Does organic acid adsorption affect alkali-feldspar dissolution rates?

Consideration of alkali-feldspar dissolution rates1The term alkali-feldspar is used in the present study to denote those feldspars that dissolve via aluminum deficient precursor complexes [Oelkers, E.H., Schott, J., Devidal, J.L., 1994. The effect of aluminum, pH, and chemical affinity on the rates of aluminosilicate dissolution reactions. Geochim. Cosmochim. Acta, 58 (1994) 2011–2024.], which includes feldspars in the system NaAlSi3O8–KAlSi3O8, as well as plagioclases of compositions ∼An80 or less [Oelkers, E.H., Schott, J., 1995b. Experimental study of anorthite dissolution and the relative mechanism of feldspar hydrolysis. Geochim. Cosmochim. Acta, 59: (1995) 5039–5053]. n from the literature suggests that in the absence of organic ligands and at far from equilibrium conditions they can be represented by r+=k(a3H+/aAl+3)n, where r+ refers to the far from equilibrium dissolution rate, k designates a rate constant, ai stands for the activity of the subscripted aqueous species, and n refers to a stoichiometric coefficient. Dissolution rates of these minerals have also been observed to increase substantially with increasing aqueous organic acid concentration. To assess if this latter effect is the result of either the change in aqueous aluminum activity due to aqueous aluminum–organic acid anion complex formation or organic acid anion adsorption to the mineral surface, dissolution rates predicted using the above equation are compared to measured alkali feldspar dissolution rates. A close correspondence between these predictions and the experimental data are found, providing strong evidence that organic acid anion adsorption does not affect alkali-feldspar dissolution rates.