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SUMMARY:Solubility of the rhabdophane\, LnPO4.0.667 H2O (Ln = La to Dy): i
 mplications for the long-term stability of monazite matrices.
DTSTART;VALUE=DATE-TIME:20171031T234500Z
DTEND;VALUE=DATE-TIME:20171101T001500Z
DTSTAMP;VALUE=DATE-TIME:20260419T082711Z
UID:indico-contribution-1130@events01.synchrotron.org.au
DESCRIPTION:Speakers: Stéphanie Szenknect (ICSM)\nMinerals belonging to t
 he monazite family\, REEPO4 (REE: Y\, Sc\, La to Dy) could incorporate in 
 their structure substantial amounts of Th and U [1]. Moreover\, monazites 
 maintain their crystallinity even after geological times of exposure to se
 lf-irradiation. Therefore\, this phase appears as a promising candidate fo
 r the specific conditioning of transuranic elements (AnIV\,AnIII). Already
 \, many thermodynamic properties of monazite are reported in the litteratu
 re [2]. However\, its hydrated form namely\, rhabdophane\, REEPO4.0.667 H2
 O could be stabilized at low temperatures\, and thus control the concentra
 tions of actinides in solution after the leaching of the monazite matrices
  [3\, 4]. \n\nFor the first time\, a systematic study of the solubility of
  the rhabdophane\, LnPO4.0.667 H2O (Ln = La to Dy) was performed using ove
 r-saturated and under-saturated experiments at different temperatures (298
  to 363 K) to demonstrate the reversibility of the solubility equilibrium 
 [5]. The structure of the neoformed phases was carefully controlled in the
  entire range of temperatures in order to attribute unambiguously the solu
 bility products and the derived thermodynamic data associated to the rhabd
 ophane phases. The obtained results indicate that the stability domain of 
 the rhabdophane phase is limited in temperature and depends on the ionic r
 adius of the lanthanide. The values of the solubility constant depend also
  slightly on the lanthanide with log KS\,0° (298 K) ranging from -25.6 ±
  0.8 (Pr) to -24.9 ± 1.7 (Eu) with minimum values for Pr to Sm. The value
 s of the standard free enery of formation at 298 K varied between -1984 ±
  2 and -2004 ± 2 kJ.mol-1 whatever the lanthanide element considered\, ex
 cept for Eu-rhabdophane that presented the highest value for the Gibbs ene
 rgy of formation (-1896 ± 2 kJ.mol-1).\nFrom the solubility data obtained
  at various temperatures\, the values of enthalpy of formation of the rhab
 dophanes were found between -2151 ± 13 and -2130 ± 12 kJ. mol-1 excepted
  for Eu for which it reached -2057 ± 9 kJ.mol-1. It is worth noting that 
 the data reported by Ushakov et al. [6] for monazites LnPO4 (Ln = La to Gd
 ) determined by oxide melt calorimetry and recalculated by taking into acc
 ount the contribution of the water molecules are in very good agreement wi
 th the data obtained for rhabdophane in this work. This result indicates t
 hat the enthalpy of formation of the rhabdophane can be simply deduced fro
 m the enthalpy of formation of the monazite by adding the contribution of 
 0.667 molecule of structural water.\n\nThese results could be used to esti
 mate the thermodynamic properties of AnIIIPO4 .0.667 H2O (with AnIII =Am\,
  Pu or Cm) by analogy with the lanthanides of close ionic radii. The rhabd
 ophane structure can also incorporate AnIV by coupled substitution of LnII
 I by MII and AnIV. In this field\, Nd0.8Ca0.1Th0.1PO4 .n H2O  were prepare
 d by wet chemistry route and preliminary solubility data were determined\,
  which showed that this phase is metastable in solution at 298 K\, Th and 
 Nd being released leading to the formation of Th2(PO4)2(HPO4). H2O (TPHPH)
  [7] and NdPO4.0.667 H2O\, respectively.\n\nReferences:\n[1] N. Clavier\, 
 R. Podor\, N. Dacheux\, (2011). Journal of the European Ceramic Society 31
 : 941-976.\n[2] F. Poitrasson\, E. Oelkers\, J. Schott\, J.-M. Montel\, (2
 004).Geochimica et Cosmochimica Acta 68: 2207-2221.\n[3] E. Du Fou de Kerd
 aniel\, N. Clavier\, N. Dacheux\, O. Terra\, R. Podor\, (2007). Journal of
  Nuclear Materials 362: 451-458.\n[4] N. Dacheux\, N. Clavier\, R. Podor\,
  (2013). American Mineralogist 98: 833-847.\n[5] C. Gausse\, S. Szenknect\
 , D.W. Qin\, A. Mesbah\, N. Clavier\, S. Neumeier\, D. Bosbach\, N. Dacheu
 x\, (2016). European Journal of Inorganic Chemistry 2016: 4615-4630.\n[6] 
 S.V. Ushakov\, K.B. Helean\, A. Navrotsky\, L.A. Boatner\, (2001). Journal
  of Materials Research 16: 2623-2633.\n[7] D. Qin\, C. Gausse\, S. Szenkne
 ct\, A. Mesbah\, N. Clavier\, N. Dacheux\, (2017). The Journal of Chemical
  Thermodynamics\, dx.doi.org/10.1016/j.jct.2017.01.003\n\nhttps://events01
 .synchrotron.org.au/event/51/contributions/1130/
LOCATION:
URL:https://events01.synchrotron.org.au/event/51/contributions/1130/
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