The Solvent Extraction of Uranium (VI) using Hydroxyalkylenediphosphonic Acids
M. Bouhoun Ali,
A.Y. Badjah Hadj Ahmed,
The solvent extraction of uranium (VI) from aqueous solutions has been investigated using hexadecylhydroxydiphosphonic acid (HHDPA, H4L1) and dodecylhydroxydiphosphonic acid (DHDPA, H4L2), which were synthesized and characterized by elemental analysis and by FT-IR, 1H NMR, 31P NMR spectroscopy. In this study, we propose a tentative assignment for the shifts of those two ligands and their specific complexes with uranium (VI). A spectroscopic analysis has showed that coordination of uranium (VI) takes place via oxygen atoms. We carried out the extraction of uranium (VI) from aqueous solutions by HHDPA and DHDPA in [carbon tetrachloride + 2-octanol (v/v: 90%/10%)] solutions at an aqueous to organic phase ratio (1/1) and at room temperature. The results showed that the extraction yields are more significant in the case of the HHDPA (92%) which is equipped with a hydrocarbon chain, longer than that of the DHDPA (88%). Logarithmic plots of the uranium (VI) distribution ratio vs. pHeq and the extractant concentration showed that the ratio of extractant to extracted uranium(VI) is 2:1. The metal complexes formed in organic phase are UO2(H3L1)2.2H2O and UO2(H3L2)2. 2H2O for HHDPA and DHDPA respectively. The equilibrium constants for the extraction of uranium(VI) were found to be 2 and 1.27 for HHDPA and DHDPA, respectively.
Received: June 02, 2010;
Accepted: August 12, 2010;
Published: October 11, 2010
Uranium plays an important role in the generation of nuclear power. For this
reason, the recovery, concentration and purification of uranium are of great
importance. Therefore, many processes have been used for uranium purification
from its ores at plant-scale. Leaching of uranium by acid or alkaline solutions,
concentration and purification by solvent extraction or ion exchange and precipitation
are the most commonly used methods; each has its merits and limitation in application
(Edwards and Oliver, 2000; Lunt
et al., 2007).
Among these, the solvent extraction method has been applied extensively in
the production and processing of uranium (Ritcey, 2006;
Black et al., 1958; Singh
et al., 2001; Girgin et al., 2002).
The solvent extraction process using acidic organophosphorus extractants is
applied world-wide for the purification of crude uranium and also thorium (Kabachnik
et al., 1974; Didi et al., 2002; Elias
et al., 1996; Didi et al., 2004; Basuki,
1991; Largman and Sifniades, 1983; Hall
et al., 2005; Chiarizia et al., 2001;
Urban et al., 1998; Horwitz
et al., 1994; Curtui et al., 2001).
In the present study we carried out the synthesis of new hydroxyalkylenediphosphonic
acids which form stable complexes with metallic species is of great importance
for improving existing hydrometallurgical processes for their recovery (Kabachnik
et al., 1974; Didi et al., 2008; Blum
et al., 1981; Largman and Sifniades, 1983).
The stability of the hydroxyalkylenediphosphonic acids with iron (III), thorium
(IV) and uranium (VI) cations is particulary high (Kabachnik
et al., 1974; Didi et al., 2002; Basuki,
1991; Tunick et al., 1982; Burgada
et al., 2003; Reed et al., 2007).
The complex-forming properties of the hydroxyalkylenediphosphonic acids are
due to their structures. The combination in one molecule of two highly acidic
phosphono-groups and also hydroxy-group, which play the role of the basic centre
in molecule, enable these ligands to form stable complexes (Kabachnik
et al., 1974). In fact, we have synthesized hexadecylhydroxydiphosphonic
(HHDPA, H4L1) and dodecylhydroxydiphosphonic (DHDPA, H4L2)
for such a purpose. The characterizations of these products were carried out
by elemental analysis and by FT-IR, 1H NMR, 31P NMR spectroscopy.
We have also tested the chelating properties of these extractants towards uranium
(VI) from aqueous solutions.
