Sorption of Organic Substances by Ion Exchangers of Various Nature

Sorption of biologically active substances by ion exchangers, characteristic features of such substances and their low stability levels are discussed. Main principles of choosing ion exchangers for isolation and purification of organic substances are developed. Investigation of ion exchange with respect to organic substances revealed specific relationships in their sorption and indicated a necessity of synthesizing special sorbents for isolation, purification and separation of organic substances, most of all biologically active substances. The main products isolated and purified with ion exchangers are biologically active substances, including antibiotics, semiproducts for their production, amino acids, peptides, alkaloids and enzymes. Natural products are usually polybasic. They are weak bases and acids. A great number of physiologically active substances belongs to the class of amphoteric compounds. Instability in solutions and inactivation under the effect of various factors are the most significant critical features practically pertinent to all the objects. This imposes definite limitations on the methods of their production. On the one hand, the sorbates greatly differ in their structure size and electrochemical properties and, on the other hand, they have a common feature, i.e. their susceptibility to the environmental factors inducing irreversible changes in the structure of the sorbates and the loss of their therapeutic activity. Such properties require specific approaches to the problem of choosing ion exchangers and conditions for isolation and purification of the organic substances with them. The statics and kinetics of ion exchange processes with respect to organic substances have been studied by us for many years and the following specific relationships were shown: (1) The sorbent solution system may be in 2 equilibrium states, i.e. true and false, (2) the sorbent counterions induce catalytic transformation of organic counter ions, (3) various mechanisms are used simultaneously for sorption of organic substances by ion exchangers. At first, the approaches to the choice of ion exchangers for sorption of organic ions are discussed. These approaches are closely connected with the structure and properties of organic ions and ion exchangers. During the usual process of ion exchange a thermodynamic equilibrium state is established in the system in spite of changes in the properties of the ion exchanger. However, under definite conditions, when organic ions are involved in the process of ion exchange, changes in the physical properties of the ion exchanger are so significant that a false equilibrium state is established which is a qualitatively new one. A distinctive feature of such a condition is availability of an infinitely large number of false equilibrium states for the same system and gradual transition to the true state. 2169 2170 E. M. SAVITSKAYA et al. When the equilibrium state is true, the counterions of the ion exchanger are uniformly distributed in the particles, while in the state of false equilibrium the distribution of the counterions of various types is not uniform. After completion of organic substance sorption from a solution, the concentration of this substance in the peripheral layers of the sorbent is much higher than that in the center of the bead. Two types of the terminal states in the process of methylene blue sorption by a carboxylic cation exchanger and the dye ion distribution in the particle after the process completion are presented in Fig.1. There exists a concentration profile of counterions in the particles of ion exchangers. Such a profile changes under the effect of various factors, i.e. temperature, the particle size, initial counterion form and crosslinking, concentration of the competing ions in solutions, concentration of organic solvents in aqueous solutions, etc. (Refs 1-4). The study on the kinetics of ion exchange with respect to organic ions showed that, when the equilibrium is established in the system, the initial diffusion coefficients of the organic ions, i.e. those calculated according to the gel kinetics mechanism, change insignificantly, while the initial content of the organic ions in the ion exchanger increases. Nevertheless, when the false equilibrium state is established in the system, the initial diffusion coefficients of the organic ions markedly decrease with an increase in the level of the organic ions in the ion exchanger and the activation energy of diffusion markedly increases (Refs 5,6) . Decrease in the diffusion rate during the ion exchange process is due to an increase in the rigidity of the ion exchanger layers mainly containing the organic counterions, which is shown in the study on the other properties of the ion exchanger, the water sorption isotherm and the specific volume of the sorbent containing different amounts of the organic counterions (Ref.7). Both true and false equilibrium states are used in development of sorption methods for industrial production of organic substances. For concentration and purification of substances isolated from fermentation broths it is advisable to use ion exchangers providing replacement of all inorganic ions by the organic ones. Figure 2 shows the results of antibiotic sorption by various carboxylic cation exchangers. When the concentration of the sodium ions in the solution is low, i.e. less than 0.3 M, the antibiotic is sorbed by the low crosslinked cation exchanger with practically completed replacement of the sodium ions by the antibiotic ions (true equilibrium state). When a more highly crosslinked ion exchanger of the same type is used, the antibiotic level in the ion exchanger appears to be much lower (false equilibrium state). This indicates that in the first case the filtrate obtained by means of desorption has a higher concentration of the antibiotic and a low concentration of the accompanying ions. The sorption rate of the main product by an ion exchanger is one of the most important characteristics defining its usefulness for the sorption of biologically active substances. As a rule, the sorption rate of organic ions is limited by diffusion inside the particles of ion exchangers. The larger the size and the higher the charge of the diffusing ion and the higher the crosslinking of the ion exchanger, the lower are the sorption and diffusion rates. It is demonstrated that the initial coefficients of the inner diffusion in the process of ion exchange sorption of organic ions are by 2-3 orders of magnitude lower than the respective coefficients for inorganic ions (Ref.8). In the synthesis of ion exchangers various means are used for increasing the sorption rate with respect to organic ions. The synthesis of ion exchangers with real pores in the ion exchanger matrix is aimed to increase the sorption rate of organic ions and improve the technological characteristics of the sorbent. Usually, for formation of the pore structure of ion exchangers, their synthesis is performed in the presence of various neutral solvents (Refs 9,10). Figure 3 presents the initial coefficients of the inner diffusion of streptomycin in the carboxylic cation exchanger synthesized by the routine methods (gel cation exchanger) and by a method using neutral solvents (modified cation exchanger). When the gel cation exchanger is used, the diffusion coefficient of the antibiotic decreases with an increase in the crosslinking of the cation exchanger.