Engineered mineralogical interfaces as radionuclide repositories

Effective capture of fugitive actinides and daughter radionuclides constitutes a major remediation challenge at legacy or nuclear accident sites globally. The ability of double-layered, anionic clay minerals known as hydrotalcites (HTC) to contemporaneously sequester a range of contaminants from solution offers a unique remedy. However, HTC do not provide a robust repository for actinide isolation over the long term. In this study, we formed HTC by in-situ precipitation in a barren lixiviant from a uranium mine and thermally transformed the resulting radionuclide-laden, nanoscale HTC. Atomic-scale forensic examination of the amorphized/recrystallised product reveals segregation of U to nanometre-wide mineral interfaces and the local formation of interface-hosted mineral grains. This U-phase is enriched in rare earth elements, a geochemical analogue of actinides such as Np and Pu, and represents a previously unreported radionuclide interfacial segregation. U-rich phases associated with the mineral interfaces record a U concentration factor of ~ 50,000 relative to the original solute demonstrating high extraction and concentration efficiencies. In addition, the co-existing host mineral suite of periclase, spinel-, and olivine-group minerals that equate to a lower mantle, high P–T mineral assemblage have geochemical and geotechnical properties suitable for disposal in a nuclear waste repository. Our results record the efficient sequestering of radionuclides from contaminated water and this novel, broad-spectrum, nanoscale HTC capture and concentration process constitutes a rapid solute decontamination pathway and solids containment option in perpetuity

New quenched-in fluorite-type materials in the Bi2O 3-La2O3-PbO system: Synthesis and complex phase behaviour up to 750 °c

New quenched-in fluorite-type materials with composition (BiO 1.5)0.94-x(LaO1.5)0.06(PbO) x, x = 0.02, 0.03, 0.04 and 0.05, were synthesised by solid state reaction. The new materials undergo a number of phase transformations during heating between room temperature and 750 °C, as indicated by differential thermal analysis. Variable temperature X-ray diffraction performed on the material (BiO1.5)0.92(LaO1.5) 0.06(PbO)0.02 revealed that the quenched-in fcc fluorite-type material first undergoes a transformation to a β-Bi 2O3-type tetragonal phase around 400 °C. In the range 450-700 °C, α-Bi2O3-type monoclinic, Bi 12PbO19-type bcc and β1/β 2-type rhombohedral phases, and what appeared to be a ε-type monoclinic phase, were observed, before a single-phase fluorite-type material was regained at 750 °C. © 2010 Elsevier Ltd © 2011 Elsevier Ltd. All rights reserved

Synthesis and formation mechanism of VO2(A) nanoplates with intrinsic peroxidase-like activity

Monocrystalline VO2(A) nanoplates have been synthesized via a one-pot hydrothermal process. In situ powder X-ray diffraction was used to monitor the hydrothermal synthesis and it was found that VO2(A) nucleates and grows directly from solution after the complete hydrolysis of a 2.0 M VO(acac)2 precursor solution, rather than involving a previously reported intermediate phase VO2(B). A hydrating-exfoliating-splitting mechanism was established to explain the formation of the nanoplate architecture. The synthesized VO2(A) nanoplates showed outstanding peroxidase-like activity and hence are a promising candidate for artificial peroxidase

Phase and morphology evolution during the solvothermal synthesis of VO2 polymorphs

The phase and morphological features of materials are often tunable by adjusting the reaction parameters of solvothermal synthesis but this versatility also poses a challenge for preparing materials with a desired phase and morphology if the behaviors of phase and morphological evolution during the solvothermal synthesis are not known. In this work, the formation and growth of VO2 nanomaterials in the solvothermal systems via the reduction of V2O5 by ethylene glycol (EG) were investigated by in situ powder X-ray diffraction (PXRD). The results show that both fast and slow heating produce the same VO2(B) final product but the phase evolution during the synthesis is very sensitive to the heating rate. Fast heating (10 °C min−1) involves an unknown intermediate while V3O7·H2O is the intermediate phase at slow heating (2 °C min−1). The formation mechanism was employed to design the synthesis of VO2(B) nanorods and the phase transformation paths were verified by large-scale batch synthesis. Furthermore, ex situ PXRD and SEM were employed to follow the structure and morphology evolution during growth. This research indicates that in situ PXRD, as a powerful tool to monitor the whole reaction process and to collect information such as phase evolution and the fate of the transient intermediates, can be used to direct the controlled synthesis of materials

