Annie B. Kersting

Lithospheric Contributions to Arc Magmatism: Isotope Variations Along Strike in Volcanoes of Honshu, Japan

Major chemical exchange between the crust and mantle occurs in subduction zone environments, profoundly affecting the chemical evolution of Earth. The relative contributions of the subducting slab, mantle wedge, and arc lithosphere to the generation of island arc magmas, and ultimately new continental crust, are controversial. Isotopic data for lavas from a transect of volcanoes in a single arc segment of northern Honshu, Japan, have distinct variations coincident with changes in crustal lithology. These data imply that the relatively thin crustal lithosphere is an active geochemical filter for all traversing magmas and is responsible for significant modification of primary mantle melts.

Stabilization of Plutonium Nano-Colloids by Epitaxial Distortion on Mineral Surfaces

The subsurface migration of Pu may be enhanced by the presence of colloidal forms of Pu. Therefore, complete evaluation of the risk posed by subsurface Pu contamination needs to include a detailed physical/chemical understanding of Pu colloid formation and interactions of Pu colloids with environmentally relevant solid phases. Transmission electron microscopy (TEM) was used to characterize Pu nanocolloids and interactions of Pu nanocolloids with goethite and quartz. We report that intrinsic Pu nanocolloids generated in the absence of goethite or quartz were 2-5 nm in diameter, and both electron diffraction analysis and HRTEM confirm the expected Fm3m space group with the fcc, PuO2 structure. Plutonium nanocolloids formed on goethite have undergone a lattice distortion relative to the ideal fluorite-type structure, fcc, PuO2, resulting in the formation of a bcc, Pu4O7 structure. This structural distortion results from an epitaxial growth of the plutonium colloid on goethite, leading to stronger binding of plutonium to goethite compared with other minerals such as quartz, where the distortion was not observed. This finding provides new insight for understanding how molecular-scale behavior at the mineral-water interface may facilitate transport of plutonium at the field scale.

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

The continuous interaction between a subducting slab, overlying mantle wedge and crustal lithosphere profoundly affects the chemical evolution of the Earth. In terms of chemical mass balance, the interactions between the subducting components (oceanic crust, sediment and fluid) and the overlying mantle wedge are far from understood and their precise role in the genesis of arc magmas much debated. Controversy surrounds: 1) whether or not oceanic sediments are subducted, and if so, whether they are melted and recycled into the arc crust via magmatism, or continue into the deep mantle, 2) whether the oceanic slab is partially melted and incorporated into the arc crust, and 3) the role and composition of metasomatic fluids resulting from both the dehydration of sediments and altered oceanic slab. Determination of the chemical fluxes in island arc systems is central to debates concerning global geochemical recycling, modeling island arc magma genesis, and ultimately understanding the differentiation of the mantle and generation of the crust. We have completed a Pb radiogenic isotope study on the volcanic rocks from Klyuchevskoy, Kamchatka as well as oceanic sediments collected near the Kamchatkan trench during Ocean Drilling Program, Leg 145. Klyuchevskoy volcano was chosen as it is well characterized and has erupted some of the most primitive basaltic magmas found in island arc settings thereby minimizing the effects of lithospheric involvement in magma generation. Continuously cored sediments collected parallel to the Kamchatkan trench provide the best analog for sediments previously subducted beneath the Kamchatkan arc. Klyuchevskoy is the worlds most active volcano, producing primarily basaltic material from high-MgO (~ 12 wt%) to high-Al203 (~ 18 wt%) basalts (Kersting and Arculus, 1994). The samples chosen for this study span the entire chemical range of erupted lavas. The overwhelming majority of sediments recovered from Leg 145 were diatomaceous oozes. The majority of sediments analyzed in this study were oozes, although two claystone and two nannofossil chalk samples were also analyzed. The North Pacific sediments analyzed represent approximately 50 million years of sedimentary deposition from the Eocene through the Pliocene. The Pb isotope ratios of the Klyuchevskoy

