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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 spectroscopy in the wavelength range of 680-1000 nm. Applying Laser induced photoacoustic spectroscopy (LPAS) in the range of 680-760 nm, the study is extended to low Np(IV) concentrations of 10-6 mol/l in DClO4-NaClO4-D2O solutions up to log [D+]=-3.3. Laser induced breakdown detection (LIBD) demonstrates the formation of Np(IV) colloids when the Np(IV) concentration exceeds 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 literature. It is found to be caused by polynucleation. In undersaturated Np(IV) solutions below 10-4 mol/l, the position and intensity of the absorption maximum at 723 nm are practically insensitive to the pH change. In oversaturated solutions the absorption band decreases significantly. The spectroscopically determined pH-dependent equilibrium concentration of mononuclear Np(IV) species above freshly formed solid or colloidal Np(IV) particles indicates that Np(OH)22+ 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 239Np(IV) trace concentrations. The solubility product of freshly formed Np(OH)4(am) particles is determined to be log K'sp = -54.4±0.4 in 0.1 M HClO4-NaClO4 and log K°sp=-56.5±0.4 (converted to I=0 by applying the SIT).