SHORT COMMUNICATION

Journal of Radioanalytical and Nuclear Chemistry, Vol. 251, No. 3 (2002) 503–506

Liquid-liquid extraction of uranium(VI) with 2-ethylhexyltolylsulfoxide(EHTSO)

Yang Yan-Zhao,1* Sun Si-Xiu,1 Feng Sheng-Yu21 Department of Chemistry, Shandong University, Jinan, P.R. China2 Institute of New Material, Shandong University, Jinan, P.R. China

(Received September 12, 2001)

The liquid-liquid extraction behavior of 2-ethylhexyltolylsulfoxide (EHTSO) towards uranium(VI) contained in nitric acid aqueous solution hasbeen investigated. It was found that the extraction increases with increasing nitric acid concentration up to 5.0 mol/l and then decreases. Extractionalso increases with increasing extractant concentration. The extracted species appears to be UO2(NO3)2

.2EHTSO. The influences of temperature,NH4NO3 and Na2C2O4 concentrations on the extraction equilibrium were also investigated and the thermodynamic functions of the extractionreaction were obtained.

Introduction

The solvent extraction method has been one ofimportant techniques in concentrating and purification ofuranium. Tri-n-butyl phosphate (TBP) has been used forseveral decades as the most successful extractant forrecovery of uranium from spent nuclear fuel(c.f. Purex Process). However, there exist two majordisadvantages: (1) the selectivity is not high, and (2)radiolytic degradations of TBP give rise to mono anddibutyl phosphates, which increase the extraction offission products resulting in a decrease in the overalldecontamination factors obtained.1 Therefore, it isnecessary to search for and develop some newextractants instead of TBP. Sulfoxides contain theS=O group and have stronger coordination ability tometal ions. The compounds without phosphorus atom areimportant, since they do not produce new solid wastebeing completely incinerable. The radiolytic degradationproducts of the sulfoxide compounds have weakercomplexing power toward fission products than those ofTBP. So sulfoxides have been considered as potentialextractants for use in nuclear applications for manyyears.2–7 We synthesized 2-ethylhexyltolylsulfoxide(EHTSO) in our laboratory. In this paper, we reportour new results on the extraction of uranium(VI)with EHTSO.

Experimental

Synthesis of 2-ethylhexyltolylsulfoxide (EHTSO)

The 2-ethylhexyltolylsulfoxide (EHTSO) in ourinvestigation was not commercially available. It wassynthesized in our laboratory (Scheme 1).8,9

The final product was obtained after the solvent wasevaporated. Its purity was checked by IR and NMRspectrometry and elemental analysis. The purity of2-ethylhexyltolylsulfoxide (EHTSO) was found to be≥95%.

Instruments and reagents

Vibrator (Yancheng Science Instrument Factory,Jiangsu Province), vibration frequency 275±5 min–1,controlling temperature precision: ±1 K; 752 type UVgrating spectrophotometer (Shanghai Third AnalysisInstrument Factory). Toluene (AR). Uranyl nitrate (AR).Nitric acid (technology superpurity). Ammonium nitrate(CP). Sodium oxalate (AR).

Experimental methods

Except for temperature influence experiment, theexperimental temperature was controlled in 298±1 K.Except for acidity influence experiment, the acidity wascontrolled at 2.0 mol/l.

The distribution ratios were determined by shakingmechanically two phases with the same volume up toequilibrium for 20 minutes. Uranium(VI) concentrationin aqueous solution was determined with aspectrophotometer,10 and uranium(VI) concentration inorganic solution was calculated by mass balance.

* E-mail: yzhyang@sdu.edu.cn

0236–5731/2002/USD 17.00 Akadémiai Kiadó, Budapest© 2002 Akadémiai Kiadó, Budapest Kluwer Academic Publishers, Dordrecht

YANG YAN-ZHAO et al.: LIQUID-LIQUID EXTRACTION OF URANIUM(VI) WITH 2-ETHYLHEXYLTOLYLSULFOXIDE

Scheme 1. Synthesis of EHTSO

Fig. 1. The effect of EHTSO equilibrium concentration on U(VI)distribution ratio; CHNO3

0 = 2.0 mol/l, CUO0

22+ = 4.00.10–3 mol/l

Results and discussion

The mechanism of the extraction of uranium(VI) withEHTSO

The effect of EHTSO concentration (0.05 to0.25 mol/l) on U(VI) distribution ratio is shown inFig. 1.

Let CUO2

2+0 and [UO2

2+] the initial concentration and

equilibrium concentration of UO22+, respectively.

Similarly C0EHTSO and [EHTSO] .

If the extraction reaction is presented as follows:

m m n

m n

KexUO NO EHTSO

UO NO EHTSO

22+

3 (o)

2 3 (o)

+ + ← →

⋅

−2

2( )(1)

then, the equilibrium constant:

Km n

ex m m n=⋅

−[ ( )

] [ ] [,

UO NO EHTSO]

[UO NO EHTSO]

2 3 (o)

22+

3 (o)

22 (2)

the U(VI) distribution ratio:

D Kexm m n= − −[ ] [ ] [ .UO NO EHTSO]2

2+3 (o)

1 2 (3)

The plot of log D vs. log [EHTSO](o) gives a slope of1.85, which is close to 2. But the distribution ratio ofuranium(VI) is independent of the concentration ofuranium(VI). So m = 1, n = 2 and the extracted compoundshould be UO2(NO3)2

.2EHTSO. The suggested structurecan be seen in Scheme 2.

