Formation of americium(III) complexes with aqueous silicic acid

Experimental and computational evidence of U(VI)-OH-Si(OH)4 complexes under alkaline conditions: Implications for cement systems

The complexation of uranyl hydroxides with orthosilicic acid was investigated by experimental and theoretical methods. Spectroluminescence titration was performed in a glovebox under argon atmosphere at pH 9.2, 10.5 and 11.5, with [U(VI)] = 10-6 and 5 × 10-6 mol kgw-1. The polymerization effects of silicic acid were minimized by ruling out samples with less than 90 % monomeric silicic acid present, identified via UV-Vis spectrometry using the molybdate blue method. Linear regression analysis based on time-resolved laser-induced fluorescence spectroscopy (TRLFS) results yielded the conditional stepwise formation constants of U(VI)-OH-Si(OH)4 complexes at 0.05 mol kgw-1 NaNO3. The main spectroscopic features - characteristic peak positions and decay-time - are reported for the first time for the UO2(OH)2SiO(OH)3- species observed at pH 9.2 and 10.5 and UO2(OH)2SiO2(OH)22- predominant at pH 11.5. Quantum chemical calculations successfully computed the theoretical luminescence spectrum of the complex UO2(OH)2SiO(OH)3- species, thus underpinning the proposed chemical model for weakly alkaline systems. The conditional stability constants were extrapolated to infinite dilution using the Davies equation, resulting in log10β°(UO2(OH)2SiO(OH)3-) and log10β°(UO2(OH)2SiO2(OH)22-). Implications for U(VI) speciation in the presence and absence of competing carbonate are discussed for silicate-rich environments expected in certain repository concepts for nuclear waste disposal.

Complexation of Cm(III) with aqueous silicic acid

The complexation of248Cm(III) with aqueous silicic acid is investigated in the pH range of 1.5–9.0 in 0.03 M NaCl with varying the silicate concentration under and over saturation in reference to the solubility of amorphous silica in order to characterize the reaction with monosilicic and polysilicic acid. Speciation is made by time-resolved laser fluorescence spectroscopy (TRLFS) in combination with radiometric quantification. Three different complexation products are observed: Cm-silicate(I), Cm-silicate(II) and Cm-silicate(III). The first and second species are formed in under-saturation of silicic acid, whereas all three species are produced in over-saturation of silicic acid. The formation of Cm-silicate(I) following the reaction of Cm3+with H3SiO4−appears in both under and over-saturation of silicic acid only as a minor fraction at pH=4–7. Cm-silicate(II) and Cm-silicate(III) are found to be colloidal. Cm-silicate(II) shows its spectroscopic characteristics varying with the experimental condition, whereas Cm-silicate(III) formed exclusively with polysilicic acid remains consistent and stable. The experimental results indicate that a species conversion takes place as Cm-silicate(I) → Cm-silicate(II) → Cm-silicate(III) with increasing pH and silicon concentration. Only the reaction leading to the formation of Cm-silicate(I) can be quantified for its stability constant as logβ°=7.74±0.08.

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 10–4M 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).