engleski

EPR determination of uranium(V) in meta-autunite

The determination of metastable and reduced uranium species in environmental matrices including minerals presents substantial problems, particularly in the case of uranium (V). Uranium in the oxidation state 5+ is stable in the solid phase but disproportionates rapidly in aqueous solution. Thus, solution based analytical techniques are inappropriate for the determination of U in this oxidation state. Uranium oxidation states in meta-autunite from pegmatites in Quinto de Baixo in Brazil were studied in order to identify the redox conditions of the environment in which the mineral crystallized. The electrochemical determination of the O/U ratio in the mineral indicated that a substantial amount of the present uranium is in the oxidation state (V). Therefore, this somewhat unexpected result had to be corroborated by an independent technique, and EPR was chosen in order to establish the presence of the unpaired electron in the 5f orbital. From the polycrystalline material of meta-autunite a single fragment (having well-developed plane along c axis was chosen for the EPR study. It can be noted that the EPR spectrum contains two prominent lines. The singlet line with higher intensity exhibits strong oriental dependence in comparison to the line with smaller intensity, which exhibits orientation-independent behavior. The obtained g-values (g|| = 2.39142, along the c-axis and g = 2.06695, perpendicular to c-axis) for the two prominent orientations show significantly larger average g = 2.17511 than giso= 2.00663 for the line with smaller intensity. The detected properties indicate that these lines belong to different paramagnetic species. In order to confirm this assumption, microwave saturation of these signals was undertaken. It is convenient to present saturation data in the plot (P1/2/Ypp)3/2 vs. P where P is microwave power in mW and Ypp is peak-to-peak amplitude of the EPR line detected in the first harmonic. In the case of homogeneous line the plot would be linear and product of spin-lattice and spin-spin relaxation times T1T2 is proportional to the slope of the plot. The EPR line, which exhibits angular dependence, shows homogeneous behavior with a unique linear slope. However, the EPR line, which is invariant on orientation (giso), shows non-linear behavior of the slope indicating non-homogeneous paramagnetic structure. The conclusion follows that at least two types of paramagnetic species can be detected in the crystal of meta-autunite at room temperature. The signal which exhibits axial g-value (g|| = 2.39142, g = 2.06695) and originates from a single paramagnetic species can be assigned to the pentavalent uranium U(V) center stabilized at octahedral sites in the crystal.

EPR; uranium(V); meta-autunite

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