By Mike Keller
Summary: Both positively and negatively charged ions can be removed using the proper ion exchange application. This introductory article represents a general review of these contaminants that fall in these categories and what resin is best used for their remediation. Proper recognition of their differences will go a long way to ensure effectiveness of an installed water softener.
Today’s water treatment challenges aren’t limited to iron and hardness removal. There are many other applications that an educated and trained water treatment professional can identify and treat by installing the proper ion exchange equipment.
In this article, we’ll be looking at applications that can be treated with ion exchange resin for potable water treatment. Ion exchange resins remove undesirable ions (charged particles) and replace them with more desirable ions. Water softeners remove hardness (calcium and magnesium) from water and replace the hardness with sodium or potassium. This exchange occurs due to a process called selectivity. In the case of a water softener, the resin reacts better with calcium and magnesium, to sodium or potassium. There are two primary factors that affect selectivity, valence and hydrated ionic radius. Calcium and magnesium, with a valence of 2 (divalent), will generally displace sodium or potassium, which have a valence of 1 (monovalent). When the valence is the same, the selectivity usually will go to the ion with the greater molecular weight. Calcium has a greater selectivity than magnesium for cation softening resin (see Figure 1).
In many applications, this reaction can be reversed by overwhelming the undesirable ion with the more desirable ion. Hardness can be displaced from a softening resin during the regeneration by introducing a high concentration of sodium chloride or potassium chloride. There will be applications where the ion selectivity is so great that regeneration will be difficult. There are excellent ion exchange reference books on this subject (see References).
When a contaminant is found in water, its charge type and valence must be identified. Some of the more high profile contaminants can be found in Figure 2.
Positively charged ions
Cation softening resins are used to remove these, which are mostly metals or metal compounds.
It can be found in municipally treated water. A compound called alum (aluminum sulfate) can be used as a coagulant to reduce suspended solids and organics in water. If the alum is overfed, soluble aluminum can find its way into the distribution system and ultimately at the tap.
A water softening resin will remove soluble aluminum very effectively. At low pH, aluminum has a valence of three positive charges (trivalent) so it has a high affinity to softening resin.*
This is a case where salt alone won’t remove the aluminum from the resin. Aluminum will accumulate on the softening resin, using up exchange sites, and will eventually cause softening capacity to decrease. A mild acid like citric or phosphoric acid can be used to strip the aluminum from the resin. An acid resin cleaner can produce a low pH in the treated water if proper steps aren’t taken before the unit is returned to service. The unit should be regenerated with a high salt dosage after the acid cleaning. Follow the resin cleaner instructions to help prevent pipe corrosion.
Barium is a naturally occurring divalent cation. It can be found in fossil fuel power plant emissions and is used by the mining industry as a lubricant. It’s used to make paints, bricks and glass. The U.S. Environmental Protection Agency (USEPA) has set a 2 milligrams per liter (mg/L) maximum contaminant level (MCL), which equates to 2 parts per million (ppm).
A water softening resin will remove soluble barium very effectively. Barium has a greater affinity to softening resin than calcium. There’s a concern when sulfate concentrations in the water or brine exceed 2 ppm. The barium can react with sulfate, resulting in barium sulfate precipitation. This precipitation can coat the resin causing the softening capacity to decrease. Barium sulfate precipitation is very difficult to dissolve and resin replacement must be considered when the decreased softening capacity is excessive.
It will generally appear in water as a corrosion product of pipes and soldier. Many older water distribution systems use lead piping. Lead solder was used in household plumbing. If the pH of the water is below 7, dissolved lead may be present in the water. The USEPA has set the lead MCL at 0.015 mg/L.
Lead is divalent and has a very high molecular weight, so it has a very high affinity to softening resin. Lead can also be found as a particulate at a pH greater than 7. This particulate can be submicron and very difficult to physically filter. Ion exchange resin isn’t manufactured to remove particulate matter; however, it will filter down to approximately 40 microns. Soluble lead can readily be removed by softening resin; however, due to its high affinity to softening resin, lead will not be stripped efficiently during regeneration. Even acid strips of the resin will be ineffective at removing the lead. The softening capacity will decrease as lead is loaded onto the resin. Lead is generally found in water at very low levels, so the decrease in softening capacity will typically occur over a long period of time.
Radium is a divalent metal and a radionuclide. It’s a decay product of uranium. The USEPA has set an MCL of 5 picocuries/L. Radium should not be confused with radon. Radon is also a decay product of uranium and radium; however, radon is a gas and is generally treated with activated carbon or aeration.
Radium has a high selectivity to softening resin. The affinity of radium to softening resin is much greater than calcium. If the softener is set up to regenerate based upon hardness, radium removal will be very effective. Salt will regenerate radium from the softening resin. Softening resin will not remove radon from water. Radon can be removed from water by aeration and some types of carbon.
Negatively charged ions
So far, we’ve discussed positively charged metals that a water softener can remove from water. There are also negatively charged contaminants in water that can be removed by an anion exchange resin. Anion resins work similar to cation softening resin except they pick up negatively charged ions from the water. Salt can be used to regenerate the anion resin. In this case, though, the anion resin is using the chloride (Cl) of the sodium chloride (NaCl) or potassium chloride (KCl) as the exchanger. The backwash flow rate will have to be reduced as compared to the softening resin, but otherwise the system works pretty much the same. The capacity of the anion resin is typically lower than the cation resin. Consult your resin manufacturer for specific applications.
It’s a hot topic currently under review by the USEPA. The current USEPA limit is 0.050 mg/L (50 ppb). The USEPA is proposing a lower limit, which will probably be in the range of 10 to 20 ppb and by the printing of this article may be fully implemented. Arsenic can be found naturally occurring in mineral deposits, it can be found in the emission products of fossil fuels and metal smelting process, and it’s used in wood preservatives and pesticides.
