LIQUID-LIQUID EXTRACTION AND REVERSED PHASE CHROMATOGRAPHIC STUDIES OF SOME 3d METAL IONS USING ALKYLPHOSPHOROUS EXTRACTANTS

Abstract

Liquid-liquid extraction is one of the premier separation techniques and commands a special place in separation science and technology. It is versatile, convenient, time saving and invariably, can be extended from micro-levels to macro concentrations. Apart from achieving a wide ranging inorganic and organic separations solvent extraction data provide useful guidelines for reversed phase chromatography. The study of the solvent extraction equilibria can give valuable information about the metal complex formation in the aqueous Phase. The yardstick, generally, used to assess the utility of an extraction system is by its efficiency to achieve separations of closely associated metal ions. Liquid-liquid extraction, no doubt, is the forerunner of reversed phase chromatography but the later has the advantage of its multistage character. The utility of the extraction chromatography to remove metal ion pollutants, like, Cd, Hg etc from various waste waters cannot be underestimated. The demands of ever increasing popularity of liquid-liquid extraction for metal ion separations is being met by a continuous search for more efficient extractants. With the advent of transplutonium era many organophosphorous compounds have emerged out as useful extractants. During the last three decades voluminous literature has accumulated on tributyl phosphate (TBP); the next being di (2-ethylhexyl) acid phosphate (DEHPA). Earlier studies on mono (2-ethylhexyl) acid phosphate (H^HP) indicated the potential of this extractant for the metal ion separations. Amongst the organophosphine extractants tri-n-octyl phosphine oxide (TOPO) has dominated the scene for a sufficiently long time. Recently American Cyanamid Company, USA, has marketed a series of organophosphines under the trade name 'CYANEX' for metal ion extraction. The structures of some of these are specifically designed to achieve better extraction power and or selectivity. Cyanex 925 is a mixture of two trialkylphosphine oxides one of which, constituting about eighty five percent, has a highly branch alkyl group. The branched structure might lead to selectivity thus making the extractant attractive to explore for separations. In the light of above discussion it was planned to study the extraction and reversed phase chromatographic behaviour of some of the 3d transition elements using HJ4EHP and Cyanex 925 extractants. The partition data have been utilized for identifying the extracting species and developing conditions for the separation of metal ions. The results thus obtained have been employed for devising separation schemes for the removal or recovery of some of the metal ions from alloys and ores. Besides achieving separation^ some of the 3d metal ions silica columns loaded with Cyanex 925 has been found useful for the removal of Hg(II). For the sake of clarity in presentation the work embodied in the thesis has been divided into the following chapters: I. GENERAL INTRODUCTION II. MATERIALS AND EQUIPMENT III. EXTRACTION BEHAVIOUR OF SOME 3d METAL IONS IN HJ4EHP IV. REVERSED PHASE CHROMATOGRAPHIC BEHAVIOUR OF SOME 3d METAL IONS USING H^MEHP AS AN IMPREGNANT. V. EXTRACTION BEHAVIOUR OF SOME 3d METAL IONS AND Cd(II) AND Hg(II) IN CYANEX 925. VI. REVERSED PHASE CHROMATOGRAPHIC BEHAVIOUR OF SOME 3d METAL IONS AND Cd(II) AND Hg(II) USING CYANEX 925 AS AN IMPREGNANT. ii Chapter I presents a brief outline of liquid-liquid extraction and extraction chromatography. The utility of the techniques for metal ion separations is highlighted and a classification of different types of extractants used for the said purpose given. It also includes an overview of literature on different organophosphorous extractants and the objectives of the present study are defined. The details about the materials and equipments used for the present study are given in chapter II. Partition studies were done using radiotracers and Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). The analysis of alloys and ores was carried out employing ICP-AES or Atomic Absorption Spectrophotometry. Chapter III gives the extraction behaviour of Cr(III), Fe(III), Mn(II), Co(II), Ni(II) and Zn(II), in H2MEHP from different concentrations of nitric acid. The extraction of Fe(III) is quantitative over the entire investigated range of acidity. The extraction of CrdlD.Mndl), Co(II) and Ni(II) decreases with increasing concentration of nitric acid. As a representative case the effect of different variables, namely, type of mineral acid used, nature of diluent, concentration of metal ion and extractant has been studied for Co(II). The extraction behaviour remains more or less the same on changing the aqueous phase from HN03 to HC1 or HSO The extractions are higher in carbon tetrachloride and toluene, the former was not used due to emulsification problem. The extraction remains unaffected in the range of metal ion concentration from 1.0 x 10~5to l.Ox 10~3M. The percent extraction increases with the increase in concentration of extractant. The pertinent equilibria have been proposed and the extracting species for Co(II) is identified. The extraction constant of ill Co(II)-H MEHP is lower than those of lanthanides. The extraction data have been utilized to achieve almost quantitative separation of Fe(III) from Cr(III), Mn(II), Co(II) and Ni(II). The separation conditions thus developed have been utilized for the removal of iron from some iron bearing metal matrices. Chapter IV deals with the reversed phase thin layer and column chromatographic behaviour of some 3d transition metal ions using H2MEHP as an impregnant and nitric acid as an eluant. The effect of concentration of nitric acid and impregnant on the retention of metal ions has been investigated. It is observed that, in general, the hRf values increase with the increasing concentration of nitric acid and decrease with impregnant concentration. Several binary and ternary separations of topical interest are conveniently achieved. Liquid-liquid extraction and reversed phase thin layer chromatographic results suggest that Fe(III) can almost be quantitatively separated from Cr(III), Mn(II), Co(II) and Zn(II) on columns loaded with HJCHP. Experimental conditions have been worked for the same. The extraction behavior of Cr(III), Fe(III), Mn(II), Co(II), Ni(II), CutII) Zn(II), Cd(II) and Hg(II) in Cyanex 925 has been presented in chapter V. The effect of various parameters, like, molarity of the mineral acid, nature of the diluent, concentration of metal ion and extractant has been studied. The results indicate that the extractant is fairly selective for Fe(III) and Hg(II) and partially selective for Zn(II). It has been possible to separate Fe(III) or Zn(II) from Cr(III), Mn(II), Co(II) and Cd(II) with high separation factors. The practical utility of the extractantjhas been demonstrated by recovering pure zinc from zinc ores and also for removing Fe(III) from a IV spent catalyst. Chapter VI embodies data on reversed phase column chromatographic studies using Cyanex 925 as an impregnant. Using these column it is possible to quantitatively separate Fe(III) or Hg(II) from Cr(III), Mn(II), Co(ll), Zn(II) and Cd(II). Hg(II) which is strongly retained on the column has to be removed by washing it with toluene. Fe(III) is eluted from the column by washing it with 0. 10M oxalic acid.