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Liquid-Liquid Extraction of Uranium(VI)from Aqueous Solution using 1-Hydroxyalkylidene-1,1-diphosphonicAcidsMustapha Bouhoun Ali a , Ahmed Yacine Badjah Hadj Ahmed b ,Mouloud Attou a , Abdelhamid Elias c & Mohamed Amine Didi da Nuclear Research Center of Draria, Draria, Algiers, Algeriab Department of Chemistry, College of Science, University of KingSaud, Riyadh, Saudi Arabiac Department of Chemistry, Faculty of Sciences, University ofMouloud Mammeri, Tizi-Ouzou, Algeriad Catalysis of Laboratory, Catalysis of Chemistry, University ofTlemcen, Tlemcen, AlgeriaAccepted author version posted online: 27 Mar 2012.Publishedonline: 05 Oct 2012.

To cite this article: Mustapha Bouhoun Ali , Ahmed Yacine Badjah Hadj Ahmed , Mouloud Attou ,Abdelhamid Elias & Mohamed Amine Didi (2012) Liquid-Liquid Extraction of Uranium(VI) from AqueousSolution using 1-Hydroxyalkylidene-1,1-diphosphonic Acids, Solvent Extraction and Ion Exchange,30:5, 469-479, DOI: 10.1080/07366299.2012.670598

To link to this article: http://dx.doi.org/10.1080/07366299.2012.670598

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Solvent Extraction and Ion Exchange, 30: 469–479, 2012Copyright © Taylor & Francis Group, LLCISSN: 0736-6299 print / 1532-2262 onlineDOI: 10.1080/07366299.2012.670598

LIQUID-LIQUID EXTRACTION OF URANIUM(VI)FROM AQUEOUS SOLUTION USING1-HYDROXYALKYLIDENE-1,1-DIPHOSPHONIC ACIDS

Mustapha Bouhoun Ali1, Ahmed Yacine Badjah Hadj Ahmed2,Mouloud Attou1, Abdelhamid Elias3, and Mohamed AmineDidi41Nuclear Research Center of Draria, Draria, Algiers, Algeria2Department of Chemistry, College of Science, University of King Saud, Riyadh,Saudi Arabia3Department of Chemistry, Faculty of Sciences, University of Mouloud Mammeri,Tizi-Ouzou, Algeria4Catalysis of Laboratory, Catalysis of Chemistry, University of Tlemcen, Tlemcen,Algeria

The extraction of uranium(VI) from aqueous solutions has been investigated using1-hydroxyhexadecylidene-1,1-diphosphonic acid (HHDPA) and 1-hydroxydodecylidene-1,1-diphosphonic acid (HDDPA), which were synthesized and characterized by elemental anal-ysis and by FT-IR, 1H NMR, 31P NMR spectroscopy. In this article, we propose a tentativeassignment for the shifts of those two ligands and their specific complexes with uranium(VI).We carried out the extraction of uranium(VI) by HHDPA and HDDPA from [carbon tetra-chloride + 2-octanol (v/v: 90%/10%)] solutions. Various factors such as contact time, pH,organic/aqueous phase ratio, and extractant concentration were considered. The optimumconditions obtained were: contact time = 20 min, organic/aqueous phase ratio = 1, pHvalue = 3.0, and extractant concentration = 0.3 M. The extraction yields are more signifi-cant in the case of the HHDPA which is equipped with a hydrocarbon chain, longer thanthat of the HDDPA. Logarithmic plots of the uranium(VI) distribution ratio vs. pHeq andthe extractant concentration showed that the ratio of extractant to extracted uranium(VI)(ligand/metal) is 2:1. The formula of the complex of uranium(VI) with the HHDPA and theDHDPA is UO2(H3L)2 (HHDPA and DHDPA are denoted as H4L). A spectroscopic analysisshowed that coordination of uranium(VI) takes place via oxygen atoms.

Keywords: Hydroxyalkylidenediphosphonic acids, uranium, liquid-liquid extraction

INTRODUCTION

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;[1] there-fore, 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 solventextraction or ion exchange, and precipitation are the most commonly used methods; eachhas its merits and limitations in application.[2,3]

Address correspondence to Mustapha Bouhoun Ali, Nuclear Research Center of Draria, Box 43 Sebala,Draria, Algiers, Algeria. E-mail: mbouhounali@gmail.com

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470 M. BOUHOUN ALI ET AL.

