STUDIES ON THE LIQUID-UQUID EXTRACTION AND REVERSE-PHASE THIN LAYER CHROMATOGRAPHY OF LANTHANIDES AND SOME OTHER ELEMENTS

Abstract

Liquid-liquid extraction enjoys a favoured position amongst the various separation techniques owing to its simplicity, versatility and less operational cost. Moreover, the extraction data is able to throw some light on metal complex formation and provides useful guidelines for reverse phase chromatographic separations. The solvent extraction studies of lanthanides have always been considered as a useful analytical exercise with sufficient topical interest. It is mainly because of the fact that the separation of lanthandies amongst themselves and with other commonly associated elements like yttrium, titanium, vanadium, zirconium, thorium and uranium poses a serious problem. It demands high analytical finesse from a technique. Eventually, many extraction systems have been employed for the purpose and the search for better extractants still continues. Over the years, three important types .of extractants which have come up on the forefront are alkylphosphorus and carboxylic acids and high molecular weight amines. A survey of literature on alkylphosphorus acids as extractants revealed that most of the work has been carried out on di-(2-ethylhexyl) phosphoric acid (DEHPA) and the studies are mainly confined to the effect of concentration of mineral acid and extractant. Several other commonly available alkylphosphorus acids have not been put to much 11 use inspite of earlier reports on their potential as extractants. On the basis of preliminary investigations a detailed study is carried out on mono-(2-ethylhexyl) acid phosphate (H2MEHP)as extractant for lanthanides and some other elements like uranium, thorium, vanadium, zirconium, titanium and yttrium. High molecular weight amines, popularly known as liquid anion exchangers, are useful extractants for the extraction of anionic metal complexes. Relatively less work has been reported on their use for the extraction of anionic metal carboxylates. Lanthanides are reported to form citrate complexes of variable compositions. Except for the reported extraction of some of the lanthanide citrate complexes in high molecular weight amines details are not available about the composition of the extracting species and the factors affecting their extraction. The author took up a systematic study to investigate the effect of various variables on the extraction of three representative lanthanides e.g., Ce(III), Gd(III) and Yb(III) and identify the probable extracting species. The carboxylic acids like caproic (c6>' caprylic (C„) and capric (C,n), Versatic and naphthenic acids have been employed as useful extractants for various metal ions. Some investigations on synergism in these systems have also been reported. However, some of these acid systems suffer from the problem of emulsion formation. In order to avoid Ill this problem studies on the synergistic extraction of some metal ions with lower carboxylic acids (< C,) in the presence of amines have been reported. However, such an extraction system has not been explored for the lanthanides and elements like uranium, thorium, zirconium, yttrium, vanadium and titanium. It was thought to be an interesting analytical exercise to investigate the mechanism of synergism and use the extraction data for separations. For convenience and clarity of presentation the subject matter of the thesis has been divided into the following chapters : I. General Introduction . II. Materials and Equipment. III. Liquid-Liquid Extraction of Lanthanides and Some Other Elements Using Mono-(2-ethylhexyl) Acid Phosphate. IV. Liquid-Liquid Extraction of Lanthanides and Some Other Elements from Citrate Solutions Using High Molecular Weight Amines. V. Reverse-Phase Thin Layer Chromatographic Behaviour of Lanthanides and Some Other Elements Using Alkylphos phorus Acids and High Molecular Weight Amines. VI. Extraction Behaviour of Lanthanides and Some Other Elements with Lower Carbox.ylic Acids in the Presence of Amines. IV Chapter I includes a brief account of the characteristic features and utility of alkylphosphorus and carboxylic acids and high molecular weight amines as extractants for metal ions. An up-to-date survey of literature on these extraction systems is also presented. Details of different materials and instruments used during the investigation are given in II chapter. The distribution studies on lanthanides were carried out using their radiotracers whereas for other elements either complexometric titrations or Inductively Coupled Plasma Atomic Emission Spectrometric (ICP-AES) technique was used. Chapter III contains the extraction behaviour of some lanthanides with mono-(2-ethylhexyl) acid phosphate. The effect of the concentration of metal and hydrogen ions and the extractant has been studied. The extraction was found to increase with the pH and extractant concentration whereas it remained more or less constant in the investigated metal ion concentration range. On the basis of the slope analysis data the extracting species has been proposed as M(HR)_ (M trivalent lanthanide ion and H„R-alkylphosphorus acid). The extraction constnats were calculated for some of the lanthanides. The extraction behaviour of UO„(II), VO(II), Y(III), Ti(IV), Zr(IV) and Th(IV) has also been studied and based on the extraction data it is possible to propose some separations of these elements from lanthanides. V Chapter IV discusses the extraction of some representative lanthanides, namely, Ce(III), Gd(III) and Yb(III) from aqueous citric acid medium in different high molecular weight amines. The effect of various variables like the nature of pretreating acid, type of amine, concentration of metal, hydrogen and citrate ions and amine on extraction has been studied. The sulphuric and citric acid pretreated amines give higher extractions. Amongst different amines used Primene JM-T, a primary amine, shows the highest extraction. It was observed that the extraction is more or less constant in the pH range 3.2-3.8. Moreover, the metal ion concentration does not have any significant -3-5 • . effect in the range 10 to 10 M. The extraction increases with increasing citrate ion concentration up to 5.0x10 M and thereafter it decreases. The extraction also shows a linear increase with increasing amine concentration. Based on the distribution data the extracting species was found to be [(RNH^] [M(Cit)2]3~. The behaviour of Y(III), Ti(IV), V(IV), Zr(IV), Th(IV) and U(VI) in chloroform solution of Primene JM-T has been studied and some separations are proposed. Chapter V deals with the reverse phase thin layer chromatographic behaviur of lanthanides and some other elements like U(VI), Th(IV), Zr(IV), V(IV), Ti(IV) and Y(III) using some alkylphosphorus acids and high molecular weight amines as impregnants on silica gel support. In the case VI of alkylphosphorus acids nitric acid was used as eluant while citric acid was employed in high molecular weight amines. The effect of concentration of impregnant and eluant on Rf values was investigated. On the basis of R_ values some binary separations of lanthanides with some other elements have been achieved. Chapter VI presents a study on the extraction of some Ln(III), Y(III), Ti(IV), V(IV), Zr(IV), Th(IV) and U(VI) with propionic, butyric and valeric acid in the presence of n-, di- and tributyl amines and fl-picoline, pyridine and quinoline. Under the conditions of investigations no significant synergistic effect was observed in the case of Ln(III) while the other elements exhibited significant enhancement in extraction. As a representative case the extraction of U(VI) was investigated in greater details. The probable composition of the synergistic adduct was found. Using the extraction data it was possible to propose the conditions for the separation of lanthanides from the other elements.