Deionization (DI)

Deionization is the process of removing ionizable solids from water using the principles of ion exchange. In a water softener, the ion exchange process is relatively simple, and consists essentially of exchanging “softer” sodium minerals/ions for “harder” calcium and magnesium minerals/ions. Deionization, as an ion exchange process, is more complicated because it involves the removal of virtually all ionizable solids from water.

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

Basics of ion exchange

The simplest form of a deionizer system involves the use of two independent columns or vessels. This form of deionization is known as a Two-Bed Deionizer. The first column/vessel contains cation exchange resin and the second column/vessel contains anion resin. The water to be treated must first pass through the cation deionizer and then the anion deionizer. As water passes down through the cation vessel 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. As the positively charged cations in the water, contact the beads, they are attracted to the negatively charged exchange sites. Since they are stronger in their positive charge than the positive hydrogen ions, they drive off the hydrogen ions and attach to the exchange sites. By doing so, they maintain a balance between positive and negative charges. The displaced hydrogen ions (H) pass down through the resin bed and exit the vessel in the water stream. Because the hydrogen ions are acidic the exchange can also be described as a displacement of acidic ions by metallic ions. As a result the water from the cation vessel is a stream of dilute mineral acid. Since the cation resin only removes positively charged ions the negatively charged ions or anions pass through the cation resin bed with the acidic water stream.

Anion exchange process
The anion exchange process is similar to the cation exchange process. A strong base anion resin is made of beads which have positive exchange sites. When the resin is in the regenerated state the positive exchange sites are occupied by negative hydroxide ions (OH). As the negatively charged non-metallic anions contact the beads, the same attraction-repulsion process takes place, as with the cations, and the 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 vessel. At the same time, the hydrogen ions (H) from the cation vessel have passed unchanged through the anion resin and they join the hydroxide ions to form HOH or H2O, i.e., water.

Cation exchange process
Cation and anion exchange resins have limited capacities and have to be periodically regenerated. Cation exchange resins are regenerated by hydrochloric or sulfuric acid. When acid is introduced to the cation resin the positively charged hydrogen acid cations, in the chemical, force the positively charged cations (calcium, magnesium, sodium, etc.) off of the resin that were attracted and held during the deionizer service cycle. The positive hydrogen ions attach to the negative exchange sites on the beads thereby restoring the resin to its regenerated hydrogen form.

Anion exchange resins are regenerated using sodium hydroxide (caustic soda). In a strong base anion resin the alkaline solution passes down through the resin bed and exchanges hydroxide ions for the anion ions (chlorides, sulfates, bicarbonates, silica, etc.) which were attracted and held by the beads during the service cycle. The negative hydroxide ions attach themselves to the positive exchange sites on the beads thereby restoring the resin to its original basic, hydroxide form.

Modern approach: mixed-bed deionizer

A two-bed deionizer provides a low dissolved solids water which results in a high quality water. If the two-bed deionization (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 percent 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 cation and anion resins are mixed together, in a single column/vessel to form a mixed-bed deionizer. With the resins thoroughly mixed the water molecules to be processed have millions of chances to contact a cation resin bead, then an anion, then another cation, another anion, and so on. The exchange process takes place, of course, only when a positive cation contacts a negative exchange site and a negative anion contacts a positive exchange site. With each exchange, the purity of the water improves because more ions are removed and held by the resin beads. The end result is a higher quality of water from the mixed-bed deionizer.

The quality of water provided by deionization is measured in a number of ways. It can be measured quantitatively in milligrams per liter (mg/L) 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 solids/impurities in the water. Thus, a measure of the 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-bed deionizers in terms of the water quality they produce. A two-bed strong base deionizer yields water of about 2.5 mg/L (2.5 ppm) TDS which equates to a conductivity of 5.0 microsiemens/cm or a specific resistivity of about 200,000 ohms/cm. Mixed-bed deionizers can yield water with less than 0.04 ppm TDS with conductivities as low as 0.06 microsiemens/cm and resistivity values as high as 18,300,000 ohms/cm (18.3 megohms/cm).

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. In addition, although 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 when addressing the problems of water treatment.

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