Study On Adsorption Of Cr(VI)in Aqueous Solution Onto Functional Graphene Aerogels

Hexavalent chromium is considered as a kind of high-risk heavy metal pollutant because of its wide source,high toxicity,strong mobility,non-biodegradability and accumulation.Therefore,the efficient treatment of Cr（VI）is of great significance.Adsorption method is widely used in water treatment because of its great advantages including high efficiency,economy and easy operation.Graphene aerogel is a kind of three-dimensional network porous material assembled by graphene layer or combined with other organic or inorganic molecules under certain conditions.This kind of material has many advantages,such as large specific surface area,unique network interpenetrating structure and high porosity.Thus,it has shown broad promising in the field of wastewater treatment.In this study,two novel and efficient graphene aerogel adsorbent materials were prepared by hydrothermal method and sol-gel method,and their adsorption properties for Cr（VI）were investigated,and the specific research contents were as follows:A new type of Tetraethyleneteramine-graphene aerogel（TEPA-GA）was prepared using graphene oxide（GO）as precursor and tetraethylenepentamine（TEPA）as nitrogen source by a simple one-step hydrothermal method.Subsequently,Fourier transform infrared spectroscopy（FT-IR）,scanning electron microscopy（SEM）,X ray diffraction（XRD）,X photoelectron spectroscopy（XPS）,Raman spectroscopy and thermogravimetric analysis（TGA）were used to characterize the morphology and chemical structure of the obtained aerogel.The results showed that a large number of amino groups have been introduced successfully into the aerogel materials through nucleophilic substitution and amination reaction between TEPA and GO layer.The density of the obtained aerogel was 6.77 mg/cm³,which indicated that it belonged to the ultra light materials.The morphology was observed by SEM.The obtained aerogel displayed three-dimensional interpenetrating network structure and possesses abundant open multilayered pore structure.TGA result showed that TEPA-GA had good thermal stability.TEPA-GA was used as adsorbent to study the adsorption of Cr（VI）in water.The effects of different factors on the adsorption performance were studied by batch studies.The results showed that when pH was 1,adsorption time was 1 h,temperature was 40 ^o C,dosage of adsorbent was 0.005 g and initial concentration of Cr（VI）was 100 mg/L,the adsorption capacity of TEPA-GA to Cr（VI）was342.48 mg/g,which indicated that the adsorbent has excellent adsorption properties for Cr（VI）.The adsorption mechanism study showed that the adsorption process of TEPA-GA on Cr（VI）was in accordance with the pseudo-second order kinetic model and Langmuir adsorption isotherm model.The results of regeneration experiments showed that TEPA-GA still exhibited excellent adsorption performance after five cycles of recycling,which had practical significance in the removal of Cr（VI）.In order to further improve the mechanical strength of aerogel materials,using GO as precursor,polyethylenimine（PEI）as crosslinking agent and main nitrogen source,introducing sodium alginate（SA）as reinforcing material,a new type of sodium alginate reinforced polyethyleneimine graphene oxide composite aerogel（SA/PEI/GA）was prepared through two steps of crosslinking under mild sol-gel method.The structure and property of SA/PEI/GA were characterized by FT-IR,SEM,XRD,XPS,Raman,TGA and physical property analyzer.The three-dimensional network structure of SA/PEI/GA with interconnected through holes was observed by SEM.The results of physical property analyzer showed that SA could effectively improved the mechanical properties of the composite areogel.The prepared composite areogel material had high strength and certain recoverability.The results of FT-IR,Raman and XPS showed that a cross-linking phenomenon appeared between PEI and GO layers,and a large number of-NH₂ was introduced into the system.SA interpenetrating network cross-linking with Ca²⁺in the system made a great contribution to the mechanical properties of the hydrogel system.XRD results showed that the crystallinity of SA/PEI/GA decreased significantly after modification and compounding.The results of TGA showed that SA/PEI/GA had good thermal stability.SA/PEI/GA was used as a new adsorbent material for adsorption of Cr（VI）.The optimum adsorption conditions were as follows:pH was 2,temperature was50℃,adsorption time was 60 min,adsorbent dosage was 0.01 g,initial concentration of Cr（VI）was 100 mg/L.Under these conditions,the maximum adsorption capacity was 174.05 mg/g,indicating that SA/PEI/GA was an excellent adsorbent with high efficiency.The adsorption process of Cr（VI）by SA/PEI/GA was in accordance with pseudo-second order kinetic model and Langmuir adsorption isotherm model,and the adsorption process belonged to spontaneous endothermic process.Furthermore,the results of regeneration experiment showed that the adsorption removal rate of Cr（VI）by SA/PEI/GA decreased by only about 10%after five cycles of recycling,which indicated that SA/PEI/GA had good cyclic adsorption stability and regeneration performance.Therefore,SA/PEI/GA could be used as a new efficient adsorbent for the removal of Cr（VI）in wastewater.