The first step of ion exchange is identical to the first stage of adsorption – movement of contaminant molecules from the bulk of the solution to the surface of the media by diffusion and Brownian motion-but the similarities end there. While adsorption almost always involves only complete, electrically neutral (unionized) molecules, ion exchange occurs only with ions-materials having a + or – electrical charge. And while the second stage of adsorption (capturing the contaminant and removing it from circulation) depends only on the chemical affinity between the contaminant and the media, the capturing step for ion exchange depends mostly on the electrical charges. And finally, after capture of a contaminant, ion exchange is much more easily reversed than adsorption, enabling many cycles of use and regeneration economically and conveniently.
Ion exchange depends on the existence of special media with inherent, permanent electrical charges that attract and hold ions with the opposite charge. Naturally-occurring and synthetic inorganic minerals with this property are called zeolites, which are special crystalline compounds of silicate and aluminate. Those chemical groups are both anions (ions with a negative electrical charge), and therefore zeolites can exchange only cations (ions with a positive electrical charge) such as Na+ or Ca+2. The ion exchange capacity of zeolites is not very great, so synthetic organic ion exchange resins were developed to make water softeners more marketable. Organic ion exchange resins can be formulated with either anionic or cationic ingredients of many types, yielding products with nearly every conceivable functionality and strength. Thus, there are resins formulated specifically to exchange nitrate ion preferentially, or fluoride ion, or uranium and plutonium from nuclear wastes, and so on. (These special resins are rare, and therefore very expensive.) Ion exchange is most efficient when the media is housed in cylindrical beds and the water flows downward through it. The exchange of ions is very rapid, and there is a zone or band of activity that slowly moves down the bed. Above the zone, all of the active sites are exhausted, and below the zone, all are still in completely regenerated form. The thickness or height of the exchange zone is determined by flow rate: the slower the flow, the narrower the zone. Just as the “unit (area) flow rate” in GPM/sq.ft. or L/min/m2 is the important factor for the hydraulic mechanical filtration, the “unit (volume) flow rate” in GPM/cu.ft. or L/min/m3 is the relevant parameter for the chemical performance of media used for adsorption, ion exchange, oxidation, etc. The usual unitary flow rate for ion exchange systems is about 2 GPM/cu.ft. (0.214 L/min/m3 ) or 16 bed volumes per hour.