AQUEOUS SI-OXALATE COMPLEXING, OXALATE ADSORPTION ONTO QUARTZ, AND THE EFFECT OF OXALATE UPON QUARTZ DISSOLUTION RATES

Previous experiments have indicated increased dissolution rates of qua rtz in the presence of oxalate, although the mechanism responsible is unclear. Possible explanations include a decrease in solution saturati on state due to the presence of an aqueous Si-oxalate complex, or liga nd promoted dissolution due to adsorption of oxalate onto the quartz s urface. Formation of an aqueous Si-oxalate complex has been investigat ed by titrating Si(OH)(4) with K2C2O4. Formation of proposed Si-oxalat e complexes, such as a silicic acid-oxalate ester, should result in pH shifts of the titration curve compared to blank titrations. At an ini tial 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 adsorpt ion onto quartz surfaces was studied for short-term (4-5 h, pH 4.3-7.1 , oxalate = 5-20 mu M) and long-term (1 week, autoclaved, pH 6.3-7.0, oxalate = 10-40 mu M) experiments. Oxalate adsorption under these cond itions is negligible, with a maximum adsorption of 3.0 x 10(-9) mol/m( 2). The lack of aqueous Si-oxalate complexing and oxalate adsorption o nto quartz indicate that these mechanisms will have no effect upon the dissolution rate of quartz in oxalate solutions. Re-evaluation of qua rtz dissolution rate data suggests that oxalate per se has little or n o effect upon quartz dissolution rates, and the observed increase in d issolution rates with increasing oxalate concentration may be due to t he corresponding increase of Na+ concentration in solution. (C) 1997 E lsevier Science B.V.