Attachment of L-glutamate to rutile (alpha-TiO(2)): a potentiometric, adsorption, and surface complexation study

Interactions between aqueous amino acids and mineral surfaces influence the bioavailability of amino acids in the environment, the viability of Ti implants in humans, and the role of mineral surfaces in the origin of life on Earth. We studied the adsorption of l-glutamate on the surface of rutile (alpha-TiO(2), pH(PPZC) = 5.4) in NaCl solutions using potentiometric titrations and batch adsorption experiments over a wide range of pH values, ligand-to-solid ratios, and ionic strengths. Between pH 3 and 5, glutamate adsorbs strongly, up to 1.4 micromol m(-2), and the adsorption decreases with increasing ionic strength. Potentiometric titration measurements of proton consumption for the combined glutamate-rutile-aqueous solution system show a strong dependence on glutamate concentration. An extended triple-layer surface complexation model of all the experimental results required at least two reaction stoichiometries for glutamate adsorption, indicating the possible existence of at least two surface glutamate complexes. A possible mode of glutamate attachment involves a bridging-bidentate species binding through both carboxyl groups, which can be thought of as "lying down" on the surface (as found previously for amorphous titanium dioxide and hydrous ferric oxide). Another involves a chelating species which binds only through the gamma-carboxyl group, that is, "standing up" at the surface. The calculated proportions of these two surface glutamate species vary strongly, particularly with pH and glutamate concentration. Overall, our results serve as a basis for a better quantitative understanding of how and under what conditions acidic amino acids bind to oxide mineral surfaces.