KINETIC ASPECTS OF BASALTIC GLASS DISSOLUTION AT 90-DEGREES-C - ROLE OF AQUEOUS SILICON AND ALUMINUM

Steady-state dissolution rates of a synthetic basaltic glass were meas ured in an open-system mixed flow reactor as a function of solution co mposition at a temperature of 90 degrees C and over the pH range 7.8 t o 8.3. The dissolution is a two-step process. The first of these steps involves the release of the cation modifier elements leading to the f ormation of a hydrated surface gel (HBG) of which the solubility contr ols the overall dissolution reaction. The glass steady-state dissoluti on rates were found to be independent of aqueous aluminium and siliciu m concentration but to depend on the chemical affinity for the overall hydrolysis reaction. The glass is a rapidly reacting solid, whose dis solution induces a dramatic change in solution concentration, which re sults readily in small chemical affinities for the dissolution reactio n. Consequently, conditions of great undersaturation have not been inv estigated (affinity max. 9.8 kJ/mol). However, our results strongly su ggest that the dissolution rates are controlled by the decomposition o f a stoichiometric silico-aluminous surface precursor. The variation o f the steady-state dissolution rates can be described using a simple e xpression based on the concept that the precursor is formed by the sim ple absorption of reactants: R (mol cm(-2) s(-1)) = 3 X 10(-10) (OH-)( 0.39) (1-Q/8.2 X 10(-5)), where Q, the ion activity quotient is equal to: Q = (H4SiO4)(Al(OH)(4)(-))(0.36) (Fe(OH)(3))(0.18) (OH-)(-0.36). ( C) 1997 Elsevier Science B.V.