Estimation of kinetic and equilibrium parameters of sorption of saponin by a natural sorbent – chitosan
Abstract
Saponins have found application in practical medicine due to a wide range of biological and pharmacological activities. The studies of sorption of glycoside by synthetic and natural origin with sorbents have shown a complex multistage interaction mechanism, including ion-coordination, hydrogen and hydrophobic bonds. It is known that an increase in temperature leads to a change in the physicochemical properties of the sorbate and sorbent. For example, a change in the degree of dissociation of functional groups, solubility, displacement of the critical concentration of micelle formation, etc. On the other hand, the increase in temperature affects the properties of the sorbent - leading to an increase in swelling and enhance the protolysis of functional groups. In addition, as the temperature increases, the thermal motion of molecules is strengthened, the network of hydrogen bonds is loosened, which facilitates the weakening of interactions in the sorbent.
The aim of the work was to consider the thermodynamic and kinetic parameters of the sorption of the triterpene saponin Quillaja Saponaria Molina on the natural sorbent chitosan. The object of the study was a sample of Quillaja Saponaria Molina saponin from the USA. Chitosan is an aminopolysaccharide, a derivative of a linear polysaccharide whose macromolecule consists of bound β-(1-4)D-glucosamine units and N-acetyl-D-glucosamine, is used as a biologically active food additive. Kinetic curves of saponin sorption from the solution were obtained using a limited volume method. Sorption equilibrium in the saponin-chitosan system was studied at a temperature of 276±2, 298±2, 353±2 K under static conditions using the variable concentration method.
The internal diffusion nature of the limitation is revealed, and the coefficients of internal diffusion are calculated. The resulting diffusion coefficients are values of the order of 10-13-10-10 cm2 /s - lower than in polymeric sorbents, which is due to the stiffness of the matrix and the pore diameter. Depending on the concentration of saponin in the solution, the kinetics of the sorption process by chitosan will be hydration-dehydration processes at low concentrations and micellization at large.
The activation energy of sorption Eakt = 76 kJ/mol is calculated. This value confirms the determining contribution of internal diffusion to the general nature of the limiting stage when triterpene saponin interacts with a natural sorbent with chitosan. The calculation of the thermodynamic parameters showed that an increase in temperature leads to a decrease in the adsorption equilibrium constant, which indicates a decrease in the energy of interaction of the saponin molecules with the surface of the sorbent. The values of the interaction energy between saponin and chitosan are in the range -ΔG = 18.1 ÷ 19.4 kJ/mol, which is typical for physical adsorption. The process of saponin absorption is exothermic, which is typical for most systems organic sorbent - sorbent. Positive changes in entropy during sorption on chitosan are related to the hydrophobic effect.
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References
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