The structure of Na sub 2 O--Al sub 2 O sub 3 --SiO sub 2 glass: impact on sodium ion exchange in H sub 2 O and D sub 2 O

The kinetics of matrix dissolution and alkali-exchange for a series of sodium aluminosilicate glass compositions was determined at constant temperature and solution pH(D) under conditions of silica-saturation. Steady state release rate for sodium was 10--50 times faster than the rate of matrix dissolution, demonstrating that alkali exchange is an important long-term reaction mechanism that must be considered when modeling systems near saturation with respect to dissolved glass components. Sodium release rates were 30% slower in D sub 2 O compared to rates in H sub 2 O; but matrix dissolution rates were unaffected. These results are consistent with rupture of the O-H bond as the rate-limiting reaction in Na exp + --H exp + exchange whereas matrix dissolution is controlled by OH exp - or H sub 2 O catalyzed hydrolysis of Si-O-Si and Si-O-Al bonds. Changes in Na exchange rate with increasing Al sub 2 O sub 3 content could not be reconciled with changes in the number of non-bridging oxygen (NBO) sites in the glass alone. A simple model was used to estimate a structural energy barrier for alkali ion exchange using Na-O bond length and co-ordination as measured by Na K-edge X-ray absorption spectroscopy, and binding energy shifts for Si-O-Na sites measured by X-ray photoelectron spectroscopy (XPS). The energy barrier was calculated to increase from 34 kJ mol exp -1 for Na sub 2 O2SiO sub 2 glass to 49 kJ mol exp -1 for a glass containing 15 mol% Al sub 2 O sub 3 , consistent with stronger bonding of Na on NBO sites and increasing mechanical stiffness of the glass network with increasing Al content. The calculated ion-exchange enthalpies were then used to calculate Na ion-exchange rates as a function of glass composition. Agreement between the calculated and measured Na ion exchange rates was excellent.