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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.


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


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,

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Highly efficient desalination using carbon nanotubes

Separating liquid compartments is not only important for generating energy in biological cells, respiration that is, but also for electrochemical cells and desalination through reverse osmosis and other processes. Therefore, scientists and engineers intensively research this field. We have already reported in several posts about promising attempts to make membranes cheaper and more effective. New nanomaterials have also been developed.

As a result of climatic changes caused by global warming, water scarcity is increasingly becoming a problem in many parts of the world. Settlements by the sea can secure their supply by desalinating water from seawater and brackish water sources. The process, however, is very energy intensive.

Now, researchers at California’s Lawrence Livermore National Laboratory (LLNL) have developed artificial pores made of carbon nanotubes that remove salt from water so efficiently that they are comparable to already available commercial desalination membranes. These tiny pores are only 0.8 nanometers in diameter. A human hair with a diameter of 60,000 nm. The researchers published the results in the journal Science Advances.

The predominant technology used to remove salt from water is reverse osmosis. A thin-film composite membrane (TCM) is used to separate water from ions. Hitherto the performance of these membranes has, however, been unsatisfactory. There is, for example, always a tradeoff between permeability and selectivity. In addition, exisiting membranes often show insufficient ion repulsion and are contaminated by traces of impurities. This requires additional cleaning stages, which again increase energy costs.

As is so often the case, the researchers got inspired by nature. Biological water channels, also known as aquaporins, are a great model for the structures that can improve performance. These aquaporins have extremely narrow internal pores that compress the water. This enables extremely high water permeability with transport rates of more than 1 billion water molecules per second per pore. Due to the low friction on the inner surfaces, carbon nanotubes represent one of the most promising approaches for artificial water channels.

The research group developed nanotube porins that insert themselves into artificial biomembranes. These engineered water channels simulate the functionality of aquaporin channels. The researchers measured the water and ion transport through their artificial porins. Computer simulations and experiments using the artificial porins in lipid membranes showed improved flux and strong ion repulsion in the channels of carbon nanotubes.

This measurement method can be used to determine the exact value of the water-salt permselectivity in such narrow carbon nanotubes. Atomic simulations provide a detailed molecular view of the novel channels. At Frontis Energy, we are excited about this promising approach and hope to see a commercial product soon.

(Image: Wikipedia)