STUDIES ON THE EXCHANGE BEHAVIOUR OF SOME INORGANIC COMPOUNDS AND THIER MEMBRANES

Abstract

There exists a big gap between the discovery of ion-exchange phenomenon in naturally occurring substances and synthetic ion-exchangers, The ion-exchange properties of clays were first demonstrated as early as 1850 by Thompson and Way and it was only after several decades that the existence of ion-exchange properties in synthetic compounds could be established. A spectacular evolution began with the incidental discovery of the English chemists, Adams and Holmes in 1935, who found that crushed phonograph records can even act as ion-exchangers. Of the numerous compounds so far studied, synthetic organic resins are the most important and widely studied ones for many reasons, e.g., easy synthesis and introduction of reactive groups thereby making them vary their properties, excellent mechanical and chemical stability, reproducibility of experimental results etc. Synthetic organic resins were developed and improved in Germany, United States and England. Today, their use as ion-exchangers is firmly established as an unit operation and is extremely valuable supplement to other procedures such as filteration, distillation and adsorption. Numerous plants accomplishing various tasks ranging from the recovery of metals from industrial wastes to decontamination of water in cooling systems of nuclear reactors, are in operation . In the laboratory, ion-exchangers are used as an aid in analytical and preparative chemistry. There are several processes of industrial and technological importance which need ion-exchangers that can be used at high temperatures. Ihspite of the many positive advantages of organic exchangers, unfortunately they fail to give response at elevated tempera tures. This need led to the inorganic ion-exchangers such as hydrous oxides of polyvalent metals, insoluble salts of heteropolyacids, insoluble ferrocyanides which have found particular use in the separation of ionic components of radioactive wastes. Inspite of the many minus points regarding reproducibility, mechanical stability, hydrolytic decomposition etc., they are specially suited where change in selectivity and capacity on exposure to radiations are the main aims. The aims of scientific research with ion-exchanger membranes extend far into physiological chemistry and biophysics. However, the most important application is still the purification and demineralization of water. Ion-exchanger membranes have not only opened up new technological possibilities but, in addition, have contributed to a better understanding of the electrochemical and kinetic phenomena in ionexchange. This, in turn, proved to be of great value to the general theory of ion-exchange materials. The large amount of interesting work published in the last decade testifies to the considerable interest existing among chemists, chemical engineers, and biologists in understanding the transport processes occurring across ion-exchanger membranes. When a membrane separates two solutions, a flow or flux of molecular or ionic species through it is caused due to difference of chemical potential, pressure or temperature. These forces when (1-4) they operate severally or in combination, may induce properties e.g., hydraulic permeability, streaming potential, membrane potential, 3 electro-osmosis etc. A pre-requisite for the success of these operations is the availability of suitable membrane systems. Therefore, considerable attention has been paid in recent years to the development of membranes with particular and predetermined specific properties. With the development of a wide variety of inorganic gels, exhibiting exchange behaviour, there is a plenty of scope for systematic investigations on their physico-chemical properties especially, the electrochemical performance of the membranes of inorganic gels vis-a-vis their exchange behaviour. From amongst the different physico-chemical properties, the electro-chemical properties control many operations in industry and biology, it was considered necessary to carry out detailed investigations on some hitherto uninvestigated inorganic systems. Since due attention had not been paid to correlate the electrochemical perfor mance with ion-exchange properties, this aspect was closely examined to give new dimension to previous work carried out in these laboratories. However before introducing the main theme of the problem under investi gation it will be worthwhile to touch some important aspects of exchange and membrane phenomena in the proceeding pages. Ion - exchange phenomena: The term 'Ion-exchange' denotes the reversible interchange of ions between a solid phase (ion-exchange material) and a liquid phase in which the exchange is carried out. Ion-exchangers are insoluble solid materials endowed with chemical properties. They carry ions (counter or contra-ions) which can be exchanged for a stoichiometrically equivalent amount of other ions of the same sign. Generally they are molecular, possess laminar or porous structure thus permitting the exchange of ions freely and rapidly. An ionexchanger may be cationic, anionic or amphoteric depending upon whether it carries exchangeable cations, anions or exhibits both cation and anion exchange behaviour. A typical cation exchange reaction may be represented as M" - A+ + B+ ^~^ M~ - B+ + A+ + + + where M - A represents the ion-exchanger, A and B are the exchanging ions. Ion-exchange process resembles sorption or merely adsorption, in both cases a dissolved species is taken up by the solid phase. The characteristic difference between the two processes is, that ion-exchange takes place stoichiometrically, by effective exchange of ions, while in adsorption process the adsorbent takes up the species present in solution without releasing others. Ion-exchange equilibrium is attained between the counter-ions of the exchanger and the electrolyte solution in which the ion-exchanger is placed. When equilibrium is attained the ionic species A and B distribute themselves on the solid as well as liquid phase. The above equilibrium obeys law of mass action and the thermodynamic equilibrium constant may be derived for the exchange process. Two practical quantities which represent the extent to which exchange takes B place are distribution coefficient, Kd, and selectivity coefficient K^ , both of which can be experimentally determined. 5 The distribution coefficient, Kd, is defined as the number of milliequivalents of that ion per ml remaining in solution at equilibrium. In cases where the quantity of the exchanger taken is not one gram, the (5) following formula is used for calculating Kd values. 100 - X V XT - X . — Kd X m where X is the percent amount remaining in solution V is the total volume of cation solution (ml) m is the quantity of the exchanger (g) Kj is a direct measure, under defined conditions, of the extent to which ion is removed from solution when exchanger is added. B Selectivity coefficient KA is due to reversible character of the ion-exchange reaction and is a direct measure of the preference of the exchanger for one ion relative to another. This is also called stoichiometric equilibrium ratio and is equal to the ratio of the concentrations of the two ions in solid phase divided by their concentra tions in solution, in equilibrium state. KA &+] [a! where barred quantities refer to equilibrium concentrations in the exchange phase, and the others to the concentrations in solution. The thermodynamic equilibrium constant is given by following equation: [5+] [a+J . 7B tA K' [B+2 it] rA . fB 6 and depicts quantitatively the selectivity of an ion-exchanger with respect to a particular pair of ions. The experimental determination of K, becomes difficult owing to the fact that the values of activity coefficients in the exchanger phase are difficult to determine. Selectivity forms the basis of separations of similarly charged metallic or non metallic elements, even with highly similar behaviour. As selectivity effect of ion-exchanger is of great practical (fi-1 T) importance, many attempts have been made to understand the basic factors contributing to it and to establish quantitative expressions that rigourly define and predict the selectivity of ion-exchange materials for various ions, under widely varying experimental conditions. Inspite of the steady progress made to explain this phenomenon, our current knowledge of these complicated systems is not yet sufficient to derive quantitative expressions with universal applications. Obviously to gain a deeper insight, further investigations in this regard are called for. The selectivity effects of ion-exchangers are described by (14-15) the following emperically established rules: 1. The extent of exchange increases with increasing valency of the exchanging ion. However at high concentrations, the normal pattern is reversed. 2. For ions of constant valence, the extent of exchange increases with increasing atomic number of exchanging ion. However, at higher concentrations, the extent of exchange does not increase with increasing atomic number and sometimes a reverse behaviour is observed. 3. The relative extent of exchange of various ions may be estimated from their activity coefficients, the higher the activity coefficients, the greater is the extent of exchange. 4. Complex metallic anions and organic ions of high molecular weight exhibit unusually high exchange affinity. 5. Selectivity is affected only slightly by temperatures except for certain cases where uptake of bivalent ions relative to univalent ions increases with temperature.