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The occurrence of plutonium dioxide (PuO2) either from direct deposition or from the precipitation of plutonium-bearing solutions in contaminated soils and sediments has been well described, particularly for the Hanford site in Washington State. However, past research has suggested that plutonium may exist in environmental samples at the Hanford site in chemical forms in addition to large size PuO2 particles and that these previously unidentified nano-sized particles maybe more reactive and thus more likely to influence the environmental mobility of Pu. Here we present evidence for the formation of nano-sized plutonium iron phosphate hydroxide structurally related to the rhabdophane group nanoparticles in 216-Z9 crib sediments from Hanford using transmission electron microscopy (TEM). The distribution and nature of these nanoparticles varied depending on the adjacent phases present. Fine electron probes were used to obtain electron diffraction and electron energy-loss spectra from specific phase regions of the 216-Z9 cribs specimens from fine-grained plutonium oxide and phosphate phases. Energy-loss spectra were used to evaluate the plutonium N4,5 (4d → 5f ) and iron L2,3 absorption edges. The iron plutonium phosphate formation may depend on the local micro-environment in the sediments, availability of phosphate, and hence the distribution of these minerals may control long-termmore » migration of Pu in the soil. This study also points to the utility of using electron beam methods for determining the identity of actinide phases and their association with other sediment phases.« less

Redox-reactive, biogeochemical phases generated by reductive microbial activity in hyporheic zone sediments from a dynamic groundwater–river interaction zone were evaluated for their ability to reduce soluble pertechnetate [ 99 Tc(VII)O 4 − ] to less soluble Tc(IV). The sediments were bioreduced by indigenous microorganisms that were stimulated by organic substrate addition in synthetic groundwater with or without sulfate. In most treatments, 20 μmol L −1 initial aqueous Tc(VII) was reduced to near or below detection (3.82 × 10 −9 mol L −1 ) over periods of days to months in suspensions of variable solids concentrations. Native sediments containing significant lithogenic Fe(II) in various phases were, in contrast, unreactive with Tc(VII). The reduction rates in the bioreduced sediments increased with increases in sediment mass, in proportion to weak acid-extractable Fe(II) and sediment-associated sulfide (AVS). The rate of Tc(VII) reduction was first order with respect to both aqueous Tc(VII) concentration and sediment mass, but correlations between specific reductant concentrations and reaction rate were not found. X-ray microprobe measurements revealed a strong correlation between Tc hot spots and Fe-containing mineral particles in the sediment. However, only a portion of Fe-containing particles were Tc-hosts. The Tc-hot spots displayed a chemical signature (by EDXRF) similar to pyroxene. The application of autoradiography and electron microprobe allowed further isolation of Tc-containing particles that were invariably found to be ca 100 μm aggregates of primary mineral material embedded within a fine-grained phyllosilicate matrix. EXAFS spectroscopy revealed that the Tc(IV) within these were a combination of a Tc(IV)O 2 -like phase and Tc(IV)–Fe surface clusters, with a significant fraction of a TcS x -like phase in sediments incubated with SO 4 2− . AVS was implicated as a more selective reductant at low solids concentration even though its concentration was below that required for stoichiometric reduction of Tc(VII). These results demonstrate that composite mineral aggregates may be redox reaction centers in coarse-textured hyporheic zone sediments regardless of the dominant anoxic biogeochemical processes.

The aqueous solubility of BaSeO4(cr) was studied at 23 ± 2 ℃ as a function of Na2SeO4 concentrations (0.0001 to 4.1 mol kg –1) and equilibration periods (3 to 596 d). The equilibrium, approached from both the under- and over-saturation directions, in this system was reached rather rapidly (≤3 d). The SIT and Pitzer's ion-interaction models were used to interpret these data and the predictions based on both of these models agreed closely with the experimental data. Thermodynamic analyses of the data show that BaSeO4(cr) is the solubility-controlling phase for Na2SeO4 concentrations –1. The log 10 K 0 value for the BaSeO4(cr) solubility product (BaSeO4(cr) ⇌ Ba2++ SeO4 2– ) calculated by the SIT and Pitzer models were very similar (− 7.32 ± 0.07 with Pitzer and − 7.25 ± 0.11 with SIT). Although the BaSeO4(cr) solubility product and Ba concentrations as a function of Na2SeO4 concentrations predicted by both the SIT and Pitzer models are similar, the models required different sets of fitting parameters. For examples, 1) interpretations using the SIT model required the inclusion of Ba(SeO4)2 2– species with log 10 K 0 = 3.44 ± 0.12 for the reaction (Ba2++ 2SeO4 2– ⇌ Ba(SeO4)2 2– ), whereas these species are not needed for Pitzer model, and 2) at Na2SeO4 concentrations >0.59 mol kg –1 it was also possible to calculate the value for log 10 K 0 for the solubility product of a proposed double salt (Na2Ba(SeO4)2(s) ⇌ 2Na++ Ba2++ 2SeO4 2– ) which for the SIT model is − (8.70 ± 0.29) whereas for the Pitzer model it is − (9.19 ± 0.19). The ion-interaction/ion-association parameters hitherto unavailable for both the SIT and Pitzer models required to fit these extensive data extending to as high ionic strengths as 12.3 mol kg –1 were determined. The model developed in this study is consistent with all of the reliable literature data, which was also used to extend the model to barium concentrations as high as 0.22 mol kg –1 and pH ranging from 1.4 to 13.8, in addition to selenium concentrations as high as 4.1 mol kg –1.

The solubility of different forms of precipitated 242PuO2(am) were examined in solutions containing aqueous Fe(II) over a range of pH values. The first series of 242PuO2(am) suspensions were prepared from a 242Pu(IV) stock that had been treated with thenoyltrifluoroacetone (TTA) to remove the 241Am originating from the decay of 241Pu. These 242PuO2(am) suspensions showed much higher solubilities at the same pH value and Fe(II) concentration than previous studies using 239PuO2(am). X-ray absorption fine structure (XAFS) spectroscopy of the precipitates showed a substantially reduced Pu–Pu backscatter over that previously observed in 239PuO2(am) precipitates, indicating that the 242PuO2(am) precipitates purified using TTA lacked the long range order previously found in239PuO2(am) precipitates. The Pu(IV) stock solution was subsequently repurified using an ion exchange resin and an additional series of 242PuO2(am) precipitates prepared. These suspensions showed higher redox potentials and total aqueous Pu concentrations than the TTA purified stock solution. The higher redox potential and aqueous Pu concentrations were in general agreement with previous studies on 242PuO2(am) precipitates, presumably due to the removal of possible organic compounds originally present in the TTA purified stock. 242PuO2(am) suspensions prepared with both stock solutions showed almost identical solubilities in Fe(II) containing solutions even though the initial aqueous Pu concentrations before the addition of Fe(II) were orders of magnitude different. By examining the solubility of 242PuO2(am) prepared from both stocks in this way we have essentially approached equilibrium from both the undersaturated and oversaturated conditions. The final aqueous Pu concentrations are predictable using a chemical equilibrium model which includes the formation of a nanometer sized Fe(III) reaction product, identified in the 242PuO2(am) suspension both by use of 57Fe Mössbauer spectroscopy and transmission electron microscopy (TEM) analysis.

