Previous work on adsorption of oxalate at aluminosilicate surfaces suggests that maximum adsorption occurs through a bidentate attachment of the organic ligand, at near-neutral pH. Rates of ligand-promoted dissolution are expected to be greatestat this pH as well. We tested this model by measuring oxalate adsorption on the surface of andesine (An{sub}47), in solutions of pH 3-5 and total oxalate concentrations of 0-8 mM. Contrary to expectation, the greatest adsorption density of 24μmolm{sup}-2 total oxalate was observed at pH 3 and 8 mM total oxalate. Adsorption is dependent upon the activities of both oxalate (C{sub}2O{sub}4{sup}2-) and bioxalate (HC{sub}2O{sub}4{sup}-) in solution and can be modeled with either a two-term Langmuir or a two-term Freundlich isotherm. A Freundlich adsorption model provided the best fit to rate data because it was not constrained to a finite number of adsorption sites, as was the Langmuir model. The two-term ligand adsorption model was incorporated into a rate model: R{sub}(tot) = k{sub}H{sup}+H{sub}(ads){sub}L + k{sub} (HOx{sup}-)(HOx{sup}-){sub}(ads) + k{sub}(Ox{sup}2-)(Ox{sup}2-){sub}(ads) where R{sub}tot is the net dissolution rate of the feldspar, i{sub}ads is the concentration of species iadsorbed to the surface, and k{sub}i is the rate constant for release of the surface complex. The model was fit to data for oxalate-promoted dissolution of andesine, resulting in estimates for the rate constants of k{sub}(HOx)-= 1.16× 10{sup}-12,k{sub}(Ox{sup}2-) = 1.05× 10{sup}-12, and k{sub}H{sup}+ = 9.61× 10{sup}-13 mol of feldspar (μmol of i){sup}{sup}-1 s{sup}-1.
展开▼
