David Fellhauer

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.

Redox behavior of Tc(VII)/Tc(IV) under various reducing conditions in 0.1 M NaCl solutions

Abstract Redox behaviour of Tc(VII)/Tc(IV) was investigated in 0.1 M NaCl solutions containing different reducing agents in the pH range 2 to 13 at 22 ºC under inert Ar atmosphere. In several samples, the 1 × 105 mol/dm3 (M) initially added TcO4- was reduced to form a Tc(IV) oxide solid phase with low solubility. The observed Tc redox transformation processes are systematized according to Eh-pH conditions in solution, indicating that a borderline for the reduction of Tc(VII) to Tc(IV), TcO4- + 3e- + 4H+⇔TcO2· xH2O(coll, hyd) + (2-x)H2O exists, independent of the reducing chemical system. This experimentally derived borderline is about 100 mV lower than the equilibrium line calculated from the reported standard redox potential of TcO2· 1.6H2O(s). This behaviour can be related to the existence of more soluble solid phase modifications, i.e. nanoparticulate Tc(IV) oxide species (TcO2· xH2O(coll, hyd)). The reaction kinetics likewise correlate to the redox potential measured in solution. Slow reduction of Tc(VII) to Tc(IV) was observed when the redox potential in the system was slightly below the above mentioned reduction borderline. Fast reduction was observed in the systems far below the borderline, but also in those systems containing Fe(II) solids, suggesting a specific surface mediated effect in the reduction process. EXAFS analysis on two magnetite samples indicate reduced Tc(IV) species which do not remain adsorbed at the reactive mineral surface and are incorporated in the magnetite structure.

The role of the 5f valence orbitals of early actinides in chemical bonding

One of the long standing debates in actinide chemistry is the level of localization and participation of the actinide 5f valence orbitals in covalent bonds across the actinide series. Here we illuminate the role of the 5f valence orbitals of uranium, neptunium and plutonium in chemical bonding using advanced spectroscopies: actinide M4,5 HR-XANES and 3d4f RIXS. Results reveal that the 5f orbitals are active in the chemical bonding for uranium and neptunium, shown by significant variations in the level of their localization evidenced in the spectra. In contrast, the 5f orbitals of plutonium appear localized and surprisingly insensitive to different bonding environments. We envisage that this report of using relative energy differences between the 5fδ/ϕ and 5fπ*/5fσ* orbitals as a qualitative measure of overlap-driven actinyl bond covalency will spark activity, and extend to numerous applications of RIXS and HR-XANES to gain new insights into the electronic structures of the actinide elements.

Np(V) solubility, speciation and solid phase formation in alkaline CaCl2 solutions. Part I: Experimental results

Abstract The aqueous chemistry of Np(V) in alkaline 0.01 to 5.5 M CaCl2 solutions at T = 23 ± 2 ℃ was thoroughly studied by long-term batch solubility experiments from both over- and undersaturation. Applying a comprehensive set of experimental and spectroscopic techniques including Vis/NIR and Np L3-edge EXAFS, the solubility controlling Np(V) solid phases and the predominant aqueous Np(V) species were identified. The results demonstrate that the solubility behavior of Np(V) in alkaline CaCl2 solutions differs completely from the one reported for alkaline NaCl and NaClO4 solutions: as solubility limiting solid phases, three so far not considered Ca-Np(V)-OH compounds were identified and their solubility and stability in CaCl2 solutions analyzed: CaNpO2(OH)2.6Cl0.4 · 2H2O(s) (I) (metastable), Ca0.5NpO2(OH)2 · 1.3H2O(s) (II) (long-term metastable) and Ca0.5NpO2(OH)2(s) (III) (stable). The considerably higher stability of these (qua)ternary phases which readily form in alkaline CaCl2 solutions from oversaturation (addition of NpO2+) and undersaturation (addition of binary NpO2OH(am)) limit the Np(V) equilibrium concentrations to values that are up to 3 log-units lower compared to those for NpO2OH(am) in NaCl and NaClO4 systems. Based on systematic evaluation of the pH dependence (solubility curve slopes), Vis/NIR and EXAFS spectroscopic information, unhydrolysed NpO2+ and innersphere chloro complexes, NpO2Cl(aq) and Caz[NpO2Cl]2z+ with z = 1 (EXAFS, Vis/NIR), were identified as prevailing Np(V) species in less alkaline solutions with pHm < 10.5 for [CaCl2] ≤ 2.0 M and [CaCl2] > 2.0 M, respectively. The steep increase of the Np(V) solubility for pHm > 11 with slopes of approximately + 2.5 (for solid phase (I)) and + 3 (for solids (II) and (III)) as well as the Np L3-edge EXAFS results for the aqueous Np(V) species in 4.5 M CaCl2/pHm ≈ 12 confirm the presence of ternary Ca-Np(V)-OH complexes Cax[NpO2(OH)2]2x–1 (x ≈ 1 is estimated within the thermodynamic evaluation in a subsequent paper (Fellhauer, D., Altmaier, M., Gaona, X., Lützenkirchen, J., Fanghänel, Th., Np(V) solubility, speciation and solid phase formation in alkaline CaCl2 solutions. Part II: Thermodynamics and implications for source term estimations of nuclear waste disposal (Radiochim. Acta, DOI 10.1515/ract-2015-2490), in the following referred to as “Part II” [1]) and Cay[NpO2(OH)5]2y–4 with y ≈ 2.4 ± 1.5 (EXAFS result) as predominant Np(V) hydrolysis species in solution for [CaCl2] ≥ 0.25 and pHm > 10.5. The thermodynamic evaluation and implications of the new findings with respect to source term estimations for nuclear waste disposal scenarios are discussed in “Part II” [1]).

