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 log10ß°(UO2(OH)2SiO(OH)3-) and log10ß°(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.

Technetium Complexation with Multidentate Carboxylate-Containing Ligands: Trends in Redox and Solubility Phenomena.

The chemistry of technetium (t1/2(99Tc) = 2.11 × 105 years) is of particular importance in the context of nuclear waste disposal and historic contaminated sites. Polycarboxylate ligands may be present in some sites and are potentially capable of strong complexing interactions, thus increasing the solubility and mobility of 99Tc under environmentally relevant conditions. This work aimed to determine the impact of five organic complexing ligands [L = oxalate, phthalate, citrate, nitrilotriacetate (NTA), and ethylenediaminetetraacetate (EDTA)] under anoxic, alkaline conditions (pH ≈ 9-13) on the solubility of technetium. X-ray absorption spectroscopy confirmed that TcO2(am,hyd) remained the solubility-controlling solid phase in undersaturation solubility experiments. Ligands with maximum coordination numbers (CN) ≥ 3 (EDTA, NTA, and citrate) exhibited an increase in solubility from pH 9 to 11, while ligands with CN ≤ 2 (oxalate and phthalate) at all investigated pH and CN ≥ 3 at pH ≈ 13 were outcompeted by hydrolysis reactions. Though most available thermodynamic values were determined under acidic conditions, these models satisfactorily explained high-pH undersaturation solubility of technetium for citrate and NTA, whereas experimental data for Tc(IV)-EDTA were highly overestimated. This work illustrates the predominance of hydrolysis under hyperalkaline conditions and provides experimental support for existing thermodynamic models of Tc-L except Tc-EDTA, which requires further research regarding aqueous speciation and solubility.

Interlink between solubility, structure, surface and thermodynamics in the ThO2(s, hyd)-H2O(l) system.

The impact of temperature on a freshly precipitated ThO2(am, hyd) solid phase was investigated using a combination of undersaturation solubility experiments and a multi-method approach for the characterization of the solid phase. XRD and EXAFS confirm that ageing of ThO2(am, hyd) at T = 80°C promotes a significant increase of the particle size and crystallinity. TG-DTA and XPS support that the ageing process is accompanied by an important decrease in the number of hydration waters/hydroxide groups in the original amorphous Th(IV) hydrous oxide. However, while clear differences between the structure of freshly precipitated ThO2(am, hyd) and aged samples were observed, the characterization methods used in this work are unable to resolve clear differences between solid phases aged for different time periods or at different pH values. Solubility experiments conducted at T = 22°C with fresh and aged Th(IV) solid phases show a systematic decrease in the solubility of the solid phases aged at T = 80°C. In contrast to the observations gained by solid phase characterization, the ageing time and ageing pH significantly affect the solubility measured at T = 22°C. These observations can be consistently explained considering a solubility control by the outermost surface of the ThO2(s, hyd) solid, which cannot be properly probed by any of the techniques considered in this work. Solubility data are used to derive the thermodynamic properties (log *K°s,0, Δf G°m) of the investigated solid phases, and discussed in terms of particle size using the Schindler equation. These results provide new insights on the interlink between solubility, structure, surface and thermodynamics in the ThO2(s, hyd)-H2O(l) system, with special emphasis on the transformation of the amorphous hydrous/hydroxide solid phases into the thermodynamically stable crystalline oxides.

Crystal Structure and Stability in Aqueous Solutions of Na0.5[NpO2(OH)1.5]·0.5H2O and Na[NpO2(OH)2].

The ternary neptunium(V) (Np(V)) hydroxides Na0.5[NpO2(OH)1.5]·0.5H2O (I) and Na[NpO2(OH)2] (II) were synthesized in aqueous NaOH solutions at T = 80 °C, and their crystal structures were determined to be monoclinic, P21, Z = 2, a = 5.9859(2), b = 10.1932(3), c = 12.1524(4) Å, ß = 98.864(1)°, V = 732.63(4) Å3 for (I) and orthorhombic, P212121, Z = 4, a = 5.856(7), b = 7.621(9), c = 8.174(9) Å, V = 364.8(7) Å3 for (II). By combining the detailed structural information with results from systematic solubility investigations, a comprehensive chemical and thermodynamic model of the Np(V) behavior in NaCl-NaOH solutions was evaluated. The results reveal a great stability of the ternary Na-Np(V)-OH solid phases that significantly enhances the predominance field of the entire Np(V) redox state to high alkalinity.

Pu(iii) and Cm(iii) in the presence of EDTA: aqueous speciation, redox behavior, and the impact of Ca(ii).