Effect of Addition of Mill Scale on Sintering of Iron Ores

Iron-rich (65 to 70 pct total Fe) mill scale generated during processing by steel mills can be recycled by using it as a ferrous raw material in the sintering process. The effect of mill scale addition on the phase formation of sintered specimens from an industrial sinter blend containing 0 to 15 wt pct mill scale was examined, and the mineral phases formed during sintering under various conditions (T = 1523 K to 1598 K [1250 °C to 1325 °C] and gas compositions of pO2 = 0.5, 5 and 21 kPa) were quantitatively measured. For samples sintered in air (pO2 = 21 kPa), there was negligible effect of mill scale addition on the phases formed. The oxidation of the mill scale was complete, and phases such as Silico-Ferrite of Calcium and Aluminum (SFCA), SFCA-I, and hematite dominated. Under lower oxygen partial pressures (pO2 = 0.5 or 5 kPa), and throughout the temperature range examined, the mill scale was converted to magnetite, with the extent of reaction controlled by the hematite-magnetite conversion kinetics. When sintered in the gas mixture with pO2 = 5 kPa, an increase in the mill scale content from 0 to 15 wt pct resulted in a decrease of hematite and total SFCA phases and a corresponding increase in the amount of magnetite which formed. The oxidation of wustite in mill scale to magnetite decreased the local partial pressure of O2 and increased sintering temperature, which promoted the decomposition of hematite

Controllable synthesis of VO2(D) and their conversion to VO2(M) nanostructures with thermochromic phase transition properties

VO2(M) nanostructures of various shapes were synthesized by a hydrothermal-calcination method. First, VO2(D) nanoparticles were synthesized by the surfactant-free hydrothermal reduction of ammonium metavanadate by oxalic acid at 160–220 °C. Then, the produced VO2(D) was further calcined at 250–600 °C to obtain the VO2(M) nanoparticles. To understand the hydrothermal reduction processes, both in situ powder X-ray diffraction (PXRD) and ex situ characterization were carried out. The results indicate a sequential process starting from the reduction of ammonium metavanadate and nucleation of the vanadium precursor, followed by the formation of intermediate VO2(B) nanosheets or nanorods, and finally phase transformation from VO2(B) to VO2(D) with a variety of morphologies. A crystal growth mechanism based on self-assembly and Ostwald ripening was proposed to explain the formation process of these unique nanostructures. The as-prepared VO2(M) nanoaggregates exhibited a lower thermochromic phase transition temperature (41.0 °C) and a narrower thermal hysteresis width (6.6 °C) than those nanopowders prepared by other methods

Raman spectroscopy of formamidinium-based lead halide perovskite single crystals

Raman spectroscopy is a powerful technique for the study of materials chemistry and nanostructure. This nondestructive technique is highly sensitive to molecular and crystal lattice vibrations, which allow for a comprehensive study of the vibrational modes of molecules incorporated in photovoltaic perovskite materials. In this study, we apply Raman spectroscopy to study FAPbX3 (X = Cl, Br, I) and FAxMA1–xPbI3 (FA stands for formamidinium; MA for methylammonium) metal halide perovskite single crystals and discuss the necessary conditions to obtain reliable data. We establish a correlation between perovskite composition and their unique Raman intensities/spectral shapes. In particular, we show that tuning of the halide content results in a spectral shift of the organic features of the Raman spectrum due to changes in the strength of hydrogen bonding, while tuning of the organic cation is related more to changes in peak intensity. Moreover, the effect of temperature on the vibrational modes corresponding to NCN bending, NH2 torsion, and NH2 wagging were studied. This enables the impact of the organic composition in FAxMA1–xPbI3 on the phase transition temperature of the material to be determined. Furthermore, we establish links between Raman spectroscopy and other conventional measurement techniques such as X-ray diffraction (XRD) and differential scanning calorimetry (DSC). This study provides insight into the interpretation of the Raman spectra of FA-based perovskites, which furthers understanding of the properties of these materials in relation to their full exploitation in solar cells.Shuai Ruan, David P. McMeekin, Rong Fan, Nathan A. S. Webster, Heike Ebendorff-Heidepriem, Yi-Bing Cheng, Jianfeng Lu, Yinlan Ruan, Christopher R. McNei

