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Principles of Deionization

Deionization is the process of removing ionizable solids from water using the principles of ion exchange. In a softener, the ion exchange process is relatively simple, and consists essentially of exchanging the minerals calcium and magnesium (and sometimes iron and manganese), for the softer mineral, sodium. Deionization is more complicated because it involves the removal of virtually all ionizable particles from water.

All dissolved minerals in water are composed of both a metallic part (a positively charged cation) and a non-metallic part (a negatively charged ion). The softener requires one resin to accomplish its job because it exchanges only cations. The deionizer requires two resins because it exchanges both cations and anions. No single resin can exchange both, because ion exchange depends on the tiny electrical charges in which like particles repel one another and unlike particles attract. A single resin can not be both positive and negative. A cation exchange resin is chemically formulated to attract positive ions; an anion exchange resin is formulated to attract negative ions.

The simplest deionizer is a two-column unit in which the cation exchange resin is held in one pressure vessel and the anion exchange resin in another. Water first passes through the cation tank, then the anion tank.

Cation Exchange Process
As water passes down through the cation tank, it encounters millions of resin beads, each of which contains a large number of negatively charged exchange sites in the pores and microscopic paths of its structure. When the resin is in the regenerated state, each exchange site is occupied by a positively charged hydrogen ion (H+). As the positively charged cations in the water contact the beads, they are attracted to the negative exchange sites. Since they (cations in the water) are stronger in their positive charge than the positive hydrogen ions in the resin, they drive off the hydrogen ions and attach to the exchange sites. By so doing, they maintain a balance between positive and negative charges. The displaced hydrogen ions (H +) pass down through the resin bed and are discharged from the tank.

Because the hydrogen ions are acidic, the exchange can also be described as a displacement of acidic ions by metallic ions, and water from the cation tank is a stream of dilute mineral acid.

At the same time, the non-metallic components, or anions, such as sulfates, chlorides and other elements pass through the cation tank unchanged. These anions, plus the hydrogen ions released from the cation exchange resin beads, are piped to the anion tank.

Anion Exchange Process
The anion exchange process is similar to the cation exchange process. There are two kinds of anion resin: strong base and weak base. A strong base anion resin is made of beads which have positive exchange sites, and which in the regenerated state are occupied by negative hydroxide ions (OH-). As the negatively charged non-metallic anions contact the beads, the same attraction-repulsion process takes place, and negative hydroxide ions are dislodged and replaced by the stronger negative non-metallic anions.

The hydroxide ions (OH-) pass down through the anion resin and are discharged from the tank. At the same time, the hydrogen ions (H+) from the cation tank have passed unchanged through the anion resin and they join the hydroxide to form HOH or H2O (water).

A weak base resin will neutralize mineral acid but does not use ion exchange. Strong base and weak base resins are used for different purposes:

Weak Base
Employed when removal of carbon dioxide or silica from water is not required. Weak base resins are generally higher in acid-removing capacity than strong base resins and are thermally stable. If silica and CO2 removal are not required, weak base resin is the logical choice.

Strong Base, Type I
Removes mineral acids most completely, including silica as silicic acid and CO2 as carbonic acid. Is stable at temperatures to 140"F.

Strong Base, Type II
Removes mineral acids efficiently, but does not remove silica as completely as Type I. Is stable at temperatures to 105?F. Is higher in capacity than Type I.

Regeneration
Cation exchange resins are regenerated by hydrochloric or sulfuric acid. As acid passes down through the resin bed the positively charged hydrogen cations in the chemical force off the positively charged cations (calcium, magnesium, sodium, etc.) that were attracted and held during the deionizer service cycle. The positive hydrogen ions attach to the negative exchange sites on the beads, restoring the resin to its regenerated hydrogen form.

Anion exchange resins are regenerated by sodium hydroxide (caustic soda). In a strong base resin, the alkaline solution passes down through the resin bed and exchanges with the mineral acids attracted and held by the beads during the service cycle, restoring the resin to its original regenerated basic form. In a weak base, resin, the alkaline solution regenerates the resin by a process of acid neutralization, not ion exchange.

Mixed Bed Deionizers
If the cation-anion exchange process could be repeated many times, the efficiency of ion exchange and removal would improve remarkably. Since no exchange process is 100% efficient, successive ion exchanges would remove even more ions, since in effect, it would be deionization of water that had already been deionized. The result would be an improvement of water purity with each successive ion exchange. This is exactly what happens when the cation and anion resins are mixed together in a mixed bed deionizer. As water passes through the mixed bed, it has millions of chances to contact a cation resin bead, then an anion, then another cation, another anion, and so on. An exchange takes place, of course, only when a positive ion contacts a negative exchange site, and vice versa. With each exchange, purity of the water improves because more ions are removed and held by resin beads.

Water Quality Measurement
Water quality can be measured quantitatively in milligrams per liter (mg/1) or parts per million (ppm) of total dissolved solids (TDS) or electrically by conductance or resistivity. Electrical measurements are based on the fact that the electrical conductance or resistance of water is directly related to the amount of ionizable impurities in the water. Thus a measure of conductance or specific resistivity is in effect a measure of the ionic content, or purity (quality) of the water.

Mixed bed deionizers are quite superior to two-column deionizers in terms of the water quality they produce. A two column deionizer yields water with specific resistivity of about 250,000 ohms/cm. Mixed bed deionizers yield water of 1,000,000 ohms/cm and up to 18,300,000 ohms/cm specific resistivity.

It should be pointed out that deionizers remove ionizable solids only, and have little or no effect on most dissolved gases, particulate matter, colloids, dissolved organic matter, or biological impurities. And even though a strong base resin will remove CO2 chemically, it may be more economical to remove it with a mechanical degasifier, especially when large amounts of CO2 are involved. Such considerations underscore the need for a systems engineering approach to the problems of water treatment. Systems engineering views the total picture in terms of the many impurities that water can contain, identifies them, and engineers a system utilizing the proper pieces of equipment and the appropriate processes for removing them. Water is a simple compound, but the impurities in it, and their removal, can be highly complex.

Electro-Deionization, (EDI)
EDI, or Electro-deionization, uses an electric field to remove ions and polar species from an aqueous stream. EDI is used with reverse osmosis to replace ion-exchange resin mixed beds (permanent or exchange-basis), which require chemical regeneration either onsite or offsite.
The primary environmental and economic benefit of EDI is the elimination of the use of resin regeneration chemicals.
The primary quality benefit of EDI is the continuous process eliminates spikes and upsets.

EDI removes ions from water using conventional ion-exchange resin, but with a key benefit. In EDI an electrical current is used to force a continuous migration of contaminant ions out of the feed water, through the resin bed, into the concentrate stream. The current also splits the water molecules into hydrogen and hydroxyl ions, continuously regenerating the resin bed. EDI replaces the primary mixed-bed in conventional water treatment systems, predictably and consistently producing water of the highest quality.