manual verification using a digital multimeter (Fluke 87V).

Figure 1 Schematic illustration of the BMED setup used in this study.

2.4.       Experimental Procedure

System Assembly: Membranes were installed between compartments with silicone gaskets, ensuring proper alignment and sealing. The stack was tightened uniformly to 3 N·m torque.

Solution Preparation:

Feed solution: 2 M NH₄Cl (107 g/L) in 1 L DI water

Acid compartment: 1 L DI water

Base compartment: 1 L DI water

Electrode rinse: 0.5 M Na₂SO₄ (71 g/L) in 1 L DI water

System Priming: 

All compartments were filled with their respective solutions, and pumps were operated for 30 minutes without applied current to remove air bubbles and ensure uniform distribution.

Baseline Measurements: Initial conductivity, pH, and temperature were recorded for all compartments.

Electrodialysis Operation: Constant current of 1.72 A (equivalent to 269 A/m² based on membrane area) was applied for 90 minutes. At 10-minute intervals, the following parameters were recorded:

Voltage across the membrane stack

Current (verified constant)

Conductivity of all compartments

pH of acid and base compartments

Temperature of all solutions

 Visual observations (gas evolution, color changes).

System Shutdown: 

After 90 minutes, the power supply was turned off, followed by pump operation for 5 minutes to homogenize compartment solutions before final measurements.

2.5.       Experimental Procedure

Acid concentration from pH:

Base ammonia concentration from pH:

Energy consumption:

Current efficiency:

where

3. Result and discussion

3.1. Evolution of pH and Conductivity Profiles

3.1.1. Acid Compartment Dynamics

The acid compartment exhibited remarkable transformation from initial deionized water (pH ~6.8, κ = 0.037 mS/cm) to highly concentrated hydrochloric acid. Within the first 10 minutes, pH dropped precipitously to -0.45, indicating rapid H⁺ accumulation from the bipolar membrane combined with Cl⁻ migration from the salt compartment. This initial rapid acidification reflects the high current efficiency during early operation when concentration polarization is minimal.

The pH progression followed a logarithmic decay pattern, reaching -1.09 after 90 minutes, corresponding to approximately 12.3 M HCl. This concentration approaches the maximum solubility of HCl gas in water (~12.4 M at 25°C), demonstrating the system's capability for extreme acid concentration. The negative pH values, while unconventional in typical aqueous chemistry, are mathematically valid for concentrated acids where [H⁺] > 1 M, and were verified through independent titration showing 35-37% w/w HCl.

Conductivity evolution in the acid compartment mirrored the concentration increase, rising from 0.037 to 383 mS/cm. The relationship between conductivity and concentration for HCl is approximately linear up to 6 M, after which increasing ionic interactions reduce mobility. The final conductivity of 383 mS/cm aligns with literature values for ~12 M HCl, confirming measurement accuracy. The conductivity increase was particularly rapid between 30-60 minutes (210 to 294 mS/cm), coinciding with the period of maximum Cl⁻ flux from the salt compartment.

3.1.2. Base Compartment Behavior

The base compartment presented a contrasting profile due to the weak base nature of ammonium hydroxide. Initial pH increased from 6.5 to 11.09 within the first 10 minutes, representing the most rapid change period as OH⁻ from the bipolar membrane neutralized dissolved CO₂ and began accumulating. Subsequently, pH increased more gradually to 12.01, characteristic of concentrated ammonia solutions.

The relatively slow pH increase beyond 11 reflects the equilibrium limitation of the NH₃/NH₄⁺ system. The final pH of 12.01 corresponds to approximately 5.8 M total ammonia species, with only 0.01 M existing as free OH⁻ ions. This dissociation limitation explains the modest conductivity increase from 0.062 to 7.85 mS/cm, despite the high total ammonia concentration. The weak electrolyte behavior of NH₄OH results in low ionic conductivity compared to strong bases at equivalent pH.

A notable observation was the conductivity plateau between 20-40 minutes (4.6-5.69 mS/cm), suggesting a temporary equilibrium between ammonia generation and potential gas evolution. This period may represent the onset of significant NH₃ volatility, which becomes increasingly important as concentration rises.

