By Greg Reyneke, CWS-VI
For eons, humanity observed that certain special sands and rocks reacted in unique ways when exposed to water. Man’s innate dissatisfaction with the status quo and pure dumb luck enabled the ion exchange industry to be born. Ion exchange is not a human invention, but humans have harnessed natural materials and developed synthetic materials that are able to cost-effectively manipulate ions in aqueous solutions. Most water treatment dealers are familiar with the concept of salt-based ion exchange softening and possibly even salt-based anion exchange to address the issues of nitrates, sulfates and other similar contaminants, but there is so much more to understand than “hard water in/soft water out.”
While many people talk about ion exchange, few actually understand the fundamentals. Simply stated, ion exchange is the reversible interchange of ions between a solid and a liquid in which there is no permanent change in the structure of the solid. Of course, an ion is an unbalanced atom or molecule that has a positive or negative charge resulting from this imbalance. Ions in water (aqueous ions) are what we work with in the field of water quality management. Aqueous ion exchange can involve anions or cations, depending on the contaminants to be addressed. Table 1 lists some common ions that are typically involved in water treatment applications.
The solid ion exchanger used in water treatment is referred to as a zeolite (or simply, resin) when discussing synthetic ion exchange media. Natural zeolites exhibit unique porous structures and contain compounds of minerals that allow for true ion exchange to occur within their structure. These zeolites have been used in commercial water treatment for almost two hundred years and until the 1930s, were the only way to consistently soften water. Natural zeolites exhibit many unique behaviors at the nano level and should not be overlooked in today’s water treatment applications, as we are only now discovering some of the benefits of these materials when properly deployed.
In the years leading up to World War II, scientists on both sides of the Atlantic experimented with various compounds to develop polymeric plastics critical to successfully prosecuting the war. One group of scientists experimented with phenolic plastics and developed the first synthetic ion exchanger. Since the phenolic exchanger didn’t contain a space-wasting mineral backbone, it immediately exhibited a massive increase in ion exchange capacity per volume, as compared to the existing zeolite materials. Today’s ion exchange resins are highly advanced polymers, manufactured for specific functionality and capacity requirements.
Anion resin is a positively charged media that can freely exchange associated anions based on differences in the selectivities of the anions. Anion resins can be sub-categorized into two types: strong-base anion (SBA) and weak-base anion (WBA).
Strong-base anion resin is most commonly deployed in chloride anion dealkalizers, nitrate reduction systems, deionizers and certain demineralization systems. When regenerated with sodium or potassium salt, the chloride ions will exchange with anions, such as bicarbonate, nitrate and sulfate anions. When regenerated with caustic soda, this resin can effectively remove both strong and weak acids from water. This resin usually receives its functional exchange capacity from quaternary ammonium groups.
Weak-base anion resin is deployed in dealkalizers and deionizers to remove mineral acids from water when silica removal is not required. When regenerated with soda ash, ammonia or caustic soda, this resin easily adsorbs hydrochloric, sulfuric and other strong acids. This resin usually receives its functional exchange capacity from tertiary amine groups.
Cation resin is a negatively charged media that can freely exchange associated cations based on differences in the selectivities of the cations. Cations can be sub-categorized into two types: strong-acid cation (SAC) and weak-acid cation (WAC) .
Strong-acid cation resin is what every water softener dealer is selling in some form. When regenerated with sodium or potassium salt, the ions inside the resin structure will effectively exchange for divalent cations, such as calcium and magnesium. When regenerated with hydrochloric or sulfuric acid, the resin can split neutral salts, converting them to their corresponding acidic compound. This resin usually receives its functional exchange capacity from functional sulfonic groups.
Weak-acid cation resin is most commonly deployed in dealkalization and desalination systems. This resin is also used in conjunction with strong-acid cation resins for deionization. When regenerated with acid, the resin will split alkaline salts, converting them to carbonic acid. This resin boasts extremely high regeneration efficiencies and usually receives its functional exchange capacity from carboxylic groups.
As entertaining as it would be to discuss the esoterica of saltsplitting, the primary focus of this series will be on ion exchange softening through the proper selection and deployment of strongacid cation ion exchange resins.
One of the biggest misconceptions in the water treatment industry is that all resin is the same. Even within a specific functional class, resins can be manufactured very differently and exhibit widely differing attributes of size, selectivity, porosity, kinetics and resistance to attrition.
In the next installment, we’ll examine softening resins, crosslinkage, structured polymer matrices, how to maximize your softening effectiveness and efficiency, and how to keep a resin bed clean and sanitary.
About the author
Greg Reyneke is the Managing Partner at Red Fox Advisors, a multidisciplinary consultancy focused on solving complex environmental problems.