Kinetic aspects of basaltic glass dissolution at 90°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°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−2s−1) = 3 × 10−10 (OH−)0.39 (1−Q/8.2 × 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.