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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 m H + ) ${{\text{m}}_{{{\text{H}}^{\text{ + }}}}})$ 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 $^ * K{^\circ _{{\text{s}},0}}$ =–(58.1±0.3) combining solubility data in acidic conditions and redox speciation by solvent extraction and CE–SF–ICP–MS. This value is in excellent agreement with the current thermodynamic selection in the NEA-TDB. Synchrotron-based in-situ XRD, XANES and EXAFS indicate that PuO2(ncr,hyd) is the solid phase controlling the solubility of Pu in hydroquinone buffered samples. Under these redox conditions and ~8≤pHm≤~13, the solubility of Pu is very low (~10−10.5 m) and pH-independent. This is consistent with the solubility equilibrium PuO2(am,hyd)+2H2O(l)⇔ Pu(OH)4(aq). Although in-situ XRD unequivocally shows the predominance of PuO2 in Sn(II)-buffered systems, XANES analyses indicate a significant contribution of Pu(III) (30±5%) in the solid phases controlling the solubility of Pu at (pe+pHm)=(2±1). For this system, EXAFS shows a systematic shortening of Pu–O and Pu–Pu distances compared to the starting Pu material and hydroquinone-buffered systems. The solubility of Pu remains very low (~10−10.5 m) at pHm>9, but shows a very large scattering (~10−9–10−10.5 m) at pHm=8. Experimental observations collected in Sn(II) buffered systems can be explained by the co-existence of both PuO2(ncr,hyd) and Pu(OH)3(am) solid phases, but also by assuming the formation of a sub-stoichiometric PuO2−x(s) phase. This extensive study provides robust upper limits for Pu solubility in alkaline, mildly to strongly reducing conditions relevant in the context of nuclear waste disposal. The potential role of Pu(III) in the solid phases controlling the solubility of Pu under these conditions is analysed and discussed in view of the current NEA-TDB thermodynamic selection, which supports the predominance of PuO2(am,hyd) and constrains the formation of Pu(OH)3(am) at pHm>8 outside the stability field of water. [ABSTRACT FROM AUTHOR]

Although the oxidative capacity of manganese oxides has been widely investigated, potential changes of the surface reactivity in dynamic anoxic/oxic environments have been often overlooked. In this study, we showed that the reactivity of layer structured manganese oxide (birnessite) was highly sensitive to variable redox conditions within environmentally relevant ranges of pH (4.0 – 8.0), ionic strength (0–100 mM NaCl) and Mn(II)/MnO 2 molar ratio (0–0.58) using ofloxacine (OFL), a typical antibiotic, as a target contaminant. In oxic conditions, OFL removal was enhanced relative to anoxic environments under alkaline conditions. Surface-catalyzed oxidation of Mn(II) enabled the formation of more reactive Mn(III) sites for OFL oxidation. However, an increase in Mn(II)/MnO 2 molar ratio suppressed MnO 2 reactivity, probably because of competitive binding between Mn(II) and OFL and/or modification in MnO 2 surface charge. Monovalent cations (e.g., Na+) may compensate the charge deficiency caused by the presence of Mn(III), and affect the aggregation of MnO 2 particles, particularly under oxic conditions. An enhancement in the removal efficiency of OFL was then confirmed in the dynamic two-step anoxic/oxic process, which emulates oscillating redox conditions in environmental settings. These findings call for a thorough examination of the reactivity changes at environmental mineral surfaces (e.g., MnO 2) in natural systems that may be subjected to alternation between anaerobic and oxygenated conditions. Surface-catalyzed oxidation of Mn(II) by dissolved oxygen controls the reactivity of nanostructured birnessite. [Display omitted] • Birnessite reactivity was highly sensitive to variable redox condition. • Surface-catalyzed oxidation of Mn(II) enabled formation of reactive Mn(III) sites. • Increase in Mn(II)/MnO 2 molar ratio suppressed MnO 2 reactivity. • Reactivity enhancement was observed in dynamic two-step anoxic/oxic process. • Oscillating redox conditions in environmental settings may affect contaminant fate. [ABSTRACT FROM AUTHOR]

