Electrodeionization: How Resins, Membranes and Current Produce Ultrapure Water Without Acid Regeneration

Ion exchange is the reversible swap of like-charged ions between a counter-ion layer and the surrounding solution: cations from the solution move into the counter-ion layer while cations already there move out. The exchange happens mainly between the diffuse layer and the bulk solution, and because clay particle surfaces usually carry a negative charge, cation exchange dominates. The process strictly obeys the law of equivalence - the equivalents of cations entering the counter-ion layer equal those displaced from it.

The phenomenon was observed in soil absorbing ammonium salts as early as 1850, but ion exchange only became a practical separation method once synthetic ion-exchange resin appeared in the 1940s. Its process and equipment resemble adsorption, but selectivity is higher and it suits high-purity separation and purification better.

What It Is Used For

Ion exchange covers water softening and purification; refining and decolorisation of solutions such as sugar liquor; extraction of uranium and rare metals from mineral leachates; extraction of antibiotics from fermentation broths; and recovery of precious metals from industrial wastewater.

Mechanistically it is a liquid-solid process combining mass transfer - external and internal diffusion - with a chemical exchange reaction. The reaction itself is usually fast, so the overall rate is set by mass transfer. Reactions are reversible: under suitable conditions exchanged ions desorb and the exchanger returns to its original state, which is what allows repeated use through exchange and regeneration. Because the reaction proceeds quantitatively, exchange capacity per unit mass of exchanger is finite.

Selectivity and Rate

Two theoretical approaches describe selectivity. One treats the exchange as a heterogeneous chemical reaction between ions in solution, assuming the resin surface behaves as a semi-permeable membrane through which exchanged ions pass freely while ions bound to the resin skeleton cannot; applying F.G. Donnan's membrane-equilibrium principle yields the Gregor formula.

On rate, the question is which phase controls. At normal temperature the exchange reaction itself is fast and not the limiting factor. If the diffusing ion moves slowly in the liquid phase, the process is externally diffusion-controlled; if it moves slowly in the solid phase, it is internally diffusion-controlled. Early work started from Fick's law, but that rate equation applies only to isotopic ion exchange. Real exchange involves at least two ions moving in opposite directions, and when their diffusion rates differ an electric field arises that affects diffusion. F.G. Helfferich derived the rate equation that accounts for it, with N as mass flux, D the diffusion coefficient, F the Faraday constant and phi the electrode potential.

Equipment Configurations

Three arrangements exist. An agitated tank handles viscous liquids and can be cascaded when single-stage exchange is insufficient. A fixed-bed ion exchanger, also called an ion-exchange column, is the most widely used. A moving-bed ion exchanger remains unapplied industrially because of technical difficulty.

Where EDI Comes In

EDI (Electro-de-ionization) is an ultrapure-water technology combining ion-exchange technology, ion-exchange membrane technology and ion electromigration, that is, electrodialysis. It uses the deep desalination of ion exchange to overcome electrodialysis polarization and incomplete desalination, and it uses electrodialysis polarization to induce water dissociation producing hydrogen and hydroxide ions for resin self-regeneration, overcoming the drawback of chemical regeneration after resin exhaustion. The technology emerged gradually from the 1980s and, after more than a decade, took a considerable share of the ultrapure-water market in North America and Europe.

Inside an EDI device, an anion-exchange membrane allows only anions through and blocks cations, while a cation-exchange membrane does the reverse. Ion-exchange resin is sandwiched between them to form a single treatment cell and dilute chamber; cells are separated by mesh to form the concentrate chamber; and DC electrodes at the ends of the stack create the electric field. Feed water flows through the dilute chamber, and its anions and cations are pulled through the membranes into the concentrate chamber. The resin packed between the membranes greatly increases the rate of ion removal, while water molecules dissociate under the field and continuously regenerate the resin, keeping it in optimal condition.

The device splits feed into three independent streams: product water at 90-95 percent, concentrate at 5-10 percent which can be recycled, and electrode water at 1 percent which is discharged.

Practical Position

EDI is a polishing unit. Its attractions are continuous production, high water quality, easy control, small footprint, no acid or alkali requirement and environmental friendliness. It is normally paired with reverse osmosis in a pretreatment-RO-EDI ultrapure-water system, replacing the mixed-bed ion-exchange equipment of conventional processes. Feed-water requirement is resistivity of 0.025-0.5 megohm-centimetre, which RO satisfies comfortably; EDI can produce ultrapure water with resistivity above 15 megohm-centimetre.

Use abroad spans more than a decade, concentrated in pharmaceutical, microelectronics, power generation and laboratory applications, with growing use in surface cleaning, surface coating, electrolytic and chemical industries. With continued equipment improvement, technical refinement and optimisation for different industries, initial investment cost will fall substantially, and it is reasonable to expect EDI to replace the mixed-bed stage in conventional water treatment entirely.