Impacts of glass composition, pH, and temperature on glass forward dissolution rate

Nuclear waste glasses dissolve at the forward dissolution rate ( r f ) in very dilute aqueous solutions, which can isolate the impact of the glass composition from solution feedback and alteration product effects. While it has long been known that pH and temperature ( T ) strongly impact r f , the impacts of glass composition have remained uncertain. In this work, r f data from 19 nuclear waste glasses were used with the aim of identifying the effect of glass composition on r f . The r f values were modeled as: r f = k 0 10 –η pH exp(− E a / RT ), with k 0 , η , E a , and R , respectively, being the intrinsic rate constant, pH coefficient, apparent activation energy, and gas constant. However, no predictive correlation could be established between the individual model parameters (log[ k 0 ], η , and E a ) and glass composition for the glasses considered in this study, an outcome that was attributed to the strong positive correlation between the log[ k 0 ] and E a parameters. Therefore, a model was fitted directly to the combined r f from all 19 glasses. This approach showed that 90% of the variation in r f data could be accounted for solely by T and pH effects. Therefore, any composition effects must be relatively small. After normalizing for differences in pH and T , the only notable differences in r f between the glasses were found to correlate with variations in the fraction of glass forming tetrahedra contributed by tetrahedral boron, f ( [4] B), with an abrupt threshold at a high value of f ( [4] B) (~0.22), where higher r f are predicted with no discernable composition effects below the threshold.