Thermodynamic study of metal silicate complexation in perchlorate media

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.

Temperature-dependent luminescence spectroscopic and mass spectrometric investigations of U(VI) complexation with aqueous silicates in the acidic pH-range

In this study the complexation of U(VI) with orthosilicic acid (H₄SiO₄) was investigated between pH 3.5 and 5 by combining electrospray ionization mass spectrometry (ESI-MS) and laser-induced luminescence spectroscopy. The ESI-MS experiments performed at a total silicon concentration of 5 · 10^-3M (exceeding the solubility of amorphous silica at both pH-values) revealed the formation of oligomeric sodium-silicates in addition to the UO₂OSi(OH)₃⁺ species. For the luminescence spectroscopic experiments (25 °C), the U(VI) concentration was fixed at 5 · 10^-6M, the silicon concentration was varied between 1.3 · 10^-4-1.3 · 10^-3M (reducing the formation of silicon oligomers) and the ionic strength was kept constant at 0.2 M NaClO₄. The results confirmed the formation of the aqueous UO₂OSi(OH)₃⁺ complex. The conditional complexation constant at 25 °C, log *β = -(0.31 ± 0.24), was extrapolated to infinite dilution using the Davies equation, which led to log *β⁰ = -(0.06 ± 0.24). Further experiments at different temperatures (1-25 °C) allowed the calculation of the molal enthalpy of reaction Δ_rH_m⁰ = 45.8 ± 22.5 kJ·mol^-1 and molal entropy of reaction Δ_rS_m⁰ = 152.5 ± 78.8 J·K^-1·mol^-1 using the integrated van't Hoff equation, corroborating an endothermic and entropy driven complexation process.

Effect of complexing anions on europium sorption on suspended silica: a TRLFS study for ternary complex formation

The sorption of europium, Eu(III), on suspended silica was studied in perchlorate media under varying concentrations of Eu(III) and silica, at pH 4 to 8, and ionic strength from 0.20 to 1.40 M NaClO4at 298 K. The complexing anions, carbonate, acetate, citrate, oxalate, phosphate and EDTA, affected the sorption of Eu(III). A synergistic enhancement in the sorption of Eu(III) was observed for oxalate and phosphate ligands. Other anions suppressed the sorption in the order: acetate < carbonate < citrate < EDTA. The results of TRLFS studies were consistent with ternary Eu/silica/ligand complexation for oxalate and phosphate anions.

Sorption of europium(III) on polymeric silica in perchlorate media

Sorption of europium, Eu(III) with polymeric silicic acid has been studied under varying experimental conditions such as pH, ionic strength (I), concentration of silicic acid and of Eu(III), the presence of carbonate ions, and aging time. Sorption studies of Eu(III) at pH 7.50 ± 0.50) andI= 0.20 M NaClO4indicate 1.32 ± 0.06 × 10-5M Eu as the Critical Coagulation Concentration, CCC. Sorption of Eu(III) on colloidal silica increases with increased ionic strength from 80% (0.20 M NaClO4) to 93% (0.89 M NaClO4). The coagulation process is favored by an increase in silica concentration. The influence of CO32-(1.00 × 10-3M) on the reaction of Eu(III) with polysilicic acid was negligible in the pH range 7.0 to 9.0. Dissolved Eu3+binds more strongly to the silicate polymers than do Cs+and Cu2+.

Sorption behavior of curium(III) on hydrous silica: a kinetic and thermodynamic study

Sorption of tracer curium, Cm3+, on SiO2· H2O (silica gel) was investigated under different experimental conditions such as time of equilibration, concentration of Sm3+, pcH, and temperature. Sm3+was used as a carrier for Cm3+ions in this study. From the sorption kinetics data, the diffusion coefficient of Cm3+ion was calculated to be 1.20(±0.05)×10-11m2s-1under particle diffusion-controlled conditions. The sorption rate was determined to be 3.47(±0.19)×10-3s-1at 298 K, pcH 5.25(±0.03) in 0.20 M (NaClO4). The sorption data were fitted in Freundlich, Langmuir and Dubinin–Radushkevich (D–R) isotherms. A synergistic enhancement in the sorption of Cm3+(Sm3+)-ligand (e.g., CO32-, F-, H2PO4-and C2O42-) complexes on silica gel was observed. The presence of these anions increase the sorption of Cm3+(Sm3+) in the following order: CO32-< F-< C2O42-< H2PO4-. Temperature variation experiments gave enhanced sorption with increased temperature for Cm3+indicating an endothermic enthalpy.