MATERIALS AND METHODS
This research project was conducted from March 2008 to March 2010 in the Nuclear Research Center of Draria, Algiers, Algeria.
Reagents and solutions: The reagents used in this work were palmitic
acid (98%, Panreac), lauric acid (BDH), phosphorus trichloride (98%, BDH), anhydrous
ethyl alcohol (Ficher), carbon tetrachloride (Labosi) and 2-Octanol (Prolabo).
Uranyl nitrate hexahydrate was purchased from Merck (99%). The aqueous solutions
of 4.2x10-2 M uranium (VI) was prepared from uranyl nitrate hexahydrate
and adjusted at pH 2 with nitric acid and NaOH. The aqueous solutions of uranium
(VI) was fixed at pH 2, because hydrolysis of uranyl ion takes place as the
pH varies from 1 to 3.0±0.1 (availability of free uranium ions). When
pH increases beyond 3.0±0.1, uranium exists in hydrolyzed form and the
following ionic species have been identified: UO22+, [(UO2)2(OH)2]2+
dimer, [(UO2)3(OH)5]+ trimer, precipitation
starts due to the formation of complexes in aqueous solution (Kadous
et al., 2009). The organic solutions were prepared from of HHDPA
and DHDPA (0.03 to 0.3 M) dissolved in the organic solvent [carbon tetrachloride
+ 2-octanol (v/v: 90%/10%)].
Instrumentation: 31P(- 1H) and 1H NMR spectra were measured on a Bruker AC 250 working at 250 MHz in a carbon tetrachloride solution. Infrared spectra were measured on a Perkin Elmer 16 PC-FTIR equipped with a thermostat to maintain the temperature of the sample cell at 25±0.1°C. Elemental analyses were performed using a ThermoQuest NC2500 elemental analyser. Potentiometric measurements were taken on a Consort C 831. In a water-acetone mixture (3:17), a known mass of each sample titrated by a solution of NaOH (2x10-3M). The amount of uranium (VI) extracted was determined from the difference between the initial and final concentrations of uranium (VI) in aqueous solution using a GBC Cintera-40 UV-Visible spectrophotometer. The water was titrated in the organic phase by the use of a Mettler DL18 Karl Fisher Titrator.
Synthesis of the extractants and characterisation: HHDPA and DHDPA were
synthesis following a method first described by Largman and
Sifniades (1983) with an original modification developed in our laboratory
(Didi et al., 2002, 2008).
The characteristics of these products are given in Table 1.
HHDPA and DHDPA were titrated by potentiometry. The pKa values indicated that in the water-acetone medium the first proton was strong and the other protons were weak.
The presence of a wide P = O band in the IR presence indicates intermolecular hydrogen bonds P = O
H-OP and C-OH
O = P. The equilibrium exists between the following two forms: C-O-H
O = P- C = O...H-O-P- which explains the presence of the 1700 cm-1 band with H2O bending.
|| HHDPA and DHDPA characteristics
|Exptl. and calcd: Experimental and calculated percentages
for the elemental analysis of the synthesized compounds, d (ppm): Chemical
shift, s: Singlet, t: Triplet, m: Multiplet, vs: Symmetric stretching, vas:
Extraction experiments: The extraction experiments were performed using
HHDPA and DHDPA as extractants. These substances were tested for uranium(VI)
extraction from aqueous solutions at pH 2. Equal volumes of organic and aqueous
phases (10 mL) were shaken together at 25°C for 15 min. Preliminary experiments
showed that equilibration was complete in 15 min. The addition of 2-octanol
in organic solution as modifier (10 vol %) was necessary to improve phase separation.
