XAFS investigation of the structure of aqueous thorium(IV) species, colloids, and solid thorium(IV) oxide/hydroxide

X-ray absorption fine structure (XAFS) spectroscopy at the Th L3 edge is applied for the characterization of crystalline, anhydrous ThO2(cr), microcrystalline ThO2.xH(2)O(s), amorphous ThOn(OH)(4-2n).xH(2)O(am), aqueous Th(IV) solutions, and colloidal suspensions up to p(c)H 3.7. The microcrystalline, possibly hydrated thorium dioxide, is formed at p(c)H 1.5-2.5 by precipitation from suspensions of 16-23 nm thorium dioxide colloids. The solubility data determined for this solid is several orders of magnitude lower than the values for amorphous Th(IV) hydroxide or hydrous oxide. The EXAFS spectrum of the isolated microcrystalline particles shows that their structure is different from that of anhydrous crystalline ThO2(cr) and amorphous ThOn(OH)(4-2n).xH(2)O(am) precipitated at higher pH and dried at room temperature. The solubility measured for the amorphous Th(IV) precipitate is comparable to that previously reported for a solid prepared in a similar manner. In other solubility studies with amorphous Th(IV) hydroxide or hydrous oxide, considerably higher thorium concentrations are measured at pH 3.5-5. The aqueous speciation is made by EXAFS for solutions prepared by careful coulometric titration under comparable conditions (p(c)H and thorium concentration). The spectra of these solutions demonstrate the presence of a large amount of Th(IV) polynuclear species or colloids of small size, having a highly asymmetric Th-O coordination. The EXAFS spectrum of these colloids is similar to that of the amorphous solid.