Feasibility and mechanism of selective enrichment of complex essential oils using alumina ceramic membranes

Essential oils (EO) constitute a class of volatile oily liquids with a wide range of applications in pharmaceuticals, foodstuffs, and household chemicals. However, because of their low contents and complex compositions in plants, especially that of some components containing hydrophilic groups that can exist in the form of dissolved oils, their extraction and separation are often difficult. In this study, the membrane-enrichment process of EOs is systematically analyzed by separating nine representative oil/water emulsions used in traditional Chinese medicine using an alumina ceramic (AC) membrane. Owing to the differences in oil content and main component properties of different herbs, AC membranes exhibit different fluxes and oil-rejection rates for different EO emulsion. The highest oil-interception rate, of up to 79.55 %, is exhibited for Eugenia caryophyllata Thunb. (EcT) EO. The droplet size and charge of the emulsions dominate the membrane-separation process. The AC membrane intercepts aldehydes, ketones, and ethers through strong adsorption, whereas highly hydrophobic olefins are intercepted via repulsion. In addition, a carbon layer is coated on the AC membrane to block the contact between alumina and EO components. The specific selective retention of volatile oil components by the AC membrane is verified. The components of EcT EO, enriched by the AC membrane, show 99.4 % similarity to those obtained by traditional steam distillation, while exhibiting comparable antibacterial activity. This study provides new insights into membrane-material design for EO enrichment and offers guidance for matching EOs with suitable membrane materials.