EFFECTS OF SALT-SOLUTIONS ON GLASS DISSOLUTION

A borosilicate nuclear waste glass was leached in deionised water and various salt solutions at 90-degrees-C. The observed glass leach rates decrease as the ionic strength of the leachant increases. These results suggest that this effect is not due to the thermodynamic ionic strength, as explained previously, but rather to a kinetic ion exchange process between the salt solutions and the glass. We suggest that kinetic ion exchange processes affect glass dissolution by suppressing destructive hydronium/alkali exchange and facilitating the competing nondestructive alkali in solution/alkali on glass exchange. This mechanism is supported by data on cation size, solution pH and salt concentration and by thermodynamic calculations using a geochemical computer code, PHREEQE. Identification of the predominant mechanism is important in accounting for the salt effects in models being developed to predict the behaviour of nuclear waste glasses in geologic repositories.