Feldspar dissolution at 25 degrees C and low pH

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Abstract
Although steady state dissolution of feldspar occurs stoichiometrically in acidified dilute solutions, leached layers form during dissolution and are maintained at constant thickness by the balance between leaching of Al3+ and M(b+) and silica network hydrolysis. The feldspar surface in contact with dilute acid solution Is penetrated by hydrogen species whose concentration is pH-dependent. Because we have observed that dissolution rate and ion exchange within the feldspar surface decrease with increasing concentration of cations in solution, but proton adsorption is not dependent upon salt concentration, we present two rate models which assume that ion exchange of H+ or H3O+ for K+, N-a+, or Ca2+ enhances the hydrolysis of Si within the leached, hydrated surface layer. Specifically, the pH-dependent ion-exchange reactions are assumed to accelerate hydrolysis of AlOSi bonds in the surface, causing an increase in the concentration of =SiOH sites throughout the leached, hydrated layer and a consequent increase in the rate of silica network hydrolysis. The silica network hydrolysis reaction (depolymerization), enhanced by ion exchange, is therefore pH-dependent and rate-limiting, in contrast to network hydrolysis of quartz at low pH. Assuming that dissolution rate is dependent upon the surface concentration of protonated exchange sites, which we model with a Langmuir isotherm, we predict the following rate equation: R = kn(S)(R)T (K-H{H+}/1 + K-H{H+} + K-M{M(b+)} +(i) Sigma K-i{C-i(+)})(n) Here, R is the area-normalized rate of feldspar dissolution at steady state, k is the rate constant, n(s) is the fraction of total sites that are AlOSi (exchange) sites at the water-feldspar interface, T is the number surface sites per unit area, KH and KM refer to the adsorption constants for H+ and M(b+) adsorption onto the AlOSi site respectively, and the braces refer to activities of dissolved species. The term Sigma(i) K-i{C-i(+)} includes the effect of adsorption of any other species C-i onto the AlOSi (exchange) site, including Al3+ adsorption. The value of n is 1 if only AlOSi sites on the surface contribute to dissolution but equals 0.5 if sites throughout the hydrated surface layer contribute. We have used the model with n = 0.5 to fit dissolution rate data of five feldspars as a function of NaCl concentration. The model prediction of rate increase with increasing hydrolysis of AlOSi bonds throughout the leached, hydrated layer suggests that condensation of bridging bonds during heating or drying cycles in natural systems might slow dissolution compared to continuously wet surfaces In laboratory reactors. Slower rate measurements derived from long duration reaction, measurements in the presence of rate-inhibiting salts, and rate measurements in alternating; wet and dry episodes might further explain the discrepancy between laboratory and held rate estimations.

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