MECHANISMS OF CHROMIUM(III) SORPTION ON SILICA .1. CR(III) SURFACE-STRUCTURE DERIVED BY EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE SPECTROSCOPY

Metal ion reactions at the solid/solution interface are important in an array of disciplines and are of environmental significance as such reactions can greatly affect the risk imposed by metals. The structural environment of metals at the solid/water interface determines their potential for remobilization to the aqueous environment and the physical/chemical modifications of the sorbent. In this study, extended X-ray absorption fine structure (EXAFS) spectroscopy was used to discern the local structural environment of Cr(III) sorbed on silica. Chromium(III) formed a monodentate surface complex on silica, with a Cr-Si distance of 3.39 angstrom. At the surface coverages investigated, a polynuclear chromium hydroxide surface phase occurred with Cr-Cr distances of 2.99 angstrom, indicative of edge-sharing Cr octahedra. Crystallographic parameters resulting from the measured atomic distances dictate that the surface phase was most likely of the gamma-CrOOH-type local structure. Environmental considerations of Cr(III) remobilization must therefore consider the chemical/physical properties of the monodentate surface-complexed Cr(III) and surface-nucleated chromium hydroxide.