Abstract: Uranium(VI) mobility in groundwater is strongly affected by sorption of mobile U(VI) species (e.g. uranyl, ) to mineral surfaces, precipitation of U(VI) compounds, such as schoepite (UO2)4O(OH)6·6H2O), and by reduction to U(IV), forming sparingly soluble phases (uraninite; UO2). The latter pathway, in particular, would be very efficient for long-term immobilization of U. In nature, Fe(II) is an important reducing agent for U(VI) because it frequently occurs either dissolved in natural waters, sorbed to matrix minerals, or structurally bound in many minerals. Redox reactions between U(VI) and Fe(II) depend not only on the availability of Fe(II) in the environment, but also on the chemical conditions in the aqueous solution. Under natural groundwater condition U(VI) forms complexes with many anionic ligands, which strongly affect its speciation. Carbonate, in particular, is known to form stable complexes with U, raising the question, if U(VI), when complexed by carbonate, can be reduced to UO2. The goal of this study was to find out if Fe(II) when structurally bound in a mineral (as magnetite, Fe3O4) or sorbed to a mineral surface (as corundum, Al2O3) can reduce U(VI) to U(IV) in the presence of . Batch experiments were conducted under anaerobic conditions to observe U removal from the aqueous phase by the two minerals depending on addition (1mM), U concentration (0.01–30μM) and pH value (6–10). Immediately after the experiments, the mineral surfaces were analyzed by X-ray photoelectron spectroscopy (XPS) to obtain information on the redox state of U bound to the solid surfaces. XPS results gave evidence that U(VI) can be reduced both by magnetite and by corundum amended with Fe(II). In the presence of the amount of reduced U on the mineral surfaces increased compared to carbonate-free solutions. This can be explained by the formation of Fe(II) carbonates on the mineral surfaces which represent an easily available Fe(II) pool for the U(VI) reduction. A facilitated U(VI) reduction is also considered possible when U is present as a carbonate complex compared to non-complexed U (e.g. uranyl). [Copyright &y& Elsevier]

The interaction of Tc(VII) with crushed crystalline rock (Äspö diorite; 1–2 mm size fraction) from the Äspö Hard Rock Laboratory (HRL) (Sweden) was studied by laboratory batch sorption and desorption experiments under Ar atmosphere using both natural and synthetic groundwater. The Äspö diorite used in the experiments was drilled, transported and handled as far as possible under anoxic conditions to preserve the in-situ rock redox capacity. For comparison, identical experiments using artificially oxidized Äspö diorite have been carried out to examine the effect of in-situ redox capacity on Tc uptake. According to the batch studies, Tc(VII) uptake on the Äspö diorite is strongly dependent on redox capacity. Uptake on un-oxidized rock is approximately 2 times higher compared to oxidized rock samples, most likely due to higher Fe(II) contents of the un-oxidized rock. Tc redox states and speciation both on the mineral surface and in the bulk were studied using X-ray photoelectron spectroscopy (XPS) and Tc K-edge X-ray absorption near edge structure (XANES) spectroscopy. The spectroscopic results verify a Tc(VII) reduction to Tc(IV) at the rock surface. Distribution coefficients (K d ) and surface normalized distribution coefficients (K a ) were determined and compared to available literature data. The formation of a Tc colloidal phase was not observed under the geochemical conditions prevailing in the experimental studies. Desorption of Tc is very low under anoxic conditions, but after artificial oxidation Tc mobility is strongly increased. The results of this work clearly highlight the effect of in-situ rock redox capacity on Tc retention. [ABSTRACT FROM AUTHOR]

Neptunium (Np(V)) sorption onto a purified illite is investigated as a function of pH (3–10) and [Np V O 2 + ] tot (3 × 10 −8 –3 × 10 − 4 M) in 0.1 M NaCl under Ar atmosphere. After about one week reaction time, only insignificant variation of Np sorption is observed and the establishment of reaction equilibrium can be assumed. Surprisingly, solid–liquid distribution ratios (R d ) are clearly higher than those measured for Np(V) sorption onto illite under aerobic conditions. The observation that R d increases with decreasing pe (pe = −log a e − ) suggests partial reduction to Np(IV), although measured redox potentials (pe values) at a first glance suggest the predominance of Np(V). Reduction to Np(IV) at the illite surface could indeed be confirmed by X-ray absorption near-edge spectroscopy (XANES). Np speciation in presence of the purified Na-illite under given conditions is consistently described by applying the 2 sites protolysis non-electrostatic surface complexation and cation exchange model. Measured pe data are taken to calculate Np redox state and surface complexation constants for Np(IV) are derived by applying a data fitting procedure. Constants are very consistent with results obtained by applying an existing linear free energy relationship (LFER). Taking Np(IV) surface complexation constants into account shifts the calculated Np(V)/Np(IV) redox borderline in presence of illite surfaces by 3–5 pe units (0.2–0.3 V) towards redox neutral conditions. Our study suggests that Np(V) reduction in presence of a sorbing mineral phase is thermodynamically favored. [ABSTRACT FROM AUTHOR]