Effect of Aging on the Reversibility of Pu(IV) Sorption to Goethite

Designing safe remediation and disposal strategies for plutonium (Pu) requires understanding the sorption affinity of Pu for soil minerals. Sorption of Pu(IV) was examined with respect to aging for a goethite system using batch sorption experiments. Sorption of Pu(IV) to iron oxides has been observed to be strong, rapid, and possibly irreversible or hysteretic. These observations may be explained by aging, a surface chemical process happening after initial sorption which causes a change in contaminant surface speciation over time. Measurements of Pu(IV) sorption are often complicated by oxidative leaching of Pu(IV) as Pu(V). Desferrioxamine B (DFOB) is a complexant capable of competing with the proposed strong surface complexes. Additionally, DFOB minimizes oxidative leaching by forming strong Pu(IV)-DFOB complexes, thereby stabilizing Pu(IV) as the dominant aqueous oxidation state. Pu(IV) was reacted in suspensions of 0.1g/L goethite and 10mM NaCl spanning pH 4–7 for various lengths of time (1,6,15,34 and 116 days). Supernatant was replaced with a 1.7µM DFOB solution and, after 34 more days, analyzed for aqueous Pu by liquid scintillation counting. Modeling sorption curves in FITEQL yielded logK values which increased from 0.078 to 0.953 over 116 days, indicating Pu(IV) sorption onto goethite becomes less reversible with aging

Experimental oxygen fugacity measurements of the Skaergaard Layered Intrusion /

Master of ScienceGeologyUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/114656/1/39015017992986.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/114656/2/39015017992986.pd

Pb isotope composition of Klyuchevskoy volcano, Kamchatka and North Pacific sediments: Implications for magma genesis and crustal recycling in the Kamchatkan arc

Pb isotope data are used to constrain the chemical contribution of the subducted components in the recycling beneath Klyuchevskoy volcano, the most active volcano in the Kamchatkan arc. The Pb isotope ratios of Klyuchevskoy basalts (206Pb/204Pb= 18.26–18.30, 207/Pb204Pb= 15.45–15.48, 208/Pb204Pb= 37.83–37.91) define a narrow range that falls within the Pacific mid-ocean ridge basalt (MORB) field and are among the least radiogenic island arc basalts measured to date. These data are similar to data from three other Quaternary Kamchatkan volcanoes: Tolbachik, Kumroch-Shish, and Maly Semiachik. In contrast, North Pacific sediments (primarily siliceous oozes) collected parallel to the Kamchatkan trench during Ocean Drilling Program Leg 145, have Pb isotope ratios (206Pb/204Pb= 18.51–18.78, 207Pb/204Pb= 15.56–15.64, 208Pb/204Pb= 38.49–38.75) that are more radiogenic than either the Klyuchevskoy basalts or Pacific MORB. Incorporation of even a small amount of sediment in the source of the Klyuchevskoy magmas would shift the Pb isotope ratios of the erupted basalts from the MORB field to more radiogenic values. The absence of 10Be and elevated Pb isotope ratios in the Kamchatkan volcanic lavas, despite the presence of distinctively radiogenic Pb in the North Pacific sediments makes it unlikely that sediments or sediment-derived fluids are involved in the source magmas beneath Kamchatka. The Kamchatkan arc thus represents an “end-member” whereby little or no sediment is involved in terms of elemental recycling and arc magma genesis. The major and trace elements, Pb, Sr and Nd isotope data of the Kamchatkan basalts are most consistently explained if derived from a fluid-fluxed, peridotitic mantle wedge source, wherein the fluid composition is dominantly controlled by dehydration of altered oceanic crust, imparting a radiogenic 87Sr/86Sr, and MORB-like Pb isotope signature to the mantle source. The erupted Klyuchevskoy lavas preserve a slab signature derived from incompatible elements that are strongly partitioned into the fluid. The 30 km of arc crust through which the Klyuchevskoy magmas traverse prior to eruption is not composed of older crust, but must be juvenile, similar in isotopic composition to MORB

Standard molar Gibbs free energy of formation for Cu2O: high-resolution electrochemical measurements from 900 to 1300 K

Oxygen-concentration cells with zirconia solid electrolytes have been used to make equilibrium measurements of the standard molar Gibbs free energy of formation for copper(I) oxide, [Delta]fGmo(Cu2O), over the temperature range from 900 to 1300 K. Compared with previous measurements, systematic errors due to thermal gradients across the zirconia solid electrolyte have been greatly reduced. Measurements with three different types of zirconia solid electrolytes have yielded results that differ by only +/-40 J[middle dot]mol-1 (+/-0.2 mV). This is the best agreement yet achieved between solid electrolytes of different composition. Our recommended value for the standard molar enthalpy of formation, [Delta]fHmo(Cu2O, 298.15 K), is -(170.59+/-0.08) kJ[middle dot]mol-1 (po = 1 x 105 Pa).Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28184/1/0000636.pd

Plutonium Desorption from Mineral Surfaces at Environmental Concentrations of Hydrogen Peroxide

