Deionized water (DI water or demineralized water) is water that has had nearly all of its dissolved mineral salts and ions removed. Standard tap water contains various charged particles, such as calcium, sodium, chloride, and sulfate, which contribute to its electrical conductivity and hardness. For industrial and scientific applications, these ions act as impurities that can interfere with processes or cause damage, such as causing scale formation in boilers or inaccurate results in sensitive laboratory research. Deionization targets these specific charged impurities, unlike reverse osmosis, which filters larger particles, or distillation, which separates water from non-volatile substances using heat.
The Chemical Process of Ion Exchange
The foundation of a deionized water system is the chemical process of ion exchange, which uses specialized resin beads to capture and replace dissolved ions in the source water. These resin beads are small, porous polymers with chemically bonded exchange sites. Source water contains positively charged ions (cations) and negatively charged ions (anions).
The process employs two types of resins: cation resin and anion resin. Cation resin beads are pre-charged with hydrogen ions (\(\text{H}^+\)) and attract positively charged cations, such as calcium (\(\text{Ca}^{2+}\)) or sodium (\(\text{Na}^{+}\)). As water flows over the resin, these cation impurities are exchanged for the \(\text{H}^+\) ions, which are released into the water.
This newly acidic water then moves to the anion resin, which is pre-charged with hydroxide ions (\(\text{OH}^-\)). The anion resin attracts negatively charged anions, such as chloride (\(\text{Cl}^-\)) or sulfate (\(\text{SO}_4^{2-}\)), and releases \(\text{OH}^-\) ions in exchange. The released \(\text{H}^+\) and \(\text{OH}^-\) ions instantly combine to form a neutral water molecule (\(\text{H}_2\text{O}\)), completing the deionization process.
System Components and Operational Types
Deionization systems use the two resin types in distinct physical arrangements to achieve different levels of water purity. The two primary configurations are the two-bed system and the mixed-bed system. Both systems utilize tanks filled with cation resin and anion resin.
A two-bed system uses two separate tanks, with water flowing sequentially through the cation tank first and then the anion tank. The initial pass through the cation resin removes positive ions, leaving the water with a high concentration of \(\text{H}^+\) ions and a lower, acidic pH. The subsequent pass through the anion resin removes negative ions and adds \(\text{OH}^-\) ions, neutralizing the water. This design is preferred for treating large volumes of water and is cost-effective for moderate purity requirements.
The mixed-bed system combines both cation and anion resins into a single pressure vessel, creating a homogeneous blend. This intimate mixture of exchange sites acts like thousands of individual two-bed units arranged in series. This multi-stage contact allows for thorough ion removal, producing the highest purity water, often referred to as ultrapure. Mixed-bed systems are used for final polishing in applications requiring maximum purity, such as pharmaceutical or microelectronics manufacturing.
Monitoring Water Purity and System Renewal
The purity of deionized water is directly measured by its inability to conduct electricity, a metric tracked using either conductivity or resistivity. Pure water is an extremely poor conductor because the ionic salts that facilitate electrical flow have been removed. Resistivity is measured in megohms per centimeter (\(\text{M}\Omega\text{-cm}\)), while conductivity is measured in microsiemens per centimeter (\(\mu\text{S}/\text{cm}\)).
Maximum theoretical water purity is achieved at a resistivity of \(18.2 \text{M}\Omega\text{-cm}\), which corresponds to a conductivity of \(0.055 \mu\text{S}/\text{cm}\). Inline monitors continuously check these values, providing a real-time measure of the system’s performance. As the resin beads reach their capacity, they begin to leak ions back into the water, causing the conductivity to rise and signaling the need for intervention.
Once the resin’s exchange sites are exhausted, the system must undergo a chemical process called regeneration to restore its functionality. Regeneration involves stripping the captured impurity ions from the resin beads and replenishing the \(\text{H}^+\) and \(\text{OH}^-\) sites. The cation resin is typically regenerated using a strong acid, such as hydrochloric or sulfuric acid, to replace the captured cations with \(\text{H}^+\) ions.
Similarly, the anion resin is regenerated using a strong base, such as sodium hydroxide, to replace the captured anions with \(\text{OH}^-\) ions. This renewal process allows the system to be reused repeatedly, making the deionization system an effective long-term solution for producing high-purity water.