Abstract
Metatorbernite [Cu(UO2)2(PO4)2·8H2O] is a promising remediation material for environmental uranium contamination. Previous X-ray diffraction studies have been unable to definitively locate hydrogen positions within metatorbernite, which are key to determining the hydrogen-bond network that helps to stabilize the structure. Here, hydrogen positions have been determined using a combination of neutron powder diffraction and the computational modelling technique ab initio random structure searching (AIRSS). Atomic coordinates determined through Rietveld analysis of neutron powder diffraction data are in excellent agreement with the minimum energy configuration predicted by AIRSS; thus, simulations confirm that our proposed model likely represents the global minimum configuration. Two groups of water molecules exist within the metatorbernite structure: free water and copper-coordinating water. Free water molecules are held within the structure by hydrogen bonding only, whilst the coordinating water molecules bond to copper in the equatorial positions to produce a 4 + 2 Jahn-Teller octahedra. The successful agreement between neutron powder diffraction data and AIRSS suggests that this combined approach has excellent potential for the study of other (trans)uranium materials in which hydrogen bonding plays a key role in phase stability.
Highlights
Autunite minerals are among the most-abundant secondary uranium minerals found in both natural and anthropogenically contaminated environments
powder X-ray diffraction (PXRD) data collected on the deuterated sample were consistent with data collected pre-deuteration and with the ideal reflection positions for metatorbernite in space group P4/n obtained from the Inorganic Crystal Structure Database (ICSD)
Results from the structural refinement of neutron powder diffraction (NPD) data and ab initio random structure searching (AIRSS) are in agreement with published work (Ross et al, 1964; Stergiou et al, 1993; Locock & Burns, 2003; Stubbs et al, 2010) to suggest that metatorbernite crystallizes in the space group P4/n with refined unit-cell parameters of a =
Summary
Autunite minerals are among the most-abundant secondary uranium minerals found in both natural and anthropogenically contaminated environments. Metatorbernite has a higher thermodynamic stability relative to other autunite minerals (Dzik et al, 2017, 2018), uranyl phosphates (GormanLewis et al, 2009; Shvareva et al, 2012), uranyl oxyhydrates (Shvareva et al, 2012), uranyl carbonates (Shvareva et al, 2012), uranyl silicates (Shvareva et al, 2012) and uranyl vanadates (Spano et al, 2017), making it one of the least mobile phases in the environment It is reportedly among the most stable of mineral phases, a better understanding of stability must be acquired before this type of remediation strategy can be implemented on a wide scale, in order to prevent the re-release of uranium into the environment
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