Effect of Adsorbed Carboxylates on the Dissolution of Boehmite Nanoplates in Highly Alkaline Solutions.

Understanding the dissolution of boehmite in highly alkaline solutions is important to processing complex nuclear waste stored at the Hanford (WA) and Savannah River (SC) sites in the United States. Here, we report the adsorption of model carboxylates on boehmite nanoplates in alkaline solutions and their effects on boehmite dissolution in 3 M NaOH at 80 °C. Although expectedly lower than at circumneutral pH, adsorption of oxalate occurred at pH 13, with adsorption decreasing linearly to 3 M NaOH. Classical molecular dynamics simulations suggest that the adsorption of oxalate dianions onto the boehmite surface under high pH can occur through either inner- or outer-sphere complexation mechanisms depending on adsorption sites. However, both adsorption models indicate relatively weak binding, with an energy preference of 1.26 to 2.10 kcal/mol. By preloading boehmite nanoplates with oxalate or acetate, we observed suppression of dissolution rates by 23 or 10%, respectively, compared to pure solids. Scanning electron microscopy and transmission electron microscopy characterizations revealed no detectable difference in the morphologic evolution of the dissolving boehmite materials. We conclude that preadsorbed carboxylates can persist on boehmite surfaces, decreasing the density of dissolution-active sites and thereby adding extrinsic controls on dissolution rates.

Disordered interfaces of alkaline aluminate salt hydrates provide glimpses of Al3+ coordination changes.

HYPOTHESIS: The precipitation and dissolution of aluminum-bearing mineral phases in aqueous systems often proceed via changes in both aluminum coordination number and connectivity, complicating molecular-scale interpretation of the transformation mechanism. Here, the thermally induced transformation of crystalline sodium aluminum salt hydrate, a phase comprised of monomeric octahedrally coordinated aluminate which is of relevance to industrial aluminum processing, has been studied. Because intermediate aluminum coordination states during melting have not previously been detected, it is hypothesized that the transition to lower coordinated aluminum ions occurs within ahighly disordered quasi-two-dimensional phase at the solid-solution interface. EXPERIMENTS AND SIMULATIONS: In situ X-ray diffraction (XRD), Raman and27Al nuclear magnetic resonance (NMR) spectroscopy were used to monitor the melting transition of nonasodium aluminate hydrate (NSA, Na9[Al(OH)6]2·3(OH)·6H2O). A mechanistic interpretation was developed based on complementary classical molecular dynamics (CMD) simulations including enhanced sampling. A reactive forcefield was developed to bridge speciation in the solution and in the solid phase. FINDINGS: In contrast to classical dissolution, aluminum coordination change proceeds through a dynamically stabilized ensemble of intermediate states in a disordered layer at the solid-solution interface. In both melting and dissolution of NSA, octahedral, monomeric aluminum transition through an intermediate of pentahedral coordination. The intermediate dehydroxylates to form tetrahedral aluminate (Al(OH)4-) in the liquid phase. This coordination change is concomitant with a breaking of the ionic aluminate-sodium ionlinkages. The solution phase Al(OH)4- ions subsequently polymerize into polynuclear aluminate ions. However, there are some differences between bulk melting and interfacial dissolution, with the onset of the surface-controlled process occurring at a lower temperature (∼30 °C) and the coordination change taking place more gradually as a function of temperature. This work to determine the local structure and dynamics of aluminum in the disordered layer provides a new basis to understand mechanisms controlling aluminum phase transformations in highly alkaline solutions.

27 Al NMR diffusometry of Al13 Keggin nanoclusters.

Although nanometer-sized aluminum hydroxide clusters (i.e., ϵ-Al13 , [Al13 O4 (OH)24 (H2 O)12 ]7+ ) command a central role in aluminum ion speciation and transformations between minerals, measurement of their translational diffusion is often limited to indirect methods. Here, 27 Al pulsed field gradient stimulated echo nuclear magnetic resonance (PFGSTE NMR) spectroscopy has been applied to the AlO4 core of the ϵ-Al13 cluster with complementary theoretical simulations of the diffusion coefficient and corresponding hydrodynamic radii from a boundary element-based calculation. The tetrahedral AlO4 center of the ϵ-Al13 cluster is symmetric and exhibits only weak quadrupolar coupling, which results in favorable T1 and T2 27 Al NMR relaxation coefficients for 27 Al PFGSTE NMR studies. Stokes-Einstein relationship was used to relate the 27 Al diffusion coefficient of the ϵ-Al13 cluster to the hydrodynamic radius for comparison with theoretical simulations, dynamic light scattering from literature, and previously published 1 H PFGSTE NMR studies of chelated Keggin clusters. This first-of-its-kind observation proves that 27 Al PFGSTE NMR diffusometry can probe symmetric Al environments in polynuclear clusters of greater molecular weight than previously considered.

Sulfidation of magnetite with incorporated uranium.