Among these, the solvent extraction method has been applied extensively in theproduction and processing of uranium.[4–7] The solvent extraction process using acidicorganophosphorus extractants is applied worldwide for the purification of crude uraniumand also thorium.[8–16]

In the present article, we carried out the synthesis of new hydroxyalkylidenediphos-phonic acids which form stable complexes with metallic species.[8,9,11,17–19] The char-acterizations 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 theseextractants towards uranium(VI). The effects of extraction parameters such as contact time,pH, organic/aqueous phase ratio, and extractant concentration has been investigated.

EXPERIMENTAL

Reagents and Solutions

The reagents used in this work were palmitic acid (98%, Panreac), lauric acid (BDH),phosphorus trichloride (98%, BDH), anhydrous ethyl alcohol (Fisher), carbon tetrachloride(Labosi), and 2-octanol (Prolabo). Uranyl nitrate hexahydrate was purchased from Merck(99%). The aqueous solutions of 4.2 × 10−2 M uranium(VI) was prepared from uranylnitrate hexahydrate at the desired pH values. The pH of the aqueous solutions was adjustedby using nitric acid or sodium hydroxide. The organic solutions were prepared fromHHDPA and HDDPA 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 at250 MHz in a carbon tetrachloride solution. Infrared spectra were measured on a PerkinElmer 16 PC-FTIR equipped with a thermostat to maintain the temperature of the samplecell at 25 ± 0.1◦C. Elemental analyses were performed using a ThermoQuest NC2500 ele-mental analyzer. Potentiometric measurements were taken on a Consort C 831. In awater-acetone mixture (3:17), a known mass of each sample titrated by a solution of NaOH(2 × 10−3M). The amount of uranium(VI) extracted was determined from the differencebetween the initial and final concentrations of uranium(VI) in aqueous solution using aGBC Cintera-40 UV-Visible spectrophotometer.

Synthesis of the Extractants and Characterization

HHDPA and HDDPA were synthesized following a method first described byLargman et al.[12] with an original modification developed in our laboratory.[9,20] Thecharacteristics of these products are given in Table 1.

HHDPA and HDDPA were titrated by potentiometry. The pKa values indicated that inthe water-acetone medium the two first protons were strong, the third weak, and the fourthextremely weak acids.[9,11,20]

The presence of a wide P = O band in the IR presence indicates intermolecularhydrogen bonds P = O. . .H-OP and C-OH. . .O = P. The equilibrium exists between thefollowing two forms: C-O-H. . .O = P- � C = O...H-O-P- which explains the presence ofthe 1700 cm−1 band with H2O bending.

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LIQUID-LIQUID EXTRACTION OF URANIUM(VI) 471

Table 1 HHDPA and HDDPA characteristics.

Products HHDPA HDDPA

Formula C15H31C(OH)(PO3H2)2 C11H23C(OH)(PO3H2)2

pKa 3.2, 6.36, 8.5, 11.1 ± 0.05 3.66, 6.45, 8.66, 11.3 ± 0.05Elemental analysis %C %H %P

Exptl. 46.75 8.60 14.68Calcd 47.76 8.95 14.42

%C %H %PExptl. 46.75 8.60 14.68Calcd 41.76 8.95 14.42

FT-IR (cm−1) 3450-3000 (νs OH), 2925 (νas CH),2854 (νs CH), 2700–2600 &2350-2190 (νs POH), 1465-1413 (δCH2), 1204 (νs P = O), 1095 (νasP-OH), 937 (νs P-OH)

3445-3000 (νs OH), 2923 (νas CH),2852 (νs CH), 2710-2620 &2340-2200 (νs POH), 1463-1415 (δCH2), 1195 (νs P = O), 1090 (νasP-OH), 940 (νs P-OH)

1H NMRδ (ppm) 0.88 (t, 3H, (CH3)), 1.25 (m,26H,(CH2)), 2.40 (s, 1H, (COH)),8.07 (m, 4H, (P(O)(OH)2))

0.90 (t, 3H, (CH3)), 1.27 (m,26H,(CH2)), 2.41 (s, 1H, (COH)),8.10 (m, 4H, (P(O)(OH)2))

31P NMRδ (ppm) 18 (s) 17.3 (s)∗Exptl. and calcd: experimental and calculated percentages for the elemental analysis of the synthe-

sized compounds, δ (ppm): chemical shift, s: singlet, t: triplet, m: multiplet, νs: symmetric stretching, νas:antisymmetric stretching.

