Hydrogen isotope effects in the absorption of water by perfluorosulphonate (Nafion-117) and polystyrene divinylbenzene sulphonate (Dowex 50W) ion-exchange resins

Abstract

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 (α^H₂O_HDO) 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 α^H₂O_HDO 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 α^H₂O_HDO 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 α^H₂O_HDO 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.