Knowledge of Pu adsorption and desorption behavior on mineral surfaces is crucial for understanding its environmental mobility. Here we demonstrate that environmental concentrations of H₂O₂ can affect the stability of Pu adsorbed to goethite, montmorillonite, and quartz across a wide range of pH values. In batch experiments where Pu­(IV) was adsorbed to goethite for 21 days at pH 4, 6, and 8, the addition of 5–500 μM H₂O₂ resulted in significant Pu desorption. At pH 6 and 8 this desorption was transient with readsorption of the Pu to goethite within 30 days. At pH 4, no Pu readsorption was observed. Experiments with both quartz and montmorillonite at 5 μM H₂O₂ desorbed far less Pu than in the goethite experiments highlighting the contribution of Fe redox couples in controlling Pu desorption at low H₂O₂ concentrations. Plutonium­(IV) adsorbed to quartz and subsequently spiked with 500 μM H₂O₂ resulted in significant desorption of Pu, demonstrating the complexity of the desorption process. Our results provide the first evidence of H₂O₂-driven desorption of Pu­(IV) from mineral surfaces. We suggest that this reaction pathway coupled with environmental levels of hydrogen peroxide may contribute to Pu mobility in the environment

Reduction of Plutonium(VI) to (V) by Hydroxamate Compounds at Environmentally Relevant pH

Natural organic matter is known to influence the mobility of plutonium (Pu) in the environment via complexation and reduction mechanisms. Hydroxamate siderophores have been specifically implicated due to their strong association with Pu. Hydroxamate siderophores can also break down into di and monohydroxamates and may influence the Pu oxidation state, and thereby its mobility. In this study we explored the reactions of Pu­(VI) and Pu­(V) with a monohydroxamate compound (acetohydroxamic acid, AHA) and a trihydroxamate siderophore desferrioxamine B (DFOB) at an environmentally relevant pH (5.5–8.2). Pu­(VI) was instantaneously reduced to Pu­(V) upon reaction with AHA. The presence of hydroxylamine was not observed at these pHs; however, AHA was consumed during the reaction. This suggests that the reduction of Pu­(VI) to Pu­(V) by AHA is facilitated by a direct one electron transfer. Importantly, further reduction to Pu­(IV) or Pu­(III) was not observed, even with excess AHA. We believe that further reduction of Pu­(V) did not occur because Pu­(V) does not form a strong complex with hydroxamate compounds at a circum-neutral pH. Experiments performed using desferrioxamine B (DFOB) yielded similar results. Broadly, this suggests that Pu­(V) reduction to Pu­(IV) in the presence of natural organic matter is not facilitated by hydroxamate functional groups and that other natural organic matter moieties likely play a more prominent role

Effect of Natural Organic Matter on Plutonium Sorption to Goethite

The effect of citric acid (CA), desferrioxamine B (DFOB), fulvic acid (FA), and humic acid (HA) on plutonium (Pu) sorption to goethite was studied as a function of organic carbon concentration and pH using batch sorption experiments at 5 mg_C·L^–1 and 50 mg_C·L^–1 natural organic matter (NOM), 10^–9–10^–10 M ²³⁸Pu, and 0.1 g·L^–1 goethite concentrations, at pH 3, 5, 7, and 9. Low sorption of ligands coupled with strong Pu complexation decreased Pu sorption at pH 5 and 7, relative to a ligand-free system. Conversely, CA, FA, and HA increased Pu sorption to goethite at pH 3, suggesting ternary complex formation or, in the case of humic acid, incorporation into HA aggregates. Mechanisms for ternary complex formation were characterized by Fourier transform infrared spectroscopy in the absence of Pu. CA and FA demonstrated clear surface interactions at pH 3, HA appeared unchanged suggesting HA aggregates had formed, and no DFOB interactions were observed. Plutonium sorption decreased in the presence of DFOB (relative to a ligand free system) at all pH values examined. Thus, DFOB does not appear to facilitate formation of ternary Pu-DFOB-goethite complexes. At pH 9, Pu sorption in the presence of all NOM increased relative to pH 5 and 7; speciation models attributed this to Pu­(IV) hydrolysis competing with ligand complexation, increasing sorption. The results indicate that in simple Pu-NOM-goethite ternary batch systems, NOM will decrease Pu sorption to goethite at all but particularly low pH conditions

Sources, seasonal cycling, and fate of plutonium in a seasonally stratified and radiologically contaminated pond

Abstract Unlike short-term laboratory experiments, studies at sites historically contaminated with radionuclides can provide insight into contaminant migration behavior at environmentally-relevant decadal timescales. One such site is Pond B, a seasonally stratified reservoir within Savannah River Site (SC, USA) has low levels (μBq L−1) of plutonium in the water column. Here, we evaluate the origin of plutonium using high-precision isotope measurements, investigate the impact of water column geochemistry on plutonium cycling during different stratification periods, and re-evaluate long-term mass balance of plutonium in the pond. New isotopic data confirm that reactor-derived plutonium overwhelms input from Northern Hemisphere fallout at this site. Two suggested mechanisms for observed plutonium cycling in the water column include: (1) reductive dissolution of sediment-derived Fe(III)-(oxyhydr)oxides during seasonal stratification and (2) plutonium stabilization complexed strongly to Fe(III)-particulate organic matter (POM) complexes. While plutonium may be mobilized to a limited extent by stratification and reductive dissolution, peak plutonium concentrations are in shallow waters and associated with Fe(III)-POM at the inception of stratification. This suggests that plutonium release from sediments during stratification is not the dominant mechanism driving plutonium cycling in the pond. Importantly, our analysis suggests that the majority is retained in shallow sediments and may become increasingly recalcitrant