Extraction Experiments

The extraction experiments were performed using HHDPA and HDDPA asextractants. These substances were tested for uranium(VI) extraction from aqueous solu-tions in mechanically agitated and thermostated beakers under constant conditions (contacttime (5–30 min), pH (0.5 ± 0.1–3.0 ± 0.1), organic/aqueous phase ratio (1/8–4/1), andextractant concentration (0.03–0.3 M)). The organic and aqueous phases were shakentogether by a mechanical stirrer with a constant stirring rate at 25◦C. The addition of2-octanol in organic solution as modifier (10 vol.%) was necessary to improve phaseseparation. This alcohol also prevents micelle formation and solvated metal-extractantcomplexes.[9,21,22] It is very important to note that no third phase or any precipitationwas observed during the extraction process. After an appropriate mixing time, the organicphase was separated from the aqueous phase. The amount of uranium(VI) extractedwas determined by complexing the uranium(VI) in the aqueous phase before and afterextraction with arsenazo III and by further visible spectrophotometric dosage of the com-plexes formed.[23,24] After phase disengagement, the aqueous phase was separated and itsequilibrium pH was measured with a pH meter.

RESULTS AND DISCUSSION

The results of the extraction experiments are discussed in term of extraction yield (Y)and distribution ratio (D) defined as follows:

Y(%) = mi − mf

mi× 100 (1)

D =(

mi − mf

mf

)× Vaq

Vorg(2)

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472 M. BOUHOUN ALI ET AL.

with the variables being as follows: mi: initial mass of metal ion in the aqueous phase; mf:mass of metal ion in the aqueous phase after extraction; Vaq: volume of the aqueous phase;Vorg: volume of the organic phase.

Effect of Various Parameters on the Extraction of Uranium(VI)

Effect of Contact Time. The extraction experiments were carried out for contacttimes ranging from 5 to 30 min. The results were represented in Fig. 1. The yield of extrac-tion of uranium(VI) increases with contact time. The extraction equilibrium was establishedafter 15 min (87.50% for HHDPA and 83.52% for HDDPA). However, in all extractionexperiments a contact time of 20 min (92.38% for HHDPA and 88.40% for HDDPA) waschosen for ensuring that the equilibrium was reached.

Effect of pH. The extraction of uranium(VI) was studied using HHDPA andHDDPA within the initial pH range 0.5 ± 0.1–3.0 ± 0.1. The yield of extraction of ura-nium(VI) increases with increase in the initial pH of the aqueous phase (Fig. 2). The yieldof extraction of uranium(VI) increased from 40.50 to 92.25% and from 32.70 to 88% forHHDPA and HDDPA, respectively. Hydrolysis of uranyl ion takes place as the pH variesfrom 1.0 to 3.0 (availability of free uranium ions). When the pH increases beyond 3.0,uranium exists in hydrolyzed form and the following ionic species have been identified:UO2

2+, [(UO2)2(OH)2]2+ dimer, [(UO2)3(OH)5]+ trimer, precipitation starts due to theformation of complexes in aqueous solution.[25]

Effect of O/A Phase Ratio. The results have shown that the yield of extrac-tion of uranium(VI) increased with increasing the O/A phase ratio (Fig. 3). The variationof the phase ratio from 1/8 to 1/1 lead to an increase in the yield of extraction ofuranium(VI) from 50.30 to 92.25% and from 46.05 to 88% for HHDPA and HDDPA,

Figure 1 Determination of the equilibrium time of uranium extraction. (operating conditions:[extractant] = 0.3 M, O/A phase ratio = 1/1, initial pH = 3.0 ± 0.1, [U(VI)] = 4.2 × 10−2 M andtemperature = 25◦C).

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LIQUID-LIQUID EXTRACTION OF URANIUM(VI) 473

Figure 2 Influence of the equilibrium pH on the yield of extraction of uranium(VI). (operating conditions: O/Aphase ratio = 1/1, [extractant] = 0.3 M, [U(VI)] = 4.2 × 10−2 M and temperature = 25◦C).

