Abstract:
Previous experiments have indicated increased dissolution rates of quartz in the presence of oxalate, although the mechanism responsible is unclear. Possible explanations include a decrease in solution saturation state due to the presence of an aqueous Si-oxalate complex, or ligand promoted dissolution due to adsorption of oxalate onto the quartz surface.Formation of an aqueous Si-oxalate complex has been investigated by titrating Si(OH)4 with K2C2O4. Formation of proposed Si-oxalate complexes, such as a silicic acid-oxalate ester, should result in pH shifts of the titration curve compared to blank titrations. At an initial pH 5-6, titrations of Si(OH)4 solutions are identical to blank titrations (after correcting for the buffering effect of silicic acid), indicating that Si-oxalate complexing is negligible.Oxalate adsorption onto quartz surfaces was studied for short-term (4-5 h, pH 4.3-7.1, oxalate=5-20 μM) and long-term (1 week, autoclaved, pH 6.3-7.0, oxalate=10-40 μM) experiments. Oxalate adsorption under these conditions is negligible, with a maximum adsorption of 3.0x10-9 mol/m2.The lack of aqueous Si-oxalate complexing and oxalate adsorption onto quartz indicate that these mechanisms will have no effect upon the dissolution rate of quartz in oxalate solutions. Re-evaluation of quartz dissolution rate data suggests that oxalate per se has little or no effect upon quartz dissolution rates, and the observed increase in dissolution rates with increasing oxalate concentration may be due to the corresponding increase of Na+ concentration in solution.
