Adsorption Behaviors Of U(Ⅵ)Onto Functional Titanate Nanomaterial And Its Applications For Water Treatment

Titanate nanomaterials are one of novel nanomaterials with strong ion exchange property,which has great application prospects not only in catalysis,ion exchange,single electron transistor,but also as a pollutant adsorption material.Compared with other ion exchange agents,titanates are more environmentally friendly and consume relatively low energy and heat energy.In this paper,the removal behavior of U（Ⅵ）from aqueous solution by prepared titanate nanomaterials with different structures by different methods were investigated.The influence factors and their potential in practical application of titanate nanomaterials on the adsorption of U（Ⅵ）were conducted by batch technique.The functional titanate nanomaterials with core-shell,yolk-shell and hollow structure can be synthesized by in situ sol-gel method on the templates of Fe₃O₄ and SiO₂,following by one step hydrothermal process.Different environmental factors can influence the U（Ⅵ）adsorption behaviors onto multi-functional titanates in the aqueous solutions.The microstructure and adsorption mechanism of titanates were investigated by means of XRD,FTIR,Raman spectroscopy,XPS,BET analysis other characterization methods before and after the adsorption of uranyl in different systems.（1）The titanate nanomaterials（TNWs）developed in low temperature demonstrated a linear structure of agglomeration,and the morphology of TNWs was remained unchanged before and after adsorption.The characterization structure showed that TNWs still had a complete crystal structure at low temperature.According to the kinetic and isotherm fitting results,the adsorption behavior of TNWs to U（Ⅵ）was in accordance with pseudo-second-order kinetics and Langmuir adsorption model,the maximum sorption capacities were calculated to be 358,384,and 410 mg g"1 at the temperatures of 298 K,313 K and 328 K,respectively.The sorption of U（Ⅵ）on TNWs was strongly affected by pH and weakly affected by ionic strength,indicating that the cation exchange and outer-sphere complexion dominated the adsorption process.The thermodynamic results revealed that the interaction process was spontaneous and endothermic.XRD analysis showed that the layered structure of TNWs will gradually disappeared with the increase of U（Ⅵ）concentration.The XPS spectrum analysis suggested that the exchange of sodium and U（Ⅵ）between layers occurs during the adsorption process.（2）For improving the recovery and utilization of titanate nanomaterials,magnetic titanate（Fe₃O₄@TNS）with different morphology by sol-gel method and hydrothermal method with magnetic Fe₃O₄ nanoparticles（NPs）as the center templates was prepared for the adsorption and removal of radioactive U（Ⅵ）in aqueous solutions.The adsorption capacity was greatly affected by the pH value and unaffected by the ionic strength,indicating that the adsorption mechanism of U（Ⅵ）on Fe₃O₄@TNS was mainly ion exchange and outer-sphere surface complexation.The sorption of U（Ⅵ）onto core-shell and yolk-shell Fe₃O₄@TNS increased in the order of Ca²⁺<Mg²⁺-Na⁺<K⁺and Mg²⁺<Ca²⁺<Na⁺<K⁺for positive ions and deceased in the order of PO4^3->Cl^->CO₃^2->SO₄^2->NO^3-≈ClO₄^-andPO₄^3->SO₄^2->Cl^->CO₃^2->NO^3-≈ClO₄^-for negative ions,respectively.The maximum sorption capacities（Qmax）calculated from the Langmuir model were 68.6 for core-shell Fe₃O₄@TNS and 82.9 mg g^-1 for yolk-shell Fe₃O₄@TNS,at 298 K,313 K and 328 K,respectively.The yolk-shell Fe₃O₄@TNS had more extraordinary sorption efficiency than core-shell Fe₃O₄@TNS since the yolk-shell structure provided internal void space inside the titanate shell to accommodate more exchangeable active sites.（3）Hollow titanate nanoparticles（H-TNNPs）with higher purity and crystallinity can be synthesized by using spherical SiO₂ as template.The investigation focus on the competive adsorption performance in the unitary system of U（Ⅵ）,binary system of U（Ⅵ）-Ni（II）and U（Ⅵ）-Sr（II）and ternary system of U-Ni（II）-Sr（II）.The adsorption was well simulated by pseudo-second-order kinetic model,and can achieve maximum adsorption within 30 min even under the competition adsorption system.The maximum adsorption capacity were calculated from the Langmuir isotherm mode to be 243.3 mg g^-1,112.6 mg g-I,114.3 mg g^-1 and138.6 mg g^-1 for the system of U（Ⅵ）,U（Ⅵ）-Ni（Ⅱ）,U（Ⅵ）-Sr（Ⅱ）and U-Ni（Ⅱ）-Sr（Ⅱ）.The results showed that the smaller ion radius of Sr（Ⅱ）and Ni（Ⅱ）in competitive system was preferentially exchange with Na⁺ in the interlayer,but different affinity and selectivity between ions and materials in ternary system slightly decreased the influence of coexisted ions.