Tori Z. Forbes

Transformation and crystallization energetics of synthetic and biogenic amorphous calcium carbonate

Amorphous calcium carbonate (ACC) is a metastable phase often observed during low temperature inorganic synthesis and biomineralization. ACC transforms with aging or heating into a less hydrated form, and with time crystallizes to calcite or aragonite. The energetics of transformation and crystallization of synthetic and biogenic (extracted from California purple sea urchin larval spicules, Strongylocentrotus purpuratus) ACC were studied using isothermal acid solution calorimetry and differential scanning calorimetry. Transformation and crystallization of ACC can follow an energetically downhill sequence: more metastable hydrated ACC → less metastable hydrated ACC⇒anhydrous ACC ∼ biogenic anhydrous ACC⇒vaterite → aragonite → calcite. In a given reaction sequence, not all these phases need to occur. The transformations involve a series of ordering, dehydration, and crystallization processes, each lowering the enthalpy (and free energy) of the system, with crystallization of the dehydrated amorphous material lowering the enthalpy the most. ACC is much more metastable with respect to calcite than the crystalline polymorphs vaterite or aragonite. The anhydrous ACC is less metastable than the hydrated, implying that the structural reorganization during dehydration is exothermic and irreversible. Dehydrated synthetic and anhydrous biogenic ACC are similar in enthalpy. The transformation sequence observed in biomineralization could be mainly energetically driven; the first phase deposited is hydrated ACC, which then converts to anhydrous ACC, and finally crystallizes to calcite. The initial formation of ACC may be a first step in the precipitation of calcite under a wide variety of conditions, including geological CO2 sequestration.

Metal–Oxygen Isopolyhedra Assembled into Fullerene Topologies

Carbon-based fullerenes have received considerable attention since their discovery and are currently being produced on an industrial scale. Fullerenes are cage structures composed of 12 pentagons and several hexagons. Many fullerene topologies are possible, but selection criteria favor a small subset. C60 is the classic and most stable fullerene. [1] No smaller fullerene can be built without its topology containing adjacent pentagons, which destabilize the structure through increased curvature. For fullerenes with less than 60 C atoms, it is thought that those with the fewest adjacent pentagons will be the most stable because they minimize strain. 5] Fullerenes with less than 60 C atoms exist, for example C50, which can be stabilized as C50Cl10 by addition of Cl ligands. This structure does follow the minimal pentagon adjacency rule, but theoretical calculations suggest that pure C50 will not because sphericity is substantially increased when the number of adjacent pentagons is increased to six, as compared to five in the case of C50Cl10. [7] There is also considerable interest in inorganic fullerenelike materials; particular emphasis is on compounds such as MoS2 that tend to form onion-skin-like structures with useful materials properties. Molybdenum–oxygen heteropolyhedra have previously been found to form clusters with fullerene topologies, termed keplerates. Our earlier discovery of spherical clusters of uranyl peroxide polyhedra containing 24, 28, or 32 hexagonal bipyramids suggests that large clusters of metal–oxygen isopolyhedra with fullerene topologies may be synthesized. Of the three uranyl peroxide clusters described earlier, the 28-metal-ion cluster U28 has a fullerene topology with 12 pentagons and 4 hexagons, whereas the other two also contain squares in their topologies. We propose that assembly of metal–oxygen isopolyhedra into nanoscale fullerene topologies may be facilitated by judicious selection of structural building units. The rules that govern the formation of metal–oxygen cages are of interest, as such materials could have a variety of applications, including catalysis and synthesis of advanced materials. Assembly of metal–oxygen isopolyhedra into conventional fullerene topologies can only occur if at least the following conditions are met: 1) Each polyhedron must link to exactly three other polyhedra, and the most stable structures will occur when the connections between the metal–oxygen polyhedra are by the sharing of polyhedral edges. 2) The polyhedra must be geometrically compatible with forming topological pentagons and hexagons. 3) The three linkages emanating from any given polyhedron should be approximately coplanar to facilitate the cage geometry. 4) Linkages between polyhedra should be consistent with the bond-valence requirements of the shared polyhedral elements within the cage.

Metal-Organic Frameworks with Direct Transition Metal-Sulfonate Interactions and Charge-Assisted Hydrogen Bonds

Seven compounds with framework structures made of divalent metal-imidazole (Im) complexes (M(II) = Co, Ni, and Mn) linked by 1,5-napthalenedisulfonate (1,5nds) were synthesized and structurally characterized. Five of these compounds, Mn(Im)(4)(1,5nds) (two forms), Co(Im)(4)(1,5nds) (two forms), and Ni(Im)(4)(1,5nds), contain direct sulfonate-metal coordination and represent the first such compounds with open d-shell transition metals without Jahn-Teller distortion. The two disulfonate ligands in these octahedrally coordinated metal centers are found in both trans- and cis-geometries and link the centers into chains. The chains are held together by charge-assisted hydrogen bonds between sulfonate and imidazole ligands from different chains. The remaining two compounds, Co(Im)(6)(1,5nds) x 2 H(2)O and Ni(Im)(6)(1,5nds) x 2 H(2)O, exhibit only charge-assisted hydrogen bonds between the octahedral M(Im)(6)(2+) cations and the disulfonate anions.

