| Uranium mining and hydrometallurgy produces a large amount of low concentration uranium containing wastewater,and its uranium concentration is around 5 mg/L,nearly 100 times the national emission standard(50 ?g/L).The wastewater poses a serious threat to the ecological environment and human health.Therefore,the separation and enrichment of uranium from the low concentration uranium containing wastewater are of great significance to the sustainable development of China's nuclear industry and the protection of the environment and human health.Corrently,adsorption is considered as one of the most effective methods for the treatment of the low concentration uranium containing wastewater.However,it is dependable on the capacity and selectivity of the adsorption materials for uranium whether the adsorption is successful or not.Therefore,it is the key problem for treating the low concentration uranium containing wastewater to improve the adsorption capacity and selectivity of adsorption materials for uranium.In this paper,the low concentration uranium containing wastewater was considered as the research fucus.From the perspectives of improving adsorption capacity,adsorption selectivity,and the separation efficiency of adsorbents,a variety of adsorption materials were designed,and the effects of p H,initial uranium concentration and adsorption time on their adsorption of uranium were systematically studied.By studying adsorption isotherms,thermodynamic equations and kinetic equations,the adsorption process of uranium by the adsorption materials was clarified,the surface morphology,microstructure,and surface chemical functional groups of the adsorption materials before and after adsorption of uranium were characterized by means of modern instruments,the mechanism for the adsorption of uranium by the adsorption materials was elucidated,and the adsorption materials were applied to treat the low concentration uranium containing wastewater.The research topics and the corresponding research results are described as follows.(1)Mg-Al-NO3-LDH adsorbent was synthesized by chemical precipitation method,and its surface morphology,microstructure,element distribution,surface functional groups and surface charging properties were analyzed by SEM-EDS,XRD,FT-IR,XPS,Zata potentiometer and other analytical methods.The effects of solution p H,initial uranium concentration,adsorption time and temperature on the uranium adsorption performance of Mg-Al-NO3-LDH were systematically studied.The surface functional groups on Mg-Al-NO3-LDH before and after adsorption of uranium and the role of oxygen-containing functional groups in uranium adsorption were analyzed by FT-IR and XPS.The results showed that the optimal p H for uranium adsorption by Mg-Al-NO3-LDH was 7.0,and the adsorption equilibrium time was 100 min.The adsorption process of uranium by Mg-Al-NO3-LDH was a spontaneous and endothermic process,which conformed to Langmuir adsorption isotherm model and pseudo-second-order kinetic model.The adsorption capacity only decreased by 1.8% after 5 adsorption-desorption cycles when using 0.2 mol/L Na HCO3 as the desorbent.Mg-Al-NO3-LDH contains oxygen-containing functional groups such as-OH,M-O-M and NO3-on its surface,which mainly adsorb uranium through surface complexation or ion exchange mechanism.(2)TPP-LDH with different amounts of sodium tripolyphosphate were synthesized by exchanging NO3-with different mass ratios of tripolyphosphate anions.Their surface morphology,microstructure,element distribution,surface functional groups and surface charging properties were analyzed by modern analytical methods(SEM-EDS,XRD,FT-IR,XPS,Zata potentiometer,etc.).Then,the effects of solution p H,initial uranium concentration,adsorption time,and temperature on the uranium adsorption performance of TPP-LDH were systematically studied by batch experiments,and the adsorption kinetics,adsorption thermodynamics were also studied.Simultaneously,the surface functional groups before and after adsorption of uranium by TPP-LDH and the role of