The impact of calcium on the solubility, redox behavior, and speciation of the An(iii)-EDTA (An = Pu or Cm) system under reducing, anoxic conditions was investigated through batch solubility experiments, X-ray absorption spectroscopy (XAS), density functional theory (DFT), and time-resolved laser fluorescence spectroscopy (TRLFS). Batch solubility experiments were conducted from undersaturation using Pu(OH)3(am) as the solid phase in contact with 0.1 M NaCl-NaOH-HCl-EDTA-CaCl2 solutions at [EDTA] = 1 mM, pHm = 7.5-9.5, and [CaCl2] ≤20 mM. Additional samples targeted brine systems represented by 3.5 M CaCl2 and WIPP simulated brine. Solubility data in the absence of calcium were well-described by Pu(iii)-EDTA thermodynamic models, thus supporting the stabilization of Pu(iii)-EDTA complexes in solution. Cm(iii)-EDTA TRLFS data suggested the stepwise hydrolysis of An(iii)-EDTA complexes with increasing pH, and current Pu(iii)-EDTA solubility models were reassessed to evaluate the possibility of including Pu(iii)-OH-EDTA complexes and to calculate preliminary formation constants. Solubility data in the presence of calcium exhibited nearly constant log m(Pu)tot, as limited by total ligand concentration, with increasing [CaCl2]tot, which supports the formation of calcium-stabilized Pu(iii)-EDTA complexes in solution. XAS spectra without calcium showed partial oxidation of Pu(iii) to Pu(iv) in the aqueous phase, while calcium-containing experiments exhibited only Pu(iii), suggesting that Ca-Pu(iii)-EDTA complexes may stabilize Pu(iii) over short timeframes (t ≤45 days). DFT calculations on the Ca-Pu(iii)-EDTA system and TRLFS studies on the analogous Ca-Cm(iii)-EDTA system show that calcium likely stabilizes An(iii)-EDTA complexes but can also potentially stabilize An(iii)-OH-EDTA species in solution. This hints towards the possible existence of four major complex types within Ca-An(iii)-EDTA systems: An(iii)-EDTA, An(iii)-OH-EDTA, Ca-An(iii)-EDTA, and Ca-An(iii)-OH-EDTA. While the exact stoichiometry and degree of ligand protonation within these complexes remain undefined, their formation must be accounted for to properly assess the fate and transport of plutonium under conditions relevant to nuclear waste disposal.

Ammonium Pertechnetate in Mixtures of Trifluoromethanesulfonic Acid and Trifluoromethanesulfonic Anhydride.

Ammonium pertechnetate reacts in mixtures of trifluoromethanesulfonic anhydride and trifluoromethanesulfonic acid under final formation of ammonium pentakis(trifluoromethanesulfonato)oxidotechnetate(V), (NH4 )2 [TcO(OTf)5 ]. The reaction proceeds only at exact concentrations and under the exclusion of air and moisture via pertechnetyl trifluoromethanesulfonate, [TcO3 (OTf)], and intermediate TcVI species. 99 Tc nuclear magnetic resonance (NMR) has been used to study the TcVII compound and electron paramagnetic resonance (EPR), 99 Tc NMR and X-ray absorption near-edge structure (XANES) experiments indicate the presence of the reduced technetium species. In moist air, (NH4 )2 [TcO(OTf)5 ] slowly hydrolyses under formation of the tetrameric oxidotechnetate(V) (NH4 )4 [{TcO(TcO4 )4 }4 ] ⋅10 H2 O. Single-crystal X-ray crystallography was used to determine the solid-state structures. Additionally, UV/Vis absorption and IR spectra as well as quantum chemical calculations confirm the identity of the species.

A Combined Study of Tc Redox Speciation in Complex Aqueous Systems: Wet-Chemistry, Tc K-/L3-Edge X-ray Absorption Fine Structure, and Ab Initio Calculations.

The combination of wet-chemistry experiments (measurements of pH, Eh, and [Tc]) and advanced spectroscopic techniques (K- and L3-edge X-ray absorption fine structure spectroscopy) confirms the formation of a very stable Tc(V)-gluconate complex under anoxic conditions. In the presence of gluconate and an excess of Sn(II) (at pe + pH ≈ 2), technetium forms a very stable Tc(IV)-gluconate complex significantly enhancing the solubility defined by TcO2(s) in hyperalkaline gluconate-free systems. A new setup for "tender" X-ray spectroscopy (spectral range, ∼2-5 keV) in transmission or total fluorescence yield detection mode based on a He flow cell has been developed at the INE Beamline for radionuclide science (KIT light source). This setup allows handling of radioactive specimens with total activities up to one million times the exemption limit. For the first time, Tc L3-edge measurements (∼2.677 keV) of Tc species in liquid (aqueous) media are reported, clearly outperforming conventional K-edge spectroscopy as a tool to differentiate Tc oxidation states and coordination environments. The coupling of L3-edge X-ray absorption near-edge spectroscopy measurements and relativistic multireference ab initio methods opens new perspectives in the definition of chemical and thermodynamic models for systems of relevance in the context of nuclear waste disposal, environmental, and pharmaceutical applications.