Understanding the Radioactive Ingrowth and Decay of Naturally Occurring Radioactive Materials in the Environment: An Analysis of Produced Fluids from the Marcellus Shale.

Background The economic value of unconventional natural gas resources has stimulated rapid globalization of horizontal drilling and hydraulic fracturing. However, natural radioactivity found in the large volumes of “produced fluids” generated by these technologies is emerging as an international environmental health concern. Current assessments of the radioactivity concentration in liquid wastes focus on a single element—radium. However, the use of radium alone to predict radioactivity concentrations can greatly underestimate total levels. Objective We investigated the contribution to radioactivity concentrations from naturally occurring radioactive materials (NORM), including uranium, thorium, actinium, radium, lead, bismuth, and polonium isotopes, to the total radioactivity of hydraulic fracturing wastes. Methods For this study we used established methods and developed new methods designed to quantitate NORM of public health concern that may be enriched in complex brines from hydraulic fracturing wastes. Specifically, we examined the use of high-purity germanium gamma spectrometry and isotope dilution alpha spectrometry to quantitate NORM. Results We observed that radium decay products were initially absent from produced fluids due to differences in solubility. However, in systems closed to the release of gaseous radon, our model predicted that decay products will begin to ingrow immediately and (under these closed-system conditions) can contribute to an increase in the total radioactivity for more than 100 years. Conclusions Accurate predictions of radioactivity concentrations are critical for estimating doses to potentially exposed individuals and the surrounding environment. These predictions must include an understanding of the geochemistry, decay properties, and ingrowth kinetics of radium and its decay product radionuclides. Citation Nelson AW, Eitrheim ES, Knight AW, May D, Mehrhoff MA, Shannon R, Litman R, Burnett WC, Forbes TZ, Schultz MK. 2015. Understanding the radioactive ingrowth and decay of naturally occurring radioactive materials in the environment: an analysis of produced fluids from the Marcellus Shale. Environ Health Perspect 123:689–696; http://dx.doi.org/10.1289/ehp.1408855

Alteration of dehydrated schoepite and soddyite to studtite, [(UO2)(O2)(H2O)2](H2O)2

Abstract The oxidation of used nuclear fuel in a geologic repository has important implications for the mobility of radionuclides and fission products in the environment. Hexavalent uranium (uranyl) minerals, including oxyhydroxides and silicates, form as alteration phases on the surface of fuel pellets in laboratory simulations. However, alpha-radiolysis of water forms hydrogen peroxide in solution, which may favor the alteration of these secondary phases to the uranyl peroxide mineral studtite. This study investigates the alteration of dehydrated schoepite, UO3(H2O), and soddyite, [(UO2)2(SiO4)] (H2O)2, in the presence of aqueous solutions containing hydrogen peroxide. Crystalline samples were reacted with various concentrations of hydrogen peroxide and the resulting material was analyzed by powder X-ray diffraction. Both dehydrated schoepite and soddyite readily convert to studtite in the presence of hydrogen peroxide following the reaction stoichiometry. These results indicate that the possible impact of peroxide buildup on the stability of alteration phases in a repository setting should not be overlooked.

Ba(NpO2)(PO4)(H2O), its relationship to the uranophane group, and implications for Np incorporation in uranyl minerals

Abstract Single crystals of Ba(NpO2)(PO4)(H2O) were obtained using hydrothermal synthesis techniques. The structure was determined using single-crystal X-ray diffraction data collected using MoKα radiation and an APEX II CCD detector and was refined on the basis of F2 for all unique data to R1 = 2.41%. Ba(NpO2)(PO4)(H2O) crystallized in monoclinic space group P21/n with a = 6.905(3), b = 7.108(3), c = 13.321(6) Å, β = 105.02°, and V = 631.4 Å3. The structure contains chains of edge-sharing neptunyl pentagonal bipyramids that link through phosphate tetrahedra to form infinite sheets. This sheet-type is identical to the anion topology of the uranophane group, in particular to that of oursinite, Co[(UO2)(SiO3OH)]2(H2O)6. Similarities between Ba(NpO2)(PO4)(H2O) and the uranophane group of minerals suggests a charge-balancing mechanism for incorporation of Np5+ into uranyl minerals.

