Independent Determination of Solute-Solid Interactions in Phytoextraction
Björn Dreisewerd, Juliane Merz, Gerhard Schembecker, TU Dortmund University, Dortmund, Germany
Phytoextraction is strongly influenced by the solute-solid interactions describing the adsorption/desorption equilibrium at the surface of the plant material. However, standard extraction experiments of plant material do not provide access to this equilibrium as various superimposing effects influence the concentration of the target component in the extract. The equilibrium at the surface is just one part of a sequence of different mass transfer steps during extraction. However, being able to differentiate between these effects is a necessary prerequisite to provide reliable predictive models for phytoextraction enabling to significantly reduce the experimental effort for designing a technical process.
Several dynamic and static procedures for the determination of the solute-solid interactions have been published, but none of them enable to determine the solute-solid interactions independently of other mass transfer steps and extraction phenomena. Therefore, misinterpretation of results is most likely.
In this study, two experimental procedures are presented to overcome this gap. Both procedures consist of two steps: first, the production of a completely leached plant material to eliminate the interference of the extraction phenomena and afterwards, adsorption experiments using the completely leached plant material and plant extracts. The first experimental procedure is used for the determination of adsorption isotherms, while the second one allows the independent measurement of the adsorption kinetics. Both experimental procedures were applied to two different model systems, namely artemisinin from Artemisia annua using ethanol, hexane or hexane with 5 mol% of ethyl acetate (referred to as hexane/ethyl acetate in the following) as extraction solvents and steviol glycosides (rebaudioside A and stevioside) from Stevia rebaudiana Bertoni using water.
The results show that artemisinin in ethanolic extracts did not adsorb onto the plant material at any temperature investigated, while for hexane and hexane/ethyl acetate linear adsorption isotherms could be determined. These results fit the expectations based on the solubility of artemisinin in the solvent systems given. The solute-solid interactions determined were validated by predicting the concentrations and accumulated masses of artemisinin in different stages of leaching experiments. This validation showed a good agreement between the experimental and the theoretical data and, therefore, proved the applicability of the experimental procedure.
Regarding the solute-solid interactions of steviol glycosides, Langmuir adsorption isotherms were determined. Since rebaudioside A has an additional β-D-glucopyranosyl group compared to stevioside, the former one is more polar and, thus, adsorbed less onto the plant material’s surface than stevioside in the presence of water. Moreover, adsorption of both rebaudioside A and stevioside strongly depended on temperature. Although adsorption usually is an exothermic process, adsorption of the steviol glycosides increased with increasing temperature. This behavior was most likely caused by solute-solvent interactions, particularly the strength of the hydrogen bond interactions between the steviol glycosides and water, which decrease with increasing temperature. Accordingly, the maximum loading of rebaudioside A was more strongly influenced by the temperature than the maximum loading of stevioside, since rebaudioside A has three additional hydroxyl groups.
However, validation of the adsorption isotherms determined was not successful, as the predicted concentrations and accumulated masses of both rebaudioside A and stevioside significantly differed from the experimental data of the leaching experiment compared with. Therefore, the surface of the plant material was analyzed by scanning electron microscopy, disclosing significant changes in the structure of the plant material during extraction. Hence, a changing adsorption behavior could be assumed. This hypothesis was proven by additional adsorption experiments after each stage of extraction, resulting in increasing loads with increasing number of stages, although the liquid concentration decreased.
The experiments performed prove that the experimental procedures proposed allow determining independently the solute-solid interactions in phytoextraction. However, the structure of the plant material has to be monitored in order to exclude changes in the adsorption characteristic due to structural changes of the extraction material.
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