Thermodynamic Studies to Support Actinide/Lanthanide Separations

17 Pages·2016·0.78 MB·English

Thermodynamic
Studies to Support
Actinide/Lanthanide
Separations
Fuel Cycle Research & Development
Prepared for
U.S. Department of Energy
Materials Recovery and Waste Form
Development
Linfeng Rao
Lawrence Berkeley National Laboratory
September 2016
FCRD-MRWFD-2016-000346
LBNL-1006035
DISCLAIMER
This document was prepared as an account of work sponsored by the
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necessarily state or reflect those of the United States Government or any
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Thermodynamic Studies to Support Actinide/Lanthanide Separations
September 2016
Revision 2
12/20/2012
APPENDIX E
FCT DOCUMENT COVER SHEET 1
Name/Title of
Deliverable/Milestone/Revision No. Thermodynamic Studies to Support Actinide/Lanthanide Separations
Work Package Title and Number Thermodynamics and Kinetics - LBNL FT-16LB03010403
Work Package WBS Number 1,02.03.01.04
Responsible Work Package Manager Linfeng Rao (Signature on file)
(Name/Signature)
Date Submitted 9/4/2016
x Lab/Participant
Quality Rigor Level for □ qrl-i QA Program (no
□ QRL-3 □ QRL-2
Deliverable/Milestone2 Nuclear Data additional FCT QA
requirements)
This deliverable was prepared in accordance with Lawrence Berkeley National Laboratory
(Participant/National Laboratory Name)
QA program which meets the requirements of
M DOE Order 414.1 □ NQA-1-2000 □ Other
This Deliverable was subjected to:
I I Peer Review
1X1 Technical Review
Technical Review (TR) Peer Review (PR)
Review Documentation Provided Review Documentation Provided
□ Signed TR Report or, □ Signed PR Report or,
I I Signed TR Concurrence Sheet or, I I Signed PR Concurrence Sheet or,
1X1 Signature of TR Reviewer(s) below I I Signature of PR Reviewer(s) below
Name and Signature of Reviewers
Zhicheng Zhang (Signature on file)
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Thermodynamic Studies to Support Actinide/Lanthanide Separations
IV September 2016
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Thermodynamic Studies to Support Actinide/Lanthanide Separations
September 2016 v
SUMMARY
Thermodynamic data on the complexation of Np(V) with HEDTA in
a wide pH region were re-modeled by including a dimeric complex
species, (Np02)2(0H)2L26" where L stands for the fully deprotonated
HEDTA ligand and better fits were achieved for the spectrophotometric
data. The presence of the dimeric complex species in high pH region was
verified for the first time by the EX AES experiments at Stanford
Synchrotron Radiation Laboratory (SSRL).
Thermodynamic Studies to Support Actinide/Lanthanide Separations
vi September 2016
CONTENTS
SUMMARY...........................................................................................................................................................v
INTRODUCTION ................................................................................................................................................1
RESULTS ..............................................................................................................................................................1
CONCLUSION......................................................................................................................................................9
ACKNOWLEDGMENTS ...................................................................................................................................9
FIGURES
Figure 1. Spectrophotometric titration of Np02 /HEDTA complexation. I = 1.0 M NaC104 , t =
25°C.......................................................................................................................................................1
Figure 2. Calorimetric titration of Np02 /HEDTA complexation (t = 25 °C,I= 1.0 M NaC104)................3
Figure 3. Stability constants vs. total ligand basicity (pKa) at t = 25°C...........................................................4
Figure 4. Possible coordination modes in the Np(V) complexes with aminopolycarboxylic acids...............5
Figure 5. Resolved molar absorbance spectra of individual Np(V) species from the
spectrophotometric titrations..............................................................................................................6
Figure 6. Np Lm EXAFS of three Np(V) solutions............................................................................................8
TABLES
Table 1. Thermodynamic parameters for the complexation of Np02 /HEDTA...............................................2
Table 2. EXAFS fitting results ofNp(V) species in solutions parameters for the complexation of
10 /III I)FA./= LOOM \a( 10: ...................................................................................................................8
Thermodynamic Studies to Support Actinide/Lanthanide Separations
September 2016 vii
ACRONYMS
DTPA diethyltriaminepentaacetic acid
EXAFS extended x-ray absorption fine structure
HDEHP bis-2-ethyl(hexyl) phosphoric acid
HEDTA N-(2-hydroxyethyl) ethylenediamine-N,N',N'-triacetic acid
HEH[EHP] 2-ethyl (hexyl) phosphonic acid mono-2-ethylhexyl ester
IDA iminodiacetic acid
NEA Nuclear Energy Agency
NEFCRD Nuclear Energy Fuel Cycle Research and Development
NTA nitrilotriacetic acid
TALSPEAK Trivalent Actinide Lanthanide Separations by Phosphorus-reagent Extraction from
Aqueous Komplexes

