Modeling Surface Area to Volume Effects on Borosilicate Glass Dissolution
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MODELING SURFACE AREA TO VOLUME EFFECTS ON BOROSILICATE GLASS DISSOLUTION WILLIAM L. BOURCIER*, W. L. EBERT**, AND X. FENG**
*Lawrence Livermore National Laboratory, L-219, Livermore CA 94550 "**ArgonneNational Laboratory, 9700 South Cass Avenue, Argonne IL 60559 ABSTRACT
We simulated the reaction of SRL-131 glass with equilibrated J-13 water in order to investigate the effects of surface area to volume ratio (SAN/) on glass dissolution. We show that glass-fluid ion exchange causes solution pH to rise to progressively higher values as SA/V increases. Because the ion exchange is rapid relative to the duration of the glass dissolution experiment, the pH effect does not scale with (SA/V)*time. Experiments compared at the same (SA/V)*time value therefore have different pHs, with higher pHs at higher SA/V ratios. Both experimental data and our simulation results show similar trends of increasing reaction rate as a function of SAN ratio when scaled to (SA/V)*time. Glasses which react in systems of differing SA/V ratio therefore follow different reaction paths and high SAN ratios cannot be used to generate data which accurately scales to long time periods unless the ion exchange effect is taken into account. We suggest some simple test designs which enable more reliable high SAN accelerated tests. INTRODUCTION
The use of high surface area to volume ratios (SAN) has been proposed as a method to accelerate glass durability tests[1,2,31. The basis for this is that because the glass dissolution rate is proportional to the amount of surface area, species build up in solution faster in high SA tests and reach conditions that would only be achieved at greater times in lower SA tests. It follows that if elemental concentrations measured in glass dissolution experiments are plotted versus (SA/V)*time rather than time, the data would all plot along a single curve if it is assumed that the glass reaction process is similar at all SA/V ratios. High SAN tests may therefore speed up the glass dissolution process and advanced reaction stages can be investigated in relatively short time periods. Some kinetic data for glass dissolution do scale along a single curve when plotted versus
(SANV)*time[4]. However, much of the experimental data do not [5,6,7,8]. The reason for a lack of systematic behavior is unclear. In some cases, data scale well with (SA/V)*time, particularly for tests at low SA/V ratios (less than 10OOm-I). Tests performed at high SAN show a trend similar to that in Figure 1,with higher SA/V tests being accelerated relative to low SAN tests at the same value of (SA/V)*time. Measured pH values are also higher in high SAN tests than in low SAN tests at the same value of (SA/V)*time. (SA/V)*time scaling should also apply under conditions where the dissolution rate is affected by the concentrations of dissolved species. Machiels and Pescatore [9] show that for a rate equation of the form
- = -k 1(I -
, where c is the concentration of a dissolved
species, co its value at saturation, SA is surface area, V is solution volum
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