OXALATE ADSORPTION AT A PLAGIOCLASE (AN(47)) SURFACE AND MODELS FOR LIGAND-PROMOTED DISSOLUTION

Previous work on adsorption of oxalate at aluminosilicate surfaces sug gests that maximum adsorption occurs through a bidentate attachment of the organic ligand, at near-neutral pH. Rates of ligand-promoted diss olution are expected to be greatest at this pH as well. We tested this model by measuring oxalate adsorption on the surface of andesine (An( 47)), in solutions of pH 3-5 and total oxalate concentrations of 0-8 m M. Contrary to expectation, the greatest adsorption density of 24 mu m ol m(-2) total oxalate was observed at pH 3 and 8 mM total oxalate. Ad sorption is dependent upon the activities of both oxalate (C2O42-) and bioxalate (HC2O4-) in solution and can be modeled with either a two-t erm Langmuir or a two-term Freundlich isotherm. A Freundlich adsorptio n model provided the best fit to rate data because it was not constrai ned to a finite number of adsorption sites, as was the Langmuir model. The two-term ligand adsorption model was incorporated into a rate mod el: R-tot = k(H)+[H-ads(+) ](L) + k(HOx)- [HOx(ads)(-)] + k(Ox)(2-)[Ox (ads)(2-)] where R-tot is the net dissolution rate of the feldspar, [i (ads)] is the concentration of species i adsorbed to the surface, and k(i) is the rate constant for release of the surface complex. The mode l was fit to data for oxalate-promoted dissolution of andesine, result ing in estimates for the rate constants of k(HOx)- = 1.16 X 10(-12), k (Ox)(2-) = 1.05 x 10(-12), and k(H+) = 9.61 x 10(-13) mot of feldspar (mu umol of i)(-1) s(-1).