| Nuclear power is the focus of the world’s energy strategy and an important choice for China’s energy strategy.At the fourth Nuclear Security Summit,President Xi Jinping pointed out it clear that efforts should be made to develop and utilize nuclear energy to fill the gap in energy demand.The operation,maintenance and decommissioning of nuclear reactors and other activities of the nuclear industry will produce a large amount of radioactive waste water,which poses a serious threat to environmental safety and human health.At present,China lacks technologically advanced and economically feasible methods for the treatment of radioactive wastewater,and it is difficult to introduce foreign technology.Therefore,it is urgent to develop cost-effective radioactive wastewater treatment technology in China.Uranium,thorium and cesium are the representative nuclide in radioactive wastewater and are the main targets for waste water prevention and control.In summary,adsorption is one of the most widely used methods in the field of radioactive wastewater treatment,due to its simple and efficient operation,economic and environmental friendliness.In recent years,graphene and its derivatives can be used as adsorbents for radioactive wastewater due to their excellent thermal stability,chemical stability,and radiation resistance stability.Based on the above background,graphene hydrogel and its composite materials were prepared by chemical reduction method,and studied the removal of nuclides in wastewater containing U(Ⅵ)/Th(Ⅳ)and Cs(Ⅰ).The corresponding adsorption mechanism and process conditions have been analyzed and optimized,and its good application prospects in radioactive wastewater treatment have been explored.The main research contents and conclusions are as follows.(1)Graphene oxide(GO)was prepared by modified Hummer’s method,and then graphene hydrogel(GH)and Prussian blue/graphene hydrogel(PB/GH)were prepared by chemical reduction method using ascorbic acid as reducing agent,and the materials were characterized by SEM,TEM,XRD,XPS and Raman spectroscopy.The results show that:1)GH and PB/GH display a 3D porous network structure,and the pore size is in the order of micrometers.2)GO is chemically reduced to form graphene,and GH is formed under the drive of π-π interaction and hydrophobic interaction between graphene sheets.3)When the PB nanoparticles are added during the reduction of GO,the PB nanoparticles are evenly loaded on the graphene sheet due to the wrapping effect of graphene.4)The specific surface areas of GH and PB/GH are 162.92 m2/g and 98.21 m2/g,respectively,and the water content is as high as more than 90%,which ensures the full contact of metal ions and hydrogel.5)Zeta potential analysis of GH shows that it is negatively charged in the range of pH=2～10.(2)Batch adsorption experiments were carried out to study the effect of contact time,initial concentration of U(Ⅵ)and Th(Ⅳ),and solution pH on the adsorption performance of GH.The adsorption mechanism of U(Ⅵ)and Th(Ⅳ)on GH was analyzed by pseudo-first-order and pseudo-second-order kinetic models,Langmuir and Freundlich adsorption isotherm models,and the reusability of GH was also investigated.The results show that:1)The adsorption of U(Ⅵ)and Th(Ⅳ)on GH reaches equilibrium in about 24 h,and the adsorption is consistent with the pseudo-second-order kinetic model,indicating that the adsorption of U(Ⅵ)and Th(Ⅵ)on GH is chemical adsorption.2)The adsorption of U(Ⅵ)and Th(Ⅳ)on GH fitted well with Freundlich isotherm model,indicating that the adsorption is heterogeneous adsorption.The maximum adsorption capacity of GH for U(Ⅵ)at pH 4.0 was 128.34 mg/g;the maximum adsorption capacity of GH for Th(Ⅳ)at pH 3.0 was 190.28 mg/g.3)The pH value of the solution has a significant effect on the adsorption of U(Ⅵ)and Th(Ⅳ)on GH,and the optimal pH value of GH for U(Ⅵ)is between 5.0-6.5.4)After 10 adsorption-desorption cycle experiments,the adsorption capacity of GH on U(Ⅵ)only decreased by 5.24%compared with the first time.After 5 adsorption-desorption cycles,the adsorption capacity of Th(Ⅳ)Compared with the first time,the adsorption capacity decreased by only 11.8%.And after repeated adsorption-desorption experiments,GH maintains a complete large-scale 3dimensional porous structure,which shows that GH has good reusability for U(Ⅵ)and Th(Ⅳ)adsorption.GH shows good removal effect and stability in U(Ⅵ)/Th(Ⅳ)removal.(3)The effects of Prussian blue and graphene oxide(PB/GO)(w/w)ratio,contact time,initial concentration of Cs(Ⅰ),and solution pH on the adsorption of PB/GH were studied through batch adsorption experiments.The results show that:1)The PB/GH prepared with PB/GO(w/w)ratio of 2 has the highest adsorption capacity for Cs(Ⅰ).2)The adsorption of Cs(Ⅰ)by PB/GH fitted well with the quasi-second-order kinetic model,proving that the adsorption is chemical adsorption.3)The adsorption of Cs(Ⅰ)by PB/GH is consistent with the Langmuir isotherm adsorption model,indicating that the adsorption of Cs(Ⅰ)on PB/GH is homogeneous adsorption.At pH 5.0,the maximum adsorption capacity of PB/GH for Cs(Ⅰ)is 50.06 mg/g.4)The pH of the solution also significantly affects the adsorption of Cs(Ⅰ).PB/GH shows good removal effect and stability in the removal of Cs(Ⅰ). |
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