| With the increasing requirement of separation and purification,the adsorption performance of adsorbent resin needed to be further improved,and chemical modification of the resin had attracted the attention of many researchers.Post-crosslinked polystyrene resins modified by functional groups had been shown to be effective polymer adsorbents for the removal of aromatic compounds,but they are rarely used for the separation and purification of natural products.Alkaloids widely distributed in higher plants are the main active components of many medicinal plants and traditional medicines,but alkaloids in nature often coexist with a large number of other types of organic or inorganic compounds and need to be further isolated and purified.The adsorption of alkaloids in water extract depends on hydrophobic action,while hydrophobic action in aqueous solution is nonspecific,resulting in low adsorption selectivity and low adsorption efficiency.In order to achieve efficient separation and purification of alkaloids,we took low-crosslinked chloromethylated polystyrene resin as the raw material,and introduced different functional groups into the non-polar resin by controlling the addition amount of functional groups.Finally,two series of post-crosslinked resin with different modification degrees were obtained.In this experiment,a series of hydroxyl-modified crosslinking resins,namely LQ-F0,LQ-F5,LQ-F10 and LQ-F15,and a series of amine-modified cross-linking resins,namely PS-N2,PS-N4,PS-N6 and PS-N8,were obtained by post-crosslinking reaction.The modified post-crosslinking resin was characterized by infrared spectroscopy,organic element analysis,X-ray photoelectron spectroscopy and Brunner-Emmet-Teller(BET)analysis,and a series of modified resins with different specific surface areas,pore structures and different degrees of functionalization were obtained through the post-crosslinking reaction.Oxymatrine,berberine,and pseudoephedrine,three alkaloids with different structure characteristics,were selected as matrices to investigate the adsorption,retention and reuse properties of the resin.Through the analysis of adsorption thermodynamics and kinetics,the adsorption mechanism of modified resin,the structure-activity relationship between adsorption and target compound and the polarity matching were discussed.Modified resins exhibited different adsorption effects on three alkaloids with different structures,and the order of adsorption capacity was: oxymatrine > berberine > pseudoephedrine.The adsorption of berberine on various modified resins indicates the importance of structural matching between target compounds and resins.The kinetic model fitting results showed that the pseudo-second-order rate equation was more suitable to describe the adsorption process of all modified resins,and the adsorption rate was controlled by both liquid film diffusion and intra-particle diffusion.The fitting results of the adsorption isotherm model showed that both Langmuir and Freundlich could fit the isotherm well,indicating that adsorption included at least two mechanisms,including single molecular layer adsorption and multi-molecular layer adsorption on non-homogeneous surface.The model parameters indicated that the adsorption process was easier to carry out.Adsorption thermodynamic analysis shows that the adsorption of two alkaloids on modified resin was a spontaneous process.For matrine,the whole adsorption was an exothermic process,and the system disorder increased.For berberine,the whole adsorption was an endothermic process,and the system disorder increased the adsorption forces between modified resin and oxymatrine include hydrophobic force,hydrogen bond and electrostatic force,while the adsorption forces between modified resin and berberine include hydrophobic force,hydrogen bond,?-? force and electrostatic force.The regeneration performance of the two series of modified resins performed well.After five cycles of use,the adsorption effect remained basically unchanged.The column experiment shows that the modified resin possessed good retention capacity.In order to further explore the adsorption selectivity of modified resin,oxymatrine and matrine were used as adsorbents for single-component system and mixed system adsorption,and it was found that there was competitive adsorption between the two alkaloids.For the adsorption of the mixed system,the largest selectivity coefficients of the two series of modified resins are LQ-F10 and PS-N2 respectively.The adsorption isotherm experiment showed that the selectivity coefficient increased with the increase of the initial concentration of the adsorbent,indicating that the higher the concentration,the more prominent the selectivity adsorption of the modified resin to matrine.The dynamic adsorption and analytical experiments proved that the modified resin has stronger adsorption and retention capacity for matrine.Ethanol solution with low concentration can eluted most of the oxymatrine,but for oxymatrine,a slightly higher concentration of ethanol solution was required to elute. |
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