Dissolution of zeolite in acidic and alkaline aqueous solutions as revealed by AFM imaging

Atomic force microscopy (AFM) makes it possible to directly detect morphological changes on the surface of a zeolite that are due to dissolution when the crystal is immersed in either an alkaline (0.1 N NaOH) or acidic (0.2 N H2SO4) aqueous solution at room temperature. The AFM images revealed for the first time that for heulandite (a natural zeolite crystal) (i) NaOH attacked the uppermost layer of aluminosilicate of the (100) surface, leaving isolated or agglomerated islands, (ii) similarly, H2SO4 attacked the (010) surface, forming pits, and (iii) step retreat did not occur for either solution. This unique dissolution pattern, in which the aluminosilicate layers of heulandite dissolve from terraces layer-by-layer, results from the characteristic pore structure of heulandite. This microscopic-level analysis should prove vital in the further development of new zeolite structures, which are critical to the activity of this important class of materials.