Solubility of tetravalent actinides in alkaline CaCl2 solutions and formation of Ca4[An(OH)8]4+ complexes: A study of Np(IV) and Pu(IV) under reducing conditions and the systematic trend in the An(IV) series

Abstract The solubility of Np(IV) and Pu(IV) hydrous oxides was studied at 22±2 °C under reducing conditions in alkaline CaCl2 solutions. Redox conditions were adjusted either with 2 mM Na2S2O4 or additions of iron powder. In 2.0 and 4.5 M CaCl2 and pHc=11–12, the neptunium and plutonium concentrations increase with a slope of +4 (log[An] vs. pHc) as expected for the formation of the complex Ca4[An(OH)8]4+ recently identified for Th(IV). At CaCl2 concentrations ≤1.0 M this effect is negligible for both Np(IV) and Pu(IV). The conditional equilibrium constants log*Ks,(4,1,8) for the reaction An(OH)4(am) + 4 H2O + 4 Ca2+ ⇔ Ca4[An(OH)8]4+ + 4 H+ are evaluated with the SIT and Pitzer model using the parameters derived from analogous data for Th(IV) in 0.5–4.5 M CaCl2. The log*K°s,(4,1,8) values and the complex formation constants logβ°(4,1,8) follow systematic trends in the series Th(IV), Np(IV) and Pu(IV) which allows the estimation of the corresponding data for U(IV).

Np(V) solubility, speciation and solid phase formation in alkaline CaCl2 solutions. Part II: Thermodynamics and implications for source term estimations of nuclear waste disposal

Abstract The results of comprehensive solubility experiments with Np(V) in dilute to concentrated CaCl2 solutions which included the spectroscopic investigation of the predominant aqueous Np(V) species and the thorough analysis of the solubility controlling Np(V) solid phases [1](Fellhauer, D., Rothe, J., Altmaier, M., Neck, V., Runke, J., Wiss, T., Fanghänel, Th., Np(V) solubility, speciation and solid phase formation in alkaline CaCl2 solutions. Part I: Experimental solubility (Radiochim. Acta, DOI 10.1515/ract-2015-2489), in the following referred to as “Part I”), showed that the concentration of Np(V) in these systems is limited by equilibrium reactions between previously unknown (qua)ternary solid compounds, CaNpO2(OH)2.6Cl0.4 · 2H2O(s) (I), Ca0.5NpO2(OH)2 · 1.3H2O(s) (II) and Ca0.5NpO2(OH)2(s) (III), and ternary aqueous complexes of Np(V) with the general formulae Cax[NpO2(OH)2]2x–1, Cay[NpO2(OH)5]2y–4 with y ≈ 2.4 ± 1.5 (EXAFS result) and innersphere Np(V) chloro complexes, Caz[NpO2Cl]2z+ with z = 0 and 1 (EXAFS result). A systematic thermodynamic evaluation of the experimental solubility data based on the specific ion interaction theory (SIT) and the Pitzer approach is performed. The stoichiometries of the ternary hydrolysis complexes are assessed as Ca[NpO2(OH)2]+ and Ca3[NpO2(OH)5]2+. The corresponding thermodynamic solubility and complex formation constants at I = 0 as well as the ion interaction parameters (SIT and Pitzer) were derived yielding a comprehensive geochemical model for the Np(V) solubility behavior in pH neutral and alkaline CaCl2 solutions over a large range of ionic strengths. This significantly improves source term estimations for scenarios with calcium dominated aquatic systems. The principally different behavior of Np(V) in NaCl and CaCl2 solutions is highlighted, and the impact of the new findings on geochemical modelling of systems relevant for nuclear waste disposal is discussed.

Redox behavior and solubility of plutonium under alkaline, reducing conditions

Abstract The solubility and redox behavior of hydrous Pu(IV) oxide was comprehensively investigated by an experimental multi-method approach as a function of different redox conditions in 0.1 M NaCl solutions, allowing a detailed characterization of Pu(IV) and Pu(III) solubility and solid phase stability in these systems. Samples were prepared at ~3≤pHm≤~6 (pHm=–log mH+)

Solubility and hydrolysis of Np(V) in dilute to concentrated alkaline NaCl solutions: formation of Na–Np(V)–OH solid phases at 22 °C

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Solubility and spectroscopic study of AnIII/LnIII in dilute to concentrated Na–Mg–Ca–Cl–NO3 solutions

and ~8≤pHm≤~13 at T=(22±2)°C under Ar atmosphere. No redox buffer was used in one set of samples, whereas mildly and strongly reducing redox conditions were buffered in two series with hydroquinone or SnCl2, respectively, resulting in (pe+pHm)=(9.5±1) and (2±1). XRD, XANES and EXAFS confirmed the predominance of Pu(IV) and the nanocrystalline character of the original, aged PuO2(ncr,hyd) solid phase used as a starting material. Rietveld analysis of the XRD data indicated an average crystal (domain) size of (4±1) nm with a mean cell parameter of (5.405±0.005) Å. The solubility constant of this solid phase was determined as log ∗K°s,0

Thermodynamic description of Np(VI) solubility, hydrolysis, and redox behavior in dilute to concentrated alkaline NaCl solutions

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