Tag: electrodialysis

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Polyelectrolyte coatings for ion-exchange membranes in electrodialysis

Reverse electrodialysis water purification

DOI: 10.13140/RG.2.2.20145.13929

Ion exchange membranes are key components for various electrochemical technologies in water treatment and energy storage, such as electrodialysis, membrane electrolysis, and flow batteries. These membranes are characterized by a high concentration of charged groups, which can be either cationic (positively charged) or anionic (negatively charged). The function of an ion exchange membrane is to facilitate the transport of counterions while limiting the loss of water and co-ions.

The efficiency of cation exchange membranes is affected by unwanted co-ion and water transport. The transport of hydroxide ions (OH−) through cation exchange membranes is of particular interest. Depending on the application, cation exchange membranes are designed either to selectively facilitate hydroxide transport or to minimize hydroxide loss. Therefore, improved ion exchange membranes must support such additional functionalities.

Researchers at Wageningen University have characterized ion exchange and water transport through both coated and uncoated cation exchange membranes. The scientists published their findings in the Journal of Membrane Science. In their study, they examined cation exchange membrane coating with polyelectrolytes made of polyallylamine and polystyrene sulfonic acid.

The researchers coated one side of commercial cation exchange membranes with double layers of these two polymers. They then studied ion and water transport in diffusion dialysis and electrodialysis. Diffusion dialysis involves passive ion transport driven by concentration gradients, while in electrodialysis, ion transport occurs actively and is powered by an applied current.

The coatings were evaluated for their selectivity for monovalent and divalent ions. This selectivity affects hydroxide transport and water permeability. Both are key factors for the efficiency of bipolar membrane electrodialysis, where solutions containing multivalent cations such as magnesium and calcium are treated.

Magnesium and calcium transport was significantly limited by the coatings, while sodium ion transport remained largely unaffected. This selectivity was attributed to the Donnan exclusion mechanism and differences in hydration shells, as multivalent ions have a higher resistance within the cation exchange membrane.

Orientation is crucial in this context. Coating alignment affected performance. Resistance increased in the direction of multivalent ion flow, which reduced the flow of magnesium ions. This finding is impotant for the design of devices for bipolar membrane electrodialysis.

Surprisingly, the coatings did not reduce water crossover. Denser layers remained the bottleneck. The hydroxide flow was somewhat higher in coated membranes exposed to extreme pH values. This was likely due to structural changes during the coating process.

The combination of a low-water-content cation exchange membrane with a coating could enable the direct use of untreated salt solutions in bipolar membrane electrodialysis. This would reduce pretreatment costs and improve sustainability. The Fuji CEM-12 proved to be a promising candidate for future designs with coatings.

Salt diffusion through uncoated cation exchange membranes was mainly determined by the type of anion, such as chloride, sulfate, or hydroxide. In addition, membrane properties, including water content and ionic charge density, had a significant influence. The ionic charge density determined the anion distribution within the cation exchange membranes.

In summary, the researchers coated various commercial cation exchange membranes multiple times on one side with polyelectrolytes. For uncoated cation exchange membranes, water permeability correlated well with  ionic membrane resistance. This correlation was due to both parameters being dependent on the water content of the membrane. Moreover, permeability for co-ions increased with a higher volume fraction of water in the membranes.

Osmotic water transport in cation exchange membranes was not affected by the multiple layers of polyallylamine and polystyrene sulfonic acid. The researchers recommended single layer coating of low-water-content cation exchange membranes to minimize the transport of hydroxides and problematic multivalent cations.

This work demonstrates that surface modification using polyelectrolyte layers can enhance the functionality of conventional membranes without significant trade-offs. Water transport remained a challenge but the ability to block multivalent ions while maintaining conductivity for sodium ions represented a major step toward more efficient and cost-effective dialysis systems.

At Frontis Energy, we are excited about the future application of multilayered membranes on an industrial scale.