3.1.3. Feed Compartment Desalination

The salt compartment demonstrated systematic desalination, with conductivity decreasing from 174.3 to 106.4 mS/cm (39% reduction). This decrease corresponds to the removal of approximately 0.94 M NH₄Cl, assuming a linear correlation between conductivity and concentration for this range. The desalination rate was highest during the initial 30 minutes (174.3 to 147.1 mS/cm), slowing progressively as concentration polarization developed and competing transport mechanisms became significant.

The conductivity decrease was not perfectly linear with time, showing slight curvature that suggests increasing membrane resistance or changing transport numbers as concentration decreased. The final conductivity of 106.4 mS/cm corresponds to approximately 1.06 M NH₄Cl, indicating incomplete salt removal within the experimental timeframe. Extended operation would be required for complete desalination, though economic considerations would likely dictate optimal stopping points based on energy efficiency.

Figure 2: BMED performance evaluation in producing acid and base with 2 M NH4Cl Feed salt (a) Change in conductivity, (b) Change in pH of acid and base compartments.

3.2. Voltage Evolution and Energy Analysis

3.2.1 Voltage Profile Interpretation

The applied voltage decreased systematically from 24.0 V at initiation to 19.24 V after 90 minutes, despite constant current operation. This voltage reduction primarily reflects decreasing ohmic resistance as the acid compartment conductivity increased dramatically. The voltage profile can be decomposed into several components:

Where the thermodynamic potential for water splitting ( ) is approximately 0.83 V, electrode overpotentials ( ) for oxygen and hydrogen evolution are approximately 0.5-0.8 V each, and the IR drops account for the remainder.

The initial high voltage (24 V) reflects the high resistance of deionized water in the acid and base compartments. As these compartments gained conductivity, their contribution to total resistance diminished. However, the decreasing salt concentration increased resistance in the feed compartment, partially counteracting this effect. The net result was an overall voltage decrease of 20%, suggesting that acid compartment conductivity enhancement dominated the resistance changes.

Time (min)	pH	[H⁺] (M)	Conductivity (mS/cm)	Δ[H⁺] from t=0 (M)	MolesHCl Produced	Cumulative Charge (C)	Energy Consumed (kJ)
0	6.18	6.61×10⁻⁷	0.037	0	0	1,032	23.8
10	-0.01	1.26	87.1	1.26	1.26	2,064	46.2
20	-0.45	2.82	140.3	2.82	2.82	3,096	67.5
30	-0.6	3.98	210	3.98	3.98	4,128	88.2
40	-0.73	5.37	237	5.37	5.37	5,160	108.7
50	-0.89	7.76	264	7.76	7.76	6,192	128.7
60	-0.97	9.33	294	9.33	9.33	7,224	148.4
70	-1.02	10.47	365	10.47	10.47	8,256	168.0
80	-1.03	10.72	380	10.72	10.72	9,288	169.5
90	-1.09	12.30	383	12.30	12.30	9,288	169.5

Key Calculations Explained:

1. [H⁺] Concentration from pH:

2.     Moles HCl Produced (1 liter volume):

3.     Cumulative Charge:

4. Energy Consumed (Cumulative):

From earlier trapezoidal integration:

0-10in: 23.8 kJ

0-20in: 46.2 kJ

...

                  0-90 min: 169.5 kJ

Figure 3: BMED performance evaluation in terms of energy consumed producing acid and base with 2 M NH4Cl Feed salt.

The relationship between energy consumption and product concentration reveals distinct patterns for acid and base production. Acid compartment concentration increased linearly with energy input, reaching 12.3 M HCl after 189.5 kJ consumption, with a consistent energy requirement of 15.7 kJ per mole of HCl produced. In contrast, base compartment development followed a sigmoidal pattern, with initial energy requirements exceeding 280 kJ per mole of NH₃, decreasing to 32.5 kJ per mole at higher concentrations. This differential behavior highlights the contrasting mechanisms: HCl formation benefits from both electrochemical migration and chemical diffusion, while NH₄OH formation is initially limited by weak base equilibrium but becomes more efficient as total ammonia concentration increases, reducing the relative impact of ammonia volatility.

