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 a n array of disciplines and are of environmental significance as such r eactions can greatly affect the risk imposed by metals. The structural environment of metals at the solid/water interface determines their p otential for remobilization to the aqueous environment and the physica l/chemical modifications of the sorbent. In this study, extended X-ray absorption fine structure (EXAFS) spectroscopy was used to discern th e local structural environment of Cr(III) sorbed on silica. Chromium(I II) formed a monodentate surface complex on silica, with a Cr-Si dista nce of 3.39 angstrom. At the surface coverages investigated, a polynuc lear chromium hydroxide surface phase occurred with Cr-Cr distances of 2.99 angstrom, indicative of edge-sharing Cr octahedra. Crystallograp hic parameters resulting from the measured atomic distances dictate th at the surface phase was most likely of the gamma-CrOOH-type local str ucture. Environmental considerations of Cr(III) remobilization must th erefore consider the chemical/physical properties of the monodentate s urface-complexed Cr(III) and surface-nucleated chromium hydroxide.