The effect of fluoride on the dissolution rates of natural glasses at pH 4 and 25°C

Abstract

Far-from-equilibrium, steady-state dissolution rates at pH 4 of a suite of natural glasses, ranging from basaltic to rhyolitic in composition, have been determined as a function of aqueous fluoride concentrations up to 1.8 × 10 −4 mol/kg in mixed-flow reactors. Dissolution rates of each of these glasses increase monotonically with increasing aqueous fluoride concentration. Measured dissolution rates are found to be consistent with both the Furrer and Stumm (1986) surface coordination model and the Oelkers (2001) multi-oxide dissolution model. Application of the latter model yields the following equation that can describe all measured rates as a function of both glass and aqueous solution composition: log ( r + , geo / ( mol / m 2 / s ) ) = [ − 0.086 ⋅ SiO 2 ( wt % ) − 2.23 ] + [ − 0.0067 ⋅ SiO 2 ( wt % ) + 0.683 ] ⋅ log ( a H + 3 / a A 1 3 + ) where r +,geo represents the far-from-equilibrium dissolution rate, normalized to geometric surface area, SiO 2(wt.%) refers to weight percent of SiO 2 in the glass, and a i denotes the activity of the subscripted aqueous species. Computed glass dissolution rates increase with increasing aqueous fluoride concentration due to the formation of aqueous Al-fluoride complexes, which decrease a Al 3+ . This rate expression can be used to predict far-from-equilibrium dissolution rates of natural glasses in a variety of natural environments. Comparison of rate predictions with the composition of natural fluids suggests that the presence of aqueous fluoride can enhance natural glass dissolution rates by an order of magnitude or more in a variety of geochemical systems.