LETTER TO THE EDITOR Journal of Radioanalytical and Nuclear Chemistry, Vol. 227, Nos 1 2 (1998) 163 165
Extraction of uranium(VI) with binary mixtures of a quadridentate Schiff base and various neutral donors S. K. Sahu, V. Chakravortty* Department of Chemistry, Utkal University, Vani Vihar, Bhubaneswar-751 004, India (Received August 13, 1997)
Liquid-liquid extraction of uranium (VI) with the binary mixture of quadridentate Schiffbase, bis-salicylidene ethylene diamine (H2Salen) (H2A) and neutral donors viz., triphenyl arsine oxide (TPAsO), triphenyl phosphine oxide (TPPO) and tri-n-octyl phosphine oxide (TOPO) has been studied. The extraction order in the equilibrium pH range 2.5 to 5.5 is found to be bis-salicylidene ethylene diamine < bis-salicylidene ethylene diamine + triphenyl phosphine oxide < bis-salicylidene ethylene diamine + tri-n-octyl phosphine oxide < bis-salicylidene ethylene diamine + triphenyl arsine oxide. The composition of the extracted species derived from slope analysis seems to be of the type (UOzA.2TOPO) and (UOzA.2TPPO).
Introduction* Bi-, tri- and tetra dentate Schiff base ligands form chelates with uranium(VI) 1 and thorium (IV). 2 The extraction efficiency of zirconium (IV), thorium (IV) and uranium(VI) with a bidentate Schiff base such as N-salicylidene-p-toluidene and N-salicylidene-p-phenetidene was found to be in order Zr+4>Th+4>UO2+2.3 The extraction efficiency of tridentate Schiff base such as N-salicylidene-2aminophenol and N-salicylidene-2-aminothiophenol with respect to molybdenum(VI)4 has also been studied. The extraction of UO2 +2 has been significantly improved by using the quadridentate Schiff base bissalicylidene ethylenediamine (H2salen)5 in the acidic pH range 1.5-4.5 as compared to the bidentate Schiff base N-salicylidene-p-toluidine.3 Therefore, the present work reports the extraction of uranium(VI) with binary mixtures of the quadridentate Schiff base bissalicylidene ethylenediamine and various neutral donors such as triphenyl arsine oxide (TPAsO), triphenyl phosphine oxide (TPPO) and tri-n-octyl phosphine oxide (TOPO). Ol-t
I Scheme 1.
Experimental The Schiff base ligand, bis-salicylidene ethylenediamine (H2salen) was prepared using a reported
method, 2,6 dried in vacuum and characterised by m.p., elemental analysis and IR spectra. Stock solution of uranium (VI) (10 4) was prepared by dissolving uranyl nitrate in distilled water. About 1 ml conc. HNO 3 was added to 100ml of the stock solution to supress hydrolysis. The pH (2-5.5) of the aqueous phase was adjusted with pyridine/dilute HC1 before extraction, since there was no extraction with pyridine alone. In the extraction studies, the concentration of the Schiff base (H2salen) in benzene was 10 3M. The concentration of U(VI) and the extractants are kept such that the condition U(VI)total << (H2salen)total, (neutral donor)total is satisfied. 5 ml each of aqueous solution and an equal volume of organic phase were mixed in a separating funnel and shaken for 15 minutes. The estimation of metal ion was done colorimetrically with Arsenazo III at )~max = 656 nm. Results and discussion Results on the liquid-liquid extraction of uranium(VI) at various pH values with the quadridentate Schiff base (H2salen), and a mixture of quadridentate Schiff base and neutral donors. (TPAsO) (TPPO) (TOPO) are presented in Fig. 1.; %E representes percentage of extraction. Extraction with TOPO (10 3M) remains almost constant (20%) with an increase in equilibrium pH. With TPAsO (10~M) extraction increases with an increase in equilibrium pH but extraction decreases after equilibrium pH 3.5. However, with Schiff base (H2salen), and a mixture of H2salen and neutral donor (TOPO), (TPPO) the extraction increases with an increase in equilibrium pH, the plot showing an S form. With the mixture of H2salen and TPAsO the extraction is found to be 86.5% at equilibrium pH 2.04 and the extraction becomes quantitative at equilibrium pH 2.5.