The reduction of PuO2(am) by Fe(II) in the presence and absence of hematite was studied over a range of pH values and oxidation/reduction potentials. In contrast to thermodynamic predictions, the presence of hematite did not have a major effect on the overall reduction of PuO2(am) to aqueous Pu(III). Instead the aqueous Pu(III) concentrations at longer time frames were accurately predicted using the measured Fe(II) concentration and existing thermodynamic data for the reaction: H2O + H++ Fe2++ PuO2(am) ⇌ Pu3++ Fe(OH)3(am) with log K =− 0.6. The accuracy of this approach in all solutions containing aqueous Fe(II), coupled with the apparent lack of oxidation of Fe(II) by O2(g), suggests that the Fe(OH)3(am) is formed by the oxidation of Fe(II) to Fe(III) by radiolysis. The continued generation of reactive amorphous iron hydroxide by radiolysis prevents thermodynamic equilibrium from being reached with more stable ferric oxide compounds, except possibly under acidic conditions where amorphous ferric hydroxide is soluble. The use of measured pe values, instead of aqueous Fe(II) measurements, also yields reasonable predictions of the final Pu(III) concentrations although the predictions are more uncertain.

Prior to this study there were no thermodynamic data for isosaccharinate (ISA) complexes of Fe(III) in the environmental range of pH (>~4.5). This study was undertaken to obtain such data in order to predict Fe(III) behavior in the presence of ISA. The solubility of Fe(OH)3(2-line ferrihydrite), referred to as Fe(OH)3(s), was studied at 22 ± 2 °C in: (1) very acidic (0.01 mol·dm−3 H+) to highly alkaline conditions (3 mol·dm−3 NaOH) as a function of time (11–421 days), and fixed concentrations of 0.01 or 0.001 mol·dm−3 NaISA; and (2) as a function of NaISA concentrations ranging from approximately 0.0001 to 0.256 mol·dm−3 and at fixed pH values of approximately 4.5 and 11.6 to determine the ISA complexes of Fe(III). The data were interpreted using the SIT model that included previously reported stability constants for $$ {{\text{Fe(ISA}})_{n}}^{3 - n} $$ (with n varying from 1 to 4) and Fe(III)–OH complexes, and the solubility product for Fe(OH)3(s) along with the values for two additional complexes (Fe(OH)2(ISA)(aq) and $$ {\text{Fe(OH)}}_{ 3} ( {{\text{ISA}})_{2}}^{2 - } $$ ) determined in this study. These extensive data provided a log10 K 0 value of 1.55 ± 0.38 for the reaction $$ ({\text{Fe}}^{ 3+ } + {\text{ISA}}^{-} + 2 {\text{H}}_{ 2} {\text{O}} \rightleftarrows {\text{Fe(OH}})_{ 2} {\text{ISA(aq}}) + 2 {\text{H}}^{ + } ) $$ and a value of −3.27 ± 0.32 for the reaction $$ ({\text{Fe}}^{ 3+ } + 2 {\text{ISA}}^{-} + 3 {\text{H}}_{ 2} {\text{O}} \rightleftarrows {\text{Fe(OH)}}_{ 3} ( {\text{ISA}})_{2}^{2 - } + 3 {\text{H}}^{ + } ) $$ and show that ISA forms strong complexes with Fe(III) which significantly increase the Fe(OH)3(s) solubility at pH

Technetium is an important environmental contaminant introduced by the processing and disposal of irradiated nuclear fuel and atmospheric nuclear tests. Under oxic conditions technetium is soluble and exists as pertechnatate anion (TcO4−), while under anoxic conditions Tc is usually insoluble and exists as precipitated Tc(IV). Here we investigated abiotic Tc(VII) reduction in mineralogically heterogeneous, Fe(II)-containing sediments. The sediments were collected from a 55 m borehole that sampled a semi-confined aquifer at the Hanford Site, USA that contained a dramatic redox transition zone. One oxic facies (18.0–18.3 m) and five anoxic facies (18.3–18.6 m, 30.8–31.1 m, 39.0–39.3 m, 47.2–47.5 m and 51.5–51.8 m) were selected for this study. Chemical extractions, X-ray diffraction, electron microscopy, and Mossbauer spectroscopy were applied to characterize the Fe(II) mineral suite that included Fe(II)-phyllosilicates, pyrite, magnetite and siderite. The Fe(II) mineral phase distribution differed between the sediments. Sediment suspensions were adjusted to the same 0.5 M HCl extractable Fe(II) concentration (0.6 mM) for Tc(VII) reduction experiments. Total aqueous Fe was below the Feaq detection limit (

The microbial reduction of Fe(III) and U(VI) was investigated in shallow aquifer sediments collected from subsurface flood deposits near the Hanford Reach of the Columbia River in Washington State. Increases in 0.5 N HCl-extractable Fe(II) were observed in incubated sediments and (57)Fe Mössbauer spectroscopy revealed that Fe(III) associated with phyllosilicates and pyroxene was reduced to Fe(II). Aqueous uranium(VI) concentrations decreased in subsurface sediments incubated in sulfate-containing synthetic groundwater with the rate and extent being greater in sediment amended with organic carbon. X-ray absorption spectroscopy of bioreduced sediments indicated that 67-77% of the U signal was U(VI), probably as an adsorbed species associated with a new or modified reactive mineral phase. Phylotypes within the Deltaproteobacteria were more common in Hanford sediments incubated with U(VI) than without, and in U(VI)-free incubations, members of the Clostridiales were dominant with sulfate-reducing phylotypes more common in the sulfate-amended sediments. These results demonstrate the potential for anaerobic reduction of phyllosilicate Fe(III) and sulfate in Hanford unconfined aquifer sediments and biotransformations involving reduction and adsorption leading to decreased aqueous U concentrations.

This study measured reductive solubilization of plutonium(IV) hydrous oxide (Pu(IV)O2·xH2O(am)) with hydrogen (H2) as electron donor, in the presence or absence of dissimilatory metal-reducing bacteria (DMRB), anthraquinone-2,6-disulfonate (AQDS), and ethylenediaminetetraacetate (EDTA). In PIPES buffer at pH 7 with excess H2, Shewanella oneidensis and Geobacter sulfurreducens both solubilized

Bioreduced anthraquinone-2,6-disulfonate (AH2DS; dihydro-anthraquinone) was reacted with a 2-line, Si-substituted ferrihydrite under anoxic conditions at neutral pH in PIPES buffer. Phosphate (P) and bicarbonate (C); common adsorptive oxyanions and media/buffer components known to effect ferrihydrite mineralization; and Fe(II)aq (as a catalytic mineralization agent) were used in comparative experiments. Heterogeneous AH2DS oxidation coupled with Fe(III) reduction occurred within 0.13–1 day, with mineralogic transformation occurring thereafter. The product suite included lepidocrocite, goethite, and/or magnetite, with proportions varing with reductant:oxidant ratio (r:o) and the presence of P or C. Lepidocrocite was the primary product at low r:o in the absence of P or C, with evidence for multiple formation pathways. Phosphate inhibited reductive recrystallization, while C promoted goethite formation. Stoichiometric magnetite was the sole product at higher r:o in the absence and presence of P. Lepidocrocite was the primary mineralization product in the Fe(II)aq system, with magnetite observed at near equal amounts when Fe(II) was high [Fe(II)/Fe(III)] = 0.5 and P was absent. P had a greater effect on reductive mineralization in the Fe(II)aq system, while AQDS was more effective than Fe(II)aq in promoting magnetite formation. The mineral products of the direct AH2DS-driven reductive reaction are different from those observed in AH2DS-ferrihydite systems with metal reducing bacteria, particularly in presence of P.

