Solubility of amorphous Th(IV) hydroxide - application of LIBD to determine the solubility product and EXAFS for aqueous speciation

The solubility of amorphous Th(IV) hydroxide at pH 3.0-13.5 and the aqueous speciation at pH < 4 are investigated in 0.5 M NaCl and 25 degreesC. The laser-induced breakdown detection (LIBD) is used to monitor the initial formation of thorium hydroxide colloids during the coulometric titration of 1.2 x 10(-1)-1.0 x 10(-5) M thorium solutions in the pH range of 2.7-4.5. The accurate solubility limit determined by this method is comparable with data measured from undersaturation with an X-ray amorphous solid precipitated at higher pH and dried at room temperature. Based on hydrolysis constants selected from the literature, the solubility product of Th(OH)(4) (am) in 0.5 M NaCl is calculated to be log K-sp(') = -44.48 +/- 0.24 and log K-sp(degrees) = -47.8 +/- 0.3 (converted to I = 0 with the SIT coefficients of the NEA-TDB). In other solubility studies with amorphous Th(IV) hydroxide or hydrous oxide, considerably higher thorium concentrations are measured at pH 3.5-5. Therefore, solutions of comparable H+ and thorium concentrations are prepared by careful coulometric titration and examined by ultrafiltration, LIBD and X-ray absorption fine structure (XAFS) spectroscopy. These measurements demonstrate the presence of a large amount of small Th(IV) colloids. The Th L-3 edge EXAFS spectra of these colloidal suspensions are similar to that of the amorphous solid.