Hydrogen isotope effects in the absorption of water by perfluorosulphonate (Nafion-117) and polystyrene divinylbenzene sulphonate (Dowex 50W) ion-exchange resins
Hydrogen/deuterium (H/D) fractionation effects in the exchanger-pore water–outer-equilibrium water have been investigated at 298 ± 1 K using H+, Li+ and Cs+ ionic forms of Nafion-117 and Dowex 50W (4%, 8% DVB) resins and waters containing deuterium from 150 to 11 000 ppm. Single-stage separation factors (αH2OHDO) evaluated show preferential enrichment of deuterium in the outer water phase in the sequences: (i) Nafion-117 > Dowex 50W × 8 > Dowex 50W × 4; (ii) H+ > Li+ > Cs+. Whereas αH2OHDO values were nearly unity in the natural D-concentration region, they were appreciably higher in deuterium-rich water systems. A linear relationship was found between log αH2OHDO and inverse deuterium concentration, which has been interpreted in terms of a Born-type equation dealing with the solvation of ions. The selective enrichment of H2O over HDO in resin-pore water conforms with similar observations reported with binary water–organic-solvent systems and with the lesser hydrogen-bonded structure of resin-pore water compared with normal water. Thermodynamic analysis of the present systems has revealed large contributions of swelling pressure terms of αH2OHDO which readily explains the sequences listed above. The role of cationic and sulphonate anion hydration has also been assessed in these fractionation effects. H+– 117, Li+– 117 and H+× 8 exchangers have been suggested as being most suited for deuterium enrichment using the simple technique of equilibrium contacting of these exchangers with waters of high D-concentrations.