Investigation of Three-liquid-phase Extraction Systems for Simultaneous Separation of Emodin and Rhein

Several three-liquid-phase extraction systems comprised of organic extractant, polymer and salt aqueous solution were applied to isolate and purify simulated herbal extract containing emodin and rhein. Some influential factors on the partitioning behaviors were studied. The results revealed that extractant type and solution pH value were crucial factors for improving the separation performance. The optimal separation results were obtained at pH 8.00 with methyl isobutyl ketone as the extractant, 10% (ω) polyethylene glycol with the molecular weight 4000 Da, and 10% (ω) ammonium sulphate. Under the optimized conditions, 95% emodin and 97% rhein were simultaneously extracted into the organic phase and polymer-rich phase from simulated herbal solution with the initial concentration of 100 × 10-6 (ω), respectively, and their separation factor of over 10000 was achieved. By adjusting pH value, back extraction rate of both emodin and rhein could be up to about 99%. With the assistance of molecular simulation program XLOGP3 and molecular orbital package, the possible mechanism of extraction was analyzed, and hydrophobic interaction and hydrogen-bonding interaction might be the major driving forces affecting the partitioning behaviors of emodin and rhein.