A spectroscopic study of the hydrolysis, colloid formation and solubility of Np(IV)

The hydrolysis, colloid formation and solubility of Np(IV) are investigated in aqueous HClO4-NaClO4 solutions (log [H+] = 0 to - 2.5) by absorption sp ectroscopy in the wavelength range of 680-1000nm. Applying Laser induced ph otoacoustic spectroscopy (LPAS) in the range of 680-760 nm, the study is ex tended to low Np(IV) concentrations of 10(-6) mol/l in DClO4-NaClO4-D2O sol utions up to log [D+] = -3.3. Laser induced breakdown detection (LIBD) demo nstrates the formation of Np(IV) colloids when the Np(IV) concentration exc eeds the solubility of Np(OH)(4)(am) at given pH. The simultaneous decrease of the Np(IV) absorption bands at 723 and 960 nm cannot be ascribed to the formation of the mononuclear complex Np(OH)(3+) as assumed in the literatu re. It is found to be caused by polynucleation. In undersaturated Np(IV) so lutions below 10(-4) mol/l. the position and intensity of the absorption ma ximum at 723 nm are practically insensitive to the pH change. In oversatura ted solutions the absorption band decreases significantly. The spectroscopi cally determined pH-dependent equilibrium concentration of mononuclear Np(I V) species above freshly formed solid or colloidal Np(IV) particles indicat es that Np(OH)(2)(2+) is the predominant species in the pH range of 1.5-3. This finding is in agreement with the Np(IV) hydrolysis constants reported in the literature from a solvent extraction study with Np-239(IV) trace con centrations. The solubility product of freshly formed Np(OH)?(am) par tides is determined to be log K ' (sp) = -54.4 +/- 0.4 in 0.1 M HClO4-NaClO4 and log K degrees (sp) = -56.5 +/- 0.4 (converted to I = 0 by applying the SIT ).