Eu(III), Sm(III), Np(V), Pu(V), and Pu(IV) sorption to calcite

Abstract The sorption behavior of Eu(III), Sm(III), Np(V), Pu(V), and Pu(IV) in the presence of calcite and as a function of pH and carbonate alkalinity was measured by batch sorption experiments. Eu(III) and Sm(III) sorption is similar, consistent with their observed aqueous speciation and precipitation behavior. For both rare earth elements, sorption decreases at the highest and lowest measured pHs. This is likely the result of speciation changes both of the calcite surface and the sorber. An increase in the equilibrium CO2(g) fugacity results in a shift in the sorption behavior to lower pH, consistent with a predicted aqueous speciation shift. Np(V) and Pu(V) sorption exhibited a strong pH dependence. For Np(V), Kds range from 0 to 217 mL/g suggesting that carbonate aqueous speciation as well as changes in the calcite surface speciation greatly affect Np(V) sorption to the calcite surface. Similar behavior was found for Pu(V). Pu(IV) sorption is also strongly pH dependent. Sorption decreases significantly at high pH as a result of Pu-carbonate complexation in solution. A surface complexation model of Sm(III), Eu(III), Np(V), Pu(V), and Pu(IV) sorption to the calcite surface was developed based on the calcite surface speciation model of Pokrovsky and Schott [1]. Sorption data were fit using one or two surface species for each sorber and could account for the effect of pH and CO2(g) fugacity on sorption. A relatively poor model fit to Pu(IV) sorption data at high pH may result from our poor understanding of Pu(IV)-carbonate aqueous speciation. While our surface complexation model may not represent a unique solution to the sorption data, it illustrates that a surface complexation modeling approach may adequately describe the sorption behavior of a number of radionuclides at the calcite surface over a range of solution conditions.

Np(V) and Pu(V) Ion Exchange and Surface-Mediated Reduction Mechanisms on Montmorillonite

Due to their ubiquity and chemical reactivity, aluminosilicate clays play an important role in actinide retardation and colloid-facilitated transport in the environment. In this work, Pu(V) and Np(V) sorption to Na-montmorillonite was examined as a function of ionic strength, pH, and time. Np(V) sorption equilibrium was reached within 2 h. Sorption was relatively weak and showed a pH and ionic strength dependence. An approximate NpO(2)(+) → Na(+) Vanselow ion exchange coefficient (Kv) was determined on the basis of Np(V) sorption in 0.01 and 1.0 M NaCl solutions at pH < 5 (Kv ~ 0.3). In contrast to Np(V), Pu(V) sorption equilibrium was not achieved on the time-scale of weeks. Pu(V) sorption was much stronger than Np(V), and sorption rates exhibited both a pH and ionic strength dependence. Differences in Np(V) and Pu(V) sorption behavior are indicative of surface-mediated transformation of Pu(V) to Pu(IV) which has been reported for a number of redox-active and redox-inactive minerals. A model of the pH and ionic strength dependence of Pu(V) sorption rates suggests that H(+) exchangeable cations facilitate Pu(V) reduction. While surface complexation may play a dominant role in Pu sorption and colloid-facilitated transport under alkaline conditions, results from this study suggest that Pu(V) ion exchange and surface-mediated reduction to Pu(IV) can immobilize Pu or enhance its colloid-facilitated transport in the environment at neutral to mildly acidic pHs.

Pu(V) and Pu(IV) Sorption to Montmorillonite

Plutonium (Pu) adsorption to and desorption from mineral phases plays a key role in controlling the environmental mobility of Pu. Here we assess whether the adsorption behavior of Pu at concentrations used in typical laboratory studies (≥10(-10) [Pu] ≤ 10(-6) M) are representative of adsorption behavior at concentrations measured in natural subsurface waters (generally <10(-12) M). Pu(V) sorption to Na-montmorillonite was examined over a wide range of initial Pu concentrations (10(-6)-10(-16) M). Pu(V) adsorption after 30 days was linear over the wide range of concentrations studied, indicating that Pu sorption behavior from laboratory studies at higher concentrations can be extrapolated to sorption behavior at low, environmentally relevant concentrations. Pu(IV) sorption to montmorillonite was studied at initial concentrations of 10(-6)-10(-11) M and was much faster than Pu(V) sorption over the 30 day equilibration period. However, after one year of equilibration, the extent of Pu(V) adsorption was similar to that observed for Pu(IV) after 30 days. The continued uptake of Pu(V) is attributed to a slow, surface-mediated reduction of Pu(V) to Pu(IV). Comparison between rates of adsorption of Pu(V) to montmorillonite and a range of other minerals (hematite, goethite, magnetite, groutite, corundum, diaspore, and quartz) found that minerals containing significant Fe and Mn (hematite, goethite, magnetite, and groutite) adsorbed Pu(V) faster than those which did not, highlighting the potential importance of minerals with redox couples in increasing the rate of Pu(V) removal from solution.

