Combined LIBD and XAFS investigation of the formation and structure of Zr(IV) colloids

The solubility of Zr(OH)4(am)--in other words hydrated Zr(IV) oxyhydroxide--is determined by means of coulometric titration (CT), and colloids are detected by laser-induced breakdown when the solubility limit is exceeded. Our results at pH 3-8 demonstrate that the solubility of Zr(OH)4(am) is several orders of magnitude higher than reported classical solubility data for acidic solutions, determined from undersaturation with a less soluble microcrystalline Zr(IV) oxide precipitate. Analysis of extended X-ray absorption fine structure (EXAFS) data shows that the microcrystalline colloids in a 0.1 mol l(-1) Zr aqueous solution at pH 0.2 contain tetrameric units, similar to those present in the structure of ZrOCl2.8H2O. Characterization of the CT solutions by means of EXAFS shows that oligomeric species form as the solubility limit is approached. The current lack of data on equilibrium constants for polynuclear hydroxide complexes prohibits the use of a realistic speciation model to describe the solubility of pH-dependent Zr(OH)4(am). However, the solubility curve is obtained using the mononuclear hydrolysis constants estimated in the present paper, along with the solubility constant (log K'sp=-49.9+/-0.5 in 0.5 mol l(-1) NaCl; log K degrees(sp)=-53.1+/-0.5 at I=0).