Uranium (U) is a radionuclide of key environmental interest due its abundance by mass within radioactive waste and presence in contaminated land scenarios. Ubiquitously present iron (oxyhydr)oxide mineral phases, such as (nano)magnetite, have been identified as candidates for immobilisation of U via incorporation into the mineral structure. Studies of how biogeochemical processes, such as sulfidation from the presence of sulfate-reducing bacteria, may affect iron (oxyhydr)oxides and impact radionuclide mobility are important in order to underpin geological disposal of radioactive waste and manage radioactively contaminated land. Here, this study utilised a highly controlled abiotic method for sulfidation of U(V) incorporated into nanomagnetite to determine the fate and speciation of U. Upon sulfidation, transient release of U into solution occurred (∼8.6% total U) for up to 3 days, despite the highly reducing conditions. As the system evolved, lepidocrocite was observed to form over a period of days to weeks. After 10 months, XAS and geochemical data showed all U was partitioned to the solid phase, as both nanoparticulate uraninite (U(IV)O2) and a percentage of retained U(V). Further EXAFS analysis showed incorporation of the residual U(V) fraction into an iron (oxyhydr)oxide mineral phase, likely nanomagnetite or lepidocrocite. Overall, these results provide new insights into the stability of U(V) incorporated iron (oxyhydr)oxides during sulfidation, confirming the longer term retention of U in the solid phase under complex, environmentally relevant conditions.

Effect of Cr(III) Adsorption on the Dissolution of Boehmite Nanoparticles in Caustic Solution.

The incorporation of relatively minor impurity metals onto metal (oxy)hydroxides can strongly impact solubility. In complex highly alkaline multicomponent radioactive tank wastes such as those at the Hanford Nuclear Reservation, tests indicate that the surface area-normalized dissolution rate of boehmite (γ-AlOOH) nanomaterials is at least an order of magnitude lower than that predicted for the pure phase. Here, we examine the dissolution kinetics of boehmite coated by adsorbed Cr(III), which adheres at saturation coverages as sparse chemisorbed monolayer clusters. Using 40 nm boehmite nanoplates as a model system, temperature-dependent dissolution rates of pure versus Cr(III)-adsorbed boehmite showed that the initial rate for the latter is consistently several times lower, with an apparent activation energy 16 kJ·mol-1 higher. Although the surface coverage is only around 50%, solution analysis coupled to multimethod solids characterization reveal a phyicochemical armoring effect by adsorbed Cr(III) that substantially reduces the number of dissolution-active sites on particle surfaces. Such findings could help improve kinetics models of boehmite and/or metal ion adsorbed boehmite nanomaterials, ultimately providing a stronger foundation for the development of more robust complex radioactive liquid waste processing strategies.

Cr(III) Adsorption by Cluster Formation on Boehmite Nanoplates in Highly Alkaline Solution.

The development of advanced functional nanomaterials for selective adsorption in complex chemical environments requires partner studies of binding mechanisms. Motivated by observations of selective Cr(III) adsorption on boehmite nanoplates (γ-AlOOH) in highly caustic multicomponent solutions of nuclear tank waste, here we unravel the adsorption mechanism in molecular detail. We examined Cr(III) adsorption to synthetic boehmite nanoplates in sodium hydroxide solutions up to 3 M, using a combination of X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), scanning/transmission electron microscopy (S/TEM), electron energy loss spectroscopy (EELS), high-resolution atomic force microscopy (HR-AFM), time-of-fight secondary ion mass spectrometry (ToF-SIMS), Cr K-edge X-ray absorption near edge structure (XANES)/extended X-ray absorption fine structure (EXAFS), and electron paramagnetic resonance (EPR). Adsorption isotherms and kinetics were successfully fit to Langmuir and pseudo-second-order kinetic models, respectively, consistent with monotonic uptake of Cr(OH)4- monomers until saturation coverage of approximately half the aluminum surface site density. High resolution AFM revealed monolayer cluster self-assembly on the (010) basal surfaces with increasing Cr(III) loading, possessing a structural motif similar to guyanaite (ß-CrOOH), stabilized by corner-sharing Cr-O-Cr bonds and attached to the surface with edge-sharing Cr-O-Al bonds. The selective uptake appears related to short-range surface templating effects, with bridging metal connections likely enabled by hydroxyl anion ligand exchange reactions at the surface. Such a cluster formation mechanism, which stops short of more laterally extensive heteroepitaxy, could be a metal uptake discrimination mechanism more prevalent than currently recognized.

Stability, Composition, and Core-Shell Particle Structure of Uranium(IV)-Silicate Colloids.