Impact of Ca(II) on the aqueous speciation, redox behavior, and environmental mobility of Pu(IV) in the presence of EDTA.

The impact of calcium on the solubility and redox behavior of the Pu(IV)-EDTA system was investigated using a combination of undersaturation solubility studies and advanced spectroscopic techniques. Batch solubility experiments were conducted in 0.1 M NaCl-NaOH-HCl-EDTA-CaCl2 solutions at constant [EDTA] = 1∙10-3 M, 1 ≤ pHm ≤ 11, and 1∙10-3 M ≤ [CaCl2] ≤ 2∙10-2 M. Additional samples targeted brine systems represented by 3.5 M CaCl2 and WIPP simulated brine. Redox conditions were buffered with hydroquinone (pe + pH ≈ 9.5) with selected samples prepared in the absence of any redox buffer. All experiments were performed at T = 22 °C under Ar atmosphere. In-situ X-ray absorption spectroscopy indicated that PuO2(ncr,hyd) was the solubility-controlling phase during the lifetime of all experiments and that aqueous plutonium was present in the +IV oxidation state across all experimental conditions except at pHm ≈ 1, where a small fraction of Pu(III) was also identified. Current thermodynamic models overestimate Pu(IV)-EDTA solubility in the absence of calcium by approximately 1-1.5 log10-units and do not describe the nearly pH-independent, increased solubility observed with increased calcium concentrations. The ternary Pu(IV)-OH-EDTA system without calcium was reevaluated using solubility data obtained in this work and reported in the literature. An updated thermodynamic model including the complexes Pu(OH)(EDTA)-, Pu(OH)2(EDTA)2-, and Pu(OH)3(EDTA)3- was derived. Solubility data collected in the presence of calcium follows a pH-independent trend (log m(Pu)tot vs. pHm), which can only be explained by assuming the formation of a quaternary complex, tentatively defined as CaPu(OH)4(EDTA)2-, in solution. The significant enhancement of plutonium solubility observed in the investigated brine systems supports the formation of a quaternary complex that is not outcompeted by Ca(EDTA)2-, even in concentrated CaCl2 solutions. Although the exact stoichiometry of the complex may need to be revisited, this new quaternary complex has a pronounced impact on plutonium predominance diagrams over a broad range of pH, pe, and calcium concentrations that are relevant to nuclear waste disposal.

Exploring the electronic structure and speciation of aqueous and colloidal Pu with high energy resolution XANES and computations.

Pu L3 HR-XANES and FEFF9 computations provide evidence for band-like 6d states in colloidal Pu contrasting to narrow 6d states in molecular Pu(iv). Pu L3 HR-XANES is valuable for bond length estimation in plutonyl, whereas Pu M5 HR-XANES is an advanced tool for analysing Pu redox states and 5f unoccupied density of states.

Systematic XAS study on the reduction and uptake of Tc by magnetite and mackinawite.

The mechanisms for the reduction and uptake of Tc by magnetite (Fe3O4) and mackinawite (FeS) are investigated using X-ray absorption spectroscopy (XANES and EXAFS), in combination with thermodynamic calculations of the Tc/Fe systems and accurate characterization of the solution properties (pHm, pe, [Tc]). Batch sorption experiments were performed under strictly anoxic conditions using freshly prepared magnetite and mackinawite in 0.1 M NaCl solutions with varying initial Tc(vii) concentrations (2 × 10-5 and 2 × 10-4 M) and Tc loadings (400-900 ppm). XANES confirms the complete reduction of Tc(vii) to Tc(iv) in all investigated systems, as predicted from experimental (pHm + pe) measurements and thermodynamic calculations. Two Tc endmember species are identified by EXAFS in the magnetite system, Tc substituting for Fe in the magnetite structure and Tc-Tc dimers sorbed to the magnetite {111} faces through a triple bond. The sorption endmember is favoured at higher [Tc], whereas incorporation prevails at low [Tc] and less alkaline pH conditions. The key role of pH in the uptake mechanism is interpreted in terms of magnetite solubility, with higher [Fe] and greater recrystallization rates occurring at lower pH values. A TcSx-like phase is predominant in all investigated mackinawite systems, although the contribution of up to 20% of TcO2·xH2O(s) (likely as surface precipitate) is observed for the highest investigated loadings (900 ppm). These results provide key inputs for an accurate mechanistic interpretation of the Tc uptake by magnetite and mackinawite, so far controversially discussed in the literature, and represent a highly relevant contribution to the investigation of Tc retention processes in the context of nuclear waste disposal.

