Anion Exchange Membranes In Electrodialysis Research Articles

Effects of silica sol on ion exchange membranes: Electrochemical characterization of anion exchange membranes in electrodialysis of silica sol containing-solutions

The fouling potential of the negatively charged silica sol in electrodialysis (ED) by adsorption on the surface of an anion exchange membrane was investigated. Since the fouling potential is related to the physical and electrochemical properties of the silica sol and anion exchange membranes, it is important to characterize the properties of silica sol and membranes. The surface charge of silica sol was investigated by the electrophoretic mobility and its isoelectric point was determined as pH 3. The commercial anion exchange membranes were characterized in terms of exchange capacity, water content, the zeta potential and the electrochemical properties of the membranes using impedance spectroscopy to predict the effects on the electrodialysis performances. Among the characterized properties, exchange capacity and some electrochemical properties of the anion exchange membranes were rather improved after ED experiments. In the electrodialysis of solution containing silica sol, deposition of the silica sol did not decrease the desalting performance of the anion exchange membranes because of loosely packed cake layer on the membrane surface.

Fouling mitigation of anion exchange membrane by zeta potential control

The feasibility of fouling mitigation of anion exchange membranes (AEMs) in the presence of humate was studied by adding three different types of water-soluble polymers, i.e., poly(acrylic acid) (PAA), poly(vinyl alcohol) (PVA), and poly(ethylene imine) (PEI), during electrodialysis (ED) desalination. Measurement of zeta potential of the humate used in this study showed highly negative potential (about −30 mV), implying that the humate had a strong fouling potential on the AEMs in ED. Of the three water-soluble polymers, PEI showed a positive zeta potential (about +14 mV) and is able to form an interpolymer complex with the humate. PAA and PVA hardly formed interpolymer complexes with humate due to electrostatic repulsion. The PEI–humate mixture with a volume ratio of 1:20 (PEI:humate) showed zero zeta potential, and a complexed humate with zero surface charge was formed, resulting in no fouling effects on the AEMs. Accordingly, the desalting ED experiments with PEI showed improved ED performance. Further, black colloids formed in the mixture did not cause the cell resistance to increase.

Change in Transport Properties of Anion-Exchange Membranes in the Presence of Ethylene Glycols in Electrodialysis

To change permselectivity between two anions through anion-exchange membranes in electrodialysis, the hydrophilicity of the anion-exchange membranes was increased by impregnation of hydrophilic compounds, ethylene glycols with varying molecular weights, into the membranes. Ethylene glycols have good affinity to the anion-exchange membranes and were selectively impregnated into a domain where anion-exchange groups exist. The change in the transport numbers of bromide, fluoride, sulfate, and nitrate ions relative to chloride ions was evaluated in the presence of the glycols (because the glycols dissolved from the membrane matrix to the salt solutions, electrodialysis was carried out using the salt solutions containing the glycols). Permeation of the strongly hydrated anions, fluoride and sulfate ions, was increased compared with chloride ions, and permeation of the less hydrated anions, bromide and nitrate ions, became difficult to achieve through the membranes in the presence of ethylene glycols. This was due to the increase in the uptake of more hydrated anions into the membrane and not due to the change in mobility ratio between anions through the membrane. Increasing the hydrophilicity of the anion-exchange membranes was an effective method for changing the permeation behavior of anions through membranes in electrodialysis.

Changing permselectivity between halogen ions through anion exchange membranes in electrodialysis by controlling hydrophilicity of the membranes

To change the permselectivity between halogen ions through anion exchange membranes in electrodialysis, the anion exchange membranes were modified: impregnation of compounds which have ether bonds into the membrane and formation of a cationic polysoap layer on the membrane surface, led to control of the hydrophilicity of the membrane. Evaluation was performed by measurement of the transport numbers of fluoride, bromide and iodide ions relative to chloride ions. The permeation of the less hydrated bromide anions decreased and that of the strongly hydrated fluoride anions increased relative to chloride ions by impregnating compounds containing ether bonds into the membranes, which was a consequence of the increase in the hydrophilicity of the membranes. Notably, permeation of chloride ions through the membrane exceeded that of bromide ions in the presence of ethylene glycols, which is the opposite behaviour of those of conventional anion exchange membranes. On the contrary, to examine the effect of decreasing the hydrophilicity on the permselectivity between halogen ions, a hydrophobic layer [poly(N-dodecyl 4-vinylpyridinium bromide)] was formed on the desalting side of the membrane surface. Although fluoride ions permeated only with difficulty through the membrane owing to formation of the layer, permeation of bromide and iodide ions was enhanced by the layer. In particular, the transport number of iodide ions relative to chloride ions reached 35.5 (without the layer 4.8). Controlling the hydrophilicity of the anion exchange membranes is an effective method to change the permeation behavior of halogen ions through anion exchange membranes.

Thermally responsive novel anion exchange membranes for electrodialysis

Anion exchange membranes containing N-isopropylacrylamide as a component showed a decrease in osmotic flux through the membrane, and an increase and decrease respectively in the transport numbers of NO3– and SO42– relative to Cl– with increasing temperature.

Macroreticular anion exchange membranes for electrodialysis in the presence of surface water foulants

In this work two new types of anion exchange membranes having pore sizes of macro dimensions are described. Static and dynamic fouling resistance to common large anions found in surface waters is evaluated. Improvements over conventional microporous anion membrane types are discussed with respect to electrodialysis.