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Ion Exchange

Ion Exchange Treatment Whole-house (POE)

Ion exchange is a water treatment method in which unwanted or undesirable contaminants are removed and exchanged for a less objectionable substance of the same ion charge.  Ion exchange is either anion (negatively "-" charged ion) exchange or cation (positively "+" charged ion) exchange.  A typical water softener is one type of ion (anion) exchange treatment which uses a specific resin to reduce water hardness by replacing calcium and magnesium (hardness) with sodium.   Examples for treatment include reduction in inorganic chemicals such as:

Ion exchange resins are typically small (1-2 mm diameter) beads, usually white or yellowish, made from an organic polymer resin.  

Highly developed structure of pores on the surface of which are sites with easily trapped and released ions. The trapping of ions takes place only with simultaneous releasing of other ions; thus the process is called ion exchange.  

There are multiple different types of ion exchange resins that are made to selectively prefer one or several different types of ions.  

There are four main types of ion exchange resins differing in their function groups:

  • strongly acidic (sulfonic acid groups, eg. Sodium polystyrene sulfonate or (polyAMPS))  
  • strongly basic, (trimethylammonium groups, eg. polyAPTAC  
  • weakly acidic (carboxylic acid groups)  
  • weakly basic (amino groups, eg. Polyethylene amine)  

Other uses for specialized treatment include mixed bed resins and specialized resin types. Ion exchange resins are designed to remove specific ions but have preferential affinity to exchange some ion over others. Therefore knowing a more complete water chemistry makeup can help provide information to the water treatment professional on how to best design a system to improve the overall treatment.  

Examples of Selectivity of Ion Exchange Resins (in order of decreasing preference).  Ion Exchange resins do not prefer all ions equally.  

Strong acid cation resins (Sodium form)             Strong base anion resins (Chloride form) 
Cation    Anion
Radium   Uranium Dioxide Carbonate 


Lead   Selenate 
Strontium   Sulfate 
Copper    Hydrogen arsenate 
Calcium    Nitrate 
Zinc    Nitrite 
Iron    Chloride 
Magnesium    Hydrogen Carbonate 
Manganese    Fluoride 

*Note:  The anion exchange resin has a preferential affinity for sulfates over nitrates and also has a preferential affinity for nitrates over arsenic.  


For nitrate treatment, anion exchange is installed for whole-house treatment. They work much the same way a water softener does. Use an anion exchange resin that has a preferential affinity for nitrates to exchange chloride for nitrates. Note that when an anion exchange treatment system is used for nitrate reduction the total combined concentration of nitrates and sulfates in the water must be known. The resins used in nitrate removal systems have a preferential affinity for sulfate. This means that nitrates that have already been removed from the water will be re-released back into the drinking water in favor of sulfates when the resin has reached capacity. Also, in order to ensure the most efficient reduction of nitrates, a water softener should precede the anion exchange system to reduce the potential for fouling the nitrate anion exchange resin. For more information about nitrate treatment see


Arsenic removal can become more complicated depending on the type of arsenic being removed and may require an oxidation step prior to the ion exchange step. For more information about the specifics of arsenic removal from water see the links below.

Additional Resource

For more information about ion exchange treatment contact the Water Quality Association (WQA). The WQA is the national professional organization for water treatment dealers and has information on treatment of aesthetic water quality issues.

Page Updated 05/30/2018