STUDIES ON SOME INORGANIC ION EXCHANGERS

Abstract

In ion-exchange chromatography, a reversible interchange of ions takes place between two compounds, one of which (the ion exchanger)is Insoluble in the medium, in which the exchange is carried out. Commonly used ion exchangers, generally, not necessarily, are large molecular polyelectrolytes having crosslinked structure which contains ionic groups. This structure gives, maximum resistance to oxidation, reduction, mechanical wear, and insolubility in common solvents. The porous structure facilitates the in and out movement of the solvent molecules. The most characteristic property of the ion exchanger is that the ions (counter ions) bound to ionic groups attached to the net work of the ion exchanger can be exchanged for other suitably charged ions. Ion-exchange processes, except for a few cases, are reversible; meaning thereby that they can be reversed, by suitably changing the concentration of the ions in solution. These processes, in many respects, are analogous to adsorption processes,but the two are not the same. The most characteristic difference between the two processes is that ion-exchange takes place stoichiometrically, by the effective exchange of ion3, whereas in adsorption the adsorbent takes up dissolved substance without releasing others into the solution. However, the two processes can not be entirely separated in practice, and they may be accompanied by each other. The scope of ion-exchange is not only limited to exchange between solid and liquid, but has been further extended to molten salts and exchange between solid-solid, and liquid-liquid (liquid ion exchangers) phases. Ion-exchange chromatography is n very potential separation technique, and can conveniently perform separations with large decontamination factors. Unlike many other techniques it can be safely extended from miero levels to maoro concentrations. Moreover, it demands less skill and judgement from the analyst. Now a days, this technique has become almost indispensable for the solution of complex problems in analytical chemistry and industry. Rapid and accurate analysis of alloys of complicated composition, pharmaceuticals, biological substances, and fission products has become possible with ion exchangers. The historic issue of 'Science* that reported the analysis of Apollo XI lunar samples, has scattered through its pages, several references to ion-exchange, being acknowledged, as a reliable standard technique tor radiochemical separations or for searching amino acldi. The first scientific studies on ion-exchange were ? 3 4 carried out in 1850 by Thompson , and Way ' on naturally occurring/inorganic materials, viz. the clay fractions of 5 6 the roil. In the early years of the 20th century, Gans • pioneered the work on the industrial production, and technical applications of synthetic permutites. The subsequent development of organic based resins, possessing greater stability in some respects, and capable of controlled synthesis to give produ. s with reproducible properties, has largely displaced their counterparts (inorganic ion exchangers) in modern technology. But during the last 10-15 years, there has been a revival of interest in Inorganic ion exchangers and they have firmly occupied their own position among the ion-exchange materials. The renewed interest in synthetic inorganic exchangers is mainly due to the following reasons: 1. The exchangers of this class, typical of whioh is zirconium phosphate, are generally, more stable than the original materials of aluminosilicate tjrpe. 2. These ion exchangers are more resistant to heat and radiations7'10 than their organic counterparts. This makes them ideally suited for processing nuclear fuels and treatment of contaminated moderator and cooling water, where higher temperature, pressure, and dose of radiations is involved. Nickel and copper ferrocyanides have been used for the treatment of liquid radioactive effluents from the Italian Nuclear Centre; at Cassaceia. 3. These materials are, generally, easy to syntheslsei owing to this ease in synthesis, the material with desired selectivity can be prepared. Therefore, a number of difficult separations can be conveniently carried out using these exchangers. 4. Recently these ion exchangers have also found 12 new applications in water desalination processes , and 1.7) in fuel cells ^ employing ion-exchange membranes for transport of hydrogen ions. Apart from these, the studies on inorganic ion exchangers throw light on the sorption of ions by precipitates, the electrophoretic behaviour of suspensions, the diffusion of ions in crystals, isotopic exchange in heterogeneous systems, and many other aspects of solid state chemistry. All this has resulted in a rapid development of this field. An idea of the rapidly growing interest in this area can be had from the fact that when the book , 'Ion Exchange' by F.Helfferioh was published in 1962, only two pages were devoted to synthetic inorganic ion exchangers. But just 15 after two years, a monograph x on Inorganic ion exchangers was published by C.B. Amphlett. This book is a classic in the field and led to a revolutionary upsurge of interests in inorganic ion exchangers. The progress in this field 16—19 has also been reviewed in a number of articles . During the last decade, a variety of amorphous and crystalline substances, which exhibit ion-exchanging properties, have been synthesised. As such, it is very difficult to classify 18 these materials. But according to Vesely and Pekarek they can be divided into the following main groupst 1. Hydrous oxides, e.g. hydrated oxides of zirconium, tin, titanium, etc 2* Acidic salts of multivalent metals, e.g. zirconium phosphate, tin antimonate, eerlo arsenate* etc. 3. Salts of heteropolyaclds, e.g. ammonium molybdophcephate, molybdoareenate, molybdosllicate, etc. 4* Insoluble ferrocyanides, e.g. ferrocyanides of zirconium, titanium, copper,etc. 5. Synthetic alumlnosilicatea, e.g. zeolites with general formula Na20. A120_. aSlOgi where & m 1-12 i.e. clays and lamellar zeolites. 