The dissimilatory metal-reducing bacterium, Shewanella oneidensis MR-1 produced γ-MnS (rambergite) nanoparticles under the concurrent reduction of synthetic MnO2 and thiosulfate coupled to H2 oxidation. Using two MR-1 mutants defective in outer membrane c-type cytochromes (ΔmtrC/ΔomcA and ΔmtrC/ΔomcA/ΔmtrF) to eliminate the direct reduction pathway for solid electron acceptors, it was determined that respiratory reduction of MnO2 was dominant relative to chemical reduction by biogenic sulfide generated from bacterial thiosulfate reduction. Although bicarbonate was excluded from the medium, incubations of MR-1 using lactate as the sole electron donor produced MnCO3 (rhodochrosite) as well as MnS in nearly equivalent amounts as estimated by micro X-ray diffraction (micro-XRD) analysis. It was concluded that carbonate released from lactate metabolism promoted MnCO3 formation and that Mn(II) mineralogy was strongly affected by carbonate ions even in the presence of abundant sulfide and weakly alkaline conditions that favor the precipitation of MnS. Formation of the biogenic MnS, as determined by a combination of micro-XRD, transmission electron microscopy, energy dispersive X-ray spectroscopy, and selected area electron diffraction analyses was consistent with equilibrium speciation modeling predictions. Although biogenic MnS likely only forms and is stable over a relatively narrow range of conditions, it may be a significant sink for Mnmore » in anoxic marine basins and terrestrial subsurface sediments where Mn and sulfur compounds are undergoing concurrent reduction.« less

The adsorption and speciation of U(VI) was investigated on contaminated, fine grained sediment materials from the Hanford 300 area (SPP1 GWF) in simulated groundwater using cryogenic laser-induced U(VI) fluorescence spectroscopy combined with chemometric analysis. A series of reference minerals (montmorillonite, illite, Michigan chlorite, North Carolina chlorite, California clinochlore, quartz and synthetic 6-line ferrihydrite) was used for comparison that represents the mineralogical constituents of SPP1 GWF. Surface area-normalized Kd values were measured at U(VI) concentrations of 5 × 10−7 and 5 × 10−6 mol L−1 that displayed the following affinity series: 6-line-ferrihydrite > North Carolina chlorite ≈ California clinochlore > quartz ≈ Michigan chlorite > illite > montmorillonite. Both time-resolved spectra and asynchronous two-dimensional (2D) correlation analysis of SPP1 GWF at different delay times indicated that two major adsorbed U(VI) species were present in the sediment that resembled U(VI) adsorbed on quartz and phyllosilicates. Simulations of the normalized fluorescence spectra confirmed that the speciation of SPP1 GWF was best represented by a linear combination of U(VI) adsorbed on quartz (90%) and phyllosilicates (10%). However, the fluorescence quantum yield for U(VI) adsorbed on phyllosilicates was lower than quartz and, consequently, its fractional contribution to speciation may be underestimated. Spectral comparison with literature data suggested that U(VI) exist primarily as inner-sphere complexes with surface silanol groups on quartz and as surface U(VI) tricarbonate complexes on phyllosilicates.

Heterogeneous reduction of actinides in higher, more soluble oxidation states to lower, more insoluble oxidation states by reductants such as Fe(II) has been the subject of intensive study for more than two decades. However, Fe(II)-induced reduction of sparingly soluble Pu(IV) to the more soluble lower oxidation state Pu(III) has been much less studied, even though such reactions can potentially increase the mobility of Pu in the subsurface. Thermodynamic calculations are presented that show how differences in the free energy of various possible solid-phase Fe(III) reaction products can greatly influence aqueous Pu(III) concentrations resulting from reduction of PuO2(am) by Fe(II). We present the first experimental evidence that reduction of PuO2(am) to Pu(III) by Fe(II) was enhanced when the Fe(III) mineral goethite was spiked into the reaction. The effect of goethite on reduction of Pu(IV) was demonstrated by measuring the time dependence of total aqueous Pu concentration, its oxidation state, and syste...

The fate of pertechnetate ((99)Tc(VII)O(4)(-)) during bioreduction was investigated in the presence of 2-line ferrihydrite (Fh) and various dissimilatory metal reducing bacteria (DMRB) (Geobacter, Anaeromyxobacter, Shewanella) in comparison with TcO(4)(-) bioreduction in the absence of Fh. In the presence of Fh, Tc was present primarily as a fine-grained Tc(IV)/Fe precipitate that was distinct from the Tc(IV)O(2)·nH(2)O solids produced by direct biological Tc(VII) reduction. Aqueous Tc concentrations (

Contaminated capillary fringe sediments are believed to function as long-term source of U(VI) to Hanford’s 300 Area groundwater uranium plume that discharges to the Columbia River. The deep vadose zone at this site experiences seasonal water table elevation and water compositional changes in response to Columbia River stage. Batch and column desorption experiments of U(VI) were performed on two mildly contaminated sediments from this system that vary in hydrologic position to ascertain their U(VI) release behavior and factors controlling it. Solid phase characterization of the sediments was performed to identify mineralogic and chemical factors controlling U(VI) desorption. Low adsorbed U(VI) concentrations prevented spectroscopic analysis. The desorption behavior of U(VI) was different for the two sediments in spite of similar chemical and textural characteristics, and non-carbonate mineralogy. Adsorption strength and sorbed U(VI) lability was higher in the near-river sediment. The inland sediment displayed low sorbed U(VI) lability (∼10%) and measurable solid-phase carbonate content. Kinetic desorption was observed that was attributed to regeneration of labile U(VI) in the near river sediment, and carbonate mineral dissolution in the inland sediment. The desorption reaction was best described as an equilibrium surface complexation reaction. The noted differences in desorption behavior appear to result from U(VI) contamination and hydrologic history, as well as sediment carbonate content. Insights are provided on the dynamic adsorption/desorption behavior of contaminants in linked groundwater–river systems.