This alcohol also prevents micelle formation and solvated metal-extractant complexes
(Didi et al., 2002; Kahlweit
et al., 1981; Strey and Jonstromer, 1992;
Mellah and Benachour, 2006). It is very important to
note that no third phase or any precipitation was observed during the extraction
process. The phases were then allowed to separate. The amount of uranium (VI)
extracted was determined by complexing the uranium (VI) in the aqueous phase
before and after extraction with arsenazo III and by further visible spectrophotometric
dosage of the complexes formed (Elias et al., 1996;
Marczenko, 1976). The water was titrated in the organic
phase by the use of a Karl Fisher Titrator.
RESULTS AND DISCUSSION
Extraction of uranium (VI) by HHDPA and DHDPA: The extraction experiments results are discussed in term of extraction yield (Y) and distribution ratio (D) defined as follows:
||Initial mass of uranium(VI) in aqueous phase
||Mass of uranium(VI) after extraction
||The volume of the aqueous phase
||The volume of the organic phase
The variable Q is the ratio of the number of moles of extractant in organic
phase versus the number of moles of metal in aqueous phase before extraction:
The variation of the extraction yield test of the 4.2x10-2M uranium (VI) from aqueous solutions as a function Q is shown in Fig. 1.
||Effect of molar ratio (Q) on the extraction yield of uranium
(VI). pH = 2; [U(VI)] = 4.2x10-2M; Vaq/Vorg
= 1; t = 25°C
Figure 1 shows the yield of extraction of uranium(VI) increases with Q. Working with extractants concentrations range 0.03 to 0.3 M (Q = 0.7 to 7), we reached a yield of 92% for HHDPA and 88% for DHDPA. The extractant with a longer alkyl group forms more hydrophobic complexes. HHDPA and DHDPA present a similar extraction power, but the hydrophobic character determines the amount of extraction. The hydrophobic character of ligand can be determined calculating log P which is defined as the partition coefficient between two phases of a substance, generally n-octanol and water. Modern molecular modeling software allows the log P values, calculated using ChemDraw Ultra (Cambridge Soft) are respectively 2.75 for DHDPA and 5.92 for HHDPA showing that HHDPA is strongly hydrophobic.
Stoichiometry of extracted species: In the work on stoichiometric relation for the extraction of uranium complex with HHDPA (H4L1) and DHDPA (H4L2), we have supposed that the solubilities of the extractant and the uranium-extractant complex in the aqueous phase are negligible; the overall reaction in the extraction of metal cations by cationic extractants, as the case of HHDPA and DHDPA, can be show as following:
where, H4L is the molecule of extractant, M is the metal (U), m valence of metal (U(VI)), n molecules of extractant engaged in the reaction, n/m number of protons exchanged by each extractant and x number of molecules of water.
The equilibrium constant of the above reaction, Kex, can be given as function of molar concentration:
||Effect of the extractant concentration on the distribution
ratio for uranium (VI). pH= 2; [U (VI)] = 4.2x10-2M; Vaq/Vorg
= 1; t = 25°C
Substitution of the distribution ratio, which is defined by the concentration of uranium (VI) in organic phase divided by that in aqueous phase, into Eq. 6 results in:
Taking logarithms of Eq. 6, one obtains:
The stoichiometry of the extracted species was determined by analysing the
experimental data. The conventional slope analyses method was used. Figure
2 shows the plots of log D versus log [extractant] which gave two straight
lines with good correlation coefficients 0.999 and 0.998 and slopes equal to
1.91, which is close to 2 and 1.96, which is close to 2 for HHDPA and DHDPA.
This result suggests that two molecules of extractant react with one uranyl
ion. Figure 2 also shows that the distribution ratio of uranium(VI)
increases with the increase in extractant concentration. The distribution ratios
are more significant in the case of the HHDPA which is equipped with a hydrocarbon
chain, longer than that of the DHDPA. Figure 3 shows the plots
of log D versus pHeq which also gave two straight lines with good
correlation coefficients 0.999 and 0.996 and slopes equal to 2.06, which is
close to 2 and 2.11, which is close to 2 for HHDPA and DHDPA. This indicates
that two protons are released during the cation exchange reaction. The plots
log D versus log [extractant] and the plots log D versus pHeq suggest
that the ratio of extractant to extracted uranium(VI) is 2:1 (Basuki,
1991; Kabachnik et al., 1974; Reed
et al., 2007; Jacopin et al., 2003;
Burgada et al., 2003; Hall
et al., 2005; Chiarizia et al., 2001).
||Effect of equilibrium pH on the distribution ratio for uranium
(VI). pH= 2; [U(VI)] = 4.2x10-2M; Vaq/Vorg
= 1; t = 25°C
||Structures of the complex HHDPA-UO2. 2H2O
(R = C15H31) and the complex DHDPA-UO2.