oxygen-containing functional groups in uranium adsorption were analyzed by FT-IR and XPS.The results showed that Mg,Al,N,O and P elements were uniformly distributed on the surfaces of TPP-LDH-1 and TPP-LDH-2.The adsorption performances of TPP-LDH-1 and TPP-LDH-2 were the best at p H 5.0 ~ 7.0.The uranium adsorption processes of TPP-LDH-1 and TPP-LDH-2 were a spontaneous and endothermic one,and the disorder degree of the solid-liquid interface increased during the adsorption process.The uranium adsorption kinetics by TPP-LDH-1 and TPP-LDH-2 conformed to the pseudo-second-order kinetic model,and the adsorption isotherms conformed to the Langmuir model.The adsorption capacities by TPP-LDH-1 and TPP-LDH-2 for uranium only decreased by 2.4% and 3.5% after 5 times adsorption-desorption cycles when using 0.2 mol/L Na HCO3 as the desorbent,and the chemical structure was also stable.The P-and O-containing functional groups such as P=O,P-O,-OH on the surfaces of TPP-LDH-2 played an important role in the uranium adsorption process.(3)Hierarchically structured Ni-Co-LDH was prepared via self-sacrifice template by high surface area of ZIF-67.The surface morphology,microstructure and surface functional groups on Ni-Co-LDH were analyzed by SEM-EDS,FT-IR and XPS.Then,the effects of reaction time on the surface morphology of Ni-Co-LDH were studied,and the adsorption properties for uranium in aqueous solution were also investigated.Finally,the application of Ni-Co-LDH in low concentration uranium containing wastewater were discussed.The results showed that Ni-Co-LDH-60 had a diamond-shaped dodecahedron structure,and a sheet-like structure was formed on the surface.As the reaction time increased, the Ni-Co-LDH-180 showed a sheet-like stacked structure.The adsorption efficiencies by Ni-Co-LDH-60,Ni-Co-LDH-120 and Ni-Co-LDH-180 for uranium were greatly affected by the p H of the solution,and the optimal p H was 6.0.Compared with Ni-Co-LDH-60 and Ni-Co-LDH-120,Ni-Co-LDH-180 showed better adsorption efficiency for uranium,and the adsorption process conformed to Langmuir adsorption isotherm model and pseudo-second-order kinetic model.According to the Langmuir model fitting,the maximum saturated adsorption capacity by Ni-Co-LDH-180 for uranium was about 1135.30 mg/g.The adsorption process by Ni-Co-LDH-180 for uranium was a spontaneous and endothermic one,and the efficient adsorption was mainly attributed to the Ni-OH,Co-OH and Ni-O-Co groups on the surface of Ni-Co-LDH-180.(4)A novel magnetic Fe3O4@PDA@TiO2 adsorbents was successively prepared by polydopamine(PDA)coated and Ti O2 film deposited on the surface of Fe3O4.The surface morphology,microstructure,surface functional groups and magnetic properties of Fe3O4@PDA@Ti O2 were analyzed by modern advanced analytical instruments(SEM-EDS,TEM,FT-IR,VSM),and the adsorption properties of uranium in aqueous solution were investigated.Finally,the application of Fe3O4@PDA@Ti O2 in low concentration uranium-containing wastewater were discussed.The results showed that the saturation magnetization of Fe3O4@PDA@Ti O2 was 7.4emu/g,which displayed good magnetic response to external magnetic field. The adsorption of uranium by Fe3O4@PDA@Ti O2 was greatly dependent on the solution p H,the optimal p H was 6.0,and the adsorption process conformed to Langmuir adsorption isotherm model and pseudo-second-order kinetic model.The adsorption process was a spontaneous,endothermic one,which was mainly controlled by chemical adsorption.Fe3O4@PDA@Ti O2 mainly interacted with uranium through the-OH,-NH2 and Ti-O groups.(5)The synthesized Mg-Al-NO3-LDH is suitable for the treatment of alkaline uranium containing wastewater,while TPP-LDH,Ni-Co-LDH and Fe3O4@PDA@Ti O2 are suitable for the treatment of weak acid uranium containing wastewater.Among which TPP-LDH-2 and Ni-Co-LDH-180 have high selectivity and removal rate of uranium for wastewater,they can be used as an effective adsorbent for treating low concentration uranium containing wastewater. |
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