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Title: Uranium coordination chemistry in Mg-rich systems
Author: Van Veelen, Arjen
ISNI:       0000 0004 5359 4172
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2014
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In the UK, large quantities of intermediate level waste pose complex radiological remediation challenges. Chemical understanding of uranium in these Mg-rich sludges is vital. Previous studies have examined uranium uptake by calcium carbonate minerals (calcite and aragonite) under conditions pertinent to both natural and anthropogenically perturbed systems. However, research on uranyl uptake by magnesium-rich minerals such as magnesite [MgCO3], brucite [Mg(OH)2], nesquehonite [MgCO3·3H2O] and hydromagnesite-[Mg5(CO3)4(OH)2·4H2O] has not, to the best of our knowledge, been previously conducted. Such experiments will improve our understanding of the mobility of uranium and other actinides in natural lithologies such as dolomitic limestones or mafic igneous emplacements, as well as provide key information applicable to nuclear waste repository strategies involving Mg-rich phases. By two EXAFS techniques, we determined: (1) where uranyl (UO22+) is adsorbed, and (2) how uranyl is attached to the mineral surface. Therefore powder experiments of U(VI) were performed with magnesite, brucite, nesquehonite and hydromagnesite. The second experiment (GIXAFS) consisted of single crystals of magnesite (10.4) and brucite (0001). The powders were reacted in solution pH ~8.5 with U(VI)nitrate for 48 hrs. under ambient PCO2 = -3.5. The single crystals were reacted under ambient and reduced PCO2 ~ -4.5 for 48 hrs. with concentrations of U(VI)chlroride above (500; 50 ppm) and below (5 ppm) solubility of schoepite [UO2(OH)2·H2O]. The GIXAFS measurements were made at χ = 0˚ and χ = 90˚ relative to the synchrotron beam polarisation to uequivocally determine the adsorbate structures. Kd values for Mg-carbonate phases were comparable to or exceeded those published for calcium carbonates. GIXAFS results clearly showed polarisation for both ambient and reduced PCO2. XANES results showed uranyl is oriented with the axial oxygens perpendicular to the mineral surface. This implies, using also X-ray reflectivity and diffuse scatter, local hydrated bayleyite [Mg2(UO2)(CO3)3∙18H2O] and possible rutherfordine-like [UO2CO3] regions, which will be useful to predict uranium behaviour in various remediation processes.
Supervisor: Not available Sponsor: EPSRC ; NDA
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Magnox sludge, mineralogy, geochemistry, uranium, EXAFS, polarization dependent GIXAFS ; X-ray reflectivity, X-ray diffuse scatter