Surface chemistry of reacted heulandite determined by SIMS and XPS

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doi: 10.1016/0009-2541(96)00065-4
Authors:Ragnarsdottir, K. Vala; Graham, Colin M.; Allen, Geoffrey C.
Author Affiliations:Primary:
University of Bristol, Department of Geology, Bristol, United Kingdom
Albert-Ludwigs-Universität Freiburg, Freiburg, Federal Republic of Germany
University of Edinburgh, United Kingdom
Volume Title:Chemical Geology
Source:Chemical Geology, 131(1-4), p.167-181. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0009-2541
Publication Date:1996
Note:In English. 67 refs.; illus., incl. 1 table
Summary: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. Abstract Copyright (1996) Elsevier, B.V. [R.E.S.]
Subjects:Applications; Chemical reactions; Experimental studies; Framework silicates; Geochemistry; Heulandite; Hydrolysis; Ion probe; Mass spectroscopy; Mineral-water interface; PH; Radioactive waste; SEM data; Silicates; Solution; Spectra; Spectroscopy; Underground disposal; Waste disposal; X-ray analysis; X-ray photoelectron spectra; X-ray spectroscopy; Zeolite group
Abstract Numbers:97M/1533
Record ID:1997005034
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands
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