ION-EXCHANGE IN ZEOLITES .2. HYDROLYSIS AND DISSOLUTION OF ZEOLITES NAX AND NAY

Hydrolysis of zeolites NaX and NaY through H3O+ exchange has been studied in deionised water and NaNO3 solutions. Hydrolysis was found to enhance the dissolution of silicon and aluminium from the zeolites. Equations based on the thermodynamic mass-action law were derived for the prediction of hydrolysis equilibria (Na+ concentration and pH) over a range of different initial conditions. The agreement between predictions and observations was generally good in moderate Na+ concentrations. At low Na+ concentrations, however, predictions failed owing to the strong decrease of the zeolite selectivities for H3O+ ions with decreasing Na+ concentration in solution. Predictions were also affected, to a small extent, by the silicate and carbonate ions present in the solutions. The carbonate ions resulted from CO2 that had been preabsorbed by the zeolites from air. Dissolution of silicon and aluminium from the zeolites was very slow, so no equilibrium parameters for this process could be established.