## 4 Discussion
Porous materials are widely and willingly used due to their properties, mainly their large specific surface, high porosity, absorptive capacity, low chemical activity, high mechanical strength, and possibility of regeneration [47]. The use of porous sorbents has often been successful in increasing microorganism production and recovery of volatile compounds from fermentation broth [32; 33; 34; 48]. However, it should be remembered that adsorption from biotransformation media causes certain difficulties which result from the fact that many components are present in cell culture supernatants. These ingredients come from both the growth medium and the extracellular products that are produced by the cells of microorganisms. Additionally, GDL adsorption takes place in the presence of unreacted lipid precursor (ricinoleic acid) (Figure 4) and lipid substrate (castor oil). Among the three tested adsorbents, the highest adsorption efficiency was obtained for Amberlite XAD-4. After 6 h of adsorption on this resin, less than 1% GDL in the solution was determined. Probably, the ability of Amberlite XAD-4 resin to adsorb GDL is higher than that of zeolite or vermiculite due to the higher specific surface area (750 m\({}^{2}\)/g) compared to other materials (700 m\({}^{2}\)/g and 394 m\({}^{2}\)/g, respectively). [PERSON] et al. [47] confirms that the adsorbents with the largest surface area seem to be the most effective in the separation of organic compounds. According to [PERSON] et al. [34], hydrophobic interactions, chemical structure, and high specific surface area are the main driving forces behind the adsorption of GDL to Amberlite XAD-4 resin in aqueous solutions. In order to better understand the mechanism of GDL adsorption from biotransformation media, which is influenced by the interactions between the components of the culture medium (especially lipids) and lactone, the sorption isotherm was drawn for the most effective Amberlite XAD-4 adsorbent. The obtained linear dependence over a certain range of GDL concentrations in the biotransformation medium (up to 4 g/L) can be attributed to the low solubility of this lactone in water. This is mentioned in the research by [PERSON] et al. [34], who obtained a linear relationship for each of the sorbents by carrying out the adsorption of GDL on the MN-100, MN-102, and MN-202 resins. The peach lactone adsorption method was closely related to the solubility of this compound in water, which is about 0.6 g/L. The same conclusions were made by [PERSON] et al. [33] while testing activated carbon and three porous polystyrene-type polymers (Porapak Q, Chromosorb 105 and Resin SM4). Desorption of GDL from individual adsorbents, carried out with ethyl alcohol, presented relatively good efficiency; 75-81% of adsorbed lactone. The chromatographic analysis of the extracts after desorption from Amberlite XAD-4 revealed the content of unreacted ricinoleic acid (Figure 4). Its amount decreased during individual cycles of the process, and the purity of the GDL after desorption was approximately 77 \(\pm\) 2%. The phenomenon of ricinoleic acid adsorption on the surface of adsorbents has been reported by [PERSON] et al. [49].