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Ion Exchange in Water Purification

Rain rings on rain puddle

Water is the universal solvent that is able to dissolve a little bit of everything it touches. It could, therefore, be said that “pure” water does not exist. Even the most highly processed and purified water still contains trace
levels of gases, salts or minerals that have been leached from its containment vessel or absorbed from the surrounding air.

Salts are formed naturally when acidic rain water is neutralized by alkaline soil strata. Common salt (also called table salt) is sodium chloride (NaCl), but salt also refers to compounds containing potassium (K), calcium (Ca), iron (Fe), carbonates (CO3 2- ), nitrates (NO3 1-) and many other combinations of elements.
When salts dissolve in water, they separate into charged particles called ions. A positively charged particle is a cation and a negative ion is an anion. Calcium (Ca2+), magnesium (Mg2+) and sodium (Na1+) are examples of
cations. Chloride (Cl1-), nitrate (NO31-) and sulfate (SO42-) are examples of anions. The number of charges on an ion indicates the number of ‘counter-ions’ it will share in any reaction. Calcium ion, for instance requires two
‘counter-ions’ of chloride (CaCl2) but only one sulfate (CaSO4) because the final salt molecule must maintain a net neutral charge . Sulfate, for example, would require two sodium ions (Na2SO4) to balance the ionic charge for sodium sulfate.
All salts and ions may be considered “contaminants” for some specific water uses.

Not all salts (contaminants) are toxic or harmful and therefore do not have to be removed to make water safe to drink. On the other hand, perfectly safe drinking water may not be suitable for many industrial needs because they contain salts that may interfere with the industrial process. Hardness, the salts of calcium and magnesium bicarbonate may precipitate out on heating coils and in boilers and cause scale to form. Yet they are safe to consume in drinking water.
Higher pressure boilers may require very “pure” or demineralized water (water from which the minerals have been removed).

To remove unwanted ions (dissolved salts) from a water, a process called “ion exchange” is most often used. Ion Exchange was first observed in the mid-1800s but did not gain wide spread use until the development of
modern polymerization technologies developed in the 1940s. With ion exchange, the less desirable or unwanted ions can be exchanged for less objectionable ions to enhance the utility of the water for its intended

The ion exchange process is widely used for the removal of hardness (as in softening), heavy metals (as in waste treatment), radionuclides (as from power plants) and municipal water feeds (removal of nitrates, arsenic,
perchlorate, hexavalent-chrome and others). Since most ion exchange processes are reversible, the ion exchangers can be regenerated (put back into their original ‘salt form’) and used over and over.
Ion exchangers (IEx) are called resins which are plastic beads made from polystyrene or acrylic. They measure 0.5 to 1.0 mm in diameter and are utilized in columns or tanks with resin beds typically about three feet deep.
Water passes through the bed of resin where the exchange takes place and when exhausted, they are regenerated with salt, acid or caustic depending upon the process. IEx resins contain functional groups that dictate which ions can be exchanged. There are four primary types of ion exchangers: strong acid cation, strong base anion, weak acid cation, and weak base anion.


Strong Acid Cation resins (SAC) typically contain sulfonic acid functionality (R-SO3 1-) that can exchange positive ions, such as calcium and magnesium, for sodium or potassium. This resin is commonly used for softening and is
regenerated with salt (sodium or potassium chloride). Water hardness is generally associated with bi-carbonate ions (HCO3 1- and is called “temporary hardness”) or chloride or sulfate ions (called “permanent hardness”). It may also contain barium, strontium, iron, or other di-valent cations (M2+). Below ‘O’ is the ion exchange resin:
Service: Ca(HCO3)2 + O Na+ O Ca++ + NaHCO3 Regeneration: O Ca++ + NaCl  O Na+ + CaCl2
Depending on the level of regeneration, the hardness can be reduced to less than 1 ppm but typically the target is simply less than 17.1 ppm or one grain of hardness per gallon (gpg). One grain is equal to 17.1 ppm of hardness expressed as calcium carbonate. Softeners do not reduce the total dissolved solids (TDS) or dissolved mineral content of water.

