T65 Radiation Effects, Mineralogy, and Materials Science of Actinides II: In Honor of Rodney C. Ewing, 2015 Roebling Medalist

Sunday, 1 November 2015
1:30 PM - 5:30 PM
Room 317

ENTHALPIES OF FORMATION OF URANYL PHOSPHATES: METATORBERNITE, METAANKOLEITE, AND METAURANOCIRCITE

Paper number: T65-8
Program: Technical Sessions
Time: 3:20 PM – 3:35 PM

Authors

Abstract

Metatorbernite (Cu[(UO2)(PO4)]2(H2O)(H2O)8, metaankoleite (K2(UO2)2(PO4)2(H2O)3, and metauranocircite (Ba[(UO2)(PO4)]2(H2O)6 are uranyl phosphate minerals that belong to the autunite group. The ubiquity of phosphorous in the environment, the uranyl phosphates are widespread in nature and are the most abundant uranyl minerals. Structurally, these minerals contain sheets with the autunite-type topology, where [(UO2)(PO4)]- sheets consist of uranyl square bipyramids and phosphate tetrahedra, with the interlayer occupied by cations and molecular water.

Polycrystalline samples of uranyl phosphate minerals have been prepared at room temperature using slow mixing by a diffusion method. Reaction products were analyzed using powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), and inductively coupled plasma optical emission spectrometry (ICP-OES). Calorimetric measurements have been performed in a Calvet-type twin calorimeter using sodium molybdate (3Na2O•4MoO3) solvent at 975K. The enthalpy of formation from the binary oxides, ΔHf-ox, at 298 K was calculated for each compound from the respective drop solution enthalpy, ΔHds. Calculated standard enthalpies of formation from the elements, ΔH0f, at 298 K are (-6768.01 ± 4.63 kJ/mol-1 kJ/mol-1) for metauranocircite, (-6799.27 ± 1.52 kJ/mol-1) for metatorbernite, (-3498.70 ± 2.08) for metaankoleite. The results allow us to describe the thermodynamic stability of uranyl phosphate minerals and can also be used for understanding their formation and fate in a geological repository for nuclear waste, as well as their existence in the subsurface of contaminated environments.