To determine the solubility product of PuPO4(cr, hyd.) and the complexation constants of Pu(III) with phosphate and EDTA, the solubility of PuPO4(cr, hyd.) was investigated as a function of: (1) time and pH (varied from 1.0 to 12.0), and at a fixed 0.00032 mol⋅L−1 phosphate concentration; (2) NaH2PO4 concentrations varying from 0.0001 mol⋅L−1 to 1.0 mol⋅L−1 and at a fixed pH of 2.5; (3) time and pH (varied from 1.3 to 13.0) at fixed concentrations of 0.00032 mol⋅L−1 phosphate and 0.0004 mol⋅L−1 or 0.002 mol⋅L−1 Na2H2EDTA; and (4) Na2H2EDTA concentrations varying from 0.00005 mol⋅L−1 to 0.0256 mol⋅L−1 at a fixed 0.00032 mol⋅L−1 phosphate concentration and at pH values of approximately 3.5, 10.6, and 12.6. A combination of solvent extraction and spectrophotometric techniques confirmed that the use of hydroquinone and Na2S2O4 helped maintain the Pu as Pu(III). The solubility data were interpreted using the Pitzer and SIT models, and both provided similar values for the solubility product of PuPO4(cr, hyd.) and for the formation constant of PuEDTA−. The log 10 of the solubility product of PuPO4(cr, hyd.) [PuPO4(cr, hyd.) \(\rightleftarrows\)\(\mathrm{Pu}^{3+}+\mathrm{PO}_{4}^{3-}\)] was determined to be −(24.42±0.38). Pitzer modeling showed that phosphate interactions with Pu3+ were extremely weak and did not require any phosphate complexes [e.g., PuPO4(aq), \(\mathrm{PuH}_{2}\mathrm{PO}_{4}^{2+}\), \(\mathrm{Pu(H}_{2}\mathrm{PO}_{4})_{2}^{+}\), Pu(H2PO4)3(aq), and \(\mathrm{Pu(H}_{2}\mathrm{PO}_{4})_{4}^{-}\)] as proposed in existing literature, to explain the experimental solubility data. SIT modeling, however, required the inclusion of \(\mathrm{PuH}_{2}\mathrm{PO}_{4}^{2+}\) to explain the data in high NaH2PO4 concentrations; this illustrates the differences one can expect when using these two different chemical models to interpret the data. Of the Pu(III)-EDTA species, only PuEDTA− was needed to interpret the experimental data over a large range of pH values (1.3–12.9) and EDTA concentrations (0.00005–0.256 mol⋅L−1). Calculations based on density functional theory support the existence of PuEDTA− (with prospective stoichiometry as Pu(OH2)3EDTA−) as the chemically and structurally stable species. The log 10 value of the complexation constant for the formation of PuEDTA− [\(\mathrm{Pu}^{3+}+\mathrm{EDTA}^{4-}\rightleftarrows \mathrm{PuEDTA}^{-}\)] determined in this study is −20.15±0.59. The data also showed that PuHEDTA(aq), \(\mathrm{Pu(EDTA)}_{4}^{5-}\), Pu(EDTA)(HEDTA)4−, Pu(EDTA)(H2EDTA)3−, and Pu(EDTA)(H3EDTA)2−, although reported in the literature, have no region of dominance in the experimental range of variables investigated in this study.

In an effort to improve the understanding of electron transfer mechanisms at the microbe-mineral interface, Shewanella oneidensis MR-1 mutants with in-frame deletions of outer membrane cytochrome genes mtrC, omcA, or both, were characterized for the ability to reduce metal oxides using a suite of microscopic, spectroscopic, and biochemicalr techniques. The results indicate that neither MtrC nor OmcA are essential for the reduction of soluble, complexed Fe(III)-citrate or Fe(III)-NTA; however, at least one of these outer membrane cytochromes is required for the reduction of Fe(III)- and Mn(III/IV)- oxides. In vitro analysis of purified, recombinant protein demonstrated that both cytochromes transfer electrons directly to metal-oxides; however, MtrC transfers electrons at a faster rate than OmcA. Immunolocalization of MtrC and OmcA reveal that both cytochromes are surface-exposed on the cell outer-membrane and co-localize with insoluble iron precipitates when respiring ferrihydrite or cultured aerobically with Fe(III)-citrate. Additionally, during prolonged incubation, wild-type cells promoted biotransformation of ferrihydrite to vivianite [Fe3(PO4)2•8H2O] while the double cytochrome mutant was unable to form any secondary mineral phases. Collectively, our results support a role for direct electron transfer from OMCs to metal oxides by establishing their in vitro electron transfer activities, confirming the requirement of either MtrC or OmcA formore » in vivo reductive biomineralization of ferrihydrite, and localizing the cytochromes to the cell exterior where they can directly contact mineral substrates.« less

Extensive studies on ThO2(am) solubility were carried out as functions of a wide range of isosaccharinate concentrations (0.0002 to 0.2 mol⋅kg−1) at fixed pH values of about 6 and 12, and varying pH (ranging from 4.5 to 12) at fixed aqueous isosaccharinate concentrations of 0.008 mol⋅kg−1 or 0.08 mol⋅kg−1, to determine the aqueous complexes of isosaccharinate with Th(IV). The samples were equilibrated over periods ranging up to 69 days, and the data showed that, in most cases, steady-state concentrations were reached in

The complexation of Np(V) with phosphate at elevated temperatures was studied by a synergistic extraction method. A mixed buffer solution of TRIS and MES was used to maintain an appropriate pH value during the distribution experiments. The distribution ratio of Np(V) between the organic and aqueous phases was found to decrease as the concentrations of phosphate were increased. Stability constants of the 1:1 and 1:2 Np(V)-HPO4 2− complexes, dominant in the aqueous phase under the experimental conditions, were calculated from the effect of [HPO4 2−] on the distribution ratio. The thermodynamic parameters including enthalpy and entropy of complexation between Np(V) and HPO4 2− at 25 °C–55 °C were calculated by the temperature coefficient method.

The extractability of uranium (U) from synthetic uranium-hydrous ferric oxide (HFO) coprecipitates has been shown to decrease as a function of mineral ripening, consistent with the hypothesis that the ripening process will decrease uranium lability. To evaluate this process, three HFO suspensions were coprecipitated with uranyl (UO2(2+)) and maintained at pH 7.0 +/- 0.1. Uranyl was added to the HFO postprecipitation in a fourth suspension. Two suspensions also contained either coprecipitated silicate (Si-U-HFO) or phosphate (P-U-HFO). After precipitation of the HFOs, at time intervals of 1 week, 1 month, 6 months, 1 year, and 2 years, aliquots of each suspension were contacted with five extractant solutions for a range of time. Uranium was preferentially extracted over Fe in varying degrees from all coprecipitates, by all extractants. The preference was dependent on the duration of mineral ripening and adjunct anion. Micro-X-ray diffraction analysis provides evidence for the transformation from amorphous material to phases containing substantial proportions of crystalline goethite and hematite, except the P-U-HFO, which remained primarily amorphous. Analysis of the U-HFO coprecipitate bythe Mössbauertechnique and scanning electron microscopy provides confirmation of an increase in particle size and evidence of mineral ripening to crystalline phases.

No thermodynamic data for Th complexes with aqueous Si are available. To obtain such data, extensive studies on ThO2(am) solubility were carried out as functions of: (1) a wide range of aqueous silica concentrations (0.0004 to 0.14 mol⋅L−1) at fixed pH values of about 10, 11, 12, and 13; and (2) and variable pH (ranging from 10 to 13.3) at fixed aqueous Si concentrations of about 0.006 mol⋅L−1 or 0.018 mol⋅L−1. The samples were equilibrated over long periods (ranging up to 487 days), and the data showed that steady-state concentrations were reached in 10. The data were interpreted using both the Pitzer and SIT models, and required only the inclusion of one mixed-hydroxy-silica complex of Th [Th(OH)3(H3SiO4) 3 2− ]. Both models provided similar complexation constant values for the formation of this species. Density functional theory calculations predict complexes of this stoichiometry, having six-fold coordination of the Th cation, to be structurally stable. Predictions based on the fitted value of log 10 K 0=−18.5±0.7 for the ThO2(am) solubility reaction involving Th(OH)3(H3SiO4) 3 2− [ThO2(am)+3H4SiO4+H2O↔Th(OH)3(H3SiO4) 3 2− +2H+], along with the thermodynamic data for aqueous Si species reported in the literature, agreed closely with the extensive experimental data and showed that under alkaline conditions aqueous Si makes very strong complexes with Th.

