Mercury extraction by the TRUEX process solvent: I. Kinetics, extractable species, dependence on nitric acid concentration and stoichiometry Page: 3 of 13
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Applicability of the Transuranic Extraction (TRUEX) process, as developed by Horwitz and co-
workers at Argonne National Laboratory [2,3], is currently being evaluated at the ICPP for removal of
trace levels of actinides from liquid SBW. The standard TRUEX solvent is typically comprised of 0.2
M n-octyl(phenyl)N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) and 1.4 M
tributylphosphate (TBP) in n-dodecane. References regarding the extraction of mercury by the TRUEX
process solvent were not identified in the literature. Due to the high mercury content and interest in the
applicability of the TRUEX process to ICPP SBW, an experimental study on the behavior of mercury in
the TRUEX process solvent was undertaken.
EXPERIMENTAL
Commercially available CMPO (97% min, Elf Atochem) was further purified by Vandegrift and
co-workers at Argonne National Laboratory using a double recrystallization from hexane. The ICPP
Quality Control Laboratory prepared fresh TRUEX process solvent from the purified material (0.2 M
CMPO and 1.4 M TBP in n-dodecane). In order to ascertain the purity and composition of the TRUEX
solvent, distribution coefficients of Am-241 were experimentally measured as a function of nitric acid
concentration. The results compared favorably with literature values [4,5] and the solvent was
considered suitably pure and of the proper composition.
A 203HgC12 standard in HCl (Amersham) was diluted to 100 mL in 2 M HNO3. This tracer
solution contained 163.5 7.5 ppm (4.61E-3 M) CL. This stock solution had an initial activity of 14,864
dps/mL. Due to the relatively short half-life of Hg203 (t1,2 = 46.8 days), all subsequent analyses were
decay corrected to the initial date and time. Aliquots of the stock tracer solution were used to prepare
aqueous feeds of the appropriate HNO3 concentration by dilution with water or addition of concentrated
nitric acid.
Non-radioactive mercury stock solutions containing 1 g/L (5.OE-3 M) Hg2' were prepared from
Hg(NO3)2 (99% anhydrous, Baker) at various concentrations of nitric acid. The stock solutions were
prepared at 0.01 M, 0.5 M, 2 M, 5 M, and 8 M HNO3, all containing 1 g/L Hg. These solutions were
subsequently used to study the effect of nitric acid concentration on mercury extractions.
All pre-equilibrations and extractions were performed in glass test tubes at an organic-to-
aqueous (O/A) phase ratio of 1:1. Mixing was facilitated with a vortex mixer, and a centrifuge was used
for phase disengagement. All contacts were conducted at room temperature, 27 2*C.
Aliquots of fresh TRUEX process solvent were pre-equilibrated thrice with nitric acid solutions
of the appropriate concentration prior to Hg extractions. This had the effect of loading the organic with
nitric acid such that the predominate species being transported between phases was the one of interest,
i.e. mercury.
A one mL sample of the appropriate phase containing Hg203 (aqueous feed, raffinate or loaded
organic) was transferred to a 5 dram glass vial with a calibrated geometry. These samples were
analyzed by y-spectroscopy using a High Purity Germanium Detector with a relative efficiency of 73%.
The Hg203 primary photopeak at 279.2 keV was used to determine mercury activities in each phase.
In Hg203 tracer experiments, mercury distribution ratios were calculated as:
Dx = [Hg].J[Hg]. , (1)
where the mercury concentrations are in dps/mL from y-scan analyses. By using tracer solutions, it is
possible to perform gamma analyses on both the organic and aqueous phases directly. Consequently,
material balances can be calculated and used as a check on the validity of the analytical results and
distribution ratio. Material balances in all traced experiments were typically within +5%.
Aliquots of the aqueous raffinates from extractions were titrated with 0.1002 N NaOH to a
phenolphthalein endpoint. The acidities determined were interpreted to be the equilibrium nitric acid
concentration at which the corresponding distribution coefficients were measured.
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Herbst, R. S.; Brewer, K. N.; Tranter, T. J. & Todd, T. A. Mercury extraction by the TRUEX process solvent: I. Kinetics, extractable species, dependence on nitric acid concentration and stoichiometry, article, December 1, 1995; Idaho Falls, Idaho. (https://digital.library.unt.edu/ark:/67531/metadc624507/m1/3/: accessed March 28, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.