Thermodynamic description of Tc(iv) solubility and hydrolysis in dilute to concentrated NaCl, MgCl2 and CaCl2 solutions.

We present the first systematic investigation of Tc(iv) solubility, hydrolysis and speciation in dilute to concentrated NaCl, MgCl2 and CaCl2 systems, and comprehensive thermodynamic and activity models for the system Tc(4+)-H(+)-Na(+)-Mg(2+)-Ca(2+)-OH(-)-Cl(-)-H2O using both SIT and Pitzer approaches. The results are advancing the fundamental scientific understanding of Tc(iv) solution chemistry and are highly relevant in the applied context of nuclear waste disposal. The solubility of Tc(iv) was investigated in carbonate-free NaCl-NaOH (0.1-5.0 M), MgCl2 (0.25-4.5 M) and CaCl2 (0.25-4.5 M) solutions within 2 ≤ pHm≤ 14.5. Undersaturation solubility experiments were performed under an Ar atmosphere at T = 22 ± 2 °C. Strongly reducing conditions (pe + pHm≤ 2) were imposed with Na2S2O4, SnCl2 and Fe powder to stabilize technetium in the +IV redox state. The predominance of Tc(iv) in the aqueous phase was confirmed by solvent extraction and XANES/EXAFS spectroscopy. Solid phase characterization was accomplished after attaining thermodynamic equilibrium using XRD, SEM-EDS, XANES/EXAFS, TG-DTA and quantitative chemical analysis, and indicated that TcO2·0.6H2O(s) exerts solubility-control in all evaluated systems. The definition of the polyatomic Tc3O5(2+) species instead of TcO(2+) is favoured under acidic conditions, consistently with slope analysis (mTcvs. pHm) of the solubility data gained in this work and spectroscopic evidence previously reported in the literature. The additional formation of Tc(iv)-OH/O-Cl aqueous species in concentrated chloride media ([Cl(-)] = 9 M) and pHm≤ 4 is suggested by solubility and EXAFS data. The pH-independent behaviour of the solubility observed under weakly acidic to weakly alkaline pHm conditions can be explained with the equilibrium reaction TcO2·0.6H2O(s) + 0.4H2O(l) ⇔ TcO(OH)2(aq). Solubility data determined in dilute NaCl systems with pHm≥ 11 follow a well-defined slope of +1, consistent with the predominance of TcO(OH)3(-) previously selected by NEA-TDB. In concentrated MgCl2 and CaCl2 solutions with pHm≥ 8, the formation of the ternary Mg3[TcO(OH)5](3+) and Ca3[TcO(OH)5](3+) species is proposed based on the slope analysis of the solubility data, model calculations and previous observations for analogous An(iv) and Zr(iv) systems. The formation and stability of these hitherto unknown Tc(iv) species are supported by DFT calculations. Based on the newly generated experimental data and previous spectroscopic observations, new comprehensive chemical, thermodynamic and activity models (SIT, Pitzer) for these systems are derived.

Oxidation state and local structure of plutonium reacted with magnetite, mackinawite, and chukanovite.

Due to their redox reactivity, surface sorption characteristics, and ubiquity as corrosion products or as minerals in natural sediments, iron(II)-bearing minerals control to a large extent the environmental fate of actinides. Pu-L(III)-edge XANES and EXAFS spectra were used to investigate reaction products of aqueous (242)Pu(III) and (242)Pu(V) reacted with magnetite, mackinawite, and chukanovite under anoxic conditions. As Pu concentrations in the liquid phase were rapidly below detection limit, oxidation state and local structure of Pu were determined for Pu associated with the solid mineral phase. Pu(V) was reduced in the presence of all three minerals. A newly identified, highly specific Pu(III)-sorption complex formed with magnetite. Solid PuO(2) phases formed in the presence of mackinawite and chukanovite; in the case of chukanovite, up to one-third of plutonium was also present as Pu(III). This highlights the necessity to consider, under reducing anoxic conditions, Pu(III) species in addition to tetravalent PuO(2) for environmental risk assessment. Our results also demonstrate the necessity to support thermodynamic calculations with spectroscopic data.