Deionized (DI) water is water that has been processed to remove virtually all dissolved mineral ions, such as salts, calcium, and magnesium. These ions are natural components of tap water, but they interfere with numerous industrial and scientific processes. Deionization is a highly effective method for achieving high-purity water quality necessary for applications where trace amounts of conductive minerals can cause problems.
The Fundamental Science of Ion Exchange
The removal of dissolved minerals relies on the chemical process of ion exchange, which targets the positively and negatively charged particles present in water. Water’s ability to conduct electricity is directly related to the concentration of these charged ions, which act as carriers for an electrical current. Eliminating these ions produces water with very low conductivity, often measured as high electrical resistivity.
The mechanism utilizes small, porous plastic beads called ion exchange resins, which are manufactured to have charged functional groups on their surface. Cation resins have a negative charge, attracting positively charged ions (cations) like sodium and calcium. Anion resins have a positive charge, attracting negatively charged ions (anions) such as chloride and sulfate.
During deionization, the cation resin exchanges its bound hydrogen ions (\(\text{H}^+\)) for the unwanted positive ions in the water. Simultaneously, the anion resin exchanges its bound hydroxyl ions (\(\text{OH}^-\)) for the unwanted negative ions. The released hydrogen and hydroxyl ions immediately combine to form a neutral water molecule (\(\text{H}_2\text{O}\)), replacing the mineral impurities with pure water.
Practical Methods Using Ion Exchange Resins
The deionization process is implemented using columns or tanks packed with ion exchange resins. The system design determines the resulting water purity and the volume that can be processed. Two primary configurations are used: two-bed and mixed-bed systems.
A two-bed deionizer utilizes two separate vessels, with water passing first through the cation resin column and then through the anion resin column. This configuration is more economical for treating large volumes of water but results in lower purity due to a small “leakage” of ions. The water leaving the first tank is slightly acidic from the displaced hydrogen ions, which is neutralized in the second tank.
For applications demanding the highest purity, a mixed-bed deionizer is employed, combining both cation and anion resins within a single vessel. This intimate mixing forces the water to encounter both resin types repeatedly, resulting in a more thorough exchange and “polishing” the water to maximum resistivity. Mixed-bed systems achieve ultrapure water quality, but separating and regenerating the exhausted resins is more complex than with the two-bed system. Regeneration involves flushing the resin beds with strong acid (for cation resin) and strong base (for anion resin) to strip away accumulated mineral ions and restore the supply of \(\text{H}^+\) and \(\text{OH}^-\) ions.
Common Applications for Deionized Water
The purity of deionized water makes it indispensable across numerous industries where dissolved ions could cause interference or damage. Its primary use is preventing scale buildup and eliminating electrical conductivity. Mineral deposits from untreated water can foul heat transfer surfaces in boilers and cooling systems, reducing efficiency and leading to corrosion.
DI water is used across several sectors:
- In electronics manufacturing, especially for semiconductor and circuit board production, for rinsing components to prevent mineral residue from interfering with delicate circuits.
- For laboratory testing and pharmaceutical production, where the absence of charged contaminants ensures accurate chemical reactions and prevents unwanted interactions in medication formulations.
- In automotive applications, such as in lead-acid batteries and engine cooling systems, where tap water minerals would accelerate corrosion and deposit scale.
- In final rinsing stages for car washes and glass cleaning, capitalizing on its lack of dissolved solids to prevent water spots and streaks from forming upon drying.
Deionized Water Versus Other Purified Types
Deionized water is often compared with other forms of purified water, such as distilled water and reverse osmosis (RO) water, but each possesses distinct purity characteristics. Distilled water is created through boiling and condensation, which leaves non-volatile contaminants behind. While this method removes ions and most minerals, it is slow, energy-intensive, and may not remove volatile organic compounds effectively.
Reverse Osmosis (RO) water is produced by forcing water through a semipermeable membrane under pressure, blocking larger particles and most dissolved solids. RO is highly effective, achieving 90 to 99 percent purity by removing contaminants like bacteria and certain chemicals, but it does not remove all ions. This final purity is insufficient for many high-tech applications.
Deionization, in contrast, is designed to remove the remaining charged ions, which is why it is often used as a “polishing” step after RO pretreatment. Using RO first significantly reduces the total dissolved solids (TDS) load, extending the lifespan of the DI resins. While RO water is cleaner in terms of microorganisms, DI water achieves a much higher electrical resistivity, making it the purest form necessary for ultra-critical ionic removal applications.