Surface chemistry of reacted heulandite determined by SIMS and XPS

The mineral heulandite ((Ca,Na,K)Al₂Si₇O₁₈ · 6H₂O, a zeolite) was reacted 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 photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Profiling 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 this observation, confirming that the surface layer at pH 2 contains only silica and oxygen and almost certainly comprises amorphous SiO₂. This surface layer is also inferred from morphology by SEM. At neutral and high pH, less change occurs in the Al profiles. At all pHs, hydrolysis 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 entire pH range, the redistribution of the channel ions (Na and K) occurs 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 increased. This suggests an ion exchange between Na/K and Ca. These observations indicate that the channel ions behave independently of the framework ions during dissolution. There appears, however, to be a direct charge-balanced exchange between Al³⁺ and H⁺ (hydrolysis), supporting the dissolution model of Oelkers et al. Also of note is that the bond strength 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.