The room temperature solubility of amorphous, hydrous technetium(IV) oxide (TcO2⋅xH2O) was studied across a broad range of pH values extending from 1.5 to 12 and in oxalate concentrations from dilute (10−6 mol⋅kg−1) to complete saturation with respect to sodium bioxalate at lower pH values, and to saturation with respect to sodium oxalate at higher pH values. The solubility was measured to very long equilibration times (i.e., as long a 1000 days or longer). The thermodynamic modeling results show that the dominant species in solution must have at least one more hydroxyl moiety present in the complex than proposed by previous investigators (e.g., TcO(OH)Ox− rather than TcO(Ox)(aq)). Inclusion of the single previously unidentified species TcO(OH)Ox− in our aqueous thermodynamic model explains a wider range of observed solubility data for TcO2⋅xH2O(am) in the presence of oxalate and over a broad range of pH values. Inclusion of this species is also supported by the recently proposed thermodynamic data for the TcO(OH)+ hydrolysis species that indicates that this species is stable at pH values as low as one.

Ethylenediaminetetracetic acid (EDTA), which was co-disposed with Pu at several US Department of Energy sites, has been reported to enhance the solubility and transport of Pu. It is generally assumed that this enhanced transport of Pu in geologic environments is a result of complexation of Pu(IV) with EDTA. However, the fundamental basis for this assumption has never been fully explored. Whether EDTA can mobilize Pu(IV) in geologic environments is dependent on many factors, chief among them are not only the complexation constants of Pu with EDTA and dominant oxidation state and the nature of Pu solids, but also (1) the complexation constants of environmentally important metal ions (e.g., Fe, Al, Ca, Mg) that compete with Pu for EDTA and (2) EDTA interactions with the geomedia (e.g., adsorption, biodegradation) that reduce effective EDTA concentrations available for complexation. Extensive studies over a large range of pH values (1 to 14) and EDTA concentrations (0.0001 to 0.01 mol⋅L−1) as a function of time were conducted on the solubility of 2-line ferrihydrite (Fe(OH)3(s)), PuO2(am) in the presence of different concentrations of Ca ions, and mixtures of PuO2(am) and Fe(OH)3(s). The solubility data were interpreted using Pitzer’s ion-interaction approach to determine/validate the solubility product of Fe(OH)3(s), the complexation constants of Pu(IV)-EDTA and Fe(III)-EDTA, and to determine the effect of EDTA in solubilizing Pu(IV) from PuO2(am) in the presence of Fe(III) compounds and aqueous Ca concentrations. Predictions based on these extensive fundamental data show that environmental mobility of Pu as a result of Pu(IV)-EDTA complexation as reported/implied in the literature is a myth rather than the reality. The data also show that in geologic environments where Pu(III) and Pu(V) are stable, the EDTA complexes of these oxidation states may play an important role in Pu mobility.

Experiments were performed herein to investigate the rates and products of heterogeneous reduction of Tc(VII) by Fe(II) adsorbed to hematite and goethite, and by Fe(II) associated with a dithionite–citrate–bicarbonate (DCB) reduced natural phyllosilicate mixture [structural, ion-exchangeable, and edge-complexed Fe(II)] containing vermiculite, illite, and muscovite. The heterogeneous reduction of Tc(VII) by Fe(II) adsorbed to the Fe(III) oxides increased with increasing pH and was coincident with a second event of Fe 2 + (aq) adsorption. The reaction was almost instantaneous above pH 7. In contrast, the reduction rates of Tc(VII) by DCB-reduced phyllosilicates were not sensitive to pH or to added Fe 2 + (aq) that adsorbed to the clay. The reduction kinetics were orders of magnitude slower than observed for the Fe(III) oxides, and appeared to be controlled by structural Fe(II). The following affinity series for heterogeneous Tc(VII) reduction by Fe(II) was suggested by the experimental results: aqueous Fe(II) ∼ adsorbed Fe(II) in phyllosilicates [ion-exchangeable and some edge-complexed Fe(II)] ≪ structural Fe(II) in phyllosilicates ≪ Fe(II) adsorbed on Fe(III) oxides. Tc-EXAFS spectroscopy revealed that the reduction products were virtually identical on hematite and goethite that were comprised primarily of sorbed octahedral TcO2 monomers and dimers with significant Fe(III) in the second coordination shell. The nature of heterogeneous Fe(III) resulting from the redox reaction was ambiguous as probed by Tc-EXAFS spectroscopy, although Mossbauer spectroscopy applied to an experiment with 56Fe-goethite with adsorbed 57Fe(II) implied that redox product Fe(III) was goethite-like. The Tc(IV) reduction product formed on the DCB-reduced phyllosilicates was different from the Fe(III) oxides, and was more similar to Tc(IV) oxyhydroxide in its second coordination shell. The heterogeneous reduction of Tc(VII) to less soluble forms by Fe(III) oxide-adsorbed Fe(II) and structural Fe(II) in phyllosilicates may be an important geochemical process that will proceed at very different rates and that will yield different surface species depending on subsurface pH and mineralogy.

The complexation of plutonium(IV) with sulfate at variable temperatures has been investigated by solvent extraction method. A NaBrO{sub 3} solution was used as holding oxidant to maintain the plutonium(IV) oxidation state throughout the experiments. The distribution ratio of Pu(IV) between the organic and aqueous phases was found to decrease as the concentrations of sulfate were increased. Stability constants of the 1:1 and 1:2 Pu(IV)-HSO{sub 4}{sup -} complexes, dominant in the aqueous phase, were calculated from the effect of [HSO{sub 4}{sup -}] on the distribution ratio. The enthalpy and entropy of complexation were calculated from the stability constants at different temperatures using the Van't Hoff equation.

Chromium(III)-carbonate reactions are expected to be important in managing high-level radioactive wastes. Extensive studies on the solubility of amorphous Cr(III) hydroxide solid in a wide range of pH (3–13) at two different fixed partial pressures of CO2(g) (0.003 or 0.03 atm.), and as functions of K2CO3 concentrations (0.01 to 5.8 mol⋅kg−1) in the presence of 0.01 mol⋅dm−3 KOH and KHCO3 concentrations (0.001 to 0.826 mol⋅kg−1) at room temperature (22±2 °C) were carried out to obtain reliable thermodynamic data for important Cr(III)-carbonate reactions. A combination of techniques (XRD, XANES, EXAFS, UV-Vis-NIR spectroscopy, thermodynamic analyses of solubility data, and quantum mechanical calculations) was used to characterize the solid and aqueous species. The Pitzer ion-interaction approach was used to interpret the solubility data. Only two aqueous species [Cr(OH)(CO3) 2 2− and Cr(OH)4CO 3 3− ] are required to explain Cr(III)-carbonate reactions in a wide range of pH, CO2(g) partial pressures, and bicarbonate and carbonate concentrations. Calculations based on density functional theory support the existence of these species. The log 10 K° values of reactions involving these species [{Cr(OH)3(am) + 2CO2(g)⇌Cr(OH)(CO3) 2 2− +2H+} and {Cr(OH)3(am) + OH−+CO 3 2− ⇌Cr(OH)4CO 3 3− }] were found to be −(19.07±0.41) and −(4.19±0.19), respectively. No other data on any Cr(III)-carbonato complexes are available for comparisons.

