Ion Exchange
The ion-exchange process percolates water through spherical, porous bead resin materials (ion-exchange resins). Ions in the water are exchanged for other ions fixed to the beads. The two most common ion-exchange methods are softening and deionization. Softening is used primarily as a pretreatment method to reduce water hardness prior to reverse osmosis (RO) processing. The softeners contain beads that exchange two sodium ions for every calcium or magnesium ion removed from “softened” water.
Benefits & Limitations
Benefits
- Removes dissolved inorganics (ions) effectively, allowing resistivity levels above 18.0 MΩ•cm @ 25 °C to be reached (corresponding roughly to less than 1 ppb total ionic contamination in water).
- Regenerable (by acid and bases in “service deionization” or by electrodeionization).
- Relatively inexpensive initial capital investment.
Limitations
- Limited capacity: once all ion binding sites are occupied, ions are no longer retained (except when operating in an electodeionization process).
- Does not effectively remove organics, particles, pyrogens or bacteria.
- Chemically regenerated DI beds can generate organics and particles.
- Single use, “virgin” resins require good pretreated water quality to be economically efficient.
Figure 1: Deionization
Deionization (DI) beads exchange either hydrogen ions for cations, or hydroxyl ions for anions. The cation-exchange resins, which are made of polystyrene chains cross-linked by divinylbenzene with covalently bound sulfonic acid groups, will exchange a hydrogen ion for any cations they encounter (e.g., Na+, Ca++, Al+++). Similarly, the anion-exchange resins, which are made of polystyrene polymer chains with covalently bound quaternary ammonium groups, will exchange a hydroxyl for any anions (e.g., Cl-, NO3-, SO4--). The hydrogen ion from the cation exchanger unites with the hydroxyl ion of the anion exchanger to form pure water.
These resins may be packaged in separate bed exchangers with separate units for the cation and anion exchange beds. Or, they may be packaged in mixed bed exchangers containing a mixture of both types of resins. This last configuration enables more efficient ion removal and provides higher water resistivity values.
The resin may be “regenerated” by strong acid and bases once it has exchanged all its hydrogen and / or hydroxyl ions for charged contaminants in the water. This regeneration reverses the purification process, replacing the contaminants bound to the DI resins with hydrogen and hydroxyl ions. However, this is a harsh chemical process that may damage the polymer chains constituting the beads, leading to contamination of the resin by organics and particulates, and creating an issue in the production of high purity water.
For the production of high purity water, two solutions exist:
- Use “virgin” mixed bed ion-exchange resin packs containing monosphere beads with low TOC (such as Millipore Jetpore ion-exchange resin) only once and discard the pack after usage. This is an economically acceptable process provided that these packs are fed by pretreated water of good quality to limit the replacement frequency. A good pretreatment should remove not only the bulk of ions to limit the burden of ionic contaminants reaching the resin pack, but also organics, particulates and colloids. This is required to prevent the build-up on the resin beads of a coating preventing the ions from accessing the ionic binding sites located mainly inside the beads’ porous structure.
- Perform the regeneration of the ion-exchange resins using a gentle and continuous procedure such as electrodeionization, in order to avoid damaging the ion-exchange resin beads and consequently generating contaminants. This process was developed by Merck in the 1980s.
Deionization can be an important component of a total water purification system when used in combination with other methods such as RO, filtration and carbon adsorption. DI systems effectively remove ions, but they do not effectively remove most organics and microorganisms. Microorganisms can attach to the resins, providing a culture media for bacteria growth and subsequent pyrogen generation over the long run. The benefits and limitations of this technology are summarized below.
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