Ion Exchange

Dissolved Contaminants in Water

Ion exchange (IE) is a water treatment method used to remove unwanted dissolved impurities from water and other liquid solutions. The process by which this is accomplished is through the exchange of ions from the contaminant with the ions from a desirable substance. These ions contain a positive electrical charge called a cation or a negative charge called an anion. The ions exchanged must be have the same type (+ or -) of electrical charge in order to maintain the neutral charge of the water.

Ions are typically formed when a contaminant dissolves in water. When dissolved substances are in the water it will still appear crystal clear. Solid substances can be removed from the water by a variety of filtering methodologies. These solid particles must first be removed from the water before replacing the dissolved ones through ion exchange.

Water Softening using Ion Exchange

A common use of ion exchange is in softening of water. Hard water primarily contains dissolved calcium and magnesium ions. These ions can precipitate and cause scaling in water pipes and plant boilers and if not removed can lead to system damage.

To soften water, we exchange hard cation ions for sodium (Na+) ions that will not lead to scale formation. The process involves exchange resin beads charged with sodium.

2 RNa + Ca++ -> R2Ca + 2 Na+

Each calcium or magnesium ion entering the resin bead is exchanged with two sodium ions. The end result is a replacement of hard ions by sodium ions although the salinity of the water remains unchanged.

Demineralization

Demineralization by ion exchange is a two-step process involving treatment with both cation and anion exchange resins. Dissolved cations are replaced with hydrogen (H+) ions and dissolved anions with hydroxide (OH-) ions. The new cations and anions recombine to form water molecules.

H+ + OH- -> HOH -> H2O

The net result is a complete removal of ionic contaminants, which attach to the exchange resins while the water becomes demineralized.