The adsorption mechanism of Sr2+ on the surface of goethite

Adsorption to iron(oxyhydr)oxide minerals such as goethite (alpha-FeOOH) may be an important retardation factor for the mobility of Sr-90 from radioactive waste repositories. EXAFS spectroscopy was used here to examine the coordination sphere around Sr2+ sorbed to goethite at pH 9.2 and 10.2. At both pHs studied, Sr was found to be coordinated to 8.0 +/- 10 atoms at a distance of 2.61 +/- 0.02. At pH 10.2, a further coordination sphere of 1.8 +/- 0.5 Fe atoms at 4.29 +/- 0.02 Angstrom demonstrated that Sr was adsorbed to goethite by an inner-sphere mechanism. From the EXAFS data, the crystallographic structure and morphology of goethite, it is proposed that the inner-sphere coordination of Sr is via two oxygen atoms (A-type hydroxyls) bound to adjacent Fe(III)) atoms on the dominant (110) surface of goethite. This is the first study of Sr sorption to an (oxyhydr)oxide mineral at an atomistic level.