Journal of Radioanalytical and Nuclear Chemistry, Vol. 25, No. 3 (2003) 597603 02365731/2003/USD 20.00 AkadØmiai Kiad, Budapest ' 2003 AkadØmiai Kiad, Budapest Kluwer Academic Publishers, Dordrecht Mechanism of uranium(IV) extraction with CYANEX 302 in kerosene from nitrate medium N. Abdel Rahman, J. A. Daoud,* H. F. Aly Hot Laboratories Centre, Atomic Energy Authority, Post Code 13759, Cairo, Egypt (Received May 21, 2003) The extraction of U(IV) by bis(2,4,4-trimethylpentyl) monothiophosphinic acid (CYANEX 302) in kerosene from nitric acid solution has been investigated under equilibrium conditions. The effects of the different parameters affecting the extraction process were studied and the stoichiometry of the extracted species was elucidated. The kinetics of this extraction was also investigated using a stirred Lewis cell. The effects of the different parameters affecting the extraction rate as well as the temperature were separately investigated. The results are interpreted by a reaction mechanism where the extraction process of U(IV) is controlled by a chemical reaction at the interface rather than in the bulk phase. Introduction Commercial dialkylphosphinic acid and its thio- substituted derivatives known as CYANEX extractants that have been made commercially available by Cytec Inc., have found useful application in the treatment of radioactive wastes. 1,2 In this respect, bis(2,2,4- trimethylpentyl) dithiophosphinic acid (CYANEX 301) was used for Am/rare earths separation from aqueous solution of high active waste (HAW). 1 The extraction of trace amounts of Am(III) and Eu(III) from nitrate medium with purified CYANEX 301 was studied by ZHU et al., 2 who found that both the extraction constants and separation factor of Am/Eu decreased with increasing extraction temperature. CYANEX 302, in which the active component is bis(2,4,4-trimethylpentyl)thiophosphinic acid, has been reported to be very efficient in removal of many cations from acidic media. 3 WANG et al. 4 have investigated the extraction of trivalent lanthanides with CYANEX 302 from acidic aqueous chloride solution and found that the commercial CYANEX 302 extracts the trivalent lanthanides more efficiently than the purified CYANEX 302. RICE and GIBSON 5 have used CYANEX 301 and CYANEX 302 for the upgrading of chloride liquors from lateritic nickel ores. Countercurrent tests on the use of CYANEX 302 and CYANEX 272 for Co/Ni separation showed that CYANEX 302 displays greater cobalt-nickel selectivity than CYANEX 272 and its use is more advantageous particularly when Ca(II) or Mn(II) are present in the feed. 6 WANG and LI 7 have studied the extraction of Sc(III), Zr(IV), Th(IV), Fe(III) and Lu(III) using CYANEX 301 and CYANEX 302 from acidic solutions and found that the separation of the investigated metals from Lu(III) with CYANEX 302 could be achieved by acidity control; however, CYANEX 301 exhibited a poor selectivity for these metals except for Lu(III). * E-mail: jadaoud@hotmail.com AWWAD et al. 8 made comparative studies on the kinetics of U(VI) and Th(IV) extraction by TBP and CYANEX 921 from nitric solution and reported that TBP is more advantageous than CYANEX 921 for the separation of uranium from thorium due to its higher extraction ability and better kinetics for U(VI) than Th(IV). IONOVA et al. 9 studied the mechanism of trivalent actinide/lanthanide separation using synergistic mixtures of CYANEX 301 with neutral O-bearing co- extractants and reported that M(III) extractabilities are correlated with the calculated effective charge on O atom in neutral organophosphorous co-extractant molecules. However, the understanding of the mechanism of metals extraction with CYANEX reagents is still limited; so far, the extraction of uranium with CYANEX 302 has not been reported. In this respect, the present work is an attempt to throw light on the mechanism of extraction of U(IV) with CYANEX 302 in kerosene from nitric acid medium. Investigations on the extraction equilibrium of the studied system were first carried out to elucidate the stoichiometry of the extracted species. Kinetic studies using a stirred Louis cell were also carried out in order to learn about the factors enhancing the extraction rate of U(IV), the propose a mechanism for the uranium transfer and to identify its controlling regime. Experimental Chemicals and reagents All chemicals and reagents used were of A.R. grade. The uranyl nitrate is a BDH product. Nitric and sulphamic acids, and zinc metal are products of Prolabo, and CYANEX 302 was kindly supplied by Cytec Inc. and used as received. Kerosene is a product of Misr Petrol Ltd., Egypt. The tetravalent uranium used in the present work was prepared by reducing uranyl nitrate by