MECHANISMS OF CHROMIUM(III) SORPTION ON SILICA .2. EFFECT OF REACTION CONDITIONS

Metal ion retention on solids present in soils and waters decreases the risk of these contaminants as they are removed from the mobile aqueous phase. Determining the stability of the sorbed metal ions and the reaction conditions which influence the structure of the sorbate are necessary to evaluate the environmental risk of the sorbed moieties. In this study, a multitude of molecular level experimental techniques, including diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and high-resolution transmission electron microscopy (HRTEM), were employed to ascertain the Cr(III) surface structure on amorphous SiO2 (silica) and reaction conditions (pH, solution Cr concentrations, and surface coverage) that affect the sorption mechanism. Chromium(III) formed a monodentate surface complex on silica at surface coverages less than 20%, while at greater surface coverages discrete chromium hydroxide surface clusters were discerned. Only the extent of surface coverage was observed to influence the atomic structure of Cr(III) on silica.