Synthesis and Structural Characterization of Hydrolysis Products within the Uranyl Iminodiacetate and Malate Systems

The interplay of hydrolysis and chelation by organic ligands results in the formation of novel uranium species in aqueous solutions. Many of these molecular complexes have been identified by spectroscopic and potentiometric techniques, but a detailed structural understanding of these species is lacking. Identification of possible uranyl hydrolysis products in the presence of organic functional groups has been achieved by the crystallization of molecular species into a solid-state compound, followed by structural and chemical characterization of the material. The structures of three novel molecular complexes containing either iminodiacetate (ida) (Na3[(UO2)3(OH)3(ida)3]·8H2O (1)) or malate (mal) (K(pip)2[(UO2)3O(mal)3]·6H2O (2a) (pip = C4N2H12), (2b) (pip)3[(UO2)3O(mal)3]·H2O, and (pip)6[(UO2)11(O)4(OH)4(mal)6(CO3)2]·23H2O (3)) ligands have been determined by single-crystal X-ray diffraction and have been chemically characterized by IR, Raman, and NMR spectroscopies. A major structural component in compounds 1 and 2 is a trimeric 3:3 uranyl ida or mal species, but different bridging groups between the metal centers create variations in the structural topologies of the molecular units. Compound 3 contains a large polynuclear cluster with 11 U atoms, which is composed of trimeric and pentameric building units chelated by mal ligands and linked through hydroxyl groups and carbonate anions. The characterized compounds represent novel structural topologies for U(6+) hydrolysis products that may be important molecular species in near-neutral aqueous systems.

Contaminant adsorption on nanoscale particles: structural and theoretical characterization of Cu2+ bonding on the surface of Keggin-type polyaluminum (Al30) molecular species.

The adsorption of contaminants onto metal oxide surfaces with nanoscale Keggin-type structural topologies has been well established, but identification of the reactive sites and the exact binding mechanism are lacking. Polyaluminum species can be utilized as geochemical model compounds to provide molecular level details of the adsorption process. An Al30 Keggin-type species with two surface-bound Cu(2+) cations (Cu2Al30-S) has been crystallized in the presence of disulfonate anions and structurally characterized by single-crystal X-ray diffraction. Density functional theory (DFT) calculations of aqueous molecular analogues for Cu2Al30-S suggest that the reactivity of Al30 toward Cu(2+) and SO4(2-) shows opposite trends in preferred adsorption site as a function of particle topology, with anions preferring the beltway and cations preferring the caps. The bonding competition was modeled using two stepwise reaction schemes that consider Cu2Al30-S formation through initial Cu(2+) or SO4(2-) adsorption. The associated DFT energetics and charge density analyses suggest that strong electrostatic interactions between SO4(2-) and the beltway of Al30 play a vital role in governing where Cu(2+) binds. The calculated electrostatic potential of Al30 provides a theoretical interpretation of the topology-dependent reactivity that is consistent with the present study as well as other results in the literature.

Evaluating Best Practices in Raman Spectral Analysis for Uranium Speciation and Relative Abundance in Aqueous Solutions.

Raman spectroscopy is emerging as a powerful tool for identifying hexavalent uranium speciation in situ; however, there is no straightforward protocol for identifying uranyl species in solution. Herein, uranyl samples are evaluated using Raman spectroscopy, and speciation is monitored at various solution pH values and anion compositions. Spectral quality is evaluated using two Raman excitation wavelengths (532 and 785 nm) as these are critical for maximizing signal-to-noise and minimizing background from fluorescent uranyl species. The Raman vibrational frequency of uranyl shifts according to the identity of the coordinating ions within the equatorial plane and/or solution pH; therefore, spectral barcode analysis and rigorous peak fitting methods are developed that allow accurate and routine uranium species identification. All in all, this users guide is expected to provide a user-friendly, straightforward approach for uranium species identification using Raman spectroscopy.

Surface Modification of Al30 Keggin-Type Polyaluminum Molecular Clusters

Keggin-type molecular clusters formed from the partial hydrolysis of aluminum in aqueous solutions have the capacity to adsorb a variety of inorganic and organic contaminants. The adsorptive capability of Keggin-type polyaluminum species, such as Al13 and Al30, lead to their wide usage as precursors for heterogeneous catalysts and clarifying agents for water purification applications, but a molecular-level understanding of adsorption process is lacking. Two model Al30 clusters, whose surface has been modified with chelated metals (Al(3+) and Zn(2+)) have been synthesized and structurally characterized by single-crystal X-ray diffraction. Al32IDA [(Al(IDA)H2O)2(Al30O8(OH)60(H2O)22)](2,6-NDS)4(SO4)2Cl4(H2O)40, IDA = iminodiacetic acid, 2,6-NDS = 2,6 napthalene disulfonate) crystallize in the triclinic space group, P1 with a = 13.952(2) Å, b = 16.319(3) Å, c = 23.056(4) Å, α = 93.31(1)°, β = 105.27(1)°, and γ = 105.52(1)°. Zn2Al32 [(Zn(NTA)H2O)2(Al(NTA)(OH)2)2(Al30(OH)60(O)8(H2O)20](2,6-NDS)5(H2O)64, (NTA = nitrilotriacetic acid), also crystallizes in P1 with unit cell parameter refined as a = 16.733(7) Å, b = 18.034(10) Å, c = 21.925(11) Å, α = 82.82(2)°, β = 70.96(2)°, and γ = 65.36(2)°. The chelated metal centers adsorb to the surface of the Al30 clusters through hydroxyl bridges located at the central belt region of the molecule. The observed binding sites for the metal centers mirror the reactivity predicted by previously reported molecular dynamic simulations and can be identified by the acidity and hydration factor of the water group that participates in the adsorption process.