Scheme 2. The assumed structure of UO2(NO3)2.2EHTSO

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YANG YAN-ZHAO et al.: LIQUID-LIQUID EXTRACTION OF URANIUM(VI) WITH 2-ETHYLHEXYLTOLYLSULFOXIDE

Fig. 2. Effect of initial nitric acid concentration on U(VI) distributionratio; C0

EHTSO = 0.25 mol/l, CUO0

22+ = 4.00.10–3 mol/l

Fig. 3. Effect of ammonium nitrate concentration on U(VI)distribution ratio; C0

EHTSO = 0.25 mol/l, CHNO3

0 = 2.0 mol/l,

CUO0

22+ = 4.00.10–3 mol/l

Influence of nitric acid concentration

In order to examine the variation of D as a functionof aqueous nitric acid concentration, the extraction ofU(VI) from 1.0 to 7.0 mol/l nitric acid into 0.25 mol/lEHTSO in toluene was systematically examined (Fig. 2).With increasing aqueous nitric acid concentration, theextraction of U(VI) exhibits a maximum. The initialincrease in D may be caused by the salting-out effect ofnitric acid. With a further increase in nitric acidconcentration, the concentration of free extratantdecreased. The decrease in D at higher acidconcentration may also be attributed to the formation ofa less extractable complex anion, such as UO2(NO3)3

–.

Fig. 4. Effect of sodium oxalate concentration on U(VI) distributionratio; C0

EHTSO = 0.25 mol/l, CHNO3

0 = 2.0 mol/l,

CUO0

22+ = 4.00.10–3 mol/l

Influence of salting-out agent concentration

Figure 3 shows that the distribution ratio ofuranium(VI) increases rapidly as ammonium nitrateconcentration increases. D0 is the distribution ratiowithout ammonium nitrate. The salting-out effect issupposed to be due both to the increase in thethermodynamic activity of uranium(VI) in the aqueousphase (decrease in free water molecules owing to thestrong hydration of NH4

+ cation)11 and to the increase inNO3

– concentration.

Influence of complex anion

Figure 4 shows that the distribution ratio ofuranium(VI) decreases rapidly as the stronglycomplexing anion (C2O4

2–) concentration in aqueoussolution raises in the extraction system. D0 is the valueof D in the absence of oxalate. This is because thecomplexing anion (C2O4

2–) and uranium(VI) form ahydrophilic complex.

Influence of temperature and calculation of thethermodynamic functions

Figure 5 shows the plot of log Kex vs. 1/T for0.25 mol/l EHTSO toluene solution.

The result shows that the extraction of uranium(VI)with EHTSO is an exothermic reaction, low temperatureis beneficial to the extraction.

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YANG YAN-ZHAO et al.: LIQUID-LIQUID EXTRACTION OF URANIUM(VI) WITH 2-ETHYLHEXYLTOLYLSULFOXIDE

Fig. 5. Effect of temperature on extraction equilibrium;C0

EHTSO = 0.25 mol/l, CHNO3

0 = 2.0 mol/l, CUO0

22+ = 4.00.10–3 mol/l

According to:

log.

KH

RTCex = − +& 0

2 303(4)

∆G0 = –RT ln Kex (5)

∆G0 = ∆H0–T.∆S0 (6)

We get:

∆H0 = –27.0±1.2 kJ.mol–1

∆G0 = –3.20±0.04 kJ.mol–1

∆S0 = –79.9±3.9 J.K–1.mol–1

References

1. S. A. PAI, J. P. SHUKLA, P. K. KHOPKAR et al., J. Radioanal.Chem., 42 (1978) 323.

2. F. A. COTTON, R. FRANCIS, J. Am. Chem. Soc., 82 (1960) 2986.3. YANG YAN-ZHAO, YANG YONG-HUI, JIANG XU-CHUAN et al.,

J. Radioanal. Nucl. Chem., 240 (1999) 335.4. YANG YAN-ZHAO, SUN GUO-XIN, BAO BO-RONG et al., Nucl. Sci.

Tech., 9 (1998) No. 3, 163.5. V. CHAKRAVORTTY, K. C. DASH, S. R. MOHANTY, Radiochim.

Acta, 40 (1986) 89.6. SHEN CHAO-HONG, BAO BO-RONG, BO YA-ZHI et al.,

J. Radioanal. Nucl. Chem., 178 (1994) 91.7. YANG YAN-ZHAO, YANG YONG-HUI, XONG JUN et al.,

J. Radioanal. Nucl. Chem., 241 (1999) 235.8. D. LANDINI , F. ROLLA, Synthesis, 8 (1974) 565.9. K. FHJIMORI, H. TOGO, SHIGERUOAE, Tetrahedron Lett., 21

(1980) 4921.10. DONG LING-YING, Analytical Chemistry of Uranium, Atomic

Energy Publishing House, Beijing, 1982, p. 158 (in Chinese).11. GAO ZI-LI, SUN SI-XIU, SHEN JING-LAN, Solvent Extraction

Chemistry, Science Publishing House, Beijing, 1991, p. 65(in Chinese).

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