Water – What are ions and ion exchange all about?

Any discussion of water quality and the purification of water will eventually gravitate to a discussion of ions.  Adding ions and/or removing ions using what is called an ion exchange process is a big and essential part of most water treatment.  Let’s take a few minutes to understand basically what these important things called ions are and how they work.

An ion, in terms that I can understand, is an atom or molecule that has lost or gained one or more electron.  Losing or gaining an electron results in the atom or molecule having an electrical charge.  Since electrons have a negative charge, an atom or molecule that has lost electron(s) has a positive charge.  Ions with a positive charge are called “cations.”  An atom or molecule that has gained electron(s) has a negative charge and is called an “anion.”  The magnitude of the charge depends on how many electrons have been gained or lost.  As is usually the case in nature, like charged particles repel one another while unlike charged particles are attracted to each other.  An ion that has been formed by losing 2 or 3 electrons is more positively charged than an ion that has been formed by losing only 1 ion.  Although all of these ions are positively charged relative to no charge at all, the degree of charge is what allows “ion exchange” to work.

Ion exchange resins found in water softeners and other ion exchange devices consist of small beads of plastic which have been treated to attract cations or anions.  In the beginning, the resin beads, which are contained in a pressure vessel with appropriate filters to contain them, are loaded up or “regenerated” using a concentrated liquid solution of ions that are not strongly charged.  When water containing ions that are more strongly charged circulates through the resin, the more strongly charged ions are more attracted to the resin beads than those with a lesser charge and replace the weakly charged ions that were deposited by the regeneration procedure.  The displaced ions are carried away in the water being processed.  This process continues until all of the ions initially attracted to the resin beads in the regeneration process are replaced with ions with a stronger affinity for the resin.  At this point, the resin is called “spent” as in can no longer remove additional ions.

The container of spent resin is replaced with a another which contains freshly regenerated resin.  The container of spent resin is regenerated either on site or sent to a facility which specializes in resin regeneration.  Regeneration is accomplished by flowing a highly concentrated solution containing the ions that were, at the beginning, only mildly attracted to the resin through the resin.  The ions in high concentration in the regenerating liquid manage to displace those more highly charged ions collected from the water with ions with a lower charge.  This is mainly due to the high concentration of replacement ions but, in some cases, is also aided by a sparging effect brought about by specific chemicals in the regenerating liquid.

A “water softener” is the ion exchange device with which most of us are at least mildly familiar.  In the water softener, Sodium ions (1 electron short of a neutral charge) are displaced from the resin by Calcium and Magnesium ions (both 2 electrons short of a neutral charge) which are more strongly attracted to the resin than are the Sodium ions.  When the resin is spent, a concentrated solution of Sodium Chloride (simple table salt) is used to displace the collected Calcium and Magnesium ions from the resin making it again ready to collect more Calcium and Magnesium ions in its next use.  The resin is re-used indefinitely in this manner.

The above, although elementary, is intended to familiarize the reader with the concept of ions and ion exchange in the water purification process.  In reality, not all ion exchange processes are quite as simple as the water softener described above but the principles are the same.  Upcoming blogs will explore ion exchange processes in somewhat more detail.

–  FJF  –

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