Synthesis Of Chitosan Bonded Silica Gel And Its Application To The Adsorption Of Cr(Ⅳ)

Chitosan is a new type biological functional material widely applied in various fields such as waste-watar treatment, daily chemicals, bioengineering and so on, because of its inherent biological activities and physicochemical properties. However, chitosan has some weakness in use, such as dissolution in acid solvent and slow sedimentation. On the other hand, silica gel is relatively stable in chemical and thermal properties, as well as has high specific area. Thus we can extend its application by chemical modification. In this work, our objective is to synthesize a new type of adsorbent:chitosan bonded silica gel, which is insoluble in aqueous acidic solution and can be sedimentated fastly. Then it was applied to the adsorption of Cr(VI) and its adsorption characteristics have been investigated.The silanized silica gel was firstly prepared by the reaction of y-aminopropyl triethoxy silane and the silica gel, followed by cross-linked reaction with glutaraldehyde between chitosan and the silanized silica gel to obtain the adsorption material of chitosan bonded silica gel. Overall,5 series,15 kinds in total, of chitosan bonded silica gels were synthesized by varying the temperature of the reaction, the pH value of the reaction, the amount of chitosan, the kinds of dialdehyde, the amount of glutaraldehyde. The morphology, dimensions, contents and structures of chitosan bonded silica gel were characterized by scanning electronic microscopy (SEM), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR) and elementary analysis (EA). The effects of some important variables in the process of preparation on the effectiveness of the chitosan bonded silica gel for the removal of Cr(Ⅵ) from wastewater were studied. The adsorption characteristics obtained from batch experiments and the recycle of it have been studied.The elementary analysis results showed that silanized silica gel was synthesized in 10 hours. The best conditions in cross-linked reaction are:reaction temperature 333 K, pH in reaction system 5, the ratio of reaction reagents is silanized silica gel:chitosan:glutaraldehyde= 2:0.8:1. The experimental results showed that the adsorption capacities of the chitosan bonded silica gel were highly pH dependent, and the adsorption efficiency of Cr(Ⅵ) increased firstly to the highest with the increase of the pH value to 5 and then it decreased. It come to adsorption equilibrium within 250 minute.Under the optimized conditions, the sample which synthsized at the best conditions in the process of preparation did the best effectiveness for the removal of Cr(Ⅵ) from wastewater. The investigation of the coexisted Cu(Ⅱ), Zn(Ⅱ), Cd(Ⅱ), Pb(Ⅱ) ions on the effectiveness of the chitosan bonded silica gel for the removal of Cr(VI) from wastewater showed that the effectiveness of the chitosan bonded silica gel for the removal of Cr(Ⅵ) can be weakened by the coexisted metal ions in varying degrees. The kinetic studies showed that the adsorption process fitted the pseudo-second-order rate equation well. The adsorption efficiency of Cr(Ⅵ) increased with the increase of the Cr(Ⅵ) concentration and was well correlated with the Langmuir isotherm equation.The highest maximum adsorption capacity of the synthsized chitosan bonded silica gel for Cr(Ⅵ) was 159.2 mg/g at 293 K. The thermodynamics studies showed that the process of the Cr(Ⅵ) adsorption onto chitosan bonded silica gel was exothermic and entropy favored in nature. The recycle of the chitosan bonded silica gel has been studied and the results showed that the adsorption capacity of 0.02 g was 30.2 mg/g after 6 cycles in the solution of 50 mL 21.28 mg/L Cr(Ⅵ).Chitosan bonded silica gel synthesized in this paper is a sort of worthwhile material because it can not only successfully and selectively adsorb Cr(Ⅵ) ion in weak acid solution, but also its adsorption capacity is large and sedimentation is fast.