Cation exchange, with synthetic resins

Kressman, T. R. E., and Kitchener, J. A. (1949). Cation exchange with a synthetic phenol-sulphonate resin, V. Kinetics. Discuss. Faraday Soc. 7, 90-103. 

Hard, attrition-resistant, insoluble synthetic polymers (typically a copolymer of styrene with divinylbenzene). The resins are manufactured in a spherical bead shape that contain either exchangeable anion or cation portions, capable of exchanging with other anions or cations and usually in an aqueous medium. Typically cation resins for water softening will have a practical operating capacity of 20,000 gpg (at 6 lb NaCl per cu ft) rising to 30,000 gpg (at 15 lb NaCl per cu ft). 

Sodium softeners are used to treated RO influent water to remove soluble hardness (calcium, magnesium, barium, and strontium) that can form scale on RO membranes. Once known as sodium zeolite softeners, zeolites have been replaced with synthetic plastic resin beads. For sodium softeners, these resin beads are strongly acidic cation (SAC) polystyrene resin in the sodium form. The active group is benzene sulfonic acid, in the sodium, not free acid, form. Figure 8.12 shows styrene-divinylbenzene gel cation resin. Equation 8.4 shows the softening reaction for calcium exchange ... 

The most important types of ion-exchange materials are the synthetic resins, which are composed of networks of relatively loosely cross-linked polymers with attached cationic or anionic groups. For example, a copolymer of styrene and p-divinylbenzene may be cross-linked to the desired extent by choosing the fraction... 

Polycarbophil Cation exchanger Synthetic hydrophilic resin copolymer of acrylic acid, loosely crosslinked with divinyl glycol. Marked water-binding capacity ... 

All ion exchangers, whether natural or synthetic, have fixed ionic groups that are balanced by counterions to maintain electroneutrality. The counterions exchange with ions in solution. As an example, consider the schematic cation-exchange resin shown in Figure 8.1. 

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