Degradation and ion exchange mechanisms in metatorbernite-type (Cu(UO2)2(XO4)2.8H2O, X=As, P) minerals; implications for remediation of radiochemical

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This project aims to understand the structure, stability and degradation mechanisms of metatorbernite type minerals. Metatorbernite (Cu(UO2)2(PO4)2 8H2O), and its arsenate analogue metazeunerite (Cu(UO2)2(AsO4)2 8H2O), are common secondary uranium minerals that result from the alteration of uranium ore (uraninite). Both minerals are considered significant hosts for uranium in natural and anthropogenically contaminated environments. Understanding the stability of these mineral phases is therefore essential in the development of retention and remediation strategies aimed to prevent the (re)release of toxic elements into the environment.Metatorbernite and metazeunerite are layered minerals, comprised of uranyl phosphate/arsenate sheets with hydrated interlayers containing water coordinated copper(II) ions. A complete solid solution exists between the two end member phases: Cu(UO2)2(PO4)2-x(AsO4)x 8H2O, 0x2. Through a series of leaching experiments we have found that the stability of the solid solution is composition dependent, with As-rich phases exhibiting a lower stability, particularly with respect to U-leaching. To understand the reason(s) for this composition-based stability, we have begun an in-depth investigation into the mineral structure. Dominant X-ray scattering exhibited by uranium and arsenic has meant that previous studies have been unable to experimentally locate the position of hydrogen atoms within the crystal lattice using X-ray techniques alone. Our analysis includes a combination of synchrotron powder X-ray diffraction, single crystal X-ray diffraction, neutron powder diffraction, infrared spectroscopy, and computational methods. Our work aims to present a new structural model for the metatorbernite metazeunerite solid solution, and apply this knowledge to the observed stability differences.