Interaction of natrolite and thomsonite intergrowths with aqueous solutions of different initial pH values at 25 degrees C in the presence of KCl: Reaction mechanisms

Natural zeolitic material composed of natrolite and thomsonite intergrowths (NAT/THO) was treated in solutions of different initial pH values at 25 degrees C under N-2 atmosphere and in 1M KCI as ionic modulator, until pH equilibration. The solid experimental products were studied by means of powder X-ray diffraction (XRD), scanning electron microscopy-energy dispersive system (SEM-EDS), Fourier-transformed infra-red (FTIR) and thermogravimetric analysis/differential thermal analysis (TGA/DTA). The liquid experimental products were analysed using atomic absorption spectroscopy (AAS) and atomic emission spectroscopy (AES). The NAT/THO material exhibited an amphoteric character with a tendency to neutralise the reacting solutions. The pH equilibration was faster for the acidic region than for the basic one. The H+ ions are chemisorbed on the bulk material, whereas the OH- ions promote a proton detachment from the exchangable cation-water complexes. No Broensted acidity, possibly responsible for the neutralisation in the basic region, was found by temperature-programmed desorption (TPD) measurements. Reaction mechanisms involving hydrolysis and degradation-dissolution are proposed. Zeolite crystals remaining at the end of the experiments showed no loss of crystallinity, phase transformation nor even framework dealumination. The insertion of K+ into the zeolites is suggested here as the reason as to why no collapse of their crystal structure occurred in the most acidic solutions causing zeolites to be more resistant to chemical weathering. (C) 1997 Elsevier Science Ltd. All rights reserved. Applied Geochemistry