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 resul ts suggest that this effect is not due to the thermodynamic ionic stre ngth, as explained previously, but rather to a kinetic ion exchange pr ocess between the salt solutions and the glass. We suggest that kineti c ion exchange processes affect glass dissolution by suppressing destr uctive hydronium/alkali exchange and facilitating the competing nondes tructive alkali in solution/alkali on glass exchange. This mechanism i s 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 a ccounting for the salt effects in models being developed to predict th e behaviour of nuclear waste glasses in geologic repositories.