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

Far-from-equilibrium, steady-state dissolution rates at pH 4 of asuite of natural glasses, ranging from basaltic to rhyolitic in composition,have been determined as a function of aqueous fluoride concentrations up to 1.8X 10(-4) mol/kg in mixed-flow reactors. Dissolution rates of each of theseglasses increase monotonically with increasing aqueous fluoride concentration.Measured dissolution rates are found to be consistent with both the Furrer andStumm (1986) surface coordination model and the Oelkers (2001) multi-oxidedissolution model. Application of the latter model yields the followingequation that can describe all measured rates as a function of both glass andaqueous solution composition: log (r(+.geo)/(mol/m(2)/s)) = [-0.086 (.)SiO2(wt%) - 2.23] + [0.0067 (.) SiO2(wt%) + 0.683] (.)log(alpha(H+)(3)/alpha(Al3+)) where r(+),(geo) represents thefar-from-equilibrium dissolution rate, normalized to geometric surface area,SiO2(wt.%) refers to weight percent of SiO2 in the glass, and alpha(i) denotesthe activity of the subscripted aqueous species. Computed glass dissolutionrates increase with increasing aqueous fluoride concentration due to theformation of aqueous Al-fluoride complexes, which decrease alpha(Al)(3+). Thisrate expression can be used to predict far-from-equilibrium dissolution ratesof natural glasses in a variety of natural environments. Comparison of ratepredictions with the composition of natural fluids suggests that the presenceof aqueous fluoride can enhance natural glass dissolution rates by an order ofmagnitude or more in a variety of geochemical systems. Copyright (C) 2004Elsevier Ltd