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Two series of Th 1- x Zr x SiO 4 phases were synthesized hydrothermally under weakly basic (pH = 8) and strongly acidic (pH = 1) conditions. Changes in pH were found to have a significant effect on experimental phase diagrams. Synthesis at pH = 8 favors the formation of Th-rich phases with resulting Th 1- x Zr x SiO 4 solid solution for x = 0 – 0.5. Contrary, synthesis at pH = 1 results in the formation of pure end-members of the ThSiO 4 -ZrSiO 4 pseudo-binary system separated by multiple miscibility gaps. Phases formed both under basic and acidic conditions were found to retain water, which can be discharged from the structure upon heating. A different high-pressure (HP) behaviour was found for Th-rich and Zr-rich solid solutions. While Th-rich Th 0.9 Zr 0.1 SiO 4 and Th 0.6 Zr 0.4 SiO 4 phases retain their stoichiometry and crystal structure upon compression at HP, a significant reduction of the Th occupancy related to a decrease of the Th-O distances is observed for the Th-poor Th 0.26 Zr 0.74 SiO 4 phase at P > 8 GPa, with the subsequent formation of a Th-rich amorphous phase. The Th diffusion between the crystalline and amorphous phases was found to be fully reversible. This unique self-healing property makes these phases promising candidates for nuclear applications under extreme pressure and temperature conditions, in particular those found in underground repositories. [Display omitted] [ABSTRACT FROM AUTHOR]

Removing f -elements from anthropogenically contaminated sites is a challenging, but ecologically important task. Some of these elements are not only radioactive, but also chemically toxic and can spread through various pathways in the environment. The present work investigates f -element sorption on biogenic silica, which may be a promising "green" material for remediation. Commercially available diatomaceous earth (DE) and the cleaned cell walls of the diatom species Stephanopyxis turris (S.t.) and Thalassiosira pseudonana (T.p.) are compared with artificial mesocellular foam (MCF) as porous silica reference material. Trivalent europium was chosen as model sorptive for chemically similar trivalent actinides. Accordingly, Eu(III) in concentrations of 10−3 M and 10−5 M was sorbed on the four silica materials at varying pH values. The zeta potentials of the implemented sorbents under the same conditions were determined. In addition, the sorption reaction and the aqueous speciation of Eu(III) in the (bio)silica suspensions were modeled using the Diffuse Double Layer (DDL) model. With time-resolved laser-induced fluorescence spectroscopy (TRLFS), two different uptake mechanisms were discerned, surface adsorption and incorporation/precipitation. [Display omitted] • The sorption of Eu(III) on biogenic and non-biogenic silica was investigated. • Sorbates were characterized with ICP-OES, SEM/EDX, TRLFS, and SCM. • Different Eu species could be discerned coinciding with the sorbent's SSA. • Origin and type of impurity elements of the silica influences the sorption. [ABSTRACT FROM AUTHOR]

Pseudomonas sp. are indigenous inhabitants of ombrotrophic bogs which can survive in acidic, nutrient-poor environments with wide temperature fluctuations. Their interactions with contaminant radionuclides can influence radionuclide biogeochemistry in boreal environment. Here, uranium (U(VI)) bioassociation by Pseudomonas sp. PS-0-L, V4-5-SB and T5-6-I isolated from a boreal bog was studied by a combination of batch contact experiments, spectroscopy and microscopy. All strains removed U from the solution and the U bioassociation efficiency was affected by the nutrient source, incubation temperature, time and pH. Highest U bioassociation occurred in the strains PS-0-L (0.199 mg U/g BDW) and V4-5-SB (0.223 mg U/g BDW). Based on in-situ attenuated total reflection Fourier transformation infrared spectroscopy (ATR FT-IR) analyses, the most likely functional groups responsible for U binding were the cell surface carboxyl groups. In addition, transmission electron microscopy with energy dispersive X-ray spectroscopy (TEM/EDX) showed dense intra-cellular round- and needle-like U accumulations in the cytoplasm and near to the inner cell membrane. The presence of U with phosphorus was indicated in elemental mapping. Modelled data showed ≡ SOOHx−1 and ≡ SOCO 2 Hx−1 as the dominant surface sites, contributing to the negative cell surface charge. The U removal efficiency depended on the U(VI) speciation under different pH conditions. At pH 5, the main species reacting with bacterial cell surfaces was UO 2 2+, while at pH 9 UO 2 (OH) 2 and UO 2 (OH) 3 - dominated the reactions. Further, U bioassociation increased with increasing aqueous U(VI) concentrations. Our data suggests U bioassociation on 1) outer cell membrane/cell wall associated carboxyl groups (e.g., proteins), and 2) intracellular phosphate groups (e.g., phospholipids). • U(VI) bioassociation by Pseudomonas strains isolated from a boreal bog was studied. • U removal was affected by incubation time, temperature, pH, and nutrient conditions. • U biosorption on outer cell membrane/cell wall associated carboxyl groups. • U bioaccumulation on intracellular phosphate groups (e.g., phospholipids). • A novel U bioassociation model approach was developed (used PHREEQC). [ABSTRACT FROM AUTHOR]