Anatomy of a complex mineral replacement reaction: Role of aqueous redox, mineral nucleation, and ion transport properties revealed by an in-situ study of the replacement of chalcopyrite by copper sulfides

The fluid-driven transformation of chalcopyrite (CuFeS2) into Cu-rich sulfides (e.g., digenite, Cu1.8S; covellite, CuS; and chalcocite, Cu2S) is a complex mineral replacement reaction where the reaction pathway is controlled by the interplay between evolving mineral make-up, texture/porosity, and solution chemistry. This transformation was investigated in CuCl2 + H2SO4 solutions under mild hydrothermal conditions (180 to 300 °C); the reaction kinetics, nature of minerals formed, and oxidation states of aqueous Fe and Cu were followed in-situ in real-time using synchrotron powder X-ray diffraction (PXRD) and X-ray absorption spectroscopy (XAS). These results are corroborated by an analysis of the textures of reaction products from comparative ex-situ quench experiments. The in-situ and ex-situ experiments revealed that: (i) aqueous Cu2+ quickly reduced to Cu+ during chalcopyrite replacement in all experiments, and Fe dissolved as Fe2+; (ii) covellite was the first mineral to form, followed by digenite-high with delayed nucleation; and (iii) a non-quenchable hydrated Fe sulfate mineral (szomolnokite, FeSO4.H2O) formed at 240 °C at relatively low concentrations of added CuCl2, which supressed the formation of digenite-high. The quantitative mineral phase evolution retrieved using in-situ PXRD was modelled using a novel dual power law (dual Avrami approach). Avrami exponents revealed that chalcopyrite replacement proceeded initially via a 3-dimensional growth mechanism, followed by diffusion-controlled growth. This is consistent with initial formation of a porous covellite rim around chalcopyrite, confirmed by the observation of the ex-situ reaction products, followed by a second reaction stage where the transport properties of aqueous Fe (released from the chalcopyrite) and aqueous Cu (added from the initial solution) to and from the reaction front become the rate-limiting step; and these two kinetic stages exist even where covellite was the only replacement product. The activation energies calculated for these two kinetic stages were 42.9 ± 7.4 kJ mol−1 and 39.3 ± 13.1 kJ mol−1, respectively. We conclude that (i) the replacement of chalcopyrite by covellite and digenite proceeds via an interface coupled dissolution and reprecipitation mechanism; (ii) availabilities of aqueous Cu+ and of Fe2+ play a critical role in covellite nucleation and on the sequence of mineral precipitation during chalcopyrite replacement; the Cu+ to Cu2+ ratio is controlled by the kinetics of Cu2+ to Cu+ reduction, which increases with increasing temperature, and by the transport of Cu2+ through the daughter mineral to the reaction front, while Fe2+ availability is limited at high temperature by the formation of insoluble ferrous sulfate; and (iii) this reaction evolves from a bulk fluid-chemistry controlled reaction (initial formation of covellite) to an interface-controlled reaction (digenite-high or further transformation to covellite). The current findings highlight the complex feedback between Cu2+/Cu+ aqueous redox, mineral nucleation, and ion transport properties during replacement reactions, and the applicability of combined in-situ PXRD and XAS techniques in deciphering complex fluid-driven mineral replacement reactions