Thermodynamic Studies to Support Actinide/Lanthanide Separations
September 2016 1
1. INTRODUCTION
In the modified TALSPEAK process, 2-ethyl (hexyl) phosphonic acid mono-2-ethylhexy 1 ester
(HEH[EHP]) is proposed to be used as the extractant in the organic phase to replace bis-2-ethyl(hexyl)
phosphoric acid (HDEHP), and a weaker aqueous complexant (N-(2-hydroxyethyl)ethylenediamine-
N,N',N'-triacetic acid, HEDTA) is to be used to replace diethyltriaminepentaacetic acid (DTPA) in the
aqueous phase. Preliminary studies have demonstrated that the combination of HEDTA with HEH[EHP]
results in an almost flat pH dependence between 2.5 and 4.5, in contrast with conventional TALSPEAK.
In addition, the HEDTA/HEH[EHP] combination has shown more rapid kinetics in phase transfer for the
heavier lanthanides without using high concentrations of a lactate buffer as in the conventional
TALSPEAK.
This milestone report summarizes the thermodynamic data on the complexation ofNp(V) with HEDTA in
a wide pH region, and the modelling of the data by including a dimeric complex species,
(Np02)2(0H)2L26 where L stands for the fully deprotonated HEDTA ligand. The model provides better
fits for the spectrophotometric titration data. The presence of the dimeric complex species in high pH
region was verified for the first time by the EX AES experiments at Stanford Synchrotron Radiation
Laboratory (SSRL).
2. RESULTS
2.1 Complexation of Np(V) with HEDTA
2.1.1 Complexation model and stability constants of Np(V)/HEDTA complexes
constants
Figure 1 shows a representative spectrophotometric titration. Figure la shows a titration covering a wide
pCH (= -log |H |) range (3.0 to 11) while Figure lb shows a titration where the highest pCH is 5.5. In
Figure la, the absorption band of free Np02+ at 980.2 nm was red-shifted as HEDTA was added.
Approximately three isosbestic points could be identified at 984, 990, and 994 nm, suggesting four
absorbing species (free Np02+ and three Np(V)/HEDTA complexes) were present in the titration
solutions. A number of models were tested to fit the spectra data in the whole range of pCH, including the
model containing Np02L2, Np02HL, and Np02(0H)L \ but none were satisfactory. In some cases, a fit
could converge but result in unreasonably large, negative absorptivities for the Np02(0H)L complex.
0.10 • W
0.08 -
0.06 ■
0 04 -
0.02 -
0 00 •
0.10 • (b)
0.08 •
0.06 •
0 04 -
0 02 -
0 00 -
Wavelength, nm
Figure 1. Spectrophotometric titrations ofNp(V) with HEDTA at 1= 1.0 mol-L"1 NaC104, i = 25 °C.
Thermodynamic Studies to Support Actinide/Lanthanide Separations
2 September 2016
To better define the Np(V)/HEDTA complexes and obtain the stability constants, the titration data shown
in Figure lb that cover a narrower range of pCH are processed first. Factor analysis of the spectra
indicates the presence of three absorbing species and an excellent fit was achieved with the model
including the formation of two Np(V)/HEDTA complexes: Np02F2" and Np02HL, represented by
reactions (1) and (2). The fit yielded the stability constants of log /?101 (Np02L2) = 6.91 ± 0.06, and log
Adi) (Np02(HF)') = 4.28 ± 0.03.
Np02 + F3 = Np02F2 (1)
Np02++ HE2 = NpOdTF (2)
With the obtained stability constants of Np02F2" and NpOdTF" as known values, the spectra from multiple
titrations covering the whole range of pCH (up to 11) were again fitted with models including an
additional species that could become significant in basic solutions. In the fitting, the hydrolysis constants
of the binary species, Np02OH(aq) and Np02(0H)2, were taken from the Nuclear energy agency (NEA)
database and held constant. The model including the Np02(OH)L complex that was reported in the
previous study did not converge. In contrast, the model including a dimeric species with the stoichiometry
of (Np02)2(0H)2L26, resulted in a good fit with reasonable molar absorptivity. The formation of
(Np02)2(0H)2L26" is represented by reaction (3), with an equilibrium constant of log /?2_22
((NpQ2)2(OH)2L26) = -(4.93 + 0.03).
2Np02+ + 2L3 + 2H20 = (Np02)2(0H)2L26 + 2H+ (3)
The equilibrium constants of reactions (1), (2), and (3) are summarized in Table 1. The equilibrium
constants for reactions (1) and (2) from this work are in reasonable agreement with the values from
previous studies, taking into consideration the difference in ionic strength. However, discrepancy exists in
the model describing the data in the high pCH region: the data in the present study are best represented by
the formation of (Np02)2(0H)2L26" (reaction (3)), while the data from the previous study suggest the
formation of Np02(0H)L'\ The EXAFS results, which are obtained in the present work and will be
discussed later, have demonstrated a dimeric Np(V) configuration in this ternary Np(V)-OH-HEDTA
complex. Also, a ternary dimeric U(VI)-OH-IDA complex (IDA: iminodiacetic acid) has been suggested
in the literature26. We thus believe that at the high pCH area, Np(V) formed the ternary dimeric Np(V)-
OH-HEDTA complex ((Np02)2(0H)2L26 ) with a similar configuration to that of the ternary dimeric
U(VI)-OH-IDA complex, in which, each Np02+ could be coordinated with one HEDTA ligand and the
two Np02+ centers are bridged by two hydroxides.
Table 1 Thermodynamic parameters of the Np(V) complexation with HEDTA at 25 C in
NaCICE. Legends: sp - spectrophotometry, cal - calorimetry, pot - potentiometry, sx - solvent
extraction, p.w.- present work.
Ligand Reaction I Method log/i AH AS ref
kJ-mol"1 J-mof-K'1
HEDTA NpOf + L3' = NpCLL2' 1.0 M sp/cal 6.91 ±0.06 -(8.0 ±2.0) 105 ±7 p.w.
NpQ2+ + HL2" = Np02HL" 1.0 M sp/cal 4.28 ±0.03 -(2.2 ±2.0) 75 ±7 p.w.
2Np02+ + 2L3" + 2H20 = (NpO2)2(OH)2L20- + 2H+ 1.0 M sp -(4.93 ±0.03) p.w.