Ion Exchangers |
The majority of ion exchange resins are based on the copolymerization of styrene and a cross-linking agent, divinylbenzene (DVB), to produce a 3-dimensional cross-linked structure, mostly supplied as spherical beads. Strongly acidic cation exchange resins are prepared by sulfonating the benzene rings in the polymer, whereas weakly acidic cation exchange resins are primarily based on acrylic acid that has been cross-linked with divinylbenzene (DVB). Ion exchangers containing quaternary ammonium groups are strongly basic, while resins with tertiary amino groups exhibit medium or weakly basic properties.
Gel-type based ion exchange resins, also called microporous resins, do not have discrete pores, also called microporous resins. Here, the ions to be exchanged diffuse through the gel structure to interact with the exchange sites. Typically, gel-type resins exhibit microporosity with pore diameters up to 30 Angstroms. Their porosity is inversely related to the DVB cross-linking. Due to this cross-linking, these ion exchangers are insoluble in concentrated acids, bases and salts, and possess resistance to oxidants, reducing agents and radioactive radiation. Gel-type resins are thermally stable and exhibit a high exchange capacity.
Since macroporous ion exchange resins are synthesized by special procedures, their macropores are more than two times larger in diameter than the micropores of gel-type based resins. The macropores facilitate the ion exchange process, and give access to high molecular weight compounds. These compounds could be natural or industrial water contaminants, such as humic acids produced by biodegradation of dead organic matter, or the waste water effluent from cellulose plants.
The macropores also facilitate the regeneration of the ion exchangers, thus preventing irreversible contamination. Due to their high porosity, these resins are opaque and exhibit little volume change (swelling) in most solvents. The unique and robust structure of macroporous resins offers greater stability with regard to chemical, osmotic, thermal, and mechanical influences. Consequently, macroporous ion exchangers can be favorably used as catalysts, particularly in non-aqueous media.
In comparison to gel-type based resins, macroporous resins have a lower ion exchange capacity. However, this is compensated for by their longer lifetime, and by the fact that chemical processes involving ion exchange reactions are made possible only by these resins.
Macroreticular ion exchangers offer several unique properties, such as a resistance to adsorption or concentration of different non-charged species. This has led to the development of a number of macroreticular polymeric adsorbents.