Strong Base Anion (SBA) resins typically contain quaternary (four) amine functionality (R-N1+(CH3)) that can be
used to exchange negative ions such as nitrates (NO31-) or alkalinity (HCO31-) for chloride. These are ideal for
residential needs where certain anionic contaminants must be reduced. SBA resins can also be regenerated with
salt. In addition to nitrates and alkalinity, SBA resins can also exchange chloride for the ‘oxy-anions’ of
hexavalent-chrome, phosphate, uranium or sulfates. Therefore, most of the ‘contaminant’ anions are removed
in exchange for chloride ion. During regeneration, the resin is converted back to its original chloride form.
Service: NaNO3 + OCl- ONO3- + NaCl Regeneration: O NO3- + NaCl  O Cl- + NaNO3

For demineralization applications, Strong Acid Cation resins can also be regenerated with acid to convert the
resins to their hydrogen (H+ ) form. This form is commonly used to remove all cations as part of a deminealiizer
system. All cations including sodium are exchanged for H+ ion which converts all the anions to their acid-salt form (decationization). When exhausted, the resins are regenerated with acid to restore capacity. Service: NaCl +Na2SO4 + O H+  O Na+ + HCl + H2SO4 Regeneration: O Na+ + HCl  O H+ + NaCl The service reaction shows a mixture of sodium chloride and sodium sulfate in the feed water and both are converted to their respective acids as hydrochloric and sulfuric. The pH of decationized water will be very low (~pH=2-3) when fed with typical city water. CAUTION: This process is not used for treating residential or drinking water alone. Caution must be used in making choices for ion exchangers and their regenerants. It is recommended that an ion exchange representative should be consulted to avoid unexpected results.

Strong base resins can also be regenerated with sodium hydroxide (NaOH) and put into the OHform which removes the acid from a demineralizer SAC system. SACation Service: NaCl +Na2SO4 + O H+  O Na+ + HCl + H2SO4 SBAnion Service: HCl + H2SO4 + O OH- O Cl- + O SO4= + H+ + OH- (= H2O)
The two bed demineralizer removes up to 99.5% of the dissolved salt content from water and produces a slightly alkaline product at a pH of 9-9.5. When mixed together (mixed bed demineralization) they reduce the TDS by 99.99% or more and produce a product water that is so low in mineral content that the only way to measure it is with electrical conductivity measurements. The unit of measure is the micro-siemen, (S). One S (also referred to as micro-mho) equals approximately 0.5 ppm of dissolved solids.
The inverse of conductivity is resistivity and the unit of measure is the ohm. Very high resistivity is stated in megohms (one million ohms) and 1 S converts to 1 megohm (0.1S = 10 megohm). With very low TDS water,
H2O dissociates into H+ and OH- and contributes to conductivity. Therefore, zero conductivity is not possible.
The ultimate measure of demineralized water is generally accepted to be 18.3 megohm or 0.055 S.

Weak Acid Cation resins or WACs will have a carboxylic acid group with an exchangeable H+ but are very different in function to the SAC. WAC resins do not split neutral salts such as NaCl or Ca(NO3)2 but do react with
alkaline salts such as Ca(HCO3)2. Alkalinity includes HCO3 -, CO3 = and OH- . Reactivity is limited to the level of the alkalinity present which gives the WAC great utility for single bed softening AND dealkalizing. Only a portion of the cation load is converted to H+(equal to the level of the HCO3 – present. This converts the HCO3- to CO2 (carbon dioxide) which can then be off-gassed to soften, dealkalize AND reduce the TDS of the process stream.
Service: Ca(HCO3)2 + NaCl + O H+  O Ca++ + NaCl + H2O + CO2 Regeneration: O Ca++ + HCl  O H+ + CaCl2 If the total hardness is lower than the alkalinity, some of the sodium will be removed so that the sum of the hardness AND sodium removed will be equal to the level of alkalinity. If the total hardness is higher than the alkalinity, only the amount of hardness equal to the alkalinity will be removed. This is a preferred process used
by the carbonated beverage industry to reduce hardness, alkalinity and TDS, all of which can interfere with flavor and carbonation.

