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.
Ion exchange resins are typically produced as small beads fabricated from a polymeric material (usually polystyrene or [less frequently] acrylic polymers). During the polymerization process, styrene is combined with divinyl benzene (DVB) to form a crosslinked polymer; this crosslinking process results in increased stability when compared to non-crosslinked materials. Active groups are added to the resin after the polymerization process to determine the type of desirable ion exchange. Each copolymer resin bead possesses a gel-like structure and is able to expand and contract in liquids; they also contain numerous surface pores for trapping ions.
Ion exchange resins are primarily used as integral parts in the water treatment process in both consumer and industrial settings. Water softening systems use resins to replace magnesium and calcium — two common elements of hard water — with sodium molecules. Resins can also be used to purify water by replacing metals such as copper, cadmium, and lead with benign sodium or potassium molecules.
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.