Analysis of the Pore Structure of Ion-Exchange Resins and the Adsorption Equilibrium of Fe3+/Cl? Complex Ions

The microstructure of ion-exchange resins has been investigated to understand more clearly the ion-exchange mechanism. Nine types of resins with different pore structures were used: all of the polystyrene family crosslinked with divinylbenzene and anionic resins, with mesh sizes ranging from 100 to 200, except for one (20–50 mesh). Various pore volumes of each resin were determined by measurements of intrusion of some chemical species (H₂O, Nd³⁺, and Hg) into the resin. The results are analyzed on the basis that an ion-exchange resin particle consists of four regions. They are: 1) the mercury intrusion region, 2) the region where coions (such as Nd³⁺) can intrude but mercury cannot, 3) the region where water or counterions can intrude but coions cannot, and 4) the polymer matrix region occupied by the polymer skeleton. The former two regions are not influenced by the resin-fixed ionic groups. While it used to be thought that specific adsorption of counterions may occur in the entire exchange resin particle, it is appropriate to consider that specific adsorption takes place only in the latter two regions. According to this point of view, the adsorption equilibrium of the Fe³⁺/Cl^? complex ions could be explained more satisfactorily.