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A  Molecular  Dynamics  Study    on  the  liquid-­‐liquid  extraction  of  

 Uranyl  ions  with  Tri-­‐Butyl-­‐Phosphate    

Mark  Thomas  

7.  Umbrella  Sampling   8.  Conclusion  

10.  Bibliography  

Nuclear  reprocessing  is  becoming  an  increasingly  important  process  in  a  time  where  there  is  a  large  focus  on  the  sustainability  of  power  generation.  Not  only  can  it  reduce  the  high-­‐level  waste  volume,  but  can  improve  the  efficiency  of  the  cycle,  by  recycling  unreacted  fuel.  The  PUREX  process  is  the  most  widely  used  method.  1)  The  fuel  rods  are  dissolved  in  nitric  acid;  2)  This   is   then   contacted   with   an   organic   phase   containing   30%   v/v                                                    

tri-­‐butyl-­‐phosphate  (TBP)  (as  seen  in  figure  1)  in  dodecane;  

3)  The  TBP  selectively  binds  to  uranyl  nitrate;  4)  These  neutral  complexes  readily  migrate  into  the  organic  phase.  

1.  Introduction  

Although  widely  used  since  the  1950’s  ,  there  are  many  aspects  of  the  process  that  are  not  understood  on  a  molecular  level.  The  TBP  tends  to  form  structures  called  reverse  micelles  around  polar  molecules   such  as  water  and  acids  due   to   its  hydrophilic  and  hydrophobic  properties,   aggregating  at   the   interface   [1].   The  particular  mechanism  of   the  extraction  around  the   interface   is  unknown.  Moreover,   there  have  been  continuous   improvements   in   the   force  field  models  used   for  Molecular  Dynamics   (MD)   simulations,  meaning   that   the   validity  of   earlier   results   are  now  bought   into  question.   The  drivers   for   third  phase   formation  are   still   unknown,   an  occurrence  that  can  seriously  impair  the  liquid-­‐liquid  extraction.  

(UO22+)aq+2(NO3

-­‐)aq+2(TBP)org  →  ???  →  (UO2(NO3)2  �  2TBP)org  

2.  Motivations  &  Objectives  Motivations  •  Increase   efficiency   of   process   by   gaining   further   understanding   on   rate   of  

mass  transfer  and  partitioning  coefficients  •  To  further  understand  the  driving  forces  of  third  phase  so  it  can  be  avoided  Objectives  •  Systematically  and  rigorously  test  the  force  field  models  for  TBP,  water  and  

dodecane  •  Gain   further   insight   into   the   kinetics,   complexation   and   extraction  

mechanisms  

3.  Methodology  •  GROMACS  4.6.7  •  NPT  –  1  atm  Berendsen  barostat.  298.15  K          

v-­‐rescale  thermostat  [2]  •  Time  step  of  1  ps  •  Energy  minimisation  followed  by  MD  

Units rdf. Read up on particular thermostats

Force  Field  System   OPLS-­‐2005  [3]  

TBP   MNDO  &  ‘DL’  Water   TIP3P  &  SPC/E  Dodecane   OPLS-­‐AA  HNO3   ESP  [4]  

UO2+   QM  [5]  

4.  Binary  System  

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Experimental

New   TBP   partial   charges   optimised   for   the   description   of   a   TBP/dodecane   system,  were  found  to  lead  to  TBP/water  immiscibility,  counter  to  experimental  observation.  It  is  known  that  saturation  point  TBP:H2O  is  1:1  [6].  •  Changing  Lennard-­‐Jones  (LJ)  potentials  had  little  effect  on  structure  •  Scaling   partial   charges   on   TBP   resulted   in   homogenous   mixture   and   better  

thermodynamic  properties  of  system  (reasonable  description  of  TBP/dodecane  also)  •  0.9  best  match  with   free  energy  of   solvation,  0.8  best  match  with  excess  enthalpy  of  

mixing.  0.84  OPLS-­‐MNDO  compromise  was  taken    

5.  Ternary  System  A  new  charged  dodecane  model  was  developed  to  prevent   liquid-­‐gel  phase  transition.  Dodecane  was  added  into  the  system  for  3  different  TBP:H2O   ratios.   Hydrophobic   dodecane   avoids   contact   with   water.  Resulted  in  single  phase  with  possible  reverse  micelles  formed.  

Nitric   acid   was   added   into  the   system.   Emulsion   like  structure   observed.   No   H2O-­‐dcc   interactions,   but   HNO3  slightly   more   dispersed.  Down   to   more   negatively  charged   H   atom.   Reverse  micelles   formed   protecting  polar  molecules  from  ddc.  