2H2O (R = C11H23)
The equations extraction equilibrium can thus be written as:
According to the above equations extraction equilibrium, the metal complexes formed in organic phase are UO2(H3L1)2. 2H2O and UO2(H3L2)2. 2H2O for HHDPA and DHDPA respectively. Proton, intervening in our extraction, corresponds to the pKa of HHDPA and DHDPA, 3.2 and 3.66, respectively. HHDPA and DHDPA extract the uranyl ions in cationic exchange mode. The two last values of pKa obtained by potentiometric measurement confirm that the extracting agent can exchange only one or two protons per molecule. We also note that the extraction of uranium (VI) by HHDPA and DHDPA is accompanied by two water molecules extraction. The equilibrium constants, Kex, for the extraction of uranium(VI) were found to be 2 and 1.27 for HHDPA and DHDPA, respectively.
The structures of the complex HHDPA-UO2. 2H2O and DHDPA-UO2. 2H2O are indicated in Fig. 4.
Figure 4 shows that the uranium complexes, UO2(H3L1)2.
2H2O and UO2(H3L2)2.
2H2O, are formed by coordinating of each uranyl ion to two phosphonate
oxygen atoms from two molecules of extractant agent. Figure 4
shows also that these complexes contain two water molecules. The HHDPA and DHDPA
form the high complexes with uranium(VI). This higher stability of the uranium
complexes is mainly attributed to the high acidity of the diphosphonic acid
group and the hydroxy-group (Kabachnik et al., 1974;
Van Hecke and Goethals, 2006; Sun
et al., 2003). The mechanism the two complex formations is based
on the employement in the interaction with uranyl ions of the electron-donating
functions of the oxygen atoms in the diphosphonic group and in the hydroxy-group
at the carbon atom joined to diphosphonic group. These electron-donating functions
of the oxygen atoms increases with increase the hydrocarbon chain of the exractant
HHDPA-UO2 and DHDPA-UO2 spectra: The solid complex HHDPA-UO2 was prepared by stirring the hydroxydiphosphonic acid in the organic solvent mixture (carbon tetrachloride + 2-octanol), with an aqueous solution of uranium (VI). After separation of the phases and evaporation of the organic solvents, the solid complex was washed with water and dried. We have observed a shift of P = O band from 1204 to 1115 cm-1. In the complex, a new IR band appears at 940 cm-1 attributed to the distortion vibration PO-UO2. Similarly, comparison of the spectra of DHDPA and DHDPA-UO2 show a shift the 1195 P = O band to 1096 cm-1. A new band in DHDPA-UO2 appears at 933 cm-1 attributed to the deformation vibration of PO-UO2. The two deformation vibrations PO-UO2 showed that the bond between P-O and UO2 for HHDPA is stronger than with DHDPA.
HHDPA and DHDPA used as extractants in solvent extraction process for extract uranium (VI) from aqueous solutions. From the obtained results the following conclusions may be drawn:
The yields of extraction of uranium (VI) increases with the ratio Q. HHDPA and DHDPA can complex and extract respectively 92 and 88% of uranium (VI) by using a large excess extractant agent (Q = 7).
HHDPA ligand has a stronger extracting power for uranium (VI) than DHDPA. This fact is related to more hydrophobic character of HHDPA vs. DHDPA.
