Feldspar dissolution rates were measured in flow-through reactors, over a pH range of 3-7, oxalic acid concentration of 0-8 mM, and feldspar compositional range of An(0)-An(76). Dissolution rates in 0 and 1 mM concentrations of oxalic acid increased linearly with increasing mole percent anorthite in the solid. Dissolution was nonstoichiometric in oxalic acid, due to a preferential release of Al at steady state. 1 mM concentrations of oxalic acid increased dissolution rates by a factor of 2-5 at pH 3 and 2-15 at pH 5-6. At pH 5-6 the rate enhancement due to oxalic acid may increase with increasing anorthite content of the feldspar. The ligand-promoted rate; R(L), defined as the observed total rate in oxalic acid minus the proton-promoted rate, R(t)-R(H), was estimated as 10(-12.8)(a(HOx-)+a(Ox2-))(0.75), suggesting that both HC2O4- (bioxalate) and C2O42- (oxalate) play a role in the dissolution mechanism.
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