Preparation Of Cyclodextrin-Based Adsorbent For Expanded Bed Adsorption And Separation Of Flavonoids From Ginkgo Biloba L.

Expanded bed adsorption (EBA) is a promising technique for bioseparations. It integrates clarification, concentration, and primary purification into a single step, allowing the capture of bio-molecules from unclarified feedstock without prior removal of particulates. EBA simplifies the technical process and shows high efficiency for protein separation. In order to expand the applications of EBA, new adsorbents would be developed and applied to separate more bio-active molecules. The purpose of this work is to develop a novel cyclodextrin-based adsorbent aiming for separating Ginkgo flavonoids with expanded bed mode. This work will focus on the preparation, characterization and application of this novel adsorbent.Fine technical process is a warrant for the adsorbent with excellent performance. In this work, a series of adsorbents named CroCD-TuC have been prepared by the methods ofβ-cyclodextrin (β-CD) pre-polymerizing and reversed phase suspension crosslinking. Tungsten carbide (TuC) powder is selected as a densifier embedded inβ-cyclodextrin-epichlorohydrin polymer skeleton. The adsorbents obtained have regular spherical shape and a proper diameter of 80μm～320μm. The reversed phase suspension process is studied in detail and the optimal technical parameters are:the mixture of paraffin liquid and pump oil GS-1 with mass ratio of 5:7 as the oil phase; the mass ratio of oil to polymer solution at 4:1; the content of dispersant tween-80 at 1.0% by weight based on the oil phase; stirring speed of 350 rpm-400 rpm. The results of five batches demonstrate that the technology shows good reproducibility with comparable physical properties of adsorbents prepared.The basic properties of adsorbents including physical properties, pore properties, mechanical strength and chemical stability are characterized. It is found that the adsorbent beads follow the logarithmic normal size distribution, and the mean diameter slightly changes within the range of 150μm～170μm, regardless of the amount of tungsten carbide used. However, their wet density and mechanical strength significantly increase as the mass ratio TuC/CD increases, providing a potential of preparing adsorbents suitable for different use by controlling the amount of TuC added in the preparation process. The effect of TuC/CD mass ratio on the porosity and pore diameter is insignificant, indicating the structure ofβ-cyclodextrin polymer skeleton is hardly changed.The hydrodynamic properties of CroCD-TuC adsorbents are determined in an expanded bed. It is found for all CroCD-TuC adsorbents tested, the expansion characteristics can be well correlated with the Richardson-Zaki equation. But the predictions of terminal settling velocity and expansion index with the Stokes' law or Martin model are somewhat deviated from that correlated, needing an improvement with empirical modification. The Bodenstein number (Bo), the axial dispersion coefficient (Dax) and the height equivalent of theoretical plate (HETP) are used to analyze and evaluate the liquid mixing in the expanded bed, based on residence time distribution (RTD) measurement. In summary, fluid velocity and viscosity are the crucial factors influencing the expansion and liquid mixing properties, while adsorbent density directly affects these properties in the form of the expansion factors. A further comparison of hydrodynamic properties of several adsorbents/matrices reveals that CroCD-TuC 3 can be selected as the most promising adsorbent for EBA use.As a new attempt, the bio-active molecule rutin is adsorbed by CroCD-TuC 3 in aqueous solutions. The isothermal adsorption equilibrium follows the Langmuir adsorption equation, and the adsorption capacity is gradually reduced as the temperature and pH of solution rises, but increases with the increase of solvent polarity. The comparisons of CroCD-TuC 3 with SephadexTM G-15 beads on isosteric enthalpy and adsorbent surface heterogeneity analysis indicate that the formation ofβ-cyclodextrin/rutin inclusion complex in CroCD-TuC 3 skeleton makes significant contribution to the adsorption. The adsorption kinetics data are fitted by pore diffusion model. The effective pore diffusivity of rutin calculated with this model (3.8×10-11 m2·s-1) is much lower than that in diluted solution, indicating the diffusion inside the pores is the rate-restricting step in the whole adsorption process. The dynamic adsorption capacities of rutin with CroCD-TuC 3 are also measured and evaluated with the breakthrough curves in packed and expanded bed. The results reveal that the dynamic adsorption capacity is notably influenced by fluid velocity and feed concentration.Finally, the practical application of CroCD-TuC 3 adsorbent is carried out in an expanded bed for separating the flavonoids from natural resources. The feedstock extracted from Ginkgo biloba L. leaves with water is directly loaded onto CroCD-TuC 3 in an expanded bed. After washed with water, the flavonoids are eluted with 50%(w/w) ethanol from the adsorbent. The recovery is 36.1% with the flavonoids purity of 36.5% and the purification factor of 20.8. The results demonstrate that CroCD-TuC 3 is suitable for flavonoids separation and purification. It is expected that the novel adsorbent CroCD-TuC 3 would be a promising candidate for flavonoids separation and purification.