Logarithmic plots of the uranium (VI) distribution ratio versus pHeq and the extractant concentration showed that the ratio of extractant to extracted uranium (VI) is 2:1. The metal complexes formed in organic phase are UO2(H3L1)2. 2H2O and UO2(H3L2)2. 2H2O for HHDPA and DHDPA, respectively. Proton, intervening in our extraction, corresponds to the pKa of HHDPA and DHDPA, 3.2 and 3.66, respectively. HHDPA and DHDPA extract the uranyl ions in cationic exchange mode. The two last values of pKa obtained by potentiometric measurement confirm that the extracting agent can exchange only one or two protons per molecule.
Extraction of uranium (VI) by HHDPA and DHDPA is accompanied by two water molecules extraction.
The equilibrium constants for the extraction of uranium (VI) were found to be 2 and 1.27 for HHDPA and DHDPA, respectively.
1: Basuki, K.T., 1991. Extraction of iron(II and III) and of uranium(IV and VI) in perchloric and phosphoric acids by octadecylhydroxydiphosphonic acid: Thermodynamic and kinetic aspects. Ph.D. Thesis, Paris 06 University, France.
2: Black, K.L., J. Koslov and J.D. Moore, 1958. Design and operation of a uranium processing mill using liquid ion exchange (solvent extraction). Geneva, Switzerland: UN., pp: 488–494.
3: Blum, H., H.U. Hempel and K.H. Worms, 1981. Hydroxyalkane diphosphonic acids. US. Patent. http://www.freepatentsonline.com/4267108.html.
4: Burgada, R., T. Bailly and M. Lecouvey, 2003. Synthese de ligands bisphosphoniques pour la complexation de l'ion uranyle du cobalt et du fer. C. R. Chimie, 7: 35-39.
Direct Link |
5: Chiarizia, R., D.R. McAlister and A.W. Herlinger, 2001. Solvent extraction by dialkyl- substituted diphosphonic acids in a depolymerizing diluent. II. Fe(III) and actinides ions. Solvent Extraction Ion Exchange, 19: 415-440.
CrossRef | Direct Link |
6: Curtui, M., I. Haiduc and L. Ghizdavu, 2001. Separation of uranium(VI) by extraction with dibutylditiophosphoric acid. J. Radioanal. Nucl. Chem., 250: 359-362.
7: Didi, M.A., A. Elias and D. Villemin, 2002. Effet of chain length of alkane-1-hydroxy-1,1- methyldiphosphonics acids on the iron (III) liquid-liquid extraction. Solvent Extraction Ion Exchange, 20: 505-513.
CrossRef | Direct Link |
8: Didi, M.A., A. Elias, L. Meddour, M. Attou and A. Azzouz, 2004. Science et Technology des Agents Extractants Organophosphores. Office des Publications Universitaires, London.
9: Didi, M.A., M. Kaid and D. Villemin, 2008. Dodecylhydroxydiphosphonic acid for solvent extraction. Solvent Extraction Ion Exchange, 26: 113-127.
CrossRef | Direct Link |
10: Edwards, C.R. and A.J. Oliver, 2000. Uranium processing: A review of current methods and technology. JOM J. Minerals Metals Materials Soc., 52: 12-20.
11: Elias, A., L. Rodehuser, A. Azzouz and M. Attou, 1996. Tetrabutylalkylenediphosphonates and Tetrabutylalkylenediphosphonates-di(2-ethylhexyl) phosphoric acid muxtures in solvent extraction of uranyle nitrate. Hydrometallurgy, 40: 189-194.
12: Girgin, S., N. Acarkan and A.A. Sirkeci, 2002. The uranium(VI) extraction mechanism of D2EHPA-TOPO from a wet process phosphoric acid. J. Radioanalytical Nucl. Chem., 251: 263-271.
13: Hall, H., J.C. Sullivan, P.G. Rickert and K.L. Nash, 2005. Kinetics of the reaction of U(VI) with benzene-1,2-diphosphonic acid. Dalton Trans., 7: 2011-2016.