The subsurface behaviour of 99Tc, a contaminant resulting from nuclear fuels reprocessing, is dependent on its valence (e.g., IV or VII). Abiotic reduction of soluble Tc(VII) by Fe(II)(aq) in pH 6–8 solutions was investigated under strictly anoxic conditions using an oxygen trap (

The speciation of U and Pu in soil and concrete from Rocky Flats and in particles from soils from Chernobyl, Hanford, Los Alamos, and McGuire Air Force Base and bottom sediments from Mayak was determined by a combination of X-ray absorption fine structure (XAFS) spectroscopy and X-ray fluorescence (XRF) element maps. These experiments identify four types of speciation that sometimes may and other times do not exhibit an association with the source terms and histories of these samples: relatively well ordered PuO2+x and UO2+x that had equilibrated with O2 and H2O under both ambient conditions and in fires or explosions; instances of small, isolated particles of U as UO2+x, U3O8, and U(VI) species coexisting in close proximity after decades in the environment; alteration phases of uranyl with other elements including ones that would not have come from soils; and mononuclear Pu-O species and novel PuO2+x-type compounds incorporating additional elements that may have occurred because the Pu was exposed to extreme chemical conditions such as acidic solutions released directly into soil or concrete. Our results therefore directly demonstrate instances of novel complexity in the A and μm-scale chemical speciation and reactivity of U and Pu in their initial formation and after environmental exposure as well as occasions of unexpected behavior in the reaction pathways over short geological but significant sociological times. They also show that incorporating the actual disposal and site conditions and resultant novel materials such as those reported here may be necessary to develop the most accurate predictive models for Pu and U in the environment.

Uranyl silicates such as uranophane and Na-boltwoodite appear to control the solubility of uranium in certain contaminated sediments at the US Department of Energy Hanford site [Liu, C., Zachara, J.M., Qafoku, O., McKinley, J.P., Heald, S.M., Wang, Z. 2004. Dissolution of uranyl microprecipitates in subsurface sediments at Hanford Site, USA. Geochim. Cosmochim. Acta 68, 4519–4537.]. Consequently, the solubility of synthetic Na-boltwoodite, Na(UO2)(SiO3OH) · 1.5H2O, was determined over a wide range of bicarbonate concentrations, from circumneutral to alkaline pH, that are representative of porewater and groundwater compositions at the Hanford site and calcareous environments generally. Experiments were open to air. Results show that Na-boltwoodite dissolution was nearly congruent and its solubility and dissolution kinetics increased with increasing bicarbonate concentration and pH. A consistent set of solubility constants were determined from circumneutral pH (0 added bicarbonate) to alkaline pH (50 mM added bicarbonate). Average log K sp o = 5.86 ± 0.24 or 5.85 ± 0.0.26; using the Pitzer ion-interaction model or Davies equation, respectively. These values are close to the one determined by [Nguyen, S.N., Silva, R.J., Weed, H.C., Andrews, Jr., J.E., 1992. Standard Gibbs free energies of formation at the temperature 303.15 K of four uranyl silicates: soddyite, uranophane, sodium boltwoodite, and sodium weeksite. J. Chem. Thermodynamics 24, 359–376.] under very different conditions (pH 4.5, Ar atmosphere).

SummaryStudies were conducted on the dissolution of Th-doped borosilicate glass, a complex assemblage of 24 different elements in various proportions, in a wide range of pH values (0.5 to 12) and carbonate (as high as 6.2 m) and bicarbonate (as high as 1.0 m) concentrations, and as a function of time to determine whether the observed thorium concentrations exhibit an equilibrium phenomenon and to ascertain whether existing thermodynamic data can be used to interpret these results. Measurable Th concentrations were observed in either the very acidic (pH 0.5 to 4) solutions or alkaline solutions containing relatively high carbonate/bicarbonate concentrations. Steady state Th concentrations were reached over time from both the oversaturation and undersaturation directions, indicating that Th concentrations are controlled by an equilibrium solubility phenomenon. The XRD, EXAFS, and thermodynamic analyses of solubility data failed to definitively identify the nature of the solubility-controlling solid in this complex system; however, the data suggests that the solubility-controlling solid most likely involves the Th-silicate phase and that the observed Th concentrations are up to many orders of magnitude lower than in equilibrium with ThO2(am). The solubility study also showed that existing thermodynamic data for carbonato complexes of Th can be used to reliably predict Th concentrations in relatively dilute to concentrated carbonate/bicarbonate solutions.

The objectives of this study were to address uncertainties in the solubility product of (UO2)3(PO4)2⋅4H2O(c) and in the phosphate complexes of U(VI), and more importantly to develop needed thermodynamic data for the Pu(VI)-phosphate system in order to ascertain the extent to which U(VI) and Pu(VI) behave in an analogous fashion. Thus studies were conducted on (UO2)3(PO4)2⋅4H2O(c) and (PuO2)3(PO4)2⋅4H2O(am) solubilities for long-equilibration periods (up to 870 days) in a wide range of pH values (2.5 to 10.5) at fixed phosphate concentrations of 0.001 and 0.01 M, and in a range of phosphate concentrations (0.0001–1.0 M) at fixed pH values of about 3.5. A combination of techniques (XRD, DTA/TG, XAS, and thermodynamic analyses) was used to characterize the reaction products. The U(VI)-phosphate data for the most part agree closely with thermodynamic data presented in Guillaumont et al.,(1) although we cannot verify the existence of several U(VI) hydrolyses and phosphate species and we find the reported value for formation constant of UO2PO−4 is in error by more than two orders of magnitude. A comprehensive thermodynamic model for (PuO2)3(PO4)2⋅4H2O(am) solubility in the H+-Na+-OH−-Cl−-H2PO−4-HPO2−4-PO3−4-H2O system, previously unavailable, is presented and the data shows that the U(VI)-phosphate system is an excellent analog for the Pu(VI)-phosphate system.

Summary The formation of a solid solution can significantly affect the solubility of a minor component. The objectives of this study were to determine the nature of U(IV) and Np(IV) solid solutions and their thermodynamic properties. For this purpose, (U x Np1- x )O2(am) precipitates with values of x varying from 0.05 to 0.95 were prepared, characterized, and used to determine their solubility in dilute to concentrated K2CO3 and KHCO3 solutions. The solubility data show that the equilibrium in these systems is reached in < 7 days, the solubility-controlling solids are amorphous, and that we were successful in maintaining U and Np in the tetravalent state. A scanning electron microscope equipped with energy dispersive spectrometry and X-ray absorption spectroscopic analyses of the solids indicated the quilibrated co-precipitates to be solid solutions. A close agreement between the thermodynamic predictions, assuming an ideal solid solution, and the experimental solubilities shows that U(IV) and Np(IV) form an ideal solid solution and that non-ideal solid solution behavior for this system can be ruled out.

Chromium(III)-phosphate reactions are expected to be important in managing high-level radioactive wastes stored in tanks at many DOE sites. Extensive studies on the solubility of amorphous Cr(III) solids in a wide range of pH (2.8–14) and phosphate concentrations (10−4 to 1.0 m) at room temperature (22±2)°C were carried out to obtain reliable thermodynamic data for important Cr(III)-phosphate reactions. A combination of techniques (XRD, XANES, EXAFS, Raman spectroscopy, total chemical composition, and thermodynamic analyses of solubility data) was used to characterize solid and aqueous species. Contrary to the data recently reported in the literature,(1) only a limited number of aqueous species [Cr(OH)3H2PO−4, Cr(OH)3(H2PO4)2−2), and Cr(OH)3HPO2−4] with up to about four orders of magnitude lower values for the formation constants of these species are required to explain Cr(III)-phosphate reactions in a wide range of pH and phosphate concentrations. The log Ko values of reactions involving these species [Cr(OH)3(aq)+H2PO−4⇌Cr(OH)3H2PO−4; Cr(OH)3(aq)+2H2PO−4⇌Cr(OH)3(H2PO4)2−2; Cr(OH)3(aq)+HPO2−4⇌Cr(OH)3HPO2−4] were found to be 2.78±0.3, 3.48±0.3, and 1.97±0.3, respectively.

