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

Abstract

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 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 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 Ox 2− ) 0.75, suggesting that both HC 2O 4 − (bioxalate) and C 2O 4 2−- (oxalate) play a role in the dissolution mechanism.