Figure 3 Influence of the O/A phase ratio on the uranium(VI) extraction. (operating conditions: initial pH = 3.0± 0.1, [extractant] = 0.3 M, [U(VI)] = 4.2 × 10−2 M and temperature = 25◦C).

respectively. Increasing the phase ratio above 1:1 had no effect on the yields of extractionof uranium(VI).

Effect of Extractant Concentration. The effect of HHDPA and HDDPA con-centration on the extraction ratio of uranium(VI) was studied in the range 0.03–0.3 M.It was observed that the yield of extraction of uranium(VI) increased with increase ofextractant concentration (Fig. 4). In the extractant concentration range 0.03–0.3 M, theincrease was from 12.40 to 91.90% in the case of HHDPA and 7 to 87.90% in the case of

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Figure 4 Effect of the extractant concentrations on the yield of extraction of uranium(VI). (operating conditions:O/A phase ratio = 1/1, initial pH = 3.0 ± 0.1, [U(VI)] = 4.2 × 10−2 M and temperature = 25◦C).

HDDPA. The extractant with a longer alkyl group forms more hydrophobic complexes.HHDPA and HDDPA present a similar extraction power, but the hydrophobic charac-ter determines the amount of extraction. The hydrophobic character of the ligand can bedetermined calculating log P which is defined as the partition coefficient between twophases of a substance, generally n-octanol and water. Modern molecular modeling soft-ware allows the log P values calculated using ChemDraw Ultra (Cambridge Soft) whichare 2.75 for HDDPA and 5.92 for HHDPA, respectively, showing that HHDPA is stronglyhydrophobic.[9,20]

Stoichiometry of Extracted Species

In the work on the stoichiometric relation for the extraction of uranium complexwith HHDPA and HDDPA, we have assumed that the solubilities of the extractant andthe uranium-extractant complex in the aqueous phase are negligible and that there is noaggregation in the organic phase; the overall reaction in the extraction of metal cations bycationic extractants, as in the case of HHDPA and HDDPA, can be shown as follows:

Mm+ + nH4L � M(H(4−n/m)L)n + mH+ (3)

where H4L is the molecule of extractant, M is the metal, m the valence of metal, nthe molecules of the extractant engaged in the reaction, and n/m the number of protonsexchanged by each extractant.

The equilibrium constant of the above reaction, Kex, can be given as a function ofmolar concentration:

Kex =[M

(H(4−n/m)L

)n

] [H+]m

[Mm+] [H4L]n (4)

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LIQUID-LIQUID EXTRACTION OF URANIUM(VI) 475

Substitution of the distribution ratio, which is defined by the concentration of ura-nium(VI) in the organic phase divided by that in the aqueous phase, into Eq. (4) resultsin:

D = Kex [H4L]n

[H+]m (5)

Taking logarithms of Eq. (5), one obtains:

log D = log Kex + n log[H4L] − m log[H+] (6)

log D = log Kex + n log[H4L] − m pH (7)

The stoichiometry of the extracted species was determined by analyzing the experimentaldata. The conventional slope analysis method was used. Figure 5 shows the plots of log Dversus log [extractant] which gave two straight lines with slopes equal to 1.91 ± 0.01 and1.96 ± 0.03 for HHDPA and HDDPA, respectively. This result suggests that two moleculesof the extractant react with one uranyl ion. Figure 5 also shows that the distribution ratioof uranium(VI) increases with the increase in extractant concentration. The distributionratios are more significant in the case of the HHDPA which is equipped with a hydrocarbonchain, longer than that of the HDDPA. Figure 6 shows the plots of log D versus pHeq

which also gave two straight lines with slopes equal to 2.06 ± 0.03 and 2.11 ± 0.05 forHHDPA and HDDPA, respectively. This indicates that two protons are released duringthe cation exchange reaction. The plots log D versus log [extractant] and the plots log Dversus pHeq suggest that the ratio of extractant to extracted uranium(VI) (ligand/metal) is2:1.[8,11,13,14,18,19,26,27] The extraction equilibrium of the equations can thus be written as:

Figure 5 Effect of extractant concentrations on the distribution ratio for uranium(VI). (operating conditions:initial pH = 3±0.1, [U(VI)] = 4.2 × 10−2 M, Vaq/Vorg = 1, temperature = 25◦C).

