Preparation Of Chitosan Microspheres@LDHs And Removal Of Nitrate From Water

In recent years,domestic,industrial and agricultural wastewater containing nitrate had been discharged into the water.The accumulation of nitrate in water not only destroyed the ecological balance,but also threatened human health.Nitrate had the characteristics of easy dissolution and high stability in water.Therefore,the efficient removal of nitrate in water became one of the environmental pollution problems to be solved urgently.In this study,new core-shell adsorption materials were prepared by in-situ growth of layered double hydroxides（LDHs）on the surface of chitosan microsphere to remove nitrate from water.The specific research was as follows:Chitosan wrinkled microspheres（GCS）was prepared by cross-linking chitosan with glutaraldehyde to improve the mechanical strength and specific surface area of chitosan.Then,Mg AL-CO₃^2--LDHs flakes with pH buffering ability were grown in situ on the surface of GCS to prepare a novel core-shell composite（GCS@LDHs）for nitrate removal in water.The effects of different influencing factors on the nitrate adsorption performance of GCS@LDHs were investigated and the adsorption mechanism was discussed.The adsorption saturation was reached within 120 min and the capacity was 29.1 mg g^-1 at pH value of 4.The adsorption of nitrate on the GCS@LDH followed the pseudo-second-order kinetic and Langmuir adsorption isothermal model,indicated that adsorption process was monolayer adsorption and mainly controlled by chemical reaction.In addition,the adsorption efficiency of nitrate still reached 80%after 7 cycles.In order to solve the pH dependence of chitosan,GCS was modified by 3-chloro-2-hydroxypropyl trimethylammonium chloride（CTA）to enhance the adsorption capacity of the composites for nitrate adsorbed.The LDHs with the best adsorption effect and Mg/Fe ratio with a similar interlayer spacing closed to the size of NO₃^-ion were screened by combination of characterization and performance experiments.Then,it was grown in-suit the surface of GCS@CTA to prepare a new composite（GCS@CTA@Mg Fe-LDHs）.GCS@CTA@Mg Fe-LDHs effectively removed nitrate at a wider pH and the adsorption capacity of GCS@CTA@Mg Fe-LDHs for nitrate can still reach more than 45 mg g^-1under neutral conditions.The adsorption capacity of nitrate was less affected with the presence of HCO₃^-and SO₄^2-anions.GCS@CTA@Mg Fe-LDHs reached the adsorption saturation within 150 min,and the saturated adsorption capacity was 71.5 mg g^-1 at pH value of 4,which was 2.4 times higher than that of GCS@LDHs.In addition,the adsorption efficiency of nitrate still reached 80%after 6 cycles.