Deionized water treatment has many purposes in the industry from semi-conductor fabrication, the manufacturing of pharmaceuticals & cosmetics, and even marine or aquarium enthusiasts. Deionized water is an ultra-pure form of water with little to no impurities that will minimize any undesired reactions when mixed with other products. A special mixture or anion and cation polymer resin is used to product deionized otherwise known as demineralized water. This mixture of polymers is called mixed bed DI resin.
The most common example to explain how mixed bed DI resin works is when it interacts with total dissolved solids (aka TDS). An example of TDS is a salt has an ionic bond between Sodium (Na) and Chloride (Cl). This ionic bond is broken down when it comes in contact with water. Water dissociates Sodium and Chloride into charged particles as Sodium (Na+) and Chloride (Cl-). Water runs through a mixed bed DI, then anion and cation resin comes into play but releasing H+ & OH- which forms water in exchange for the Na+ & Cl- . Of course, this works with more than just regular table salt. Ion exchange can occur anytime there is a charged particle in the water – not just salt. Removal will occur for Magnesium, Calcium, Phosphates, Sulfates, Silica, etc. As long as there is a charge, than mixed bed DI resin will remove it.
Mixed bed DI resin has a short life if it is used by itself without any pretreatment. Deionization resin should be combined with reverse osmosis as a post treatment or at a bare minimum with sediment and chlorine removal as this will hinder the life of the filter cartridge or media. If reverse osmosis is used, the ro system will do the major of the TDS removal while the DI filter will be used as a polisher where it removes the rest of the TDS. This will extend the life of the DI media and still give you ultrapure water.
On the internet, there has been debate whether or not water filtered through DI media is unsafe to drink. The answer is “yes” and “no”. Food grade DI media is safe to drink because it releases Hydrogen and Hydroxide ions which is essentially water. Depending on the type, non-food grade media may release Lithium and Hydroxide ions in exchange for TDS. Lithium Hydroxide is hazardous to our health and should not be consumed.
For aquarium enthusiasts, this means the removal of Phosphates, Silica, or Sulfates in the water that can grow algae; for semi-conductor manufacturers, it means getting up to 18 mega ohm-cm resistivity and little to no unwanted reactions; it means different things to different people but the end result is the same – ultrapure deionized water.