Description of the sorption isotherm of triterpene saponin on natural sorbents

Abstract

There are quite a number of papers focusing on the sorption of bioactove compounds on syntheric and
natural sorbents. These compounds include triterpenoid saponins which are commonly used as natural medicinal products. In this study we analysed, whether the existing models can be applied to interpret the experimental results of the adsorption of saponin on natural sorbents.
The object of the study was a saponin sample Quillaja Saponaria Molina produced in the USA. Chitosan,
produced by OOO Pharmakon Production (St.Petersburg), is an aminopolysaccharide, a derivative of a linear polysaccharide, whose macromolecules are composed of β-(1-4)-linked D-glucosamine and N-acetyl-Dglucosamine.
It is often used as biologically active food supplements. Activated charcoal produced by OAO Pharmastandart-Leksredstva was used as a carbon-based material. The sorption equilibrium in the «saponin – chitosan» and «saponin – charcoal» systems were studied at a temperature of 298±2 К under static conditions using variable concentrations.
During the experiments the isotherms of saponin on chitosan and on activated charcoal were obtained.
The first regions of both isotherms are linear. The interaction between saponin and chitosan is based on the ion coordination interaction with the ammonium group of the polysaccharide. The interaction with the carbon-based material is determined by long-range hydrophobic forces which cause the association of saponin molecules and attract them to the hydrophobic surface of the sorbent. The second regions are different for the two isotherms.
There is an inflection point followed by a plateau on the chitosan isotherm and a local minimum in the areas of concentration on the carbon-based material isotherm. This is followed by an increase in the sorption parameter on both isotherms. The nonlinearity of the isotherm in this region demonstrates that there are two opposite processes taking place at the same time: the formation of micelles and adsorption on the surface of the sorbent, which are characterised by the energy of adsorption and the energy of association. Depending on the values of these energies, there is either a step to a plateau or a local minimum on the curve. The latter is observed when the energy of micelle formation prevails over the adsorption energy. In the third region there is a sharp increase in the sorption parameters, which is accounted for by the formation of lamellar aggregates in the adsorption layers.
Taking into account the shape of the equilibrium curves, the experimental results were modelled using
basic adsorption isotherms. The values of sorption parameterswere calculated. Since the correlation coefficients (R2) are large, we can use Henry’s law to conduct a quantitative description of the first region of the chitosan isotherm and the BET method to analyse the whole isotherm, as it takes into account the multilayer character of the adsorption process.
The adsorption of saponin on the carbon-based material is described by Langmuir equation in the range
of 0.02-1 mmol/dm3 and the BET equation in the range of 2-17 mmol/ dm3. The obtained results comply with the proposed models of adsorption of saponin on natural sorbents, and can serve as an explanation for a possible change in its pharmacokinetic properties in the immobilized state.

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