An ion-exchange resin or ion-exchange polymer is a resin or polymer that acts as a medium for ion exchange. It is an insoluble matrix (or support structure) normally in the form of small (0.5–1 mm diameter) microbeads, usually white or yellowish, fabricated from an organic polymer substrate.
The beads are typically porous, providing a large surface area on and inside them. The trapping of ions occurs along with the accompanying release of other ions, and thus the process is called ion exchange. There are multiple types of ion-exchange resin. Most commercial resins are made of polystyrene sulfonate.
Types of resins:
Four main types of ion-exchange resins differ in their functional groups:
- strongly acidic, typically featuring sulfonic acid groups, e.g. sodium polystyrene sulfonate or polyAMPS,
- strongly basic, typically featuring quaternary amino groups, for example, trimethylammonium groups, e.g. polyAPTAC),
- weakly acidic, typically featuring carboxylic acid groups,
- weakly basic, typically featuring primary, secondary, and/or tertiary amino groups, e.g. polyethylene amine.
Strong Acid Cation Resins:
Strong acid resins are so named because their chemical behavior is similar to that of a strong acid. The resins are highly ionized in both the acid (R-SO3H) and salt (R-SO3Na) form of the sulfonic acid group. They can convert a metal salt to the corresponding acid by the reaction:
The hydrogen and sodium forms of strong acid resins are highly dissociated and the exchangeable Na+ and H+ are readily available for exchange over the entire pH range. Consequently, the exchange capacity of strong acid resins is independent of solution pH.
These resins would be used in the hydrogen form for complete deionization; they are used in the sodium form for water softening (calcium and magnesium removal). After exhaustion, the resin is converted back to the hydrogen form (regenerated) by contact with a strong acid solution, or the resin can be convened to the sodium form with a sodium chloride solution. For the above reaction, hydrochloric acid (HCl) regeneration would result in a concentrated nickel chloride (NiCl) solution.
Weak Acid Cation Basins:
In a weak acid resin the ionizable group is a carboxylic acid (COOH) as opposed to the sulfonic acid group (SO3H) used in strong acid resins. These resins behave similarly to weak organic acids that are weakly dissociated.
The degree of dissociation of a weak acid resin is strongly influenced by the solution pH. Consequently, resin capacity depends in part on solution pH. A typical weak acid resin has limited capacity below a pH of 6.0, making it unsuitable for deionizing acidic metal finishing wastewater.
Strong Base Anion Resins:
Like strong acid resins, strong base resins are highly ionized and can be used over the entire pH range. These resins are used in the hydroxide (OH) form for water deionization. They will react with anions in solution and can convert an acid solution to pure water:
Regeneration with concentrated sodium hydroxide (NaOH) converts the exhausted resin to the hydroxide form.
Weak Base Anion Resins:
Weak base resins are like weak acid resins in that the degree of ionization is strongly influenced by pH. Consequently, weak base resins exhibit minimum exchange capacity above a pH of 7.0. The weak base resin does not have a hydroxide ion form as does the strong base resin. These resins merely sorb strong acids, they cannot split salts.
Consequently regeneration needs only to neutralize the absorbed acid, it need not provide hydroxide ions. Less expensive weakly basic reagents such as ammonia (NH3) or sodium carbonate can be employed.
Applications of Ion-Exchange Resins:
Positively charged cation resins remove positively charged ionic water contaminants. Included in this category of resins are strong acid/strong cation (SAC) and weak acid/weak cation (WAC) resins.
SAC resin is effective for water softening, which removes hardness ions. It has been used in residential, commercial and industrial applications for more than 100 years. Like tiny magnets, SAC resin beads remove scale-forming calcium (Ca2+) and magnesium (Mg2+) ions by exchanging them for sodium ions. Hardness levels are reduced and sodium levels are increased.
Softening and dealkalization:
Weak acid cation (WAC) resin can remove hardness and alkalinity simultaneously. It also provides some degree of total dissolved solids (TDS) removal. Generally, WAC resin removes about 80 percent of the temporary hardness (hardness associated with dissolved bicarbonate minerals). TDS are reduced by about 17.1 parts per million (ppm) for each grain of hardness removed.
Since WAC resin exchanges hardness and alkalinity ions for hydrogen ions, the treated water will be acidic (or lower pH). The degree to which TDS are reduced and pH lowered largely depends on the incoming hardness levels.
Barium and radium removal:
Barium and radium, two divalent cations, are regulated by the U.S. Environmental Protection Agency (EPA) for National Primary Drinking Water Standards and can be removed by standard SAC resin. However, when regenerating the resin, efficiency is reduced because of the slow diffusion of their larger atomic mass deep into the resin matrix.
Special types of SAC resin with properties that enhance barium and radium reduction are commercially available and tested/certified by the National Science Foundation (NSF).
Negatively charged anion resins remove negatively charged ionic contaminants in water. Included in this category of resins are strong base/strong anion (SBA) and weak base/weak anion (WBA) resins. These anion resins can be used to remove the contaminants described in this section.
SBA resin can remove nitrate (NO3-). If the ratio of sulfate to NO3– concentration in the water is high, the resin must be regenerated early to avoid the sulfate anion acting as a regenerant and discharging NO3-. In situations with elevated sulfate concentrations, a selective SBA resin can be used as well.
SBA resins exist that selectively remove perchlorate (ClO4-). These resins can be single-use and/or regenerable.
In water, arsenic is related to arsenate, As(V) and arsenite, As(III). Only negatively charged arsenate (HAsO42-) anions can be removed using SBA resins. Arsenite (H3AsO3) is normally neutral in aqueous solution. Therefore, pre-oxidation is needed to convert As(III) to As(V) anion. Once this oxidation is complete, the residual must be removed before contacting the SBA resin.
Total organic carbon (TOC):
Total organic carbon (TOC) or naturally occurring organic matter can be oxidized by secondary chlorine disinfection and create DBPs, such as THMs and HAAs. These DBPs are suspect carcinogens and regulated by the EPA in drinking water. Municipal treatment plants sometimes remove TOC to limit the formation of DBPs. TOC is typically negatively charged and removed using SBA resin.
SBA resin can be used to remove uranium, which typically exists as anionic uranyl carbonate/sulfate complexes.
SAC and SBA resins employed in combination either individually or mixed together can be used to reduce minerals and TDS in water. Minerals in the water are exchanged with hydrogen cations (H+) and hydroxide anions (OH–) from the resin beads to form highly purified water (H2O).
SBA resin is used to ionically bind halogens as an antimicrobial disinfectant and is commercially available for use in different treatment applications.