Preparation Of Modified Manganese Dioxide Nanocomposites And The Studies On Their Properties And Mechanisms Of U(Ⅵ) Adsorption

Under the increasingly serious environmental pressure,the development of nuclear energy,which is clean,stable and has high energy density,is of great significance to ensure energy security and reduce the pollution of fossil energy.However,with the rapid development of nuclear energy,the treatment and disposal of radioactive waste has become an important environmental topic that many countries in the world are facing and need to solve urgently.As the most important nuclear fuel,uranium is abundant in radioactive nuclear waste water,posing a serious potential threat to biological and ecological balance.The development of functional nanoparticles with high adsorption capacity and high adsorption efficiency is one of the important issues in the treatment of radioactive U(Ⅵ)-containing wastewater.Manganese dioxide(MnO2)is an environment-friendly metal oxide,which has excellent properties such as wide source,low cost,diverse morphology,rich crystal form,stable structure,controllable particle size and so on.It shows great potential in the application of radionuclides removal.Unfortunately,excessive pressure drop,poor mechanical rigidity and certain agglomeration limit the practical application of MnO2.Therefore,the modification of MnO2 with organic or inorganic materials to improve its adsorption performance has attracted extensive attention.In this paper,a-MnO2 nanowires with unique properties,good morphology and stable structure were prepared by a simple hydrothermal method.Based on the idea of designing MnO2 composite materials with good dispersibility,rich active sites and excellent tolerance,the adsorbents(MnO2@M)composed of α-MnO2 and organic polymer,metal oxide,graphene substance or inorganic clay mineral were prepared through a simple synthesis method for the adsorption of U(Ⅵ)in wastewater.The influence factors,practical application potential of MnO2@M in U(Ⅵ)enrichment were investigated,and the interaction mechanism between MnO2@M and U(Ⅵ)was discussed.Finally,the effect of different kinds of modified materials on the properties of α-MnO2 and the mechanism of enhancing the adsorption performance of α-MnO2 were summarized and compared.(1)α-MnO2@PDAs composites were synthesized by coating a-MnO2 nano wires with polydopamine(PDA).α-MnO2@PDAs possessed excellent dispersibility and abundant surface functional groups.The adsorption of U(Ⅵ)on α-MnO2@PDAs was obviously influenced by pH rather than ionic strength.The adsorption reaction could reach equilibrium within 5 min.According to Langmuir model parameters,the order of maximum adsorption capacity at 298 K and different pH was pH 5.0(383.1 mg/g)>8.0(213.2 mg/g)>3.0(158.7 mg/g),indicating that α-MnO2@PDAs could effectively remove U(Ⅵ)under harsh acidic and alkaline conditions.Spectroscopic analyses suggested that favorable adsorption occurred at active binding sites such as phenolic hydroxyl and amide functional groups.The high adsorption capacity ofα-MnO2@PDAs for U(Ⅵ)in natural or synthetic water proved its potential applicability in actual wastewater treatment.(2)Titanium dioxide-modified manganese dioxide(MnO2@TiO2)was prepared by immobilizing anatase titanium dioxide(TiO2)on the surface of α-MnO2 nanowires.The basic material α-MnO2 acted as the physical template of the guest TiO2,and the formed MnO2@TiO2 composite exhibited a highly uniform core-shell nanorod structure.The maximum adsorption capacity of MnO2@TiO2(105.3 mg/g)for U(Ⅵ)was much higher than that of MnO2(13.3 mg/g).The U(Ⅵ)adsorption by MnO2@TiO2 involved monolayer chemisorption.During the adsorption process,there might be three kinds of interactions between U(Ⅵ)and MnO2@TiO2,namely inner-sphere surface complexation,chemical precipitation and electrostatic interaction.(3)Interpenetrating 3-dimensional MnO2@GO composites(MGs)were rationally constructed by integrating α-MnO2 nanowires with graphene oxide(GO)via a simple ultra-sonication process.Preliminary experiments showed that compound M1G2 with a mass ratio of MnO2 to GO of 1:2 had a relatively optimal adsorption capacities for U(Ⅵ)(271.7 mg/g)at pH～5.0(298 K).M1G2 had high selectivity for U(Ⅵ),which could remove>90%of target ions in the presence of NO3-,Cl-,CO32-,HCO3-,Mg2+,K+or Na+.M1G2 exhibited excellent stability under a wide pH range(3.0-10.0)and great resistant to high ionic strength.M1G2 could be reused for 4 times without any significant decrease in performance,thus realizing the sustainability of the adsorbent.Kinetic studies exhibited that M1G2 could effectively capture target ions within ultra-short kinetic equilibrium time(<1 min).The interaction mechanism was clearly visualized by analyzing characterization data,and oxygen-containing functional groups took a maj or part in binding target ions.(4)The composite adsorbent α-MnO2@LDHs composed of α-MnO2 and layered double hydroxides(LDHs)was constructed.The maximum removal capacity of U(Ⅵ)by α-MnO2@LDHs was 135.5 mg/g at 298 K through the formation of inner-sphere surface complex and redox reaction.At 328 K,the removal amount reached 565.0 mg/g,indicating α-MnO2@LDHs had potential to treat high-temperature radioactive wastewater.In addition,α-MnO2@LDHs behaved wide ionic strength(0.001-0.1 M)and pH(5.0-12.0)availability,strong resistance to foreign ions interference,and rapid adsorption.In-depth and systematic spectral analysis revealed that the active functional groups were regarded as Al-and Mg-OH.Mn3+ and CO32-also made important contributions to the combination of U(Ⅵ).To sum up,MnO2@M nanocomposites with excellent properties were synthesized by simple method.The prepared MnO2@M combined the respective advantages of single α-MnO2 and modified substances,and remarkably improved the adsorption performance,which had potential application value for treating wastewater containing U(Ⅵ).By comparing the effects of different kinds of modified substances on the properties and adsorption properties of α-MnO2,it is found that the introduction of modified substances had certain improvement on the dispersibility,specific surface area and functional group content of the adsorbents.Groups such as-OH and-COOH had strong affinity for U(Ⅵ)and made great contribution to the increase of U(Ⅵ)removal.Compared with the increase of specific surface area,the introduction of a large number of specific groups was more conducive to the enhancement of adsorption effect.