Water Treatment via Ion-exchange Resin
An ion-exchange resin or ion-exchange polymer is an insoluble matrix (or support structure) normally in the form of small (0.5-1 mm diameter) beads, usually white or yellowish, fabricated from an organic polymer substrate. The beads are typically porous, providing a high surface area. The trapping of ions occurs with concomitant releasing of other ions; thus, the process is called ion-exchange. There are multiple types of ion-exchange resin. Most commercial resins are made of polystyrene sulfonate.
Ion-exchange resins are widely used in different separation, purification, and decontamination processes. The most common examples are water softening and water purification. In many cases, ion-exchange resins were introduced in such processes as a more flexible alternative to the use of natural or artificial zeolites. Also, ion exchange resins are highly effective in the biodiesel filtration process.
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Types of resins
Most typical ion-exchange resins are based on crosslinked polystyrene. The actual ion exchanging sites are introduced after polymerisation. Additionally, in the case of polystyrene, crosslinking is introduced via copolymerisation of styrene and a few percent of divinylbenzene (non-crosslinked polymers are soluble in water). Crosslinking decreases ion-exchange capacity of the resin and prolongs the time needed to accomplish the ion exchange processes but improves the robustness of the resin. Particle size also influences the resin parameters; smaller particles have larger outer surface, but cause larger head loss in the column processes.
Besides being made as bead-shaped materials, ion exchange resins are produced as membranes. The membranes, which are made of highly cross-linked ion exchange resins that allow passage of ions, but not of water, are used for electrodialysis.
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 base, typically featuring primary, secondary, and/or ternary amino groups, e.g. polyethylene amine.
Specialised ion exchange resins are also known such as chelating resins (iminodiacetic acid, thiourea-based resins, and many others.)
Anion resins and cation resins are the two most common resins used in the ion exchange process. While anion resins attract negatively charged ions, cation resins attract positively charged ions.
Anion resins may be either strongly or weakly basic. Strongly basic anion resins maintain their positive charge across a wide pH range, whereas weakly basic anion resins are neutralized at higher pHs. Weakly basic resins do not maintain their charge at a high pH because they undergo deprotonation. They do, however, offer excellent mechanical and chemical stability. This, combined with a high rate of ion exchange, make ‘Weakly’ base anion resins well suited for the organic salts.
For anion resins, regeneration typically involves treatment of the resin with a strongly basic solution, e.g. aqueous sodium hydroxide. During regeneration, the regenerant chemical is passed through the resin and trapped negative ions are flushed out, renewing the resins’ exchange capacity.