The Studies On The Preparation Of Manganese Dioxide Nanocomposites And Their Properties Of Uranium Adsorption

With economic and social development,the demand for energy is increasing,and non-renewable traditional fossil energy sources have failed to meet the needs of mankind,making the problem of energy shortage increasingly prominent.As an excellent clean energy source,the vigorous development of nuclear energy is of great importance to the maintenance of a country’s energy security.However,the development of nuclear energy inevitably generates radioactive wastewater containing uranium,and adsorption is a common and cost-effective method of wastewater treatment.As a common metal oxide,manganese dioxide（MnO₂）has a variable crystalline shape and can provide binding sites for heavy metal ions in water.However,due to limitations such as its tendency to agglomerate in aqueous solutions,poor solubility under acidic conditions and the small number of functional groups it contains,the adsorption capacity of MnO₂ for heavy metals needs to be improved.Improving and optimizing the physicochemical properties of MnO₂ to enhance its ability to treat wastewater is a key research problem for scholars.In this paper,MnO₂ was modified by three monomers,namely chitosan（Chitosan,CTS）,metal organic framework material ZIF-8 and UiO-66,to form three composites to improve the adsorption capacity of uranium U（Ⅵ）.（1）CTS was used as a template material to synthesize α-MnO₂@CTS,and the effects of pH,ionic strength,interference of co-existing ions,time,temperature,number of cycles and stability on the adsorption of U（Ⅵ）by the material were investigated.The results showed that the removal of U（Ⅵ）by α-MnO₂@CTS was strongly influenced by pH,while the concentration of NaNO3 had less effect on this process,indicating that the adsorption was a complexation process existing on the inner surface of the spheres.The adsorption of U（Ⅵ）by α-MnO₂@CTS was as high as 326.54 mg/g at pH=6.0±0.1,T=298 K,which is consistent with the results of the Langmuir adsorption isotherm model.In various studies,the high removal rate of α-MnO₂@CTS for U（Ⅵ）was attributed to electrostatic attraction.The material also maintained a good structure with stable performance in strong acids and bases.α-MnO₂@CTS could efficiently and rapidly adsorb U（Ⅵ）under acidic conditions,indicating its promising application in radioactive wastewater treatment.（2）α-MnO₂ and zeolitic imidazole material ZIF-8 were used as a medium for the deposition of dodecahedral nanoparticles ZIF-8 on nanowires α-MnO₂,forming a composite α-MnO₂@ZIF-8 with a surface area of 1056 m2/g.The maximum adsorption of U（Ⅵ）on α-MnO₂@ZIF-8 was 802.36 mg/g,which was much higher than the adsorption amount on α-MnO₂ or ZIF-8 under the same conditions.High selectivity for U（Ⅵ）could be achieved in the presence of interfering ions,and also good recovery performance could be maintained,and the absorption efficiency was still about 90%after 5 cycles.The possible adsorption mechanisms for the reaction of U（Ⅵ）with αMnO₂@ZIF-8 during adsorption were analyzed in combination with adsorption experiments and X-ray photoelectron spectroscopy（XPS）,i.e.,surface complexation inside the sphere,chemisorption and electrostatic interaction.The simple preparation and high removal rate of α-MnO₂@ZIF-8 indicated that the adsorbent has some potential applications in the treatment of wastewater.（3）The composite α-MnO₂@UiO-66 was synthesized using the solvothermal method.α-MnO₂@UiO-66 was shown to have a higher adsorption capacity than both monomers α-MnO₂ and UiO-66 by the Langmuir isothermal adsorption model,reaching 237.82 mg/g under optimal conditions.α-MnO₂@UiO-66 could reach the adsorption equilibrium within 5 min,which was more consistent with the proposed secondary kinetic model.Ca2+and SO42-interfere with the adsorption of U（Ⅵ）,while U（Ⅵ）showed good selectivity in the presence of Mg2+,K+,Zn2+,Co2+,Ni+,CO32+,Cl-,and NO3-.The high selectivity and fast and efficient adsorption process exhibited by the composite α-MnO₂@UiO-66 was expected to provide some reference value for other materials for the removal of radioactive elements from wastewater.