Effect of Fulvic Acid Surface Coatings on Plutonium Sorption and Desorption Kinetics on Goethite

The rates and extent of plutonium (Pu) sorption and desorption onto mineral surfaces are important parameters for predicting Pu mobility in subsurface environments. The presence of natural organic matter, such as fulvic acid (FA), may influence these parameters. We investigated the effects of FA on Pu(IV) sorption/desorption onto goethite in two scenarios: when FA was (1) initially present in solution or (2) found as organic coatings on the mineral surface. A low pH was used to maximize FA coatings on goethite. Experiments were combined with kinetic modeling and speciation calculations to interpret variations in Pu sorption rates in the presence of FA. Our results indicate that FA can change the rates and extent of Pu sorption onto goethite at pH 4. Differences in the kinetics of Pu sorption were observed as a function of the concentration and initial form of FA. The fraction of desorbed Pu decreased in the presence of FA, indicating that organic matter can stabilize sorbed Pu on goethite. These results suggest that ternary Pu-FA-mineral complexes could enhance colloid-facilitated Pu transport. However, more representative natural conditions need to be investigated to quantify the relevance of these findings.

Development and Testing of Diglycolamide Functionalized Mesoporous Silica for Sorption of Trivalent Actinides and Lanthanides.

Sequestration of trivalent actinides and lanthanides present in used nuclear fuel and legacy wastes is necessary for appropriate long-term stewardship of these metals, particularly to prevent their release into the environment. Organically modified mesoporous silica is an efficient material for recovery and potential subsequent separation of actinides and lanthanides because of its high surface area, tunable ligand selection, and chemically robust substrate. We have synthesized the first novel hybrid material composed of SBA-15 type mesoporous silica functionalized with diglycolamide ligands (DGA-SBA). Because of the high surface area substrate, the DGA-SBA was found to have the highest Eu capacity reported so far in the literature of all DGA solid-phase extractants. The sorption behavior of europium and americium on DGA-SBA in nitric and hydrochloric acid media was tested in batch contact experiments. DGA-SBA was found to have high sorption of Am and Eu in pH 1, 1 M, and 3 M nitric and hydrochloric acid concentrations, which makes it promising for sequestration of these metals from used nuclear fuel or legacy waste. The kinetics of Eu sorption were found to be two times slower than that for Am in 1 M HNO3. Additionally, the short-term susceptibility of DGA-SBA to degradation in the presence of acid was probed using (29)Si and (13)C solid-state NMR spectroscopy. The material was found to be relatively stable under these conditions, with the ligand remaining intact after 24 h of contact with 1 M HNO3, an important consideration in use of the DGA-SBA as an extractant from acidic media.

Sorption interactions of plutonium and europium with ordered mesoporous carbon

Both 3d-cubic FDU-16-type and 2d-hexagonal C-CS-type ordered mesoporous carbons (OMCs) were synthesized to test their application as radionuclide sorbent materials. A portion of each OMC was oxidized with acidic ammonium persulfate (APS), and the physicochemical properties of all four OMCs were characterized with several techniques. Based on plutonium (Pu) sorption and desorption tests with FDU-16, oxidized FDU-16-COOH, C-CS, and oxidized C-CS-COOH, the C-CS-COOH was the most effective OMC for sorption of Pu over a wide pH range. Batch sorption interactions of C-CS and C-CS-COOH were further explored with Pu(VI) and Eu(III) to determine the uptake capacities, sorption kinetics, and effects of ionic strength. The nature of the Pu sorption reaction was also probed via X-ray absorption spectroscopy (XAS) and transmission electron microscopy (TEM). The highly oxidized surface, large pores, and high surface area of C-CS-COOH make it a very effective general scavenger for actinide and lanthanide cations. Pu and Eu uptake by C-CS-COOH appears to be dictated by chemisorption, and the Langmuir Eu capacity (138 mg g−1 from pH 4 solution) is higher than those previously reported for many other adsorbents. Pristine C-CS has a low affinity for Eu(III), but is an excellent sorbent of PuO2 nanocrystals (∼3 nm diameter), which are formed because the carbon reduces Pu(VI) and Pu(V) to Pu(IV). Plutonium is also reduced by C-CS-COOH, but PuO2 colloid formation in pH 4 solution is prevented by carboxyl complexation of Pu(IV) at the C-CS-COOH surface.

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.