What Is Ion Exchange?
Some cations in an aqueous solution enter the counter-ion layer, while the cations originally in the counter-ion layer enter the aqueous solution; this exchange of like-charged ions between the counter-ion layer and the normal-concentration aqueous solution is called the ion-exchange action. Ion exchange mainly occurs between the diffuse layer and the normal aqueous solution. Since clay particle surfaces usually carry a negative charge, ion exchange is dominated by cation exchange, and is thus also called cation exchange. Ion exchange strictly obeys the law of equivalence, i.e., the equivalents of cations entering the counter-ion layer equal those displaced from it.
The ion-exchange phenomenon in soil absorbing ammonium salts was discovered as early as 1850, but ion exchange as a modern separation method came only after the synthetic ion-exchange resin was developed in the 1940s. The process and equipment of ion exchange are basically the same as adsorption, but ion exchange has higher selectivity and is more suitable for high-purity separation and purification. [1]
Ion exchange is mainly used for water treatment (softening and purification); refining and decolorization of solutions (e.g., 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, etc. [2]
Ion exchange is the process of using ion exchangers (most commonly ion-exchange resins) to separate liquid mixtures containing electrolytes. The ion-exchange process is a mass-transfer (including external and internal diffusion) and chemical-reaction (ion-exchange reaction) process between liquid and solid phases; usually the ion-exchange reaction is fast, so the process rate is mainly determined by the mass-transfer rate.
Ion-exchange reactions are generally reversible; under certain conditions the exchanged ions can be desorbed (reverse exchange), restoring the ion exchanger to its original state, i.e., the ion exchanger can be used repeatedly through exchange and regeneration. Meanwhile, the ion-exchange reaction proceeds quantitatively, so the exchange capacity of the ion exchanger (the equivalents or moles of ions exchangeable per unit mass of exchanger) is limited. [3]
Two theories can be used to study the selectivity of the exchange process:
(1) Heterogeneous chemical reaction theory
Assume the following exchange reaction between ions A1 and A2:
It is assumed that the resin surface acts as a semi-permeable membrane, through which the exchanged ions can pass freely, while ions bound to the resin skeleton cannot. According to F.G. Donnan's membrane-equilibrium principle, the Gregor formula is derived:
Ion exchange is a liquid–solid phase reaction process that inevitably involves the diffusion of substances in the liquid and solid phases. At normal temperature the exchange reaction is fast and not the controlling factor. If the diffusing ion is slow in the liquid phase, it is called external-diffusion control; if it is slow in the solid phase, it is called internal-diffusion control.
Early research started from Fick's law (see molecular diffusion), and the derived rate equation applies only to the exchange of isotopic ions. In fact, ion exchange involves at least two ions diffusing in opposite directions. If their diffusion rates differ, an electric field is generated, which inevitably affects ion diffusion. Taking this field into account, F.G. Helfferich derived the corresponding rate equation:
where N is the mass flux; D is the diffusion coefficient; F is the Faraday constant; φ is the electrode potential.
(1) Agitated tank, suitable for treating viscous liquids. When single-stage exchange does not meet the requirement, multiple stages can be cascaded.
(2) Fixed-bed ion exchanger, also called ion-exchange column, is a fixed-bed mass-transfer device for ion exchange and the most widely used.
(3) Moving-bed ion exchanger is a moving-bed mass-transfer device for ion exchange, not yet industrially applied due to technical difficulties.
EDI (Electro-de-ionization) is an ultrapure-water technology combining ion-exchange technology, ion-exchange membrane technology, and ion electromigration technology (electrodialysis). It uses ion exchange's deep desalination to overcome electrodialysis polarization and incomplete desalination, and uses electrodialysis polarization to induce water dissociation producing H⁺ and OH⁻ ions for resin self-regeneration, overcoming the drawback of chemical regeneration after resin exhaustion. It is a technology that emerged gradually since the 1980s. After more than a decade of development, EDI has captured a considerable share of the ultrapure-water market in North America and Europe.
An EDI device includes anion/cation exchange membranes, ion-exchange resin, DC power supply, etc. The anion-exchange membrane allows only anions to pass and blocks cations, while the cation-exchange membrane allows only cations to pass and blocks anions. Ion-exchange resin is sandwiched between the anion and cation membranes to form a single treatment cell and the dilute chamber. Cells are separated by a mesh to form the concentrate chamber. DC electrodes at the two ends of the cell stack create an electric field. Feed water flows through the dilute chamber; the water's anions and cations are removed through the membranes under the field and enter the concentrate chamber. The ion-exchange resin filled between the membranes greatly increases the rate of ion removal. Meanwhile, water molecules dissociate into H⁺ and OH⁻ ions under the field, continuously regenerating the resin to keep it in optimal condition. An EDI device divides the feed water into three independent streams: product water, concentrate, and electrode water. Product water (90%–95%) is the final output; concentrate (5%–10%) can be recycled; electrode water (1%) is discharged.
An EDI device is a water polishing unit with the advantages of continuous production, high water quality, easy control, small footprint, no acid/alkali requirement, and environmental friendliness, and has broad application prospects. With equipment improvement, technical refinement, and optimization for different industries, the initial investment cost will be greatly reduced. It is believed that in the near future it will completely replace the mixed-bed stage in conventional water-treatment processes.
EDI technology has been widely used abroad for over a decade, mostly in the pharmaceutical, microelectronics, power-generation, and laboratory industries. Its use in surface cleaning, surface coating, electrolytic, and chemical industries is also growing. In China it has been applied for only 2–3 years, mainly for ultrapure-water treatment in the pharmaceutical and microelectronics industries, while its use in power-plant chemical water-treatment systems is just emerging.
An EDI device belongs to a polishing water system and is generally used with reverse osmosis (RO) to form a pretreatment–RO–EDI ultrapure-water treatment system, replacing the mixed-bed ion-exchange equipment of conventional processes. EDI feed-water requirement is resistivity 0.025–0.5 MΩ·cm, which RO fully satisfies. EDI can produce ultrapure water with resistivity above 15 MΩ·cm.
