The following excerpts are taken from Water Conditioning and Purification Magazine’s Feb. 2007 article – ‘The Basics of Ion Exchange and Water Chemistry’, by C. F. ‘Chubb’ Michaud, CEO and Technical Director of Systematix Company
Introduction: Look around: thousands of heavenly bodies in the night sky (comprising a mere fraction of one percent of the known universe), hundreds of cities, millions of houses filled with tens of millions of people. It is somewhat amazing to realize that all of it – every single thing is made up of only three components: electrons, protons and neutrons. Each grouping of these components forms a unique Structure we call an element. There are barely 100 naturally occurring elements here on Earth or in stellar space and their collective study is called chemistry. Each compound (water, air, steel, rubber) has its own chemistry. We can predict the properties of most things by studying the unique make up of their components. The chemistry of water is basic but, nonetheless, it is still chemistry. Some people shy away from trying to understand this subject because they feel it’s over their heads. Understanding the fundamentals of chemistry, however, is necessary in order to grasp the full breadth of how certain aspects of water filtration work – particularly ion exchange… The building blocks: In the worlds around us, there are barely 100 elements that occur naturally and, by definition, they are all separate and distinct from one another. Sodium, calcium, sulfur and oxygen are all elements. Elements are made up of a balanced number of positively and negatively charged particles called protons (+) and electrons (-), which, along with neutrons (which are neutral), form an atom of that element. The atom was first theorized by Democritus in the 5th century BC and derives from the Greek word for ‘un-cutable’. It is the smallest particle still identifiable as having the properties of the element. Modern science finally accepted this theory but not until the development of nuclear weapons in the 1940s…
Recap: By way of a quick summary, all matter is made up of elements (which are made up of electrons (–), protons (+) and neutrons (=). When elements combine, they form compounds. When compounds combine, they can form new compounds or mixtures. Acids and bases neutralize each other to form salts (and water). When salts are dissolved in water, they separate into cations (+) and anions (–) which carry charges (and are, therefore, attracted to other charged substances such as ion exchange resins). Water (H2O) does not ionize as H+ and O– –. Instead, it becomes H+ and OH– (called hydrogen and hydroxyl). These two ions are the backbone of the ion exchange demineralizer reaction (see Reaction 5). In reality, when salts are dissolved in water, they are no longer salts and they are no longer associated with their original partners. It is sort of like a junior high dance. It doesn’t matter who you came in with or with whom you go home, while you are on the dance floor, you’re on your own. Thus, if we add sodium carbonate and calcium chloride to water, we produce six different ions: Ca++ (calcium), Na+ (sodium), Cl– (chloride) and CO3 – (carbonate) plus the H+ and OH–. Each ion is free to associate with whatever it feels the most strongly attracted… Introduction to ion exchange: In the above case, the ‘unused’ part of the exchange reaction remains in the water and raises the total dissolved solids (TDS—that would be the Na and the Cl ions). But, what if we could anchor the reactive ions to a solid matrix so we didn’t have to filter them out and their partners would not go into solution to add to the TDS? That is exactly what ion exchange resin does. Ion exchange resins are plastic beads with a built-in reactive partner and an exchangeable ‘soluble’ partner. While the exchangeable partner is free to jump on and off the bead, the fixed reactive partner remains attached. In the case of a softening exchange resin, the partners are sodium (Na+ free to jump) an sulfonate (HSO3– which is fixed). When a calcium salt is introduced (as hard water), the calcium replaces the sodium on the bead and sodium replaces the calcium in the water (on a one-for-one equivalent basis) and there is no increase in TDS and no further filtration needed. The reason this reaction takes place is because the calcium from the hardness has a higher attraction (divalent) to the exchange resin than does the sodium (monovalent). This is known as ion selectivity and is the backbone of the ion exchange process. As shown by Reaction 5, certain elements or compounds in water can be made to undergo specific selec- tive reactions and these reactions are predictable to some degree according to the element’s family association in the Periodic Table. Divalent ions (those with a double positive charge) such as calcium and magnesium, will react with soap and cause ‘bathtub ring.’ They will also react with the carbonate ion to form scale in pipes and heaters. Although we could precipitate these salts with the addition of carbonate ions (see Reaction 6), we have no easy way to remove the resulting solid except in an industrial setting with large tankage. With ion exchange resins, only the exchangeable ion is soluble or free to move. The counter ion, which is the resin bead itself, is not. This makes the separation after the exchange very easy. In the case of a softener, the resin has an exchangeable Na+. The hardness (Ca++ and Mg++) combined with the resin forms a very strong bond. The water, minus the hardness, passes on through because the resin is retained in the exchange column. Sodium replaces the hardness on an equivalent basis. This means that it will take two sodium ions from the exchange bead to replace a single calcium or magnesium ion. However, on a ppm as CaCO3 basis, this is a one-for-one exchange with no change in TDS (more on this in Part 2)… Conclusion: The Periodic Table of the Elements places all elements into families that help us predict properties and determine similarities. We have shown that there is a preferred coupling of certain elements to form reactions (such as CaCO3 precipitation) that lead us to methods of removing those elements from water. This can be done either selectively (such as in softening) or completely (as in demineralization) utilizing ion exchange resins.
Common elements found in tap water
Aluminum, Calcium, Carbon, Chloride, Fluoride, Iron
Magnesium, Manganese, Nitrogen, Oxygen, Potassium
Silica, Sodium, Sulfur