PubMed | Direct Link |
14: Horwitz, E.P., R.C. Gatrone and K.L. Nash, 1994. Extraction metal ions with diphosphonic acid, or derivative thereof. US Patent 5332531. http://www.patentstorm.us/patents/5332531/claims.html.
15: Jacopin, C., M. Sawicki, G. Plancque, D. Doizi and F. Taran et al., 2003. Investigation of the interaction between 1-hydroxyethane-1,1-diphosphonic acid (HEDP) and uranium(VI). Inorg. Chem., 42: 5015-5022.
16: Kabachnik, M.I., T.Y. Medved, N.M. Dyaltova and M.V. Rudomino, 1974. Organophosphorus complexones. Russian Chem. Rev., 43: 733-733.
CrossRef | Direct Link |
17: Kadous, A., M.A. Didi and D. Villemin, 2009. Extraction of Uranium(VI) using D2EHPA/TOPO based supported liquid membrane. J. Radioanalytical Nucl. Chem., 280: 157-165.
Direct Link |
18: Kahlweit, M., R. Strey and G. Busse, 1991. Effect of alcohol on phase behaviour of microemulsion. J. Phys. Chem., 95: 5344-5352.
19: Largman, T. and S. Sifniades, 1983. Extraction of uranium from phosphoric acid using supported extractants. US. Patent 4,402,917. http://www.freepatentsonline.com/4402917.html.
20: Lunt, D., P. Boshoff, M. Boylett and Z. El-Ansary, 2007. Uranium extraction: The key process drivers. J. Southern Afr. Inst. Min. Metallurgy, 107: 419-426.
21: Marczenko, Z., 1976. Spectrophotometric Determination of Elements. Ellis Horwood Ltd., Chichester, pp: 305-321.
22: Mellah, A. and D. Benachour, 2006. Solvent extraction of heavy metals contained in phosphoric acid solutions by the 7-(4-ethyl-1-methyloctyl)-8-hydroxyquinoline in kerosene diluent. Hydrometallurgy, 81: 100-103.
23: Reed, W.A., L. Rao, P. Zanonato, A.Y. Garnov, B.A. Powell and K.L. Nash, 2007. Complexation of UVI with 1-hydroxyethane-1,1-diphosphonic acid in acidic to basic solutions. Inorg. Chem., 46: 2870-2876.
Direct Link |
24: Ritcey, G.M., 2006. Solvent extraction in hydrometallurgy: Present and future. Tsinghua Sci. Technol., 11: 137-152.
25: Singh, H., R. Vijayalakshmi, S.L. Mishra and C.K. Gupta, 2001. Studies on uranium extraction from phosphoric acid using di-nonyl phenyl phosphoric acid-based synergistic mixtures. Hydrometallurgy, 59: 69-76.
26: Strey, R. and M. Jonstromer, 1992. Role of medium-chain alcohols in interfacial films of noionic microemulsions. J. Phys. Chem., 96: 4537-4542.
CrossRef | Direct Link |
27: Tunick, A.A., T. Largman and S. Sifniades, 1982. Extraction of uranium values from phosphoric acid. US. Patent 4,316,877. http://www.freepatentsonline.com/4316877.html.
28: Urban, V., R. Chiarizia, A.W. Herlinger, C.Y. Ku and P. Thiyagarajan, 1998. SANS study of dialkylsubstituted diphosphonic acids and complexes with Ca, Fe, La, Th and U in toluene. Phys. B: Phys. Condensed Matter, 241: 355-357.
Direct Link |
29: Van Hecke, K. and P. Goethals, 2006. Research on Advanced Aqueous Reprocessing of Spent Nuclear Fuel: Literature Study. Belgian Nuclear Research Centre, Boeretang.
30: Sun, Z.M., B.P. Yang, Y.Q. Sun, J.G. Mao and A. Clearfield, 2003. Hydrothermal syntheses, characterizations and crystal structures of three new cadmium (II) amino-diphosphonates: Effects of substitute groups on the structures of metal phosphonates. J. Solid State Chem., 176: 62-68.