Solubility studies on UO2(c), precipitated at 90°C from low-pH U(IV) solutions, were conducted under rigidly controlled redox conditions maintained by EuCl2 as a function of pH and from the oversaturation direction. Samples were equilibrated for 24 days at 90°C and then for 1 day at 22°C. X-ray diffraction (XRD) analyses of the solid phases, along with the observed solubility behavior, identified UO2(c) as the dominant phase at pH≳1.2 and UO2(am) as the dominant phase at pH≳1.2. The UV-Vis-NIR spectra of the aqueous phases showed that aqueous uranium was present in the tetravalent state. Our ability to effectively maintain uranium in the tetravalent state during experiments and the recent availability of reliable values of Pitzer ion-interactionparameters for this system have helped to set reliable upper limits for the log Ko value of ≤ −60.2 + 0.24 for the UO2(c) solubility [UO2(c) + 2H2O ⇌ U4+ + 4OH−] and of >−11.6 for the formation of U(OH)4(aq) [U4++ 4H2O ⇌ U(OH)4(aq) + 4H+]

The main objective of this study was to develop a thermodynamic model for predicting Cr(III) behavior in concentrated NaOH and in mixed NaOH–NaNO3 solutions for application to developing effective caustic leaching strategies for high-level nuclear waste sludges. To meet this objective, the solubility of Cr(OH)3(am) was measured in 0.003 to 10.5 m NaOH, 3.0 m NaOH with NaNO3 varying from 0.1 to 7.5 m, and 4.6 m NaNO3 with NaOH varying from 0.1 to 3.5 m at room temperature (22 ± 2°C). A combination of techniques, X-ray absorption spectroscopy (XAS) and absorptive stripping voltammetry analyses, were used to determine the oxidation state and nature of aqueous Cr. A thermodynamic model, based on the Pitzer equations, was developed from the solubility measurements to account for dramatic increases in aqueous Cr with increases in NaOH concentration. The model includes only two aqueous Cr species, Cr(OH)4− and Cr2O2(OH)4− (although the possible presence of a small percentage of higher oligomers at >5.0 m NaOH cannot be discounted) and their ion–interaction parameters with Na+. The logarithms of the equilibrium constants for the reactions involving Cr(OH)4− [Cr(OH)3(am) + OH− ⇌ Cr(OH)4−] and Cr2O2(OH)42− [2Cr(OH)3(am) + 2OH− ⇌ Cr2O2(OH)42− + 2H2O] were determined to be −4.36 ± 0.24 and −5.24 ± 0.24, respectively. This model was further tested and provided close agreement between the observed Cr concentrations in equilibrium with Cr(OH)3(am) in mixed NaOH–NaNO3 solutions and with high-level tank sludges leached with and primarily containing NaOH as the major electrolyte.

PuO2(am) solubility was investigated as a function of time, for pH from 0.5 to 11, and in the presence of 0.001 M FeCl2 or 0.00052 M hydroquinone to determine the effect of environmentally important reducing agents on PuO2(am) solubilization under geological conditions. Equilibrium was reached in

Obtaining reliable thermodynamic data for Pu(V) is difficult because of its redox and/or disproportionation reactions in most aqueous systems. The known stability of Pu(V) in PuO2(am) suspensions in slightly acidic to near neutral conditions was used to study the solubility of PuO2(am) in 0.4 and 4.0 M NaCl or NaClO4solutions ranging in pcH+values from 4 to 9 as a function of time. The close agreement between the observed solubility and the predicted solubility using Pitzer ion-interaction parameters of Np(V) with Cl-or ClO4-for Pu(V) indicates that Pu(V), as expected, behaves in an analogous fashion to Np(V) and confirms the value of using Np(V) data to model Pu(V) behavior.

Data are extremely limited on the effects of temperature on crystallinity and the resulting changes in solubility products of thermally transformed thorium oxide phases. Such data are required to reliably predict thorium behavior in high-level waste repositories where higher than ambient temperatures are expected. Solubility studies were conducted as a function of pH and time and at 0.1 M NaCl for 1) ThO2(am) at 23°C, 2) ThO2(am→c), a thermally transformed amorphous thorium hydrous oxide at 90°C, and 3) ThO2(c) at 23°C and 90°C. Results show that when ThO2(am) is heated to 90°C, it transforms to a relatively insoluble and crystalline solid [ThO2(am→c)]. At a fixed pH, the observed solubility of ThO2(am) at 23°C is more than 11 orders of magnitude greater than those for ThO2(c) at 23°C or of ThO2(am→c) and ThO2(c) at 90°C. Solubility data were interpreted using the Pitzer ion-interaction model. The log of the solubility product for the thorium dioxide dissolution reaction [ThO2(s) + 2 H2O ↔ Th4++ 4 OH-] was determined to be -44.9 for ThO2(am) at 23°C, ≥-56.9 for ThO2(c) at 23°C, and -51.4 for ThO2(c) at 90°C. At 90°C, a relatively less crystalline phase, ThO2(am→c), showed slightly higher solubility (log Ksp= -49.2) than crystalline ThO2(c).

The aqueous solubility of Fe2(SeO3)3·6H2O(c) was studied in deionized water adjusted to a range in pH values from 0.77 to 5.1 and in Na2SeO3 solutions ranging in concentrations from 0.0002 to 0.02 mol-dm−3. The studies were conducted from both the undersaturation and oversaturation directions, with equilibration periods ranging from 7 to 1725 days. Stoichiometric dissolution of the solid was observed in solutions with pH values up to nearly 4. In general, concentrations of both Se and Fe decreased as pH increased from 1 to 4. Analyses of the equilibrated suspensions confirmed the equilibrium solid to be Fe2(SeO3)3·6H2O(c) and the aqueous Se to be selenite. Pitzer's ion-interaction model was used with selected ion pairs to interpret the solubility data. The logarithm of the solubility product of ferric selenite $$Fe_2 (SeO_3 )_3 .6H_2 O(c) \begin{array}{*{20}c} \to \\ \leftarrow \\ \end{array} 2Fe^{3 + } + 3SeO_3^{2 - } + 6H_2 O$$ was found to be −41.58±0.11. This value is less than any reported in the literature for a ferric selenite by more than 10 orders of magnitude. The solubility data and calculations show an extremely strong interaction between aqueous Fe3+ and SeO32−; interpretation of these data requires the inclusion of FeSeO3+ i.e. $$Fe^{3 + } + SeO_3^{2 - } \begin{array}{*{20}c} \to \\ \leftarrow \\ \end{array} FeSeO_3^ + , log K = 11.15 \pm 0.11$$

Time-dependent reduction of PuO2(am) was studied over a range of pH values in the presence of aqueous Fe(II) and magnetite (Fe3O 4) nanoparticles. At early time frames (up to 56 days) very little aqueous Pu was mobilized from PuO2(am), even though measured pH and redox potentials, coupled to equilibrium thermodynamic modeling, indicated the potential for significant reduction of PuO2(am) to relatively soluble Pu(III). Introduction of Eu(III) or Nd(III) to the suspensions as competitive cations to displace possible sorbed Pu(III) resulted in the release of significant concentrations of aqueous Pu. However, the similarity of aqueous Pu concentrations that resulted from the introduction of Eu(III)/Nd(III) to suspensions with and without magnetite indicated that the Pu was solubilized from PuO2(am), not from magnetite. © 2012 American Chemical Society.

