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Procedia Chemistry 7 ( 2012 ) 239 – 244
ATALANTE 2012 International Conference on Nuclear Chemistry for Sustainable Fuel Cycles The Extraction of Silver and the effect of Diluent, Ligand side group and Solvent composition
Elin Löfström-Engdahla*, Emma Aneheima, Christian Ekberga, Mark Foremana, Gunnar Skarnemarka , Zuzana Hájkováb, Bohumír GrĦnerb
aChalmers University of Technology, Nuclear Chemistry & Industrial Materials Recycling, SE-41296 Göteborg, Sweden, bInstitute of Inorganic Chemistry AS CR, v.v.i., CZ 250 68, ěež, Czech Republic
Abstract
Solvent extraction and the so called BTBP class of ligands can be used for the separation of the actinides from the rest of used nuclear fuel. One troublesome co-extracting element in this separation is silver.
Therefore, two different BTBP molecules, having different side groups have been investigated. It was shown that the silver distribution ratio is higher using the CyMe4-BTBP than theC2 -BTBP ligand. In additional experiments, it was shown that no water soluble silver complex is formed in the CyMe4 system and that the complex is one ligand / metal. No effect of varying the diluent/solvent was proven.
©© 20122012 ElsevierThe Authors. B.V...Selection Published and/orby Elsevier peer-review B.V. under responsibility of the Chairman of the ATALANTE 2012 ProgramSelection Committee and/or peer-review under responsibility of the Chairman of the ATALANTE 2012 Program
Keywords: Silver, BTBP ligands, Diluent effects, Solvent effects Introduction
1. Introduction
The separation of the trivalent minor actinides from trivalent lanthanides and some corrosion products is rather complicated task due to their similar chemical behaviour. However, it can be performed using solvent extraction.
* Corresponding author. Tel.: +46317722920; fax: +46317722931 E-mail address: [email protected]
1876-6196 © 2012 Elsevier B.V...Selection and/or peer-review under responsibility of the Chairman of the ATALANTE 2012 Program Committee doi: 10.1016/j.proche.2012.10.039 240 Elin Löfström-Engdahl et al. / Procedia Chemistry 7 ( 2012 ) 239 – 244
The bis-triazin-bi-pyridine (BTBP) class of ligands (Figure 1) is well known for its ability to selectively extract minor actinides from lanthanides [1, 2]. One of the most promising molecules of this class is the CyMe4-BTBP [3-7]. Together with the well-known extractant tributylphospate (TBP) and cyclohexanone used as the diluent, a process called Group ActiNide EXtraction (GANEX) solvent has been designed [3]. The GANEX solvent has shown promising results for the separation of target nuclides from spent nuclear fuel. However, when using this class of ligands one of the troublesome fission products identified that may, in principle, be co-extracted, is silver [3, 4]. Therefore, gaining some basic understanding concerning the silver extraction is of interest.
Complementary HPLC and MS measurements have also been made in order to rule out the formation of a water soluble complex between silver and CyMe4-BTBP.
Figure 1. The bis-triazin-bipyridine, or BTBP, core molecule.
2. Experimental
2.1. Extraction
The extractions were performed using aliquots (300-500 μl) of the organic and aqueous phase. The phase contact was facilitated using a mechanical device (1500 rpm) for 60 minutes. That time was sufficient for reaching the extraction equilibrium. The phases were pre saturated with the corresponding phase before extraction due to the mutual solubility of cyclohexanone and the acidic aqueous phase. The aqueous phase used in all experiments was 1 M HNO3 (Sigma Aldrich, 69 %). The concentrations of ligand (CyMe4-BTBP and C2-BTBP, synthesized in at the spot) were varied and the diluents were cyclohexanone (Acros organics, 99.8 %) and octanol (Acros organics, 99 %). TBP (Fluka, >97 %) was added to cyclohexanone in one series of experiment. After phase contact the samples were left to settle by gravitation. Samples of 50-200 μl were withdrawn and analyzed by HPGe (657.8 keV (branching ratio for the line; 94,6 %)) (Ortec, GEM 15180-S). The uncertainties are 3 % and based on error propagation, mainly from the pipetting during sample preparation.
