The dissolution rates of natural glasses as a function of their composition at pH 4 and 10.6

Far-from-equilibrium dissolution rates of a suite of volcanic glasses that range from basaltic to rhyolitic in composition were measured in mixed flow reactors at pH 4 and 10.6, and temperatures from 25 to 74°C. Experiments performed on glasses of similar composition suggest that dissolution rates are more closely proportional to geometric surface areas than their BET surface areas. Measured geometric surface area normalized dissolution rates (r,geo) at 25°C were found to vary exponentially with the silica content of the glasses. For pH 4 solutions this relation is given by: log r,geo(mol/m2/s)= -0.03 · [SiO2(wt%)]-7.58, (A1) and at pH 10.6 this relation is given by: log r,geo(mol/m2/s)= -0.02 · [SiO2(wt%)]-7.02. (A2) These equations can be used to estimate lifetimes and metal release fluxes of natural glasses at far-from-equilibrium conditions. The lifetime at pH 4 and 25°C of a 1 mm basaltic glass sphere is calculated to be 500 yr, whereas that of a 1 mm rhyolitic glass sphere is 4500 yr. Estimated nutrient release rates from natural glasses decrease exponentially with increasing silica conten