Uranium is typically the most abundant radionuclide by mass in radioactive wastes and is a significant component of effluent streams at nuclear facilities. Actinide(IV) (An(IV)) colloids formed via various pathways, including corrosion of spent nuclear fuel, have the potential to greatly enhance the mobility of poorly soluble An(IV) forms, including uranium. This is particularly important in conditions relevant to decommissioning of nuclear facilities and the geological disposal of radioactive waste. Previous studies have suggested that silicate could stabilize U(IV) colloids. Here the formation, composition, and structure of U(IV)-silicate colloids under the alkaline conditions relevant to spent nuclear fuel storage and disposal were investigated using a range of state of the art techniques. The colloids are formed across a range of pH conditions (9-10.5) and silicate concentrations (2-4 mM) and have a primary particle size 1-10 nm, also forming suspended aggregates <220 nm. X-ray absorption spectroscopy, ultrafiltration, and scanning transmission electron microscopy confirm the particles are U(IV)-silicates. Additional evidence from X-ray diffraction and pair distribution function data suggests the primary particles are composed of a UO2-rich core and a U-silicate shell. U(IV)-silicate colloids formation correlates with the formation of U(OH)3(H3SiO4)32- complexes in solution indicating they are likely particle precursors. Finally, these colloids form under a range of conditions relevant to nuclear fuel storage and geological disposal of radioactive waste and represent a potential pathway for U mobility in these systems.

Reaction of U(VI) with titanium-substituted magnetite: influence of Ti on U(IV) speciation.

Reduction of hexavalent uranium (U(VI)) to less soluble tetravalent uranium (U(IV)) through enzymatic or abiotic redox reactions has the potential to alter U mobility in subsurface environments. As a ubiquitous natural mineral, magnetite (Fe3O4) is of interest because of its ability to act as a rechargeable reductant for U(VI). Natural magnetites are often impure with titanium, and structural Fe(3+) replacement by Ti(IV) yields a proportional increase in the relative Fe(2+) content in the metal sublattice to maintain bulk charge neutrality. In the absence of oxidation, the Ti content sets the initial bulk Fe(2+)/Fe(3+) ratio (R). Here, we demonstrate that Ti-doped magnetites (Fe3 - xTixO4) reduce U(VI) to U(IV). The U(VI)-Fe(2+) redox reactivity was found to be controlled directly by R but was otherwise independent of Ti content (xTi). However, in contrast to previous studies with pure magnetite where U(VI) was reduced to nanocrystalline uraninite (UO2), the presence of structural Ti (xTi = 0.25-0.53) results in the formation of U(IV) species that lack the bidentate U-O2-U bridges of uraninite. Extended X-ray absorption fine structure spectroscopic analysis indicated that the titanomagnetite-bound U(IV) phase has a novel U(IV)-Ti binding geometry different from the coordination of U(IV) in the mineral brannerite (U(IV)Ti2O6). The observed U(IV)-Ti coordination at a distance of 3.43 Å suggests a binuclear corner-sharing adsorption/incorporation U(IV) complex with the solid phase. Furthermore, we explored the effect of oxidation (decreasing R) and solids-to-solution ratio on the reduced U(IV) phase. The formation of the non-uraninite U(IV)-Ti phase appears to be controlled by availability of surface Ti sites rather than R. Our work highlights a previously unrecognized role of Ti in the environmental chemistry of U(IV) and suggests that further work to characterize the long-term stability of U(IV) phases formed in the presence of Ti is warranted.

Pore-scale characterization of biogeochemical controls on iron and uranium speciation under flow conditions.

Etched silicon microfluidic pore network models (micromodels) with controlled chemical and redox gradients, mineralogy, and microbiology under continuous flow conditions are used for the incremental development of complex microenvironments that simulate subsurface conditions. We demonstrate the colonization of micromodel pore spaces by an anaerobic Fe(III)-reducing bacterial species (Geobacter sulfurreducens) and the enzymatic reduction of a bioavailable Fe(III) phase within this environment. Using both X-ray microprobe and X-ray absorption spectroscopy, we investigate the combined effects of the precipitated Fe(III) phases and the microbial population on uranium biogeochemistry under flow conditions. Precipitated Fe(III) phases within the micromodel were most effectively reduced in the presence of an electron shuttle (AQDS), and Fe(II) ions adsorbed onto the precipitated mineral surface without inducing any structural change. In the absence of Fe(III), U(VI) was effectively reduced by the microbial population to insoluble U(IV), which was precipitated in discrete regions associated with biomass. In the presence of Fe(III) phases, however, both U(IV) and U(VI) could be detected associated with biomass, suggesting reoxidation of U(IV) by localized Fe(III) phases. These results demonstrate the importance of the spatial localization of biomass and redox active metals, and illustrate the key effects of pore-scale processes on contaminant fate and reactive transport.

Aerobic microbial manufacture of nanoscale selenium: exploiting nature's bio-nanomineralization potential.

The potential of the environment to yield organisms that can produce functional bionanominerals is demonstrated by selenium-tolerant, aerobic bacteria isolated from a seleniferous rhizosphere soil. An isolate, NS3, was identified as a Bacillus species (EU573774.1) based on morphological and 16S rRNA characterization. This strain reduced Se(IV) under aerobic conditions to produce amorphous alpha Se(0) nanospheres. A room-temperature washing treatment was then employed to remove the biomass and resulted in the production of clusters of hexagonal Se(0) nano-rods. The Se(0) nanominerals were analyzed using electron microscopy and X-ray diffraction techniques. This Bacillus isolate has the potential to be used both in the neutralizing of toxic Se(IV) anions in the environment and in the environmentally friendly manufacture of nanomaterials.