* Author for correspondence. 0236 5731/98/USD 17.00 9 1998 Akaddmiai Kiad& Budapest All rights reserved
Elsevier Science B. K, Amsterdam Akad~miai Kiad& Budapest
S. K. SAHU, V. CHAKRAVORTTY:EXTRACTIONOF URANIUM(VI)WITH BINARYMIXTURES
Q A 9 9 
103M 103M 103M 103M 103M
Schiff base + 103M TPASO in benzene (Schiff base + TOPO) in benzene (Schiff base + TPPO) in benzene Schiff base in benzene TPASO in benzene
1 0 3 M TOPO
ECluil. DH Fig. 1. Plot o f E vs.equilibrium, pH for the extraction of U(VI) with TOPO, TPAsO, Schiffbase and their binary mixtures
A 9 9
10"3M (Schiff base + TOPO) in benzene 103M (Schiff base + TPPO) in benzene 103M Schiff base in benzene Slope = 2.0
Slope = 2.13
Equil. pH Fig. 2. Plot of logD vs.equilibrium pH for the extraction of U(VI) with Schiff base and its mixture with TOPO and TPPO
S. K. SAHU,V. CHAKRAVORTTY:EXTRACTIONOF URANIUM(VI)WITH BINARYMIXTURES
0.2 - -
-0.2 - -
O-Slope = 1.86
oO/ o o/
Slope = 1.8
/ I -2.0
I ~ 0.0
log ( [ T O P O ] / M)
Fig. 4. Plot of logDmix vs.log ((TPPO)/M) for the extraction of U(VI) with the binary mixture of Schiffbase and TPPO
log ( [ T O P O ] / M)
Fig. 3. Plot of logDmix vs.log((TOPO)/M) for the extraction of U(VI) with binary mixture of Schiff base and TOPO
The extraction order in the equilibrium pH range 2-5.5 with H2salen, and with mixtures of H2salen and neutral donor (TPAsO) (TPPO) (TOPO) is found to be H2salen, <H2salen + TPPO < H2salen + TOPO < H2salen + TPAsO. The PH0.5 values (pH at which 50% extraction is obtained) for the same metal ions are 4.04 (H2salen), 3.63 (H2salen + TPPO) and 3.35(H2salen + TOPO). The composition of the extracted species has been deduced from the plot of logD vs. Equilibrium pH (Fig. 2) in the pH range 2-5.5, where D is the distribution ratio of the uranium (VI). The plot shows a slope of 2 for H2salen, 2.13 for H2salen + TPPO and 2.26 for H2salen + TOPO. From the above experimental results and slope analysis data the following extraction equilibrium is proposed, where H2salen is represented as H2A. UO2ยง + H2A(org) = UO2A(org) + 2Hยง The plots of logDmix (Dmix is the distribution coefficient for the mixture) vs. log((TOPO)/M) and log((TPPO)/M) due to variation in neutral donor concentration in the range 2.10 3 to 2.10 2M and 2.10 4 to 9.10 2, keeping the concentration of H2salen constant (1.10 3) at pH 2.5 show slopes of 1.86 and 1.8, respectively (Figs 3 and 4). These features indicate that two molecules of neutral donors form chelates with
UO2 ยง and the following equilibrium may be proposed, in which the neutral donor is represented as B: UO2
+ + (aq) H2A(org) 2B(org)
However, with varying concentration of TPAsO quantitative extraction was observed in the range 2.10 3Mto 8.10 3M atpH 2.5. Conclusions
Synergism was observed in the extraction of uranium(VI) from dilute nitric acid with binary mixtures of a Schiff base, namely bis-salicylidene ethylenediamine and neutral donor such as tri-n-octyl phosphine oxide, triphenyl phosphine oxide and triphenyl arsine oxide.
Grateful thanks are due to CSIR for funding the project.
References 1. H. N. MAHANTA,K. C. DASH, Indian J. Chem., 15A (1977) 657. 2. K. C. DASH, H. N. MAHANTA, Transition Metal Chem., 2 (1977) 229. 3. C. R. PANDA, V. CHAKRAVORTTY,K. C. DASH, Indian J. Chem., 24 (1985) 807. 4. P. K. HATH, V. CHAKRAVORTTY,K. C. DASH, Annali di chimica, 77 (1987) 961. 5. C. R. PANDA, V. CHAKRAVORTTY, K. C. DASH, J. Radioanal. Nucl. Chem., 108 (1986) 65. 6. H. N. MAHANTA, K. C. DASH, J. Inorg. Nucl. Chem., 39 (1977) 1345. 7. A. I. BUSEV, V. G. TIPTSOVA, V. M. IVANOV, Analytical Chemistry of Rare Elements, Mir Publisher, Moscow, 1981.