Study On Ionic Liquids And Chitosan-loaded Activated Carbon For Removing Cr(Ⅵ) Under Alkaline Conditions

As a highly toxic heavy metal,Cr（Ⅵ）is more toxic than Cr（Ⅲ）and has been widely used in electroplating,paint,and metal processing,etc.Cr（Ⅵ）compound is difficult to degrade and easily absorbed by the human beings,which can cause diarrhea,vomiting and other serious consequences such as liver and lung cancer.The discharge of Cr（Ⅵ）in industrial wastewater is limited within 0.5 mg·L^-1 according to the"Comprehensive Wastewater Discharge Standard of China"（GB 8978-1996）and the World Health Organization,while the concentration of Cr（Ⅵ）is limited within0.05 mg·L^-1 according to the"Sanitary Standard for Drinking Water of China"（GB5749-2006）.As national statistical results showed,more than 15%of the industrial product may be related to chromium,which brought serious chromium pollution,especially hexavalent chromium pollution issue in China.At present,the method of the rapid and efficient degradation of Cr（Ⅵ）has become more significant and urgent than ever.Activated carbon has been widely applied as adsorbent in many industrial processes.Most studies had shown that the activated carbon exhibited the best adsorption performance of Cr（Ⅵ）ions at the pH of 2-3.However,the Cr（Ⅵ）ions adsorption performance of the activated carbon decreased rapidly due to the existence form of CrO4^2-at the alkaline circumstance,for example,marine environment.Studies have shown that it is feasible to improve the Cr（Ⅵ）adsorptio n ability of the activated carbon at the alkaline condition by regulating the surface physicochemical properties of the activated carbon.Among many modifications,ionic liquid and chitosan modification were chosen in this study because of their better per formance under alkaline circumstance,which not only greatly improve the activated carbon’s application,but also overcome the agglomeration tendency of the nano particles.In this study,imidazole-type ionic liquid was loaded onto activated carbon by impregnation and bonding methods.Properties of the series adsorbent were characterized by specific surface area analyzer,Fourier infrared analysis,scanning electron microscopy,and Zeta potential.The effects of pH value,dosage,coexisting ions,initial concentration of Cr（Ⅵ）,reaction temperature and reaction time on the adsorption performance of Cr（Ⅵ）were investigated,and the dynamics data were fitted with the Lagergren and Frendlich models.Sequentially,chitosan was selected to modify the oxygen-rich functional Sargassum Horneri based activated carbon,and the modified material was applied in Cr（Ⅵ）adsorption,performance characterization and the dynamics model were performed as those of ionic liquid modified material.The results indicated that:（1）Immersion-based adsorbent has greatly improved the adsorption capacity in alkaline environment,but seriously blocked the pore.The pore blockage is improved by that of the bond-based adsorbent.The loaded ionic liquid has an amino group,which further enhances its ability to adsorb Cr（Ⅵ）.The pH _pzc of the adsorbent after loading is significantly increased,which is favorable for the adsorption of Cr（Ⅵ）under alkaline conditions.（2）Combined with the BET results of the adsorbent and the changes in the amount of Cr（Ⅵ）adsorption,the specific surface area and the pores maybe not critical factor affecting the amount of adsorption.Cr（Ⅵ）adsorption is mainly influenced by the anion exchange and the synergistic effect of HCl in an alkaline environment,which makes pH the most important factor affecting the adsorption.（3）The pseudo-second-order kinetic model can simulate the adsorption kinetics of Cr（Ⅵ）on the adsorbent.The adsorption of Cr（Ⅵ）on the adsorbent is dominated by chemical adsorption.The Freundlich model is suitable for describing Cr（Ⅵ）adsorption equilibrium process on an adsorption machine.（4）Sargassum Horneri based activated carbon（SAC）was synthesized by using Sargassum Horneri and zinc chloride as raw material and activator,respectively,following with modification by Cross-linked chitosan.Under the condition of pH=8,the adsorption of Cr（Ⅵ）was significantly increased.The equilibrium adsorption capacities of SAC and CSAC were 5.513 mg/g and 10.27 mg/g at 30°C,pH=8,and 20mg/L Cr（Ⅵ）solution,respectively,and the removal rates were 22.17%a nd 95.64%,respectively.The pseudo-second-order equation fits well with the adsorption kinetics process,and the Frendlich model provided the best correlation of the equilibrium data.