Study on the Process and Performance of Separation of Oleuropein by Anionic Resin
Olea europaea L. is an evergreen tree belonging to the Oleaceae family and the Oleaceae genus. It is widely planted in China and Mediterranean countries and is one of the main agricultural cash crops in these countries and regions. At present, the development and utilization of olive resources mainly focus on the utilization of fruits, while olive leaves are abandoned as waste in plantations, causing great resource waste. Modern research has found that olive leaves contain abundant iridoid glycosides, phenols, pentacyclic triterpenoids, and flavonoids. Among them, oleuropein (OL) is the most active substance, which has antioxidant, antiviral, bactericidal and anti-inflammatory, hypoglycemic, antihypertensive, anti-cancer and other effects, and is widely used in food, medicine, cosmetics and other fields. OL belongs to the class of iridoid glycosides, which are easily soluble in solvents such as water, ethanol, and methanol. They can be easily decomposed into hydroxytyrosol under conditions such as acid, alkali, high temperature, and light.
At present, the main methods for separating OL include macroporous resin adsorption, high-speed countercurrent extraction, and supercritical extraction. Among them, high-speed countercurrent extraction and supercritical extraction have better separation effects and can obtain high-purity OL. However, these two methods require high equipment and personnel operation, and are not suitable for large-scale industrial production. Macroporous resin has a porous structure and excellent adsorption performance, and is widely used for the enrichment, separation, and purification of plant active ingredients. The main mechanism of enrichment and separation of OL by macroporous resin is that the two can undergo reversible interactions through hydrogen bonding and van der Waals forces under certain conditions, thereby selectively enriching and separating them from the extraction solution. In recent years, many scholars have focused on the enrichment and separation of OL in olive leaf extracts using non-polar and weakly polar resins. Tian et al. used macroporous resin D101 to enrich and separate OL, and obtained OL product with a purity of 63.43% through ethyl acetate extraction. Wang et al. used AB-8 resin to enrich and separate OL, and the purity increased from 9.16% to 20.38%. Sahin et al. found that the mechanism of XAD 7HP resin adsorbing OL conforms to quasi second order kinetics. Liu et al. found that the mechanism of enrichment of OL by macroporous adsorption resin BMKX-4 conforms to quasi second order kinetics and Freundlich model, and increases the purity of OL to 47.59%. In theory, OL belongs to polyphenolic compounds, and the adsorption mechanism of anionic resin can achieve its enrichment and separation. However, there have been no reports on the research of anion resin separation of OL. Therefore, we used anion resin (LX-68M) to study its static and dynamic adsorption and desorption properties of OL, and further investigated the leakage curve, elution curve, and stability of anion resin regeneration of OL, in order to provide theoretical reference for the enrichment, separation, purification of OL and similar natural products by anion resin.

The isothermal adsorption experiment results show that the Freundlich isothermal adsorption model can well describe the process of LX-68M resin adsorbing OL at the experimental temperature, with a correlation coefficient R2=0.9642, indicating that adsorption is favorable. The adsorption of OL by LX-68M resin is more in line with pseudo second order kinetics, with R2=0.9934, indicating that the adsorption of OL by LX-68M resin occurs simultaneously through physical adsorption and chemical adsorption, with chemical adsorption being the main mechanism. The mechanism may be the formation of chemical bonds between the functional groups of the resin and OL. The optimal separation conditions obtained through static and dynamic adsorption and desorption experiments are: extraction concentration of 0.25g/mL, eluent of 45% ethanol, loading rate of 2.0BV/h, loading volume of 6.0BV, desorption rate of 2.0BV/h, and desorption volume of 10.0BV; At this time, the adsorption rate and desorption rate were 86.86% and 87.08%, respectively, and the purity of OL obtained was 52.33%. After 4 cycles of resin adsorption desorption, the adsorption efficiency was 54.9%. After regeneration, the adsorption rate reached 93.1% of the initial adsorption rate, indicating that the resin still has good adsorption performance after regeneration. The above results indicate that LX-68M resin has good enrichment, separation, and purification efficiency for OL, and compared with existing processes, this process operates at room temperature and pressure, is safe and simple, and uses green and non-toxic solvents. It can provide theoretical research basis and technical support for the application of anionic resin in the industrial separation and purification of OL and similar natural products. Although this study investigated the adsorption process of LX-68M resin on OL, the competitive adsorption behavior between impurities and OL can also have a certain impact on the separation efficiency. Therefore, further research is needed to determine the corresponding adsorption mechanism and provide theoretical basis for the further development and utilization of LX-68M resin.