Stability and structure of the Cu(UO2)2(PO4)2-x(AsO4)x.8H2O solid solution and its environmental significance
2019-03-05T09:35:12Z (GMT) by
Radioactive samples have been taken from two locations at the South Terras site close to the disused mine and analysed by powder X-ray diffraction and ICP-OES to determine their composition. Stability studies were carried out using acidic (sulfuric and citric) and basic (sodium hydroxide and sodium, potassium and ammonium bicarbonate solutions) media to investigate the stability of the phases and the mobility of uranium from these materials in the natural environment. X-ray diffraction data indicated a solid-solution existed between Cu(UO2)2(PO4)2.8H2O (metatorbernite) and Cu(UO2)2(AsO4)2.8H2O (metazeunerite) reflecting the natural abundance of phosphorus and arsenic at the site. Stability studies showed that the majority of the uranium was contained in the non-exchangeable residual fraction of the soil at near neutral pH meaning the solubility of uranium is low and therefore active species are unlikely to migrate away from the mine site under standard environmental conditions. Based on the discovery of the Cu(UO2)2(PO4)2-x(AsO4)x.8H2O solid solution at South Terras, work was undertaken to produce a synthetic solid solution to further investigate the stability and structure of these novel phases. Cu(UO2)2(PO4)2-x(AsO4)x.8H2O (x=0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0, 1.2, 1.4, 1.5, 1.6, 1.8, 2.0) were synthesised by precipitation from solution using phosphoric/arsenic acid mixtures of the appropriate molar proportions. The phases were characterised using PXRD, ICP-OES, TGA and Raman and IR spectroscopy. Refined cell parameters determined by Pawley analysis, showed a linear variation in the a cell parameter according to Vegard s Law which allows the composition of these phases to be determined from measurement of the cell parameters. Changes in the relative intensities of the ν3 Raman absorptions at 990cm-1 for the tetrahedral phosphate species were also shown to be able to approximate the PO43-/AsO43- content and hence the empirical formula of the phase. High-resolution PXRD data were collected on five solid solution phase members (where x = 0, 0.5, 1, 1.5 and 2). The four solid solution member phases containing phosphate (x = 0, 0.5, 1 and 1.5) were found to crystallise in the P4/n space group. Metazeunerite (Cu(UO2)2(AsO4)2.8H2O) crystallised in the P4/ncc space group. Upon cooling to temperatures of ~110K a reversible phase transition was observed for the Cu(UO2)2(AsO4)2.8H2O phase to a lower symmetry orthorhombic space group, Pccn. Stability studies were carried out on the synthetic samples to investigate the effect of acid/base on the solid solution as a function of temperature, time and concentration. Increasing the concentration of arsenate in the crystal structure of the Cu(UO2)2(PO4)2-x(AsO4)x.8H2O was found to have a profound influence on reactivity. In strongly basic solutions and in the presence of NaHCO3, greater degradation of the crystalline structure and more dissolution of the material into solution is observed as the value of x increases. Metazeunerite (Cu(UO2)2(AsO4)2.8H2O) and Cu(UO2)2(PO4)0.5(AsO4)1.5.8H2O undergo an ion-exchange type reaction in KHCO3 and NH4HCO3 where the copper cations located in the interlayer spaces either exchange or undergo dissolution/reprecipitation with the ammonium or potassium ions in solution to form hydrated phases of NH4(UO2)(AsO4) and K(UO2)AsO4. The differences in reactivity may have important implications for the stability of the Cu(UO2)2(PO4)2-x(AsO4)x.8H2O solid solution in the environment both at South Terras and at other locations. The work reported here has the potential to influence the choice of remediation solutions employed at this or other sites.