Weak Acid Cation resins can also be run in the Na+ form to provide softening and/or metals removal. However, they do not regenerate with salt. Regeneration is a two-step process that uses acid to convert back to the H+ form and then caustic (sodium hydroxide, NaOH) to convert to the Na+ form. WAC resins are highly selective for hardness and di-valent metals when run in Na+ form and have the ability to operate in much higher TDS
background than do SAC resins. They can be used to soften brackish water ranging up to about 10,000 ppm and are often used as metal scavengers in the plating industry and in small cartridges for lead and other heavy
metals removal from drinking water. Regeneration is a two-step process done by first regenerating back to the H+ form and then converting to the Na+ form with NaOH. Service: Ca(HCO3)2 + O N+  O Ca++ + 2NaHCO3 Regeneration 1: O Ca++ + HCl  O H+ + CaCl2 Regeneration 2: O H+ + NaOH  O Na+ + H2O

Weak Base Anion resins or WBAs are produced by a similar process to SBA except they have tertiary (three) amine functionality and operate in what is called the free base (FB) form. WBAs do not split neutral salts but only react with free acids by sorption. WBAs only react with strong acids such as HCl, HNO3 and H2SO4 so they do not remove CO2 (carbonic acid) or silica (silicic acid) which are weak acids. WBA resins are often used in two-bed
demineralizers to neutralize the acid produced by the SAC unit. Operations such as car wash spotless rinses find great utility in WBA because they have high capacity and very good chemical efficiency. Regeneration is with
sodium hydroxide, NaOH. Service: HCl + H2CO3 + silica + O FB  O HCl + H2CO3 + silica Service: H2CO3  H2O + CO2 + silica Regeneration: O HCl + NaOH  O FB + NaCl On drying, the H2CO3 evaporates leaving only silica behind. Before employing the SAC/WBA for spotless rinses, the user should check the level of silica. Levels much above 10-15 ppm may leave damaging residue on the surface. In this case, the use of SAC/SBA mixed beds are dictated for true spot free rinsing.
Weak Base Anion resins can also be run in the acid form to remove more strongly selective acid radicals from solution. An example of this is the use of WBAs run in HCl form to remove hexavalent-chrome from well water.
Regeneration is a two-step process done by first converting the resin back to the free base form and then converting that to the acid form. Service: Na2CrO4 + O HCl  O H2CrO4 +2 NaCl Regeneration 1: O H2CrO4 + NaOH  O FB + Na2CrO4 Regeneration 2: O FB + HCl  O HCl + H2O

Ion Exchange operations and the choice of resins to use are highly dependent on the water analysis, what has to be removed and to what level it has to be reduced. The primary driving force is selectivity. Selectivity is
determined by the ionic charge on the specific ion and the resin type. Typically, resins have a higher selectivity for multiply charged ions than for singly charged ions (see below). Also, this is highly influenced by the other ions in solution that might compete for the reactive sites. No resin is so highly selective that it is exclusive for a specific contaminant. Even if the selectivity is low compared to the selectivity of the ionic form the resin, ion exchange can still take place on a mass action basis. If the resin is very clean and free from the contaminant, it will react to pick up the
contaminant from solution. An example is with the use of a SBA to reduce fluoride. The Fion is below the selectivity of the Clion on the resin and even though the equilibrium may be unfavorable, the SBA resin can be
used to remove F- from the feed. Selectivity dictates that the resin will absorb all other anions as well as the Fso the capacity is greatly reduced. In general, selectivity for an ion will increase with its molecular weight and
ionic charge. For SAC one can expect Ra2+>Ba2+>Sr2+>Ca2+>Mg2+ and Al3+>Mg2+>Na+ . For SBA one can expect I1->Br1->Cl1->F1- and SO4
In order to work with ion exchange systems and design, it is very important to have a complete and accurate water analysis. The water analysis is generally presented in units of weight per unit volume such as ppm or ppb
(mg/l or g/l). Labs differ in their reporting so one must not confuse the data. The local lab that deals with agricultural issues may report only those constituents that affect plants rather than people. In addition, reports
may present data based on how it was generated. For instance the nitrate content as NO3-N meaning that the analysis is only telling you the nitrogen (N) content and not the nitrate (NO3) value. Metals are often reported as
ppb (parts per billion) rather than ppm (parts per million). Hardness and alkalinity values are frequently reported in units as ppm “as CaCO3” even though the sample may contain neither calcium nor carbonate. The
unit “calcium carbonate” is not a measure of hardness contained but is a unit of measure based on the number of atomic sites it represents. With ion exchange, we have to work with the number of ions rather than with
milligrams of contaminant so there is a need to be familiar with the conversion process.

For further reading:
Kunin, Robert, Ion Exchange Resins, Krieger Publishing Co, Huntington, NY, 1972 Applebaum, Samuel B., Demineralization by Ion Exchange, Academic Press, NY, NY 1968 Owens, Dean L., Practical Principles of Ion Exchange, Tall Oaks Pub, Littleton, CO 1995 Water Quality Association, Deionization Basics, WQA, Lisle, IL 1996

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