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476 M. BOUHOUN ALI ET AL.

Figure 6 Effect of equilibrium pH on the distribution ratio for uranium(VI). (operating conditions:[U(VI)] = 4.2 × 10−2 M, Vaq/Vorg = 1, temperature = 25◦C).

UO2+2 + 2H4L � UO2(H3L)2 + 2H+ (8)

According to the above equations, the composition of the complex of uranium(VI)with the HHDPA and the DHDPA is UO2(H3L)2 (HHDPA and DHDPA are denotedas H4L). Protons, intervening in our extraction, correspond to the pKa of HHDPA andHDDPA, 3.2 ± 0.05 and 3.66 ± 0.05, respectively. HHDPA and HDDPA extract the uranylions in cationic exchange mode. The structures of U(VI)-HHDPA and U(VI)-HDDPAcomplexes are indicated in Fig. 7.

Figure 7 show that the uranium complexes U(VI)-HHDPA and U(VI)-HDDPA, areformed by coordinating of each uranyl ion to two molecules of the extractant agent. TheHHDPA and HDDPA form strong complexes with uranium(VI). This higher stability ofthe uranium complexes is mainly attributed to the high acidity of the diphosphonic acidgroup and the hydroxy-group.[8,27–29] The mechanism of the complex formation is based onuranyl coordination by the electron-donating functions of the oxygen atoms in the diphos-phonic group and in the hydroxy-group at the carbon atom joined to diphosphonic group.These electron-donating functions of the oxygen atoms increase with the increase of thehydrocarbon chain of the extractant agent.

RU

O

OOH

PO

O OH

OH

PO

HO

OH

PO

OHO

HO

PO

OH

R

Figure 7 Suggested structure of U(VI)-HHDPA and U(VI)-HDDPA complexes.

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LIQUID-LIQUID EXTRACTION OF URANIUM(VI) 477

HIDMP-UO2 and DIDMP-UO2 Spectra

The solid complex U(VI)-HHDPA was prepared by stirring the hydroxydiphosphonicacid in the organic solvent mixture (carbon tetrachloride + 2-octanol), with an aqueoussolution of uranium(VI). After separation of the phases and evaporation of the organicsolvents, the solid complex was washed with water and dried. We have observed a shift ofP = 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, a comparison of the spectra ofHDDPA and HDDPA-UO2 show a shift the 1195 P = O band to 1096 cm−1. A new bandin HDDPA-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 forHHDPA is stronger than with HDDPA.

CONCLUSIONS

From the obtained results the following conclusions may be drawn:

� The yield of extraction of uranium(VI) increases with contact time. The extraction equi-librium was established after 15 min. However, in all extraction experiments a contacttime of 20 min (92.38% for HHDPA and 88.40% for HDDPA) was chosen for ensuringthat the equilibrium was reached.

� In the pH range 0.5 ± 0.1–3.0 ± 0.1, the yield of extraction of uranium(VI) increaseswith increase in the initial pH of the aqueous phase. The yield of extraction of ura-nium(VI) increased from 40.50 to 92.25% and from 32.70 to 88% for HHDPA andHDDPA, respectively.

� The variation of the O/A phase ratio from 1/8 to 1/1 lead to an increase in the ratio ofextracted uranium(VI) from 50.30 to 92.25% and from 46.05 to 88% for HHDPA andHDDPA, respectively. Beyond ratio 1/1, the phase ratio had no effect on the uranium(VI)extraction.

� The yield of extraction of uranium(VI) with the increase of extractant concentration.In the extractant concentration range 0.03–0.3 M, the increase was from 12.40 to 91.90%in the case of HHDPA and 7 to 87.90% in the case of HDDPA.

� HHDPA ligand has a stronger extracting power for uranium(VI) than HDDPA. This factis related to a more hydrophobic character of HHDPA vs. HDDPA.

� Logarithmic plots of the uranium(VI) distribution ratio vs. pHeq and the extractant con-centration showed that the ratios of extractant to extracted uranium(VI) (ligand/metal)is 2:1. The formula of the complex of uranium(VI) with the HHDPA and the DHDPAis UO2(H3L)2. The last two values of pKa obtained by potentiometric measurementconfirm that the extracting agent can exchange only one or two protons per molecule.

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