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).