Sorption Mechanisms on Oxide Surfaces

Retention reactions on particulate of soils and waters are of environmental and agricultural significance because these reactions can greatly affect the risk imposed by contaminants and the availability of plant nutrients. The surface structure of the sorbate determines their potential for remobilization to the aqueous environment and the physicaUchemical modifications of the sorbent. Therefore, it is essential to employ experimental techniques that provide direct and accurate information on sorption mechanisms. In this study, the use of multiple surface probing techniques for investigating sorption reactions is demonstrated for Cr(I1!) retention on a commonly occurring natural sorbent, amorphous Si02 (silica). Extended X-ray absorption fine structure (EXAFS) spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), high-resolution transmission electron microscopy (HRTEM), and surface probing microscopy (SPM) were used to discern the structural environment of Cr(Ill) on silica. Chromium(III) formed a monodentate surface complex on silica, which with increased Cr(III) levels resulted in a nucleated y-CrOOH type structure. The initial nucleation expanded on the silica surface, but as precipitation progressed discrete highly-ordered y-CrOOH surface clusters formed rather than a precipitate that was distributed over the surface. ' Consequently, when modeling the retention reaction surface complexation and precipitation must be considered; desorption would be dominated by the dissolution of the chromium hydroxide surface clusters