Kinetic aspects of basaltic glass dissolution at 90 degrees C: role of aqueous silicon and aluminium

Steady-state dissolution rates of a synthetic basaltic glass were measured in an open-system mixed flow reactor as a function of solution composition at a temperature of 90 degrees C and over the pH range 7.8 to 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 formation of a hydrated surface gel (HBG) of which the solubility controls the overall dissolution reaction. The glass steady-state dissolution rates were found to be independent of aqueous aluminium and silicium concentration but to depend on the chemical affinity for the overall hydrolysis reaction. The glass is a rapidly reacting solid, whose dissolution induces a dramatic change in solution concentration, which results readily in small chemical affinities for the dissolution reaction. Consequently, conditions of great undersaturation have not been investigated (affinity max. 9.8 kJ/mol). However, our results strongly suggest that the dissolution rates are controlled by the decomposition of a stoichiometric silico-aluminous surface precursor. The variation of the steady-state dissolution rates can be described using a simple expression based on the concept that the precursor is formed by the simple 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.