Covalent Immobilization Of Urease In Coconut Shell Activated Carbon For Urea Wastewater Treatment

ACs were prepared from Hainan coconut shell employing microwave radiation heating with zinc chloride as activating agent, the effects of microwave power, radiation time, impregnation time and concentration of ZnCl2aqueous solution on the yield and adsorption properties of ACs were systematically studied.. An orthogonal test was designed to optimize the preparation conditions. When the radiation power was800W, activation time, concentration of ZnCl2and impregnation time were800W,9min respectively, was9min, concentration of ZnCl2was50(wt)%and impregnation time was48h, the resulting product had the best iodine adsorption value of1258.34mg/g and a methylene blue adsorption value of200.00mL/g. Its BET surface area and total pore volume are1395.46m2/g and0.7021mL/g, respectively. The pore size predominantly ranged from4to9nm.Coconut shell ACs were used as support to covalently immobilize enzyme after successive oxidation, aminosilylation and aldehyde grafting modification.20(wt)%HNO3aqueous solution was used to treat ACs at80℃under reflux for3h. Its FTIR spectrum indicates that hydroxyl groups were generated on the surface of ACs. Subsequently, the oxidized ACs were aminosilylated by refluxing with (3-aminopropyl)trimethoxysilane (APTMS) in toluene for24h. The product was characterized by FTIR and XPS. Glutaraldehyde was connected to the amine groups of the aminosilylated ACs prior to immobilization of enzyme.Relative activities of the immobilized and free enzymes were investigated as functions of temperature and pH. The optimum temperature was found to be60℃for free urease, and70℃for the immobilized urease. In term of a relative activity higher than70%, the immobilized urease shows a wider temperature range of42-80℃, compared to45-75℃for the free enzyme. Meanwhile, an optimum enzyme activity was observed at pH7.4for the free urease, and pH6.8for the immobilized urease. The pH range at which the immobilized enzyme had a relative activity higher than70%was slightly widened compared to that of the free one.Degradation of urea aqueous solution was used to evaluate the immobilized urease’s performance in a packed bed. The immobilized urease worked20h in3days. The results showed that the relative activity was maintained over80%and the average degradation ratio was83.3%. It has a better catalytic stability than those reported in references.