2.2. HPLC
HPLC (High Performance Liquid Chromatography) was applied in order to quantify the content of CyMe4-BTBP in cyclohexanone (initial concentration 4.5 mM). The analyses were made before and after being contacted with different acidic aqueous phases (1 M HNO3) containing various concentrations of silver nitrate (1.9, 1.4, 0.95, 0.49 and 0.1 mM). A Merck-Hitachi HPLC system LaChrom 7000 series was used. The HPLC conditions for CyMe4-BTBP analyses were: column: Gemini C18 RP (150 x 3 mm I.D.), mobile phase 10 mM triethylamine carbonate buffer in 85% aqueous acetonitrile (Gradient grade, Aldrich), flow rate 0.6 ml/ min: detection DAD 235-300 nm and selected wavelengths were 238, 239, 241 and 288 nm. The injection volume was 6 ȝl. To ensure reliable determination of the BTBP ligand in the samples, a reduction method was applied in order to release the silver from the BTBP complexes. The samples from the organic phase were prepared for the removal of silver by adding 55 ȝL of the sample to 45 ȝL of freshly prepared 1.1 M solution of NaBH4. The mixture was shaken for 20 min (Heindolpf MultiReax shaker at 1200 rpm) and after this, 900 ȝL of MeOH (Aldrich, Chromasolv,
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Gradient Grade) was added and the samples were shaken for an additional time of 20 min. The released silver metal was then filtered off with 0.45 ȝm nylon micro-filter (Tessek Ltd). Similarly, the samples from the aqueous phases were treated with an aqueous solution of 1.1 M NaH2PO2.
2.3. MS measurements
MS (Mass spectrometry) was used in order to identify the composition of the silver: BTBP-complexes. Measurements were performed using a Finnigan LCQ FleetTM spherical Ion Trap LC/MS instrument (Thermo Scientific). Mass Spectra from direct infusion were measured with APCI (Atmospheric Pressure Chemical Ionization) in positive mode. Conditions used for the APCI interface: Flow Rate from a syringe infusion pump: 20 ȝL/min; Sheath gas flow 20 L/min, Auxiliary gas flow: 9 L/min, Source voltage: 3.75 kV, Vaporizer temperature 400°C, Capillary Temperature 250°C, Capillary Voltage 48V, Tube lens Voltage 100 V and Mass range from 100 to 2000.
3. Results and discussion
3.1. Pure diluents
The silver extractions from the pure diluents (cyclohexanone and cyclohexanone + 30 % TBP, without BTBP's) were investigated in order to see whether the diluent/solvent is able to extract the target nuclides alone, or if the extraction is only a function of the CyMe4-BTBP content. As has been found, the pure diluent does not extract silver to any higher extent (log D=-1.3) and after addition of TBP to the solvent the extraction is even more suppressed (log D=-2.8).
3.2. The Effect of Ligand side group
The effect of the side group of the ligand was investigated using CyMe4 and C2 –BTBP. As can be seen in Figure 2 the CyMe4-BTBP gives overall higher D values than the C2 ligand. The slope (log D versus log [ligand]) of the C2-BTBP system is lower (slope=0.27) than that of the CyMe4 system (slope=1). However, the slope of 1 in the CyMe4 system is only achieved at lower ligand concentrations (<1 mM). This indicates that the side group can have an effect on the silver extraction both according to extraction and complexation.
3,5 3
2,5
D 2 CyMe4
log 1,5 C2 1 Slope1 0,5 0 Ͳ4 Ͳ3,5 Ͳ3 Ͳ2,5 Ͳ2 log[ligand]
Figure 2. The extraction of silver using CyMe4- and C2–BTBP in cyclohexanone. The diluent is cyclohexanone. Aqueous phase is 1 M HNO3. The line is added in order to guide the eye, only.
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3.3. The Effect of Solvent composition
When introducing TBP in considerable amounts to the diluent several parameters are changed, such as the density and the rate of phase separation. The addition of TBP does not, however, affect the extraction of silver Figure 3).
