SURFACE-CHEMISTRY OF REACTED HEULANDITE DETERMINED BY SIMS AND XPS

The mineral heulandite ((Ca,Na,K)Al2Si7O18. 6H(2)O, a zeolite) was rea cted with solutions ranging in pH from 2 to 12. Surface morphology and chemistry of the minerals both before and after the experiments were examined with secondary ion mass spectrometry (SIMS), X-ray photoelect ron spectroscopy (XPS), and scanning electron microscopy (SEM). Profil ing by SIMS has revealed that a depletion in Al can be detected in all samples, particularly those reacted at pH 5 and below. XPS supports t his observation, confirming that the surface layer at pH 2 contains on ly silica and oxygen and almost certainly comprises amorphous SiO2. Th is surface layer is also inferred from morphology by SEM. At neutral a nd high pH, less change occurs in the Al profiles. At all pHs, hydroly sis occurs to the same depth to which the framework ion Al is depleted . Ca is noted to be lowered to the same depth as Al. Throughout the en tire pH range, the redistribution of the channel ions (Na and K) occur s to a greater depth than the framework ion Al. Of note is that where Na and K are depleted below the depth of Al depletion, Ca has increase d. This suggests an ion exchange between Na/K and Ca. These observatio ns indicate that the channel ions behave independently of the framewor k ions during dissolution. There appears, however, to be a direct char ge-balanced exchange between Al3+ and H+ (hydrolysis), supporting the dissolution model of Oelkers et al. Also of note is that the bond stre ngth of the Si and Al framework elements is weakened by 0.5 eV on the hydrated mineral surface. It is postulated here that this weakening of the bonds facilitates the dissolution of the mineral framework.