Environmental conditions in deep geological repositories for radioactive waste may involve high pH values due to the degradation of concrete. However, the U(VI) sorption at such (hyper)alkaline conditions is still poorly understood. In this study, batch sorption experiments with Ca-bentonite in the pH range 8–13 at different carbonate concentrations were combined with spectroscopic investigations in order to gain insight into the underlying retention mechanisms. It was found that U(VI) sorption strongly correlates with the aqueous U(VI) speciation determined by time-resolved laser-induced luminescence spectroscopy (TRLFS). Increasing retention with increasing pH was accompanied by a change in aqueous speciation from uranyl carbonates to uranyl hydroxides. The occurrence of luminescence line-narrowing and a decreased frequency of the symmetric stretch vibration, deduced from site-selective TRLFS, indicate the presence of adsorbed U(VI) surface complexes. X-ray absorption fine structure (EXAFS) spectroscopy confirms that surface precipitation does not contribute significantly to the removal of U(VI) from solution but that retention occurs through the formation of two non-equivalent U(VI)-complexes on the bentonite surface. The present study demonstrates that in alkaline environments, where often only precipitation processes are considered, adsorption can provide effective retention of U(VI), despite the anionic character of prevailing aqueous species. Unlabelled Image • U(VI) sorption on clay minerals at (hyper)alkaline conditions is poorly understood. • In this study, spectroscopic techniques revealed underlying retention mechanisms. • At pH 10–13, U(VI) is retained by adsorption of two independent species. • Adsorption is realized despite an anionic character of predominant aqueous species. • With increasing pH, a shift to a 4-fold coordination of U(VI) is observed. [ABSTRACT FROM AUTHOR]

• A log β0 for the UO 2 OSi(OH) 3 + complex was determined with luminescence spectroscopy. • The molal enthalpy and entropy of reaction were determined in the T-range 1–25 °C. • Silicate monomers and oligomers were detected in solution using ESI-MS. • The abundance of polysilicates was influenced by pH and the U/Si concentrations. In this study the complexation of U(VI) with orthosilicic acid (H 4 SiO 4) 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 · 10−3 M (exceeding the solubility of amorphous silica at both pH-values) revealed the formation of oligomeric sodium-silicates in addition to the UO 2 OSi(OH) 3 + species. For the luminescence spectroscopic experiments (25 °C), the U(VI) concentration was fixed at 5 · 10−6 M, the silicon concentration was varied between 1.3 · 10−4–1.3 · 10−3 M (reducing the formation of silicon oligomers) and the ionic strength was kept constant at 0.2 M NaClO 4. The results confirmed the formation of the aqueous UO 2 OSi(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 r H m 0 = 45.8 ± 22.5 kJ·mol−1 and molal entropy of reaction Δ r S m 0 = 152.5 ± 78.8 J·K−1·mol−1 using the integrated van't Hoff equation, corroborating an endothermic and entropy driven complexation process. [ABSTRACT FROM AUTHOR]