Deionization (DI) is a water purification process that removes dissolved mineral salts from water. It targets and removes charged particles, known as ions, which are the primary conductive impurities. The goal is to achieve extremely high-purity water, measured by its electrical resistivity. Pure water resists the flow of electricity, and the presence of ions lowers this resistance. Ultra-pure water approaches the theoretical maximum resistivity of \(18.2 \text{M}\Omega\cdot\text{cm}\) (megohm-centimeters) at \(25^\circ\text{C}\).
Understanding the Targets: What are Ions?
Ions are atoms or molecules that carry a net electrical charge. These dissolved solids are classified into two groups: cations and anions. Cations are positively charged ions, typically originating from metals, including calcium (\(\text{Ca}^{2+}\)), magnesium (\(\text{Mg}^{2+}\)), and sodium (\(\text{Na}^{+}\)). Anions are negatively charged ions, such as chloride (\(\text{Cl}^{-}\)), sulfate (\(\text{SO}_{4}^{2-}\)), and bicarbonate (\(\text{HCO}_{3}^{-}\)). The presence of these charged particles allows water to conduct electricity, and they can lead to issues like scale formation or interference with laboratory analysis. Deionization is necessary because simple filtration methods do not effectively remove these dissolved ions.
The Principle of Operation: Ion Exchange Resins
The core of a deionizer is the use of synthetic ion exchange resins, which are small, porous polymer beads. These beads have fixed functional groups that are chemically prepared to exchange their own ions for the contaminant ions in the water. The process is a chemical substitution, not a physical filtration, where an undesirable ion is swapped for a more acceptable one.
Two distinct types of resins are required for complete deionization. The cation exchange resin is charged with a hydrogen ion (\(\text{H}^{+}\)). As water flows through the resin bed, the cation resin captures positively charged contaminant ions, such as \(\text{Na}^{+}\) or \(\text{Ca}^{2+}\), and releases an equivalent amount of \(\text{H}^{+}\) into the water. This removes the metallic impurities from the water stream.
The second type is the anion exchange resin, which is pre-loaded with a hydroxyl ion (\(\text{OH}^{-}\)). This resin has a positive functional group that attracts and captures negatively charged contaminant ions like \(\text{Cl}^{-}\) or \(\text{SO}_{4}^{2-}\). In exchange, the anion resin releases a hydroxyl ion (\(\text{OH}^{-}\)) into the water.
The final step is the reaction of the exchanged ions. The hydrogen ions (\(\text{H}^{+}\)) released by the cation resin combine instantly with the hydroxyl ions (\(\text{OH}^{-}\)) released by the anion resin. This combination forms a pure water molecule (\(\text{H}_{2}\text{O}\)), neutralizing the acidity and basicity introduced by the resins and producing deionized water.
System Configurations: Separate vs. Mixed Beds
The physical arrangement of the cation and anion resins determines the final purity level of the treated water.
Separate-Bed Configuration
A separate-bed configuration, often called a dual-bed system, employs two distinct vessels. Water flows sequentially, first through the cation exchange vessel and then into the anion exchange vessel. This two-stage approach is simpler to operate and regenerate, making it a cost-effective choice for moderate purity requirements. However, the separate steps allow for ion “slippage,” resulting in a lower overall resistivity.
Mixed-Bed Configuration
For applications demanding the highest quality water, a mixed-bed deionizer is used. In this configuration, the cation and anion resins are thoroughly intermixed within a single pressure vessel. This mixing simulates thousands of sequential ion exchange steps as the water passes through the bed. The intimate contact ensures that ions released by one resin are immediately captured by the neighboring opposite resin. This continuous, simultaneous exchange mechanism produces ultra-pure water with a resistivity that can reach the maximum \(18.2 \text{M}\Omega\cdot\text{cm}\).
Sustaining the Process: Resin Exhaustion and Regeneration
The ion exchange process is finite because the resin beads have a limited number of exchange sites. Over time, contaminant ions gradually replace all the available \(\text{H}^{+}\) and \(\text{OH}^{-}\) ions loaded onto the resin. Once all sites are occupied, the resin is exhausted and can no longer purify the water; instead, it begins to release the captured contaminant ions, a phenomenon known as breakthrough.
The deionizer must then be regenerated to restore its capacity. Regeneration involves reversing the chemical reactions by flushing the resin with highly concentrated solutions of acid and base. A strong acid, such as hydrochloric acid, is used to strip the captured cations from the cation resin and reload it with \(\text{H}^{+}\) ions. Similarly, a strong base, typically caustic soda (sodium hydroxide), is applied to the anion resin to displace the captured anions and replenish the \(\text{OH}^{-}\) sites. After the chemical regeneration, the resin is thoroughly rinsed to remove excess regenerant chemicals, preparing the deionizer for another service cycle.