  O   Aq  

nH2O   4   1247  

nHNO3   35   -  

nTBP   38   -  

nDDC   108   -  

nH3O+   -   210  

nNO3-   -   210  

•  The  TBP  scaling  factor  0.6  underestimated  intermolecular  forces.  A  factor  of  0.84  or  0.9  OPLS-­‐MNDO  is  recommended  

•  The  unchanged  OPLS-­‐AA  dodecane  prevents  liquid-­‐gel  phase  transition  •  Current  force  field  models  do  not  favour  uranyl  extraction,  despite  formation  

of  reverse  micelles  Recommendations  •  Add  polarizabilities  to  force  field  models,  in  particular  phosphate  head  •  Increase  LJ  potential  between  uranyl  and  TBP  •  Carry  out/compare  results  to  additional  experimental  data  (neutron  scattering  

&  x-­‐ray  diffraction)  

The   author  would   like   to   thank   Junju  Mu,   Karl  Fairhurst  and  Gareth  Myers  for  their  results  and  continuing   cooperation,   and   Andrew   Masters  for   his   l imit less   help   and   knowledge  throughout.  

Systems   Free energy of solvation of H2O in TBP (kJ mol-1)  

Experimental   -19.58112  0.60 MNDO   -6.12 ± 0.29  0.70 MNDO   -10.14 ± 0.79  0.80 MNDO   -15.47 ± 0.57  0.81 MNDO   -15.18 ± 0.25  0.82 MNDO   -16.66 ± 0.39  0.83 MNDO   -14.38 ± 0.52  0.84 MNDO   -17.05 ± 0.96  0.85 MNDO   -14.54 ± 0.15  0.86 MNDO   -17.71 ± 0.81  0.87 MNDO   -18.99 ± 0.61  0.88 MNDO   -20.93 ± 1.04  0.89 MNDO   -19.29 ± 1.50  0.90 MNDO   -19.75 ± 0.41  1.00 MNDO   -22.84 ± 0.92  

0.60  MNDO  0.84  MNDO  0.9  MNDO  LJ  –  1.25  LJ  –  1.75  

LJ  –  2  LJ  -­‐  5  

Gibbs  Free  Energy  Profile  “Double  Pull”  

1.  C. A. Hawkins, L. X. Dang, M. Nilsson, and H. D. Nguyen Q. N. Vo, The Journal of Physical Chemistry, vol. 119, no. 4, pp. 1588–1597, 2015.

2.  J. Postma H. Berendsen, The Journal of Chemical Physics, vol. 81, no. 8, pp. 3684-3690, 1984. 3.  H. Beard, Y. Cao J. Banks, J. Comput. Chem., vol. 26, pp. 1752–1780, 2005. 4.  M. Burgard, and G. Wipff M. Baaden, J. Phys. Chem., vol. 105, pp. 11131-11141, 2001. 5.  S. P. Tiwari, and E. J. Maginn N. Rai, The Journal of Physical Chemistry, vol. 116, p.

10885−10897, 2012. 6.  L. Donadieu, and M. Benedict D. R. Olander, A.1.Ch.E. Journal, vol. 7, no. 1, pp. 152-157, 1961.

Table  1:  Force  fields  used  for  system  and  each  molecule.  

Figure  2:  0.6  MNDO  TBP  (PINK)  &  TIP3P  H2O  (PURPLE).   Figure  3:  0.84  MNDO  TBP  (PINK)  &  TIP3P  H2O  (PURPLE).   Figure  4:  TBP  (RIGHT)  &  H2O  (LEFT)  dimer  formed  in  system.  

Figure  5:  Effect  of  partial  charges  on  enthalpy  of  system.  

Table  2:  Effect  of  charge  of  free  energy  of  solvation.  

Figure  6:  RDF  of  OW-­‐OW  interactions.  Effect  of  partial  charges  &  LJ  scaling.  

Figure  7:  Uncharged  and  charged  dodecane  model  (PURE).  

Figure  8:  RDF  of  ddc-­‐H2O  &  P-­‐OW  for  ternary  system.  

Figure  9:  TBP  (GREEN)  &  H2O  (BLUE)  for  ternary  system.  

Figure  10:  TBP  (RIGHT)  &  HNO3  (LEFT)  dimer.   Figure  11:  TBP  (GREEN),  HNO3  (RED)  H2O  (BLUE).  

Figure  12:  Gibbs  free  energy  curve  for  pulling  &  “double”  pulling  simulation.  

kJ  mol-­‐1  

Figure  13:  Pulling  of  uranyl  ion  (RED)  from  the  aqueous  phase  consisting  of  NO3-­‐  (GREEN),  H3O+  (ORANGE)  &  H2O  (BLUE).  

Table  3:  Composition  of  pulling  simulations.  

Figure  1:  Structure  of  TBP  molecule.  

Uranyl   nitrate   ion   was   pulled   across   interface,  e x p e c t e d   t o   l o s e   2 ( H 2O )   a n d   f o rm  (UO2(NO3)2�2TBP).   Uranyl   nitrate   in   fact   pulled  water   across   interface   and   did   not   bind  with   TBP.  Gibbs   free   energy   profile   indicates   that   extrachon  process   is   unfavourable,  with   an   achvahon   energy  of  60  kJ  mol-­‐1.  This  result  can  not  be  true  otherwise  the  PUREX  process  wouldn't  work,   and  we  believe  we   have   seriously   undereshmated   the   anrachons  between  TBP  and  uranyl.  “Double  Pull”  simulahon,  showed  that  organic  phase  is  more  favourable  than  interface.    

P-­‐OW  H2O-­‐ddc  

9.  Acknowledgements  

6.  Quaternary  System