6. Certain other substances, e.g. synthetic apatites, sulphides, alkaline earth sulphates, etc. Amongst all these, the aeldio salts of multivalent metals form one of the most extensively studied series of Inorganic ion exchangers. A wide range of oompounds has been described to this end. The metals studied are titanium(IV), slrconium(IV), tin(IV), oerium(IV), thorium(IV), aluminium(III), chromium(III), iron(III), vanadium(Vl), uranium(VI), etc.* the anions include silicate, phosphate, vanadate, arsenate, molybdate, antimonate, tungstate, tellurate, etc. There has been a special emphasis on the salts of quadrivalent metals, which is probably due to their better chemical stability compared to the salts of bivalent and tervalent metals. Zirconium phosphate20'21 was the first Insoluble polybaaic metal salt to be used as an ion exchanger. It is the most studied, and prob&bly the most useful of all the materialB of this type. Alongwith a few others, it is now commercially available. These materials, acting mostly as cation - exchangers, are gel-like microcrystalline or amorphous substances. Their composition and ion-exchange properties largely depend upon the method of precipitation. As pointed out earlier, they mostly possess high chemical, temperature, and radiation stability. The mechanism of uptake of ions by these materials is very complicated. However, the cation-exchange properties of these exchangers, generally, arise from the presence of readilyexchangeable hydrogen ions, associated with the anionic groups present in the salt. Amphlett22 suggested that the ion-exchange in zirconium phosphate HZr(HPQ4)2.H2O^J occurs by the displacement of hydrogen ion from P-OH groups, which are bonded to water molecules; viz. P-OH 0H2 +M+ 3$ P-O-M4* +H30 j- A similar mechanism has also been proposed for the hydrous 23 24 zirconium oxide 0% ^t Kn+ + (-Zr - 0H)„ *—* Mn+ + (-Zr - 0)„ + nH+ i " | n '(2) (-Zr - 0)nlP+ +nH4- The uptake of cation will not only depend on the ionisation constant for the hydroxyl group in step(l), but also on the relative affinities of -Zr-0 for H+ and Mn+ in step (2), i.e. upon the readiness with which M*14" associates with the oxygen in the matrix. In many cases, the precipitation mechanism is also 25 operative for the uptake of ions. Pekarek and Benesova , and Vesely et ajL.2 6 have studied the sorption of di-and tervalent cations on uranyl hydrogen phosphate, and explained the sorption in terms of the formation of simple metal phosphates. A similar mechanism has also been reported for other phosphates of bivalent metals, e.g. calcium phosphate ,ammonium magnesium phosphate? zinc ,575 ^A 34 phosphate JJ*J*t and copper phosphate . Another course of sorption may be ascribed to the surface adsorption phenomenon as such or by some electrostatic forces. Murry and Fuerstenau55 carried out many interesting studies on the surface properties of zirconium phosphate. They found, that the charge on the gel, which depends on the exchanger composition and the pH of the solution, is responsible for the uptake of ions in many cases. The uptake of electrolytes, such as K and Li , is lowest at the zero point of charge and the pH values where the solid is positively charged. The cations are not adsorbed until the gel has attained a negative surface charge. This suggests, that the surface charge is one of the important contributing factors towards the sorptive, and exchange properties of these gels. But when the exchange leads to M formation of an insoluble precipitate the surface charge is less prominent. In such cases, the cations are chemisorbed55, even when the solid is positively charged. Sometimes more than one mechanism may be operative simultaneously, for the uptake of ions. However, it is a bit easier to understand the type of the exchange mechanism, if the material has a definite crystalline structure. No doubt, the crystallisation of inorganic ion exchangers is tedious yet it offers many additional advantages. In view of this, a number of inorganic ion exchangers5 47 have been successfully orystallised. It was found, for instance, that on crystallisation thorium arsenate becomes specific for lithium ions , while zirconium phosphate and tin(IV) phosphate become more stable towards hydrolysis27*57"*59. Almost in all the cases, 8 crystallisation adds to the stability of the ion exchanger. Further, crystallisation ensures the purity of the product, so it facilitates many theoretical studies. Despite the outstanding theoretical achievements, a stage has not reached where the properties of these Ion exchangers can be predicted from chemical considerations only. Therefore, different ion exchangers have to be studied, and their ion-exchange behaviour be elucidated before their practical utility can be established. A survey of literature on inorganic ion exchangers* involving quadrivalent metal ions, showed that the following ion exchangers were either scarcely investigated or not studied at all