Figure 3. The extraction of silver using various concentrations of CyMe4-BTBP in pure cyclohexanone and in the GANEX solvent (70 % cyclohexanone, 30 % TBP). Aqueous phase is 1 M HNO3.
3.4. The Effect of Diluent
It has several times been concluded that the exchange of the diluent effects the extraction of several elements when using BTBP- class of ligands [5, 6, 7]. Whether this is the case also for silver is not known. Therefore an exchange of the cyclohexanone for octanol was made. The slight difference between the extraction into the two diluents is in this case not statistically significant (Figure 4).
3,5
3
2,5 D
log 2 cyclohexanone octanol 1,5
1 Ͳ4 Ͳ3,5 Ͳ3 Ͳ2,5 Ͳ2 log[BTBP CyMe4]
Figure 4. The extraction of silver using CyMe4-BTBP in pure cyclohexanone and in octanol. Aqueous phase is 1 M HNO3.
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3.5. The formation of water soluble complexes and Stoichiometry
Practically no decrease of the content of the CyMe4-BTBP in the organic phase was observed after being in contact with the acidic aqueous phases containing silver (Figure 5). In accordance with these observations, no BTBP ligand was found in the aqueous phase by HPLC after treatment with silver ions in various concentrations. The use of MS (APCI+ ionization) revealed the molecular ion [M+Ag]+, m/z= 641.33, corresponding to a complex with the stoichiometry silver to CyMe4-BTBP 1: 1. This complex was only observed in the organic phase (Figure 6, left). Only traces of free, uncomplexed ligand were observed in the aqueous phase (Figure 6, right).
Figure 5. Plot of the CyMe4-BTBP concentration in organic (cyclohexanone) phase determined by HPLC. Samples of original concentration (see Experimental) were in contact with aqueous phases containing various concentration of Ag+, detection at 239 nm. Uncertainties are one standard deviation.
APCI_HAJ_Ag_I_org_111123091906 #1-83 RT: 0.00-1.00 AV: 83 NL: 6.14E4 APCI_HAJ_Ag_aq_I_pnorm_111129125202 #1-60 RT: 0.00-1.00 AV: 60 NL: 7.32E1 T: ITMS + p APCI corona Full ms [100.00-2000.00] T: ITMS + p APCI corona Full ms [100.00-2000.00] 663.25 2.2 535.50 179.08 20 [M+H]+ 2.0 18 557.42 1.8 535.50 + 16 [M+Na] [M+H]+ 1.6 14 483.42 1.4 371.33 12 551.33 1.2 632.17 + 721.33 [M+C6H10O] 10 1.0 577.42 Relative Abundance
Relative Abundance 8 0.8 208.17 641.33 6 603.33 0.6 [M+Ag]+ 390.33 574.33 0.4 [2M]+ 4 649.33
735.33 834.42 862.42 0.2 355.33 2 890.33 792.25 963.42 663.33 1068.75 757.33 873.33 929.42 1187.75 1019.42 1063.33 1122.83 0.0 0 200 400 600 800 1000 1200 600 700 800 900 1000 1100 1200 m/z m/z
Figure 6. Expanded region (right) of MS (APCI+) spectra of the sample I-Aq of organic (left) and the aqueous phase (right) containing + CyMe4-BTBP (4.5 mM in the cyclohexanone) in contact with Ag nitrate (initially, 1.9 mM in 1M HNO3) with the assignment of the main identified peaks.
