3.2.5 Study of isothermal absorption models
In this study, to evaluate the isotherms of adsorption on the desired adsorbent under optimal conditions, the three absorption isotherms of Langmuir, Freundlich and Tamkin were examined (Fig. 9).
Fig. (9): Investigation of adsorption isotherms, a) Langmuir isotherm b) Freundlich isotherm c) Temkin isotherm.
In isothermal absorption, it is assumed that the drug is placed on the adsorbent as a single-layer coating. According to the calculation, the R2 obtained from the isotherm is the absorption of Langmuir (0.948), which indicates that there is a single molecular absorption of curcumin on the target adsorbent. Freundlich adsorption isotherm is widely used for multilayer absorption modeling on heterogeneous adsorbent surfaces and it is not limited to the formation of a single layer on the surface in addition absorption functions can also be described using the Freundlich isotherm parameters. As can be seen from the calculations, R2 is obtained from the calculations of this isotherm (0.995). Also, the value (1/n) obtained indicates that the adsorption conditions of curcumin on the adsorbent are optimal and the curcumin groups are adsorbed on the heterogeneous adsorbent surface. Using the Temkin adsorption isotherm, the ratio of the surface covered to the total available surface can be obtained for the adsorbent and in this isotherm, R2 is equal to (0.945). This indicates that almost a large area of the adsorbent is available to absorb curcumin and is covered with curcumin.
Table (7): Investigation of different types of isotherms of absorption and presentation of their parameters