A Kinetic Model for Borosilicate Glass Dissolution Based on the Dissolution Affinity of a Surface Alteration Layer
- PDF / 454,978 Bytes
- 8 Pages / 420.48 x 639 pts Page_size
- 104 Downloads / 187 Views
A KINETIC MODEL FOR BOROSILICATE GLASS DISSOLUTION BASED ON THE DISSOLUTION AFFINITY OF A SURFACE ALTERATION LAYER
WILLIAM L. BOURCIER, DENNIS W. PEIFFER, KEVIN G. KNAUSS, KEVIN D. McKEEGAN, and DAVID K. SMITH Lawrence Livermore National Laboratory Livermore, CA 94550
ABSTRACT A kinetic model for the dissolution of borosilicate glass, incorporated into the EQ3/6 geochemical modeling code, is used to predict the dissolution rate of a nuclear waste glass. In the model, the glass dissolution rate is controlled by the rate of dissolution of an alkalidepleted amorphous surface (gel) layer. Assuming that the gel layer dissolution affinity controls glass dissolution rates is similar to the silica saturation concept of Grambow [1] except that our model predicts that all components concentrated in the surface layer, not just silica, affect glass dissolution rates. The good agreement between predicted and observed elemental dissolution rates suggests that the dissolution rate of the gel layer limits the overall rate of glass dissolution. The model predicts that the long-term rate of glass dissolution will depend mainly on ion concentrations in solution, and therefore on the secondary phases which precipitate and control ion concentrations. INTRODUCTION Borosilicate glass dissolution in near-neutral pH solutions initiates through ion exchange and hydration of the glass resulting in a hydrated alkali-depleted surface layer. With time, three layers develop on the glass surface (Figure 1). Nearest the fresh glass is a partially hydrated diffusion layer in which diffusion gradients for alkali elements and water are observed [2,3]. Next outwards is a more hydrous, x-ray amorphous gel layer enriched in silicon, aluminum, and other elements more insoluble relative to the alkalis. In this zone the tetrahedral network breaks down and Si and other elements are released into solution. No diffusion gradients are observed in the gel layer. Both the gel and diffusion layers are commonly a few tenths of a micron in thickness. Coating the gel layer is a rind of secondary phases that may be precipitates from solution or end results of aging and crystallization of the gel layer. Surface analytical methods have shown that the thickness of the diffusion layer remains nearly constant after an initial developmental period [2]. Glass dissolution occurring through matrix dissolution (as opposed to diffusion controlled dissolution which takes Work performed under the auspices of the U. S. Department of Energy by the Lawrence Livermore National Laboratory under contract no. W-7405-ENG-48.
Mat. Ras. Soc. Symp. Proc. Vol. 176. @1990 Materials Research Society
210
dissolution
Ffront
glass
I.I
layer
(•
I
layer
•
Calcium
Sodium
Hydrogen
Figure 1. Schematic diagram showing surface layers that develop on glass during dissolution and concentration profiles of calcium, sodium, and hydrogen in these layers. Adapted
from [2]. place under acidic conditions) can therefore be treated as a steady-state process where the diffusion layer of constant
Data Loading...
