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Basics of Deionization

Mario C. Uy
Domingo A. Mesa
Deionization, also referred to as demineralization, is an ion-exchange process wherein virtually all of the dissolved ions in the water can be removed, producing pure water.
Depending on the type and combination of equipment, deionization can produce a purity from 100,000 ohms/cm to 18 million ohms/cm (or mega-ohms/cm). 
 Measurement of purity
Water purity is measured in units of resistivity (ohms/cm) or conductivity (microsiemens/cm).  Resistivity and conductivity are reciprocal of each other, meaning as resistivity goes up, conductivity goes down, and vice versa.   In pure water application, purity is typically measured in resistivity because conductivity would be too low to be measured accurately.   Fig. 1 shows an example of this relationship.   Purity is relative.  For example, a pharmaceutical application may only require a 100K (100,000 ohms/cm) purity whereas a circuit board manufacturer may need at least a 10 meg (10 million ohms/cm) purity.
Types of solids
Water dissolves minerals by dissociating the minerals to cations (+) and anions (-).  For example, sodium chloride (NaCl), a table salt, will dissolve in water by dissociating to sodium (Na+) cation and chloride (Cl-) anion.  The dissociated ions, having ionic charges, are easily removed by deionization.
Some gasses like carbon dioxide (CO2 ) reacts with water to form carbonic acid (HCO3) which dissociates to 2H+ and CO3-2 Likewise, these ions are easily removed by deionization.
Other solids, when dissolved in water, do not dissociate to ions, but they have enough density of charges that allow them to bridge with the water molecule.  Examples are organic compounds like sugar, alcohol, and solvents.   Although they do not dissociate to ions, they impart some ionic charges, and thus can be removed by deionization.
Lastly, other compounds like hydrophobic organics and some biological matters, when mixed in water, are non-ionic, and thus cannot be removed by deionization, but can be removed by other equipment such as multi-media filters, activated carbon filters, and reverse osmosis.   
Types of resin
A deionizer (also referred to as DI) uses 2 opposing charged resins (cationic and anionic).   The cationic resin removes the cations.  The anionic resin removes the anions.
The cationic resin is typically made from styrene containing sulfonic acid groups which is negatively charged.  Although the resin is actually negatively charged, it is called a “cationic” resin, referring to the cations that it will exchange.  This resin typically comes in the hydrogen ion (H+) form, meaning it is precharged with hydrogen ions on its exchange sites.
The anionic resin is typically made from styrene containing quaternary ammonium groups which is positively charged.  Likewise, despite its positive charge, it is called an “anionic” resin, referring to the anions that it will exchange.  This resin typically comes in the Hydroxide ion (OH-) form, meaning it is precharged with hydroxide ions on its exchange sites.
Both resins can be strongly or weakly ionized.  Cationic resin is referred to as strong acid or weak acid.  Anionic resin is referred to as strong base or weak base.  The basic difference is the weakly ionized resin will exchange only the weak ions, whereas a strongly ionized resin will exchange both weak and strong ions, but at the expense of a reduced capacity.
The strong base anion comes in type 1 or type 2.   The basic difference is type 1 can remove silica and CO2 better than type 2, but likewise at a reduced capacity.
There are other variations of resins to consider, with each providing its own advantages and disadvantages, such as, thermal, physical, and chemical stability, oxidation and organic fouling resistance, kinetics, and costs.  Suffice it to say that one must choose the proper type of resin to fit the application.
Dual and mixed beds
Deionizers come in dual-beds or mixed-beds.   In a dual-bed system, the cationic resin and the anionic resin are in separate vessels.  Whereas in a mixed-bed, the cationic and the anionic resins are mixed in a single vessel.  This is also called a single-bed DI.
In a dual-bed DI (Fig. 2), water is first passed through the cationic vessel where the cations in the water are exchanged with the hydrogen ions from the resin.   The cationic resin, having a greater affinity for the cations, releases the hydrogen ions while grabbing the cations.  The released hydrogen ions form acid with the remaining anions in the water.  This water is then passed through the anionic vessel where the anions are now exchanged with the hydroxide ions from the resin.  Similarly, the anionic resin, having a greater affinity for the anions, releases the hydroxide ions while grabbing the anions.    The released hydrogen ions (H+) from the cationic vessel and hydroxide ions (OH-) from the anionic vessel now combine to form HOH or H2O.  
Not all of the ions will be exchanged.  Some of the ions will slip by.  If the DI water can be passed through the dual-bed again, the DI will have another chance to remove the ions that slipped by the first time.  Each time this pass is repeated, more ions are removed and exchanged, producing an even purer water.  In a mixed-bed, the water passes through the cationic resin and the anionic resin repeatedly virtually for an infinite time.  As such, the resulting water is usually ultra pure.  A mixed-bed can produce over 10M-ohm water,  whereas a dual-bed typically produces only a 50K-100K water.  Other variation is a combination of dual-bed and mixed-bed, where the mixed-bed is used as a polisher.  Such set-up can produce ultra pure water up to 18 mega-ohm/cm. 
Recycling the DI water
If the flow through the DI is temporarily halted, the water in the vessels will begin to take back the impure ions from the resin.   When the flow is resumed, the first few gallons will usually be below specs.  To prevent this, the DI water can be recycled back at a fraction of the service flow rate to the front of the DI to keep the water moving and the purity up.  
See Fig. 3  
This method is also used where the DI water is stored after the deionizer.   If the water is not used immediately, it will absorb gasses like CO2 and other air-borne particles, reducing the purity of the water.  To prevent the water from falling below specs, it can also be recycled back to front of the DI for similar benefits.
Eventually the resins will expire, as all the hydrogen and/or hydroxide ions are expelled, and all the exchange sites are filled with impure ions.   Thereafter, the DI will no longer remove any subsequent impure ions, allowing them to leak through, reducing water purity.   
The most common indicator of a DI expiration is a sudden and significant drop in the  resistivity of the DI water.  
There are other indicators. When the cationic resin expires, the first ion to leak through will be Sodium (Na+).  When the anionic resin expires, the first ions to leak through will be Silica (SiO2) and/or carbonate alkalinity (CO3-2). 
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 mixedbed, 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 with less than one ppm TDS 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 can be highly complex; systems engineering is the best approach to solve these complex problems.
For a discussion on the basic principles of ion exchange, see Technical Bulletin on “Principles of Ion Exchange Softening.”
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