Elozeiri et al. 2026, Water and co-ion transport across ion-exchange membranes coated with PAH/PSS polyelectrolyte multilayer in electrodialysis and diffusion dialysis, Journal of Membrane Science,741, 125072, DOI: 10.1016/j.memsci.2025.125072

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Water desalination and fluoride ions removal from water using electrodialysis

Clean freshwater is of the utmost importance for our health. Despite its central role for our lives, progressing global industrialization threatens freshwater resources around the world. Albeit a vital trace element, fluoride is a serious public health threat. Absorbed in larger quantities for a long time, fluoride causes fluorosis, a form permanent poising responsible for irreparable bone damage.

Fluoride bearing rocks are particularly common in India. Fluoride is leached into adjacent aquifers and contaminates the soil. Sometimes, the concentration of fluoride ions in Indian aquifers exceeds 30 mg/L. Toxic concentrations of 20-80 mg / day over a period of 10 to 20 years cause irreparable damage to the human body.

Fluoride ions in groundwater are removed for water treatment using membranes. However, such membranes foul easily, for example by bacteria present in wastewater or other deposits.  Fouling can become a serious threat to public health. Therefore, a particular focus in membrane research is on the development of fluoride removing membranes that prevent fouling. It can be accomplished when bacterial growth is slowed down or inhibited entirely. For water treatment, antimicrobial surface modifications are used in high-quality membranes for ultrafiltration, nanofiltration, reverse osmosis and electrodialysis.

Electrodialysis is often used to remove water contamination, because only little energy is needed for the process. For electrodialysis membranes, salt deposits are an economic risk that is to be avoided. Precipitates can occur when the concentration of bivalent ions in the water is too high. Added to precipitates comes the risk of biofouling caused by microbial growth. Both affect the performance of electrodialysis membranes, causing economic losses as the membranes must be cleaned or replaced. For efficient water treatment, it is therefore important to improve the thermal and mechanical properties of the membranes.

A group of scientists have synthesized a composite anion exchange membrane for water-salt altitude and fluoride ion removal by electrodialysis that has improved antimicrobial properties. She published her results in the journal ACS ES&T Water. The consortium consisted of researchers of the Academy of Scientific and Innovative Research in Ghaziabad, India and the University of Tokyo.

Their anion exchange membranes are based on cross-linked terpolymers with built-in silver nanoparticles to slow microbial growth. The membranes are suitable for water desalination and fluoride ion removal by electrodialysis. The preparation of the terpolymers and polyacrylonitrile copolymers was carried out by N-alkylation using various alkyl halides. N-alkylation of the terpolymer through various alkyl groups affected the water absorption, hydrophobicity, ion transport and ionic conductivity of the membrane. Long alkyl groups increased the effectiveness of fluoride removal as well as the oxidative and physical stability of the membranes. The suitability of the composite membranes was verified by testing removal efficiency of fluoride ions (5.5 and 11 mg/L) from a sodium chloride solution (2 g/L) by electrodialysis at an applied voltage of 2 V.

The incorporation of 0.03% silver nanoparticles in the quaternized polymer caused the desired antimicrobial effect. The uniform distribution of silver nanoparticles in the liquid and solid phases was detected by transmission electron microscopy and atomic force microscopy. The attachment of bacteria was quantified counting colony forming units and 100x lower when silver nanoparticles were present in the membrane. The reduced microbial attachment to the membrane surface is therefore due to the antimicrobial effect of the silver nanoparticles. The small amount of 0.03% silver nanoparticles was sufficient to achieve desired antimicrobial effect in the membrane.

After 15 days and at a water temperature of 50°C, no detectable silver leaching occurred. The novel membranes are thus an improved anion exchange solution with antimicrobial properties for efficient removal of fluorine and desalination by electrodialysis.

