Hydrolysis of Np(IV) and Pu(IV) and their complexation by aqueous Si(OH)4

Summary The hydrolysis and interaction of Np(IV) and Pu(IV) with orthosilicic acid, Si(OH)4, were studied in 0.1–1.0 M ionic strength aqueous solutions. Spectrophotometry was used to study these reactions at about 10–4M Np(IV) and Pu(IV) concentrations. The first hydrolysis constants,Khydr, agree with the majority of earlier spectrophotometric and potentiometric data. The absorption spectra of NpOH3+and PuOH3+were obtained by spectral deconvolution. Reasons to explain the overestimation ofKhydrobtained by other methods [by extraction of trace amounts of Np(IV) and Pu(IV) and by solubility] are discussed. Formation of the complexes NpOSi(OH)33+and PuOSi(OH)33+is demonstrated in the presence of 0.005–0.016 M Si(OH)4in the p[H+] range 1.0–2.2 and 0.3–1.4, respectively. Measured values of equilibrium constants of the reaction M4++ Si(OH)4↔ MOSi(OH)33++ H+ at ionic strengthI=1.0 are lgK1= 0.41±0.02 and 1.04±0.04, respectively, for Np(IV) and Pu(IV). The stability constants of the NpOSi(OH)33+and PuOSi(OH)33+complexes, recalculated to zero ionic strength, are lg β10= 11.2 and 11.8, respectively. The correlation betweenKhydrandK1, as observed for all earlier studied metal ions, also occurs for both Np(IV) and Pu(IV).