Arsenic in water can generally be found in two different forms—arsenite with a negative 3 valence and arsenate with a negative 5 valence. If no oxygen is present, arsenite will be predominant. If there is oxygen, arsenate will be predominant. Anion ion exchange will effectively remove arsenate. It will not be effective at removing arsenite. Any arsenite in the water must be oxidized to the arsenate form. The oxidation can be accomplished by treating the water with chlorine, ozone, manganese dioxide or permanganate regenerated manganese greensand. Once the arsenate is formed, a strong base anion resin will effectively remove it from water. Salt is used to regenerate the anion resin. Please note that when an anion resin is used, there will be a pH drop on the treated water for a portion of the service cycle. If neutralization is required, the water must be treated after the anion unit.
It’s a negatively charged ion that enters the water due to fertilizers, septic systems, feed lots and industrial pollution. Nitrates cause a condition in infants called methemoglobinemia or blue baby syndrome. Nitrate will inhibit the release of oxygen in the blood. Nitrates are frequently found in shallow wells and surface water sources. The USEPA has set the MCL for nitrate at 10 mg/L.
Nitrate is a monovalent anion. It will be removed from water with a strong base anion. With regular type I strong base anion resins—below a total dissolved solids (TDS) of about 2,000 parts per million (ppm)—nitrate has an apparent selectivity to standard anion below sulfate, which can cause a phenomenon called “dumping.” Dumping occurs when the resin becomes saturated with sulfate and additional sulfate ions begin to displace nitrate ions that have been previously removed by the resin. This nitrate, in combination with the influent nitrate, will cause the nitrate level in the treated water to be higher than the raw water.
Resins that are less selective for sulfate have been developed to reduce the chance of dumping from occurring in potable drinking water applications. Salt is used to regenerate the resin. The water analysis, to calculate run length and leakage for the system, must include the sulfate and nitrate concentrations, as well as chloride and alkalinity. Nitrate can be expressed as nitrate as nitrogen (NO3 as N) or nitrate as nitrate (NO3 as NO3). The MCL is expressed as nitrogen. Make sure you know how the nitrate is expressed on the water analysis, as this will impact on how the run length is calculated.
It’s frequently used to sequester iron. Sodium tripolyphosphate and hexametaphosphate are used by municipal water treatment facilities to keep iron in solution and prevent iron precipitation and staining. Polyphosphate will break down over time and due to high temperature. Hot water heaters will break down the iron complex allowing the iron to stain fixtures. Polyphosphate sequestered iron cannot be removed with a standard softening resin. The iron complex has a negative charge and a strong base anion has been effective in its removal.
Testing has indicated that a strong base acrylic anion, similar to what is used in many tannin removal applications, has generally the best removal properties—fish-like smell notwithstanding. Salt is used to regenerate the resin. An iron-reducing compound like sodium hydrosulfite and/or sodium bisulfite should be used to keep the anion resin from fouling with iron.
Sodium silicate can also be used as a sequesterant; however, this compound isn’t easily removed from water. A chloride form anion resin will not be effective.
Uranium is a naturally occurring metal that generally forms a negatively charged ion in water. Uranium is a radionuclide and the USEPA has set an MCL of 15 picocuries of gross alpha particle activity.
Uranium can be removed from water with a strong base anion resin regenerated with salt. The capacity of the anion for uranium is extremely high, so run lengths of over 10,000 bed volumes (1 BV = 8 gallons) can be achieved, depending upon TDS, uranium concentration and the mix of other ions present. It’s recommended the units be regenerated once a week to prevent the unit from becoming radioactive hot.
There you have it, a fundamental approach to some sticky problems using ion exchange. Keep in mind that there are other factors that influence selectivity and contact your resin supplier when questions arise.
* NOTE: Aluminum is an insoluble precipitant at neutral pH and a monovalent anion at high pH (>9). The precipitated form is removed by filtration but the percent of removal is variable and often inconsistent. Anionic aluminum is not removed by cation resin. For these reasons, removal of aluminum by softening resin is problematic and guarantees for low leakage of aluminum are sometimes difficult to ensure.
- Applebaum, Samue, Demineralization by Ion Exchange, Academic Press, 1968.
- Clifford, Dennis, “Computer Prediction of Arsenic Ion Exchange,” Journal of the AWWA, page 10.
- Clifford, Dennis, “Arsenic, Part 2 of 2: Removing Arsenic from Water—The Importance of pH, Background Contaminants & Oxidation,” WC&P, August 2000.
- Dimotsis, George, and Frank McGarvey, “Studies on the Effect of Polyphosphates on the ion exchange Process”, Given at 55th Annual International Water Conference, Pittsburgh, Pa., 1994.
- Hunt, Jim, “Arsenic—Dealer Develops Simple, Reliable Treatment Method,” WC&P, p. 104-107, October 1997.
- Jelinek T. Robert, and T. Sorg, “Operating a Small Full-Scale Ion Exchange System for Uranium Removal,” Journal AWWA, July 1988, pp. 79-83.
- Keller, Michael, “Basic Ion Exchange for Residential Water Treatment,” Sybron Chemicals Inc., 1996.
- Simon, George, Ion Exchange Training Manual, Van Nostrand Reinhold, New York, New York, 1991.
About the author
Mike Keller is a marketing specialist for Sybron Chemicals, an ion exchange resin producer that’s based in Birmingham, N.J., and is a subsidiary of Bayer AG. With more than a dozen years in water treatment, Keller has been with Sybron since 1988 and works in the Household Ion Exchange Division. He holds a bachelor’s degree in environmental science from California University of Pennsylvania. He can be reached at (800) 678-0020, (609) 894-8641 or email: http://[email protected].