244 Elin Löfström-Engdahl et al. / Procedia Chemistry 7 ( 2012 ) 239 – 244
4. Conclusions
The extraction of silver using two BTBP class ligands has been investigated. It has been shown that the distribution ratios are affected by the side group of the ligand. CyMe4 is a more efficient extractant for silver when compared to C2-BTBP. HPLC and MS results indicate that no water soluble complex is formed in the CyMe4-BTBP system and that the complex in the organic phase is of the stoichiometry 1: 1. This could be verified using slope analyses and low ligand concentrations (<1 mM). The addition of TBP to the CyMe4-BTBP/ cyclohexanone extraction system seems not to affect the extraction. The exchange of the diluent cyclohexanone for octanol shows a small decrease in the distribution ratio, however, it is hard to tell whether it is statistically significant. In future studies the existence of diluent effects in the CyMe4-BTBP system will be further studied. Silver, together with palladium, nickel and cadmium have been identified as troublesome fission products for a future process development aimed at separation the actinides as a group from the rest of the used fuel [3]. The effect of Pd2+ on extraction was already studied in more details [8] and has been found to affect both, extraction and significant looses of the ligand into aqueous phases. More detailed results on the influence of silver we present here have shown comparatively less limiting behavior of Ag+ in the system, consisting of the complex formation in the organic phase.
Acknowledgements
Finally, the Swedish Nuclear Fuel and Waste Management company (SKB) and the European Union FP7 Project ACSEPT (Project No: 211267) are acknowledged for the funding of this work.
References
[1] Foreman, M., Hudson, M., Geist, A., Madic, C., Weigl, M., An Investigation into the Extraction of Americium(III), Lanthanide and
D-Block Metals by 6,6’-Bis-(5,6-dipentyl-[1,2,4]triazin-3-yl)-[2,2’]bipyridinyl (C5-BTBP), Solvent Extraction and Ion Exchange, 23(5), 645-662, 2005. [2] Nilsson, M., Ekberg, C., Foreman, M., Hudson, M., Liljenzin, J-O., Modolo, G., Skarnemark, G.: Separation of Actinides(III) from Lanthanides(III) in Simulated Nuclear Waste Streams using 6,6’-Bis-(5,6-dipentyl-[1,2,4]triazin-3-yl)-[2,2’]bipyridinyl (C5-BTBP) in Cyclohexanone, Solvent Extraction and Ion Exchange 24(6), 823-843, 2006. [3] Aneheim, E., Ekberg, C., Fermvik, A., Foreman, M., Retegan, T., Skarnemark. G. A TBP/BTBP-based GANEX Separation Process – Part 1: Feasibility, Solvent Extraction and Ion Exchange vol. 28(4), pp. 437–458, 2010. [4] Andreas Wilden, Michal Sypula, Christian Schreinemachers, Paul Kluxen, Giuseppe Modolo.: 1-cycle SANEXprocess development studies performed at Forschungszentrum Jülich, Proceedings of the First ACSEPT International Workshop, Lisbon, Portugal, 31 March – 2 April 2010 (Actinide reCycling by SEParation and Transmutation). [5] Retegan, T., Ekberg, C., Fermvik, A., Skarnemark, G. The Effect of Diluents on Extraction of Actinides and Lanthanides, in Scientific Basis for Nuclear Waste Management XXX, edited by D.S. Dunn, C. Poinssot, B. Begg (Mater. Res. Soc. Symp. Proc. 985, Warrendale, PA), 0985-NN14-05, 2007. [6] Nilsson, M., Andersson, S., Drouet, F., Ekberg, C., Foreman, M., Hudson, M., Liljenzin, J-O., Magnusson, D., Skarnemark, G., Extraction Properties of 6,6'-Bis-(5,6-dipentyl-[1,2,4]triazin-3-yl)-[2,2'] bipyridinyl Solvent Extr Ion Extr. 24: 299-318, 2006. [7] Elin Löfström-Engdahl, Emma Aneheim, Christian Ekberg, Nathalie Mabile, Gunnar Skarnemark, Solvent effects on the extraction rate in proposed GANEX processes, Proceedings of the 19th International Solvent Extraction Conference (ISEC), paper 70, October 3-7, Santiago, Chile, 2011. [8] Aneheim, E., Ekberg, C., Foreman, M. R.StJ., Löfström-Engdahl, E., Grüner, B., Hájková, Z., Palladium Complexation Behaviour with Nitrogen Donor Ligands used for Spent Nuclear Fuel Treatment, Solv. Extr., Ion Exch., submitted.