Flavins are secreted by the dissimilatory iron-reducing bacterium Shewanella and can function as endogenous electron transfer mediators. To assess the potential importance of flavins in Fe(III) bioreduction, we investigated the redox reaction kinetics of reduced flavin mononucleotide (i.e., FMNH(2)) and reduced riboflavin (i.e., RBFH(2)) with ferrihydrite and lepidocrocite. The organic reductants rapidly reduced and dissolved ferrihydrite and lepidocrocite in the pH range 4-8. The rate constant k for 2-line ferrihydrite reductive dissolution by FMNH(2) was 87.5 ± 3.5 M(-1)·s(-1) at pH 7.0 in batch reactors, and k was similar for RBFH(2). For lepidocrocite, k was 500 ± 61 M(-1)·s(-1) for FMNH(2) and 236 ± 22 M(-1)·s(-1) for RBFH(2). The surface area normalized initial reaction rates (r(a)) were between 0.08 and 77 μmol·m(-2)·s(-1) for various conditions in stopped-flow experiments. Initial rates (r(o)) were first-order with respect to iron(III) oxide concentration, and r(a) increased with decreasing pH. Poorly crystalline 2-line ferrihydrite yielded the highest r(a), followed by more crystalline 6-line ferrihydrite and crystalline lepidocrocite. Compared to a previous whole-cell study with Shewanella oneidensis strain MR-1, our findings suggest that the reduction of electron transfer mediators by the Mtr (i.e., metal-reducing) pathway coupled to lactate oxidation is rate limiting, rather than heterogeneous electron transfer to the iron(III) oxide.

This study measured reductive solubilization of plutonium(IV) hydrous oxide (Pu(IV)O(2)·xH(2)O((am))) with hydrogen (H(2)) as electron donor, in the presence or absence of dissimilatory metal-reducing bacteria (DMRB), anthraquinone-2,6-disulfonate (AQDS), and ethylenediaminetetraacetate (EDTA). In PIPES buffer at pH 7 with excess H(2), Shewanella oneidensis and Geobacter sulfurreducens both solubilized0.001% of 0.5 mM Pu(IV)O(2)·xH(2)O((am)) over 8 days, with or without AQDS. However, Pu((aq)) increased by an order of magnitude in some treatments, and increases in solubility were associated with production of Pu(III)((aq)). The solid phase of these treatments contained Pu(III)(OH)(3(am)), with more in the DMRB treatments compared with abiotic controls. In the presence of EDTA and AQDS, PuO(2)·xH(2)O((am)) was completely solubilized by S. oneidensis and G. sulfurreducens in ∼24 h. Without AQDS, bioreductive solubilization was slower (∼22 days) and less extensive (∼83-94%). In the absence of DMRB, EDTA facilitated reductive solubilization of 89% (without AQDS) to 98% (with AQDS) of the added PuO(2)·xH(2)O((am)) over 418 days. An in vitro assay demonstrated electron transfer to PuO(2)·xH(2)O((am)) from the S. oneidensis outer-membrane c-type cytochrome MtrC. Our results (1) suggest that PuO(2)·xH(2)O((am)) reductive solubilization may be important in reducing environments, especially in the presence of complexing ligands and electron shuttles, (2) highlight the environmental importance of polynuclear, colloidal Pu, (3) provide additional evidence that Pu(III)-EDTA is a more likely mobile form of Pu than Pu(IV)-EDTA, and (4) provide another example of outer-membrane cytochromes and electron-shuttling compounds facilitating bioreduction of insoluble electron acceptors in geologic environments.

A large set of sediment samples from a 1600 m² experimental plot within a 2.2 km² vadose zone and groundwater uranium (VI) plume was subject to physical, chemical, and mineralogic characterization. The plot is being used for field experimentation on U(VI) recharge and transport processes within a persistent groundwater plume that exists in the groundwater-river interaction zone of the Columbia River at the U.S. DOE Hanford site. The samples were obtained during the installation of 35 tightly spaced (10 m separation) groundwater monitoring wells. The characterization measurements for each sample included total contaminant concentrations (U and Cu primarily), bicarbonate extractable U(VI), sequential ²³⁸U(VI) contaminant desorption Kd, ²³³U(VI) adsorption K(d), grain size distribution, surface area, extractable poorly crystalline Fe(III) oxides, and mineralogy. The characterization objective was to inform a conceptual model of coupled processes controlling the anomalous longevity of the plume, and to quantify the spatial heterogeneity of the contaminant inventory and the primary properties effecting reactive transport. Correlations were drawn between chemical, physical, and reaction properties, and Gaussian simulation was used to compute multiple 3-D realizations of extractable U(VI), the ²³³U(VI) adsorption K(d), and the distribution of the reactive

The dissimilatory metal-reducing bacterium, Shewanella oneidensis MR-1 produced γ-MnS (rambergite) nanoparticles during the concurrent reduction of MnO₂ and thiosulfate coupled to H₂ oxidation. To investigate effect of direct microbial reduction of MnO₂ on MnS formation, two MR-1 mutants defective in outer membrane c-type cytochromes (ΔmtrC/ΔomcA and ΔmtrC/ΔomcA/ΔmtrF) were also used and it was determined that direct reduction of MnO₂ was dominant relative to chemical reduction by biogenic sulfide generated from thiosulfate reduction. Although bicarbonate was excluded from the medium, incubations of strain MR-1 with lactate as the electron donor produced MnCO₃ (rhodochrosite) as well as MnS in nearly equivalent amounts as estimated by micro X-ray diffraction (micro-XRD) analysis. It was concluded that carbonate released from lactate metabolism promoted MnCO₃ formation and that Mn(II) mineralogy was strongly affected by carbonate ions even in the presence of abundant sulfide and weakly alkaline conditions expected to favour the precipitation of MnS. Formation of MnS, as determined by a combination of micro-XRD, transmission electron microscopy, energy dispersive X-ray spectroscopy, and selected area electron diffraction analyses was consistent with equilibrium speciation modelling predictions. Biogenic manganese sulfide may be a manganese sink in the Mn biogeochemical cycle in select environments such as deep anoxic marine basins within the Baltic Sea.

Heterogeneous reduction of actinides in higher, more soluble oxidation states to lower, more insoluble oxidation states by reductants such as Fe(II) has been the subject of intensive study for more than two decades. However, Fe(II)-induced reduction of sparingly soluble Pu(IV) to the more soluble lower oxidation state Pu(III) has been much less studied, even though such reactions can potentially increase the mobility of Pu in the subsurface. Thermodynamic calculations are presented that show how differences in the free energy of various possible solid-phase Fe(III) reaction products can greatly influence aqueous Pu(III) concentrations resulting from reduction of PuO₂(am) by Fe(II). We present the first experimental evidence that reduction of PuO₂(am) to Pu(III) by Fe(II) was enhanced when the Fe(III) mineral goethite was spiked into the reaction. The effect of goethite on reduction of Pu(IV) was demonstrated by measuring the time dependence of total aqueous Pu concentration, its oxidation state, and system pe/pH. We also re-evaluated established protocols for determining Pu(III) {[Pu(III) + Pu(IV)] - Pu(IV)} by using thenoyltrifluoroacetone (TTA) in toluene extractions; the study showed that it is important to eliminate dissolved oxygen from the TTA solutions for accurate determinations. More broadly, this study highlights the importance of the Fe(III) reaction product in actinide reduction rate and extent by Fe(II).