Rates of feldspar dissolution at pH 3-7 with 0-8 m M oxalic acid

Feldspar dissolution rates were measured in flow-through reactors, over a pH range of 3-7, oxalic acid concentration of 0-8 m M, and feldspar compositional range of An₀-An₇₆. Dissolution rates in O and l m M 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 m M 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 HC₂O^-₄ (bioxalate) and C₂O^2-₄ (oxalate) play a role in the dissolution mechanism.