Methodology

The entire synthesis was carried out in four steps:

  • Step 1: Silver nitrate was diluted with deionized water to produce a 30 mm solution
  • Step 2: Terpolymer and quaternized terpolymers were prepared by free radical polymerization
  • Step 3: Composite additives were prepared by the reduction of silver nitrate with sodium borohydrite in the presence of dimethylformamide
  • Step 4: The membrane was networked with the silver nanoparticles

Characterization of the anion exchange membrane

The membrane was characterized using several analytical methods:

  • UV-VIS and IR spectroscopy
  • Incorporation of silver nanoparticles by scanning electron microscopy, atomic force microscopy and transmission electron microscopy
  • Thermal stability, tensile properties, solubility and further physicochemical and electrochemical properties of the silver nanoparticle composite
  • Desalination and fluoride removal
  • The effectiveness of silver nanoparticles on microbial attachment
  • Energy consumption and efficiency during water desalination and fluoride removal by the composite membrane
  • Membrane stability with respect to pH, temperature and Fenton’s Reagent was evaluated

Reference:

Pal et al. 2021 “Composite Anion Exchange Membranes with Antibacterial Properties for Desalination and Fluoride Ion Removal” ACS ES&T Water 1 (10), 2206-2216, https://doi.org/10.1021/acsestwater.1c00147

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Lead removal from water using shock electrodialysis

Lead was widely used in water pipes during the industrial revolution that triggered urbanization and exponential growth of the population in metropolitan centers. The reason for its popularity was the plasticity of the material used in service lines near the end user. The negative health effects have been known since the 1920s, but infrastructure modernization in industrialized countries remains an enormous economic challenge. Lead service lines therefore continue to circulate water in our supply systems. The city of Flint in the northwest of Detroit, for example, received much press attention due to its long struggle with lead poisoning (e.g. Flint Water Crisis). Dissolved lead is highly toxic in a very small concentration and accumulates in body tissues.

The biggest challenge when removing lead from the water cycle is that it is usually dissolved in very low concentrations. Other compounds “mask” the dissolved lead, which makes its removal difficult. Sodium, for instance, is concentrated ten thousand times higher than lead. While nowadays lead can be removed from water by reverse osmosis or distillation, these processes are not selective and thus ineffective. They consume a lot of energy, which in turn is an environmental issue in itself. High energy consumption makes water treatment also very expensive. At the same time, other minerals dissolved ion water are beneficial and therefore desired ingredients that should not be removed.

MIT engineers have developed a much more energy-efficient method to selectively remove lead from water and published their results in the journal ACS EST Water. The new system can remove lead from water in private households or industrial plants and hence from the water cycle. Through its efficiency, it is economically attractive and offers its users the clear advantage of not being poisoned.

The method is the most recent of a number of development steps. The researchers started with desalination systems and later developed it into radioactive decontamination method. With lead the engineers have found an attractive market. It is the first system that is also suitable for private households. The new approach uses a process that was named shock electrodialysis by the MIT engineers. It is essentially very similar to electrodialysis as we know it, as charged ions migrate into an electric field through the electrolyte. As a result, ions are enriched on one side while being depleted on the other.

The difference of the new method is that the electric field moves as a sort of shock wave through the electrolyte and drags dissolved ions along. The shock wave traverses from one side to the other is the voltage increases. The process leads to a lead reduction of 95%. Today, similar methods are also used to clean up aquifers or soil contaminated by solvents. In principle, the shock wave makes the process much cheaper than existing processes because the electrical energy is targeted to remove specifically lead while leaving other minerals in the water. Hence, a lot less energy is consumed.

As usual for bench top prototypes, shock electrodialysis is still too ineffective to be economically viable. Its up-scaling will take time. But the strong interest of potential users will certainly accelerate its industrialization. For a household whose water supply is contaminated by lead, the system could be placed in the basement and slowly clean the water carried by the supply pipes because high rates occur only during peak hours. For this purpose, a water reservoir is necessary, keeping a stock of purified water. This can be a fast and cheap solution for communities such as Flint.

The process could also be adapted for some industrial purposes. The mining and oil industries produce much heavily contaminated wastewater. One imagine to reclaim dissolved metals and sell them to the market. This would create economic an incentives for wastewater treatment. However, a direct comparison with currently existing methods is difficult because the longevity of the developed system is yet to be demonstrated.

At Frontis Energy we are thrilled by the idea of ​​creating economic incentives to help implementing environmentally friendly processes and are already looking forward to a commercial product.

(Photo: Wikipedia)