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

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 10-4 M 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 of Khydr obtained 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)4 in 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 strength I = 1.0 are lg K1 = 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 between Khydr and K1, as observed for all earlier studied metal ions, also occurs for both Np(IV) and Pu(IV). (orig.)