Deionization (DI) / Ion Exchange
Deionization (DI) is a water filtration process whereby total dissolved solids (TDS) are removed from water through ion exchange. In simple terms, by controlling the electric charge of ions in the water, it is possible to remove the TDS. Much like a positively charged magnet will attract a negatively charged magnet (and vice-versa), DI resins attract non-water ions and replace them with water ions, leaving a more pure water form.
The process of deionization uses two resins that are opposite in charges – the cationic (negative) and the anionic (positive). The cationic resin is typically made from styrene containing negatively charged sulfonic acid groups, and will be pre-charged with hydrogen ions. This resin will attract the positively charged ions in the water (Ca++, Mg++, Na+, etc.) and releases an equivalent amount of hydrogen (H+) ions.
Like the cationic, the anionic resin is also made from styrene, but contains positively charged quaternary ammonium groups, and will be pre-charged with hydroxide ions. This resin will attract the negatively charged ions (HCO3-, Cl-, SO4--, etc.) and releases an equivalent amount of hydroxide (OH-). The hydrogen and hydroxide ions then combine to form water. (H+ + OH- = HOH or H2O.)
The two resins can be ionized at a certain level, usually weak or strong. The cationic can be either a strong or weak acid. Likewise, the anionic resin can be either a strong or weak base. A weaker ionization will exchange only the weak ions, providing for a greater capacity (meaning longer filter cartridge life), while a stronger ionization will provide a higher degree of ion exchange, but at the cost of reduced capacity (shorter filter cartridge life).
As with many other types of filtration or purification processes, a single deionization cycle may not remove all the TDS. Some of the ions will not be attracted by the resins, so running the DI water through a second cycle will provide for additional purification. In other words, the more you run the deionized water through the more pure the yielding water will be. However, it is important to test the filtered water with a TDS meter after each cycle to determine the effectiveness of your DI system. Compared with other filtration and purification methods, DI has a relatively short filter cartridge life and once it begins to fail, the TDS level of the purified will “rise” exponentially.
Water Treatment Fundamentals, Seventh Edition, Joseph F. Harrison
Water Technology Magazine (http://www.watertechnoline.com/)
Our Education Center is your resource for all things water. This knowledge base includes numerous articles on water, water quality and water filtration.