Template Synthesis Of Mesoporous Materials For Uranium U(?) Adsorption

As a relatively mature electricity supply technology,nuclear power is a potential power source to solve the energy crisis without serious harm to human beings and environment.However,the increasing development of nuclear energy consumes huge resources and causes a large amount of nuclear waste.Uranium is an important raw material for spent fuel and also a major radioactive contamination element.Therefore,one of focus problems of environmental protection and nuclear development is to effectively separate and enrich uranium in nature and purify uranium-containing wastewater.Based on the design and synthesis of functional mesoporous materials with high specific surface area,high pore volume,uniform mesoporous size and abundant active sites,mesoporous carbon materials(MCMs),mesoporous g-C3N4(g-C3N4-T)and mesoporous silica microspheres(SiO2-COOH)were prepared by template method to overcome the bottleneck in the real treatment.The adsorption behavior and mechanism between U(VI)and mesoporous materials were investigated by batch experiments,characterization technologies,EXAFS spectrum analysis andDFT theory calculation.(1)Mesoporous carbon material(MCMs)was prepared by hard template method using ordered mesoporous silicon SBA-15 as template,sucrose and glucose as carbon source.The specific surface area of MCMs was as high as 682 m2/g.MCMs had ordered and uniform mesoporous channels and a large number of oxygen-containing functional groups to provide effective active sites.The batch experimental results showed that the maximum adsorption capacity of U(VI)on MCMs was 293.95 mg/g.The adsorption kinetics were well-fitted by pseudo-second-order kinetic model,while the adsorption isotherms were well-fitted by Langmuir model.MCMs with excellent cycling performance could still maintain good adsorption performance in simulated water.Combined with FTIR and XPS spectroscopy,the hydroxyl and carboxyl functional groups on the surface and mesoporous channels of MCMs played a major role in the adsorption process.(2)Mesoporous g-O3N4 with different polymerization temperatures were prepared by hard template method,using natural crab shell as template and urea as carbon source.The ordered mesoporous structure and abundant adsorption sites(such as C-N=C,N-(C)3 and C-N-H groups)of g-C3N4 were confirmed by multi-scale characterization.The experimental results showed that the g-C3N4-550 with the highest degree of polymerization reached the adsorption equilibrium rapidly within 120 minutes and showed superior uptake performance(149.70 mg/g)at pH 5.0.The existence of U-N shell(R=2.80 A)in the analysis of EXAFs spectra revealed that the adsorption of U(VI)on g-C3N4-550 was mainly the inner-sphere surface complexation at pH 5.0.DFT calculation further proved the strong interaction between U(VI)and g-C3N4(Ead=156.83 kcal/mol).The most eff-ective adsorption sites were inside the holes of g-C3N4.(3)Mesoporous silica microspheres(SiO2)were prepared by soft template method and modified with carboxyl functional groups to improve the adsorption properties of high valence radionuclides.The ordered mesoporous structure and abundant active sites made SiO2-COOH have better U(VI)removal performance than unmodified SiO2 under various conditions.The adsorption experiments showed that the equilibrium time of U(VI)adsorption on SiO2-COOH was less than 15 minutes,and the maximum adsorption capacity was 46.53 mg/g(pH 3.0),167.79 mg/g(pH 5.0)and 328.95 mg/g(pH 8.0).SiO2-COOH with good cycle performance could effectively immobilize U(VI)from solution under different conditions.More fundamentally,it is demonstrated by spectroscopic analysis that carboxyl groups on the surface of SiO2-COOH played a combined role in the adsorption process:(1)hydrophilic group;(2)electron withdrawing group;(3)binding site.In this paper,mesoporous materials were successfully prepared by template methods,and the adsorption behavior and mechanism between U(VI)and mesoporous materials were investigated from macroscopic,microcosmic and molecular levels.A series of mesoporous materials synthesized in this paper have shown certain application potential in the treatment of uranium-containing wastewater,which provided some references for the later basic research and technology development in the separation and purification of radioactive wastewater.