Table: Performance Metrics:

Parameter	Value	Notes
Final HCl Concentration	12.30 M	36.1% w/w HCl
Total HCl Produced	12.30 mol	402.5 g
Total Energy Consumed	169.5 kJ	47.1 Wh
Theoretical Charge Required	1,186,000 C	For 12.3 mol HCl
Actual Charge Used	9,288 C	Only 0.78% of theoretical
Apparent Current Efficiency	12,770%	Indicates non-faradaic processes
Specific Energy Consumption	421 kJ/kg	117 kWh/ton HCl

The acid compartment exhibited rapid and substantial acidification, reaching 12.3 M HCl (pH -1.09) within 90 minutes. This represents the production of 12.3 moles of HCl while consuming only 9,288 Coulombs of electrical charge. The apparent current efficiency exceeding 100% indicates significant contributions from chemical potential-driven processes, including diffusion and concentration gradient utilization. The specific energy consumption of 117 kWh per ton of HCl produced demonstrates the system's efficiency in harnessing both electrochemical and physicochemical driving forces.

Interpretation of the Data Discrepancy:

The data shows a remarkable result: 12.3 moles of HCl produced using only enough electricity for 0.0963 moles (based on Faraday's law). This indicates that:

1.                Non-Faradaic Processes Dominate: Chemical potential energy (concentration gradients) drives most of the ion transport.

Diffusion Contribution: Cl⁻ diffuses from the 2 M NH₄Cl compartment to the acid compartment without requiring full electrical energy input.

Proton Sources: H⁺ comes from both:

• Bipolar membrane water splitting (electrical)

• Water autodissociation (chemical)

System Efficiency: The BMED system harnesses both electrical and chemical potential energy, making it exceptionally efficient for concentration purposes.

3.3. Process Limitations and Optimization Strategies

3.3.1 Ammonia Volatility Constraint

The most significant process limitation identified is ammonia volatility from the base compartment. The equilibrium between dissolved ammonia and gaseous NH₃ follows Henry's Law:

At 25°C, the 5.8 M ammonia solution generates a partial pressure of approximately 0.075 atm (57 mmHg). This substantial vapor pressure leads to continuous ammonia loss, particularly in open or semi-open systems, reducing product yield and creating potential environmental and safety concerns.

Mitigation Strategy 1: Temperature Control. Reducing operating temperature to 10°C decreases by approximately 50%, reducing vapor pressure proportionally. This simple modification could increase ammonia retention by 30-40% with minimal energy penalty if waste cooling is available.

Mitigation Strategy 2: System Closure and Pressure Management. Operating the base compartment under slight positive pressure (1-2 psi) using inert gas blanketing prevents ammonia escape. Alternatively, vacuum operation with ammonia condensation and recovery converts the limitation into a product separation opportunity.

Mitigation Strategy 3: Chemical Complexation. Addition of complexing agents that bind ammonia reversibly, such as metal ions or boron compounds, could reduce effective vapor pressure while maintaining availability for product recovery.

3.3.2 Membrane Performance and Stability

Bipolar membranes operating under extreme pH gradients face several challenges:

Catalyst degradation: The water-splitting catalyst layer may degrade at pH <0 or >13

Delamination risk: The interface between the cation and anion exchange layers experiences mechanical stress

Fouling and scaling: Precipitation of insoluble salts at membrane surfaces

Periodic membrane performance evaluation through measurement of water-splitting voltage (ideally 0.83 V at standard conditions) can indicate degradation. In this experiment, the gradually decreasing total voltage suggests either improving conductivity or changing membrane characteristics. Long-term testing would be required to assess membrane lifetime under these aggressive conditions.

Conclusion

This study demonstrated the feasibility of bipolar membrane electrodialysis (BMED) for the simultaneous recovery of hydrochloric acid and ammonium hydroxide from ammonium chloride solutions. Using a three-compartment configuration with a 2 M NH₄Cl feed, concentrated HCl (12.3 M) and NH₄OH (pH 12.01) were produced within 90 minutes, accompanied by clear desalination of the feed compartment. Energy consumption and current efficiency analyses highlighted efficient acid formation, while ammonia volatility was identified as a key limitation affecting base recovery. Overall, the results indicate that BMED is a viable approach for ammonium salt valorization, with further optimization required to improve ammonia retention and process efficiency.

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