Organo-modification Of Palygorskite And Its Application

Palygorskite was characterized and then organo-modified to prepare two novel materials. Octodecyl trimethyl ammonium chloride (OTMAC) modified palygorskite was used as an adsorbent, and 3-aminopropyltriethoxysilane modified palygorskite was used as a support for lipase immobilization.An extensive study was performed on the mineral characteristics and modification mechanism of palygorskite in terms of thermal properties (TG-DTG-DSC), chemical composition (FT-IR spectroscopy), morphology (AFM), specific surface area (BET method), pore size distribution (gas adsorption method), basal spacing (XRD).Cation surfactant was mainly grafted on the surface of palygorskite without changing its crystal structure, because of its nano aggregation and low cation exchange capacity. OTMAC was grafted on the surface of palygorskite because it is electronegative. And ultrasonic treating improved the efficiency. The grafting ratio was 44.66%. The OTMAC modified palygorskite had a surface area of 63.96 m2/g, a large reduction from that of acid activated palygorskite (152.85 m2/g). The pore size distributions of the palygorskite had no significant change after OTMAC modification.There were lots of water and hydroxy groups in the palygorskite, with which 3-aminopropyltriethoxysilane was hydrolyzed to become silanol group. The 3-amino- propyltriethoxysilane was then grafted or covered on the palygorskite through dehydration condensation of the silanol group and hydroxyl group of palygorskite. The pores were formed and the surface areas were improved during the modification process. The 3-aminopropyltriethoxysilane modified palygorskite had more pores with the size from 1.3 to 3.5 nm than the acid activated palygorskite. The 3-aminopropyltriethoxysilane modified palygorskite had a surface area of 182.09 m2/g, a large increasing from that of acid activated palygorskite (152.85 m2/g).The OTMAC modified palygorskite adsorbed phenol and active dyes more effectively than the acid activated palygorskite, natural palygorskite and commercial palygorskite adsorbent. Adsorption of phenol on OTMAC modified palygorskite was a physical adsorption. Heat for the adsorption was low because of the small force between them. Heat was 12 kJ/mol when the concentration of phenol was 100mg/L. The adsorption mechanism of phenol was controlled by the surface diffusion. Second-order adsorption kinetics was observed in the case, the rate constant was 1.367 g/(mg min) and the initial adsorption raet was 0.8534 mg/(g min). Adsorption of active dyes on the OTMAC modified palygorskite was a chemical-physical process, and the chemical adsorption was the main one. The heat was about 40 kJ/mol at lower concentration (<150mg/L). All of the initial adsorption rates were larger than 250 mg/(g min) at three different temperatures (30℃,50℃,70℃). Chemical interaction was the main force for the adsorption at low concentration. And the physical adsorption also happened with the increasing of the concentration.Tannin was selectively removed by OTMAC modified palygorskite from model tannin/soybean isoflavones and tannin/puerarin mixtures. Adsorption of tannin/tea polyphenols on OTMAC modified palygorskite was also detected. And the results showed that tannin was removed thoroughly. The adsorption capacity of adsorbent for different components in tea polyphenols was as follows: CG(ECG)>GCG(EGCG)>C(EC)>GC (EGC). The selective adsorption of tannin on OTMAC modified palygorskite was driven by the collaboration of hydrogen bonding, electrostatic force and hydrophobic interactions of tannin molecular with adsorbent.Adsorption of tannin on OTMAC modified palygorskite was a physical process. In adsorption isotherm experiments of tannin adsorption on OTMAC modified palygorskite, obtained data fitted well to the Freundlich model. The enthalpy value of adsorption of tannin on OTMAC modified palygorskite was about 20 kJ/mol. Studies found the high velocity and large capacity for the adsorption of tannin on OTMAC modified palygorskite. The velocity of adsorption was increased with the increasing of temperature and the decreasing of concentration of tannin in a certain range. The tannin molecular contacting with the adsorbent was increased with the decreasing of the concentration of tannin.Lipase from Candida lipolytica was covalently immobilized on 3-aminopro- pyltriethoxysilane modified palygorskite support through glutaraldehyde. The optimized immobilization protocol was as follows: 0.4 g of glutaraldehyde-activated 3-amino- propyltriethoxysilane modified palygorskite supports were added to 40 mL enzyme solution in phosphate buffer pH 7.0, with a protein content of 1 mg/mL. The covalently bond process was performed over 6 h at 25-30℃. Scanning electron micrographs and atomic force micrographs proved the covalently immobilization of lipase on the palygorskite support through glutaraldehyde. The activity of 3,100-3,300 U/g per gram immobilized lipase was obtained. Immobilized lipase retained activity over wider ranges of temperature and pH than those of the free enzyme. The optimum pH and temperature of the immobilized lipase was different from those of free lipase. The immobilized enzyme retained high activity after 8 cycles.