Ion exchange membranes
Ion exchange membranes transport dissolved ions across a conductive polymeric membrane.[1] The membranes are often used in desalination and chemical recovery applications, moving ions from one solution to another with little passage of water.[2]
Ion exchange membranes are made of a polymeric material attached to charged ion groups. Anion exchange membranes contain fixed cationic groups with predominantly mobile anions; because anions are the majority species most of the conductivity is due to anion transport. Cation exchange membranes contain fixed anionic groups with predominantly mobile cations; because cations are the majority species most of the conductivity is due to cation transport. The selectivity of the membranes is due to Donnan equilibrium and Donnan exclusion and not due to physically blocking or electrostatically excluding specific charged species.
There are two primary classes of membranes: (1) heterogeneous, and (2) homogeneous. Heterogeneous membranes are low cost, have a thicker composition with higher resistance and a rough surface that can be subject to fouling. Homogeneous membranes are more expensive, but have a thinner composition with lower resistance and a smooth surface less susceptible to fouling. Homogeneous membrane surfaces can be modified to alter the membrane permselectivity to protons, monovalent ions, and divalent ions.[3]
The permselectivity of ion exchange membranes describes their charge selectivity. This charge selectivity reflects the membrane’s ability to discriminate between ions of opposite charge. A higher selectivity leads to increased recovery and performance of the membrane.[3]
Traditional applications
Ion exchange membranes are traditionally used in electrodialysis or diffusion dialysis by means of an electrical potential or concentration gradient, respectively, to selectivity transport cationic and anionic species. When applied in an electrodialysis desalination process, anion and cation exchange membranes are typically arranged in an alternating pattern between two electrodes (an anode and a cathode) within the electrodialysis stack. A galvanic potential is supplied as a voltage generated at the electrodes.[3]
A typical industrial electrodialysis stack consists of two chambers: a product water chamber and a concentrate reject chamber. During stack operation, salts are transferred from the product to the concentrate. As a result, the reject stream is concentrated up while the product stream is desalted.[3]
Exemplary applications of ion exchange membranes utilized in electrodialysis and EDR include seawater desalination, industrial wastewater treatment of highly scaling waters, food and beverage production, and other industrial wastewaters.[3]
Modern ion exchange membrane applications
Recent advancements in ion exchange membrane polymer science allow modifications to homogeneous membrane surfaces, altering their permselectivity to specific ions and enabling advanced membrane applications. In 2014, Saltworks Technologies Inc., a Canadian membrane supplier and wastewater treatment company, developed membranes capable of selective ion removal, such as protons for acid recovery applications.[4] Proton selective membranes enable high recovery acid applications, with EDR providing many benefits over traditional diffusion dialysis for acid recovery.
In addition, advanced membranes allow EDR to treat waters with organic content and oil and gas produced waters. These waters are not suited for pressurized membrane systems such as reverse osmosis, leading to fouling of the membranes and poor recoveries.[5]
Other recent membrane advancements have enabled monovalent selectivity, splitting monovalent and divalent ions. These membranes permanently split these scaling divalent ions (ex. CaSO4) into two non-scaling streams (ex. Na2SO4 and CaCl2). The monovalent rich stream is then sent to a Reverse Osmosis unit. Reverse Osmosis is highly efficient in removing monovalents when the scaling ions are removed. This multivalent separating EDR – RO hybrid set-up allows high recovery on scaling waters, greatly reducing brine reject volumes.[6]
References
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