Rates of glass dissolution from laboratory and field studies are often considered to be irreconcilable, although potential causes for the difference, such as solution saturation state and increasing surface area from progressive weathering, have not been explored in depth. The dissolution rate of SON68 glass, the non-radioactive analog of the French R7T7 composition, was determined in a single-pass flow-through (SPFT) system at 90. °C and pH 9 over a silica-saturation interval. Dissolution rates were determined on both powdered and monolithic specimens by assaying the concentration of elements released from glass to effluent solution. In addition, rates of 12 monolithic specimens were quantified using a Vertical Scanning Interferometry (VSI) method. The method entails measuring the difference in height between a reference and reaction surface. The height difference is proportional to the dissolution rate. By adjusting the relative position of the reacted surface to average surface roughness, the effects of surface area on the dissolution rate can be minimized. Values of the dissolution rate, based upon chemical assay of the effluent solution on the one hand, and VSI methods on the other, were compared. In general, rates determined by the two methods are within a factor of 2×. The difference in rates may be due to the presence of a reaction layer that develops on the glass surface, resulting in an underestimation of the height difference measurement. The dissolution rates of SON68 glass in silica-saturated solutions were then compared to rates previously determined on basalt glass in natural weathering environments (Gordon and Brady, 2002, Chem. Geol. 190, 113-122). When adjusted for differences in temperature and pH, the ranges of borosilicate and basalt glass dissolution rates overlap, indicating that laboratory and field rates can be reconciled and that the principal control on glass dissolution is solution saturation with respect to amorphous silica.
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