The Adsorption Of Hexavalent Chromium From Wastewater By Modified ACF

Water pollution has become a severe threat to human beings increasingly.Herein,hexavalent chromium in wastewater is the most fatal pollutant to humans for the large amount of emissions and the difficulty to handle.Activated carbon adsorption method is used by industries to handle chromium-containing wastewater for the advantages of convenient operation,high adsorption efficiency,low investment and no secondary pollution.Activated carbon fibers?ACFs?,as the third generation of activated carbons,have the structure of micropores in dominant,small pore size and narrow pore distribution.Hence,ACFs are often used as adsorbents for heavy metal ions by physical adsorption.However,activated carbon fibers have limitations in adsorption because of fewer active groups on the surface and weaker chemical adsorption capacity.Therefore,it is necessary to modify the surface of ACFs by introducing active groups to expand the total adsorption capacity for hexavalent chromium.Polyethyleneimine?PEI?is a non-toxic amorphous compound,rich in the amount of active amino groups,and has a good effect on hexavalent chromium removal in liquid phase.Based on this,in this paper,PEI@ACF and the photocatalyst TiO₂@PEI@ACF were prepared for highly effective removal of Cr?VI?.The experimental results were as follows:1.Chemical modified ACFs with abundant amino groups were prepared using PEI as the modification reagent to increase the adsorption capacity for hexavalent chromium.It was shown that the prepared PEI@ACFs had the best adsorption capacity for hexavalent chromium when the molecular weight of the PEI was 10,000,and the mass fraction of the PEI was 10%.In this case,the adsorption capacity of PEI@ACF increased 0.4%?15.9%?18.4%?15.6%and 21.8%compared to unmodified ACF in the conditions of pH=1,2,3,5,7 respectively,at the initial concentration of100 mg L^-1.And the adsorption of hexavalent chromium was greatly influenced by temperature and pH value.Increasing the temperature and decreasing the pH of the solution were benefit to the adsorption of hexavalent chromium.The adsorption of Cr?VI?by PEI@ACFs was obeyed Langmuir adsorption isotherm model,which belonged to monolayer surface adsorption and the maximum adsorption capacity was 532.1 mg g^-1.The adsorption-desorption process indicated PEI@ACFs could still retain 80%of the original adsorption capacity after 6 cycles,showing an excellent recycling performance and high reliability as adsorption materials.The adsorption breakthrough curve was also established,and the curve showed PEI@ACFs had great continuous adsorption performances.The breakthrough point was 1400 mL and the terminal point was2900mL when the hydraulic retention time was 35min.For 100 mg L^-1,pH=2 hexavalent chromium solution,it was guaranteed that the removal rate had been over 99.5%within 24 hours by continuous influent,and the effluent met the national emission standards.2.The TiO₂ were loaded on PEI@ACFs by hydrothermal method to further enhancing the removal capacity for hexavalent chromium.The loading amount of TiO₂ had a great influence on the photocatalytic removal of hexavalent chromium.It was found that the removal of hexavalent chromium was best when the mass ratio of TiO₂ to PEI@ACFs was 1:1.Batch experiments found that the main factors affecting the photocatalytic removal of hexavalent chromium were pH value,illumination time and initial concentration of hexavalent chromium.Lower pH value,longer illumination time and lower initial concentration of hexavalent chromium were favorable for the Cr?VI?removal.The removal efficiency of TiO₂@PEI@ACFs in photocatalysis-adsorption of Cr?VI?in static situation was significantly higher than that of the adsorption by PEI@ACFs under the same conditions.For pH=2,100 mg L^-1 hexavalent chromium solution,the removal rate of99.6%was achieved by photocatalytic-adsorption process,which met the national emission standards.For 10 mg L^-1 hexavalent chromium solution,the removal rate was maintained over95%in moderate pH values,and the effluent met the national emission standards by photocatalysis-adsorption process in static condition,which simplified industrial purification processes and saved costs.In this paper,activated carbon fibers were modified by grafting PEI to improve the removal capacity for hexavalent chromium from wastewater.Furthermore,TiO₂ were loaded on the surface of PEI@ACFs,and the removal of hexavalent chromium from wastewater was further improved by photocatalytic-adsorption method.Therefore,PEI@ACFs and TiO₂@PEI@ACFs can be used as potential adsorbents for the treatment of practical industrial wastewater.