Open Access

Show Abstract ▼

Abstract: Carbon monoxide (CO) purification from syngas impurities is a highly energy and cost intensive process. Adsorption separation using metal–organic frameworks (MOFs) is being explored as an alternative technology for CO/nitrogen (N2) and CO/carbon dioxide (CO2) separation. Currently, MOFs' uptake and selectivity levels do not justify displacement of the current commercially available technologies. Herein, we have impregnated a leading MOF candidate for CO purification, i.e. M-MOF-74 (M = Co or Ni), with Cu+ sites. Cu+ allows strong π-complexation from the 3d electrons with CO, potentially enhancing the separation performance. We have optimised the Cu loading procedure and confirmed the presence of the Cu+ sites using X-ray absorption fine structure analysis (XAFS). In situ XAFS and diffuse reflectance infrared Fourier Transform spectroscopy analyses have demonstrated Cu+–CO binding. The dynamic breakthrough measurements showed an improvement in CO/N2 and CO/CO2 separations upon Cu impregnation. This is because Cu sites do not block the MOF metal sites but rather increase the number of sites available for interactions with CO, and decrease the surface area/porosity available for adsorption of the lighter component.

Show Abstract ▼

Abstract: Metaschoepite is commonly found in U contaminated environments and metaschoepite-bearing wastes may be managed via shallow or deep disposal. Understanding metaschoepite dissolution and tracking the fate of any liberated U is thus important. Here, discrete horizons of metaschoepite (UO3●nH2O) particles were emplaced in flowing sediment/groundwater columns representative of the UK Sellafield site. The column systems either remained oxic or became anoxic due to electron donor additions, and the columns were sacrificed after 6- and 12-months for analysis. Solution chemistry, extractions, and bulk and micro-/nano-focus X-ray spectroscopies were used to track changes in U distribution and behavior. In the oxic columns, U migration was extensive, with UO22+ identified in effluents after 6-months of reaction using fluorescence spectroscopy. Unusually, in the electron-donor amended columns, during microbially-mediated sulfate reduction, significant amounts of UO2-like colloids (>60% of the added U) were found in the effluents using TEM. XAS analysis of the U remaining associated with the reduced sediments confirmed the presence of trace U(VI), non-crystalline U(IV), and biogenic UO2, with UO2 becoming more dominant with time. This study highlights the potential for U(IV) colloid production from U(VI) solids under reducing conditions and the complexity of U biogeochemistry in dynamic systems.

Open Access

Show Abstract ▼

Abstract: Np(V) behaviour in alkaline, calcite containing systems was studied over a range of neptunium concentrations (1.62 × 10−3 μM–1.62 μM) in two synthetic, high pH, cement leachates under a CO2 controlled atmosphere. The cement leachates were representative of conditions expected in an older (pH 10.5, Ca2+) and younger (pH 13.3, Na+, K+, Ca2+) cementitious geological disposal facility. These systems were studied using a combination of batch sorption and solubility experiments, X-ray absorption spectroscopy, and geochemical modelling to describe Np behaviour. Np(V) solubility in calcite equilibrated old and young cement leachates (OCL and YCL) was 9.7 and 0.084 μM, respectively. In the OCL system, this was consistent with a Np(V)O2OH(am) phase controlling solubility. However, this phase did not explain the very low Np(V) solubility observed in the YCL system. This inconsistency was explored further with a range of pH 13.3 solubility experiments with and variable Ca2+(aq) concentrations. These experiments showed that at pH 13.3, Np(V) solubility decreased with increasing Ca2+ concentration confirming that Ca2+ was a critical control on Np solubility in the YCL systems. X-ray absorption near-edge structure spectroscopy on the precipitate from the 42.2 μM Np(V) experiment confirmed that a Np(V) dioxygenyl species was dominant. This was supported by both geochemical and extended X-ray absorption fine structure data, which suggested a calcium containing Np(V) hydroxide phase was controlling solubility. In YCL systems, sorption of Np(V) to calcite was observed across a range of Np concentrations and solid to solution ratios. A combination of both surface complexation and/or precipitation was likely responsible for the observed Np(V) reaction with calcite in these systems. In the OCL sorption experiments, Np(V) sorption to calcite across a range of Np concentrations was dependent on the solid to solution ratio which is consistent with the formation of a mono-nuclear surface complex. All systems demonstrated slow sorption kinetics, with reaction times of weeks needed to reach apparent equilibrium. This could be explained by slow recrystallisation of the calcite surface and/or the presence of Np(V) colloidal species. Overall, these data provide valuable new insights into Np(V) and actinide(V) behaviour in alkaline conditions of relevance to the disposal of intermediate level radioactive wastes.

Show Abstract ▼

Abstract: Technetium is a problematic contaminant at nuclear sites and little is known about how repeated microbiologically-mediated redox cycling impacts its fate in the environment. We explore this question in sediments representative of the Sellafield Ltd. site, UK, over multiple reduction and oxidation cycles spanning ~ 1.5 years. We found the amount of Tc remobilised from the sediment into solution significantly decreased after repeated redox cycles. X-ray Absorption Spectroscopy (XAS) confirmed that sediment bound Tc was present as hydrous TcO2-like chains throughout experimentation and that Tc’s increased resistance to remobilisation (via reoxidation to soluble TcO4-) resulted from both shortening of TcO2 chains during redox cycling and association of Tc(IV) with Fe phases in the sediment. We also observed that Tc(IV) remaining in solution during bioreduction was likely associated with colloidal magnetite nanoparticles. These findings highlight crucial links between Tc and Fe biogeochemical cycles that have significant implications for Tc’s long-term environmental mobility, especially under ephemeral redox conditions.

Show Abstract ▼

Abstract: The speciation and fate of neptunium as Np(V)O2+ during the crystallization of ferrihydrite to hematite and goethite was explored in a range of systems. Adsorption of NpO2+ to iron(III) (oxyhydr)oxide phases was reversible and for ferrihydrite, occurred through the formation of mononuclear bidentate surface complexes. By contrast, chemical extractions and X-ray absorption spectroscopy (XAS) analyses showed the incorporation of Np(V) into the structure of hematite during its crystallization from ferrihydrite (pH 10.5). This occurred through direct replacement of octahedrally coordinated Fe(III) by Np(V) in neptunate-like coordination. Subsequent XAS analyses on mixed goethite and hematite crystallization products (pH 9.5 and 11) showed that Np(V) was incorporated during crystallization. Conversely, there was limited evidence for Np(V) incorporation during goethite crystallization at the extreme pH of 13.3. This is likely due to the formation of a Np(V) hydroxide precipitate preventing incorporation into the goethite particles. Overall these data highlight the complex behaviour of Np(V) during the crystallization of iron(III) (oxyhydr)oxides, and demonstrate clear evidence for neptunium incorporation into environmentally important mineral phases. This extends our knowledge of the range of geochemical conditions under which there is potential for long term immobilisation of radiotoxic Np in natural and engineered environments.