F. A. Shehata et al.: Mechanism of Extraction of Trivalent Europium from Perchlorate Medium zyx 753 Mechanism of Extraction of Trivalent Europium from Perchlorate Medium by a Chloroform Mixture of Thenoyltrifluoroacetone and 18C6 F.A. Shehata, J.A. Daoud zyxwvu *), and H.F. Aly Hot Laboratories Centre, Atomic Energy Authority, 13758, Egypt zyxw Key Words: Chemical Kinetics / Interfaces / Radiation Chemistry / Reaction Mechanisms The extraction of Eu (111) from perchlorate medium by a chloroform solution of thenoyltrifluoroacetone (HTTA) and i8-crown-6 (1 8C6) was kinetically studied. The effect of the extractants concentration, interfacial surface area as well as other parameters affecting the rate of extraction was separately investigated. The effect of tempera- ture was also studied and the thermodynamic functions were calculated. A mechanism of extraction of Eu(1II) was proposed to account for the experimental results. Introduction Crown ethers are relatively novel extractants used for the selective extraction of alkaline earth [ 11, lanthanide [2] and actinide metal cations [3]. The synergic effect of crown ethers as neutral adducts in the extraction of f-elements has been reported [4, 51. Aly et al. [6] studied the extraction of Eu3+, Tm3+ and Yb3+ by a mixture of HTTA and 15-crown-5 and obtained a large enhancement (synergism) of extraction of these lanthanides by the addition of the crown. Shehata et al. [7] studied the synergic extraction of Gd(III), Eu(II1) and Am(II1) by 15-crown-5 and 18-crown-6mixed by HTTA from perchlorate medium. The authors reported that when 18C6 was used as synergic ligand the adduct extraction constant decreased in the se- quence Am>Eu>Gd, while in case of 15C5 the sequence changed to Eu > Am > Gd. Mathur and Choppin [8] studied the extraction of UO?', Eu3+, La2+ and Th4+ complexes of TTA with several crown ethers. They reported that, al- though the crown ethers exhibited a synergic effect, the fit of the size of the cation and crown cavity was not an impor- tant factor in the formed TTA complexes. Change et al. [9, 101 studied the equilibrium and kinetics of extraction of selected lanthanides by zyxwvuts 1,7-diaza-4,10,13-tri-oxacyclopen- tadecane N, N'-diacetic acid (DAPDA) and 1,10, -diaza- 4,7,13,16-tetra-oxacyclooctadecane N, N'-diacetic acid (DACDA) in conjunction with TTA in chloroform. These authors reported the variation in the extraction kinetics with the pH of the medium. In a recent review paper on the basic chemistry and recent developments in trivalent f-elements separation, Nash [ 1 I] reported that the exploitation of the differences in the reac- tion kinetics of the metal ions and the use of crown ethers are among the most important future trends in lan- thanide/actinide separation. In this concern, the present work is directed to study the kinetics of extraction of Eu(II1) by HTTA and 18C6 in chloroform from aqueous perchlorate medium, which was previously investigated at equilibrium [7]. The aim of this study is to get more infor- mation concerning the factors affecting the rate determin- ing steps of extraction and to propose a mechanism for the *) Author to whom correspondence should be addressed. synergic Eu(II1) extraction by HTTA and 18C6 in chloroform from perchlorate medium. Experimental (i) Chemicals and Reagents All chemicals used were of analytical reagent grade (AR) and used without purification. The 18-crown-6 was supplied by Alfa chemicals, while thenoyltrifluoroacetone (HTTA) was obtained from Fluka. Chloroform was a Prolabo prod- uct. The aqueous phases were acetate buffers of sodium acetate (Mallinckrodt) and acetic acid (BDH) adjusted to the constant ionic strength of 0.1 M with sodium per- chlorate (Aldrich). The radioactive europium, Eu(152 + 154), was prepared by neutron irradiation of the spectroscopically pure europium oxide and used in concen- trations lower than 10-6M. (ii) Procedure The concentrations of HTTA and 18C6 were kept con- stant at 0.09 M and 0.06 M, unless otherwise stated. The pH of the aqueous media was 3.52k0.10 and varied when stu- dying the effect of hydrogen ion concentration. The temper- ature was kept constant at 25 zyx f 0.01 "C by using a Julabo circulator connected to a stirred Lewis cell. The cell descrip- tion as well as the measurements are given in a previous publication [12]. A stirring speed of 250 rpm was used for all the experiments carried out in this work, unless other- wise stated. The activity concentration measured in the organic phase at time (t) is denoted by A while the activity at equilibrium is denoted by Aeq. The plots of In (Aeq -A) versus time (t) yielded straight line relations indicating a pseudo-first order reaction. The slopes of these lines gave the observed reaction rate constant (/cobs). The deviation in the repeated experiments was < +4%. Results and Discussion Equilibrium Investigations Previous equilibrium investigations on the extraction of Eu(II1) by HTTA and 18C6 mixture in chloroform from Ber. Runsenges. Phys. Chem. 99, 753 - zyxwvu 7S8 (1995) zyxwvutsrqpo No. 5 zyxwvutsrqp 0 VCH Verlagsgesellschaft mbH, 0-49451 Weinheim, 1995 0005-9021/9S/OSO5-07S3 $ 10.00+.25/0