Preparation Of Nano Titania Hollow Microspheres And Nano-titanate Salts As Well As Adsorption Kinetics And Thermodynamics Of Heavy Metal Ions

This article reports the researches on the preparation of environmentally friendly titanium-based micro-nano adsorbents with different structures and their adsorption performance on heavy metal ions in aqueous solution.Firstly,hollow TiO₂microspheres with a mesoporous shell structure were prepared by sacrificial template method and 3-aminopropyltriethoxysilane(APTES)was used to modify the hollow TiO₂ microspheres.Secondly,micron-sized Ca TiO₃ particles with different sizes and morphologies were prepared by hydrothermal method and the effects of organic modifiers(polyethylene glycol,cetyltrimethylammonium bromide,trisodium citrate)on the structure and morphology of Ca TiO₃ particles were investigated.Thirdly,layer-structured titanate nano whiskers were prepared by hydrothermal method.Finally,potassium hexatitanate nano whiskers were prepared by calcination method.The relationship between the structure of titanium-based micro-nano materials and the preparation conditions has been investigated.At the same time,the adsorption characteristics of this series of titanium-based micro-nano materials for Cd(II),Pb(II),Cu(II),Cr(III)and Cr(VI)has been also investigated,revealing the structure-activity relationship between the structure of titanium-based adsorbent and the adsorption performance.The research results obtained in this article are summarized as follows.Hollow TiO₂ microspheres with a mesoporous shell structure,which was constructed with anatase TiO₂ nanoparticles,were prepared by the sacrificial template method using methyl acrylic acid/styrene latex as the template.The hollow TiO₂microspheres with a mesoporous shell structure were modified with 3-aminopropyl triethoxysilane(APTES)through the Ti-O-Si bonding.According to the Langmuir adsorption isotherms,the maximum adsorption capacities of APTES-modified hollow TiO₂ microspheres for Cu(II),Cd(II)and Pb(II)ions were 12.7,17.5,and 1.8 times those of naked hollow TiO₂ microspheres,respectively.However,when Cr(VI)was adsorbed on the surface of APTES-modified hollow TiO₂ microsphere,the strong oxidation ability of Cr(VI)decayed its adsorption capacity.With the use of Ca Cl₂ and titanium(IV)isopropoxide as raw materials at the hydrothermal temperatures of 120?and 140?,Ca TiO₃ nanoparticles were mainly produced,whereas at 180?,cuboid and micron-sized Ca TiO₃ particles were mainly produced.During the hydrothermal synthesis process,the addition of polyethylene glycol helps to form small-sized Ca TiO₃ nanoparticles;the addition of cetyltrimethylammonium bromide and trisodium citrate can induce the formation of cuboid and micron-sized Ca TiO₃ particles.The adsorption of Cd(II),Pb(II)and Cu(II)ions on micron-sized Ca TiO₃ powder conforms to the pseudo-secondary adsorption kinetic model;Langmuir adsorption isotherm well fits the equilibrium adsorption data of these heavy metal ions;The adsorption process is spontaneous and endothermic.The Ca TiO₃ powders synthesized by the hydrothermal method have higher adsorption capacities for Cd(II)(82.6 mg·g^-1)and Pb(II)(261.8 mg·g^-1)cations than the porous Ca TiO₃ powders synthesized by the solid-state calcination method reported in literatures.Layer-structured titanate nano whiskers were prepared by hydrothermal method starting from industrial metatitanic acid and potassium hydroxide.When the concentration of potassium hydroxide is in the range of 6-12 mol·L^-1,the titanate whiskers were formed by combining nanobelts with a layered structure.The molecular formula of titanate whiskers is H_2-xK_xTi₃O₇·n H₂O(x=0.6,n=3.8-4.0).After being treated with hydrochloric acid,the potassium content and interlayer spacing were reduced due to the replacement of potassium ions by protons.The adsorption of Cu(II),Pb(II)and Cr(III)ions on the titanate whiskers follows a pseudo-second-order adsorption kinetic model.The Langmuir adsorption isotherm fits the adsorption equilibrium data of Cu(II)and Pb(II)ions well.The Freundlich adsorption isotherm fits the adsorption equilibrium data of Cr(III)ions better.According to the Langmuir adsorption isotherms,the maximum adsorption capacities of titanate nanowhiskers for Cu(II),Pb(II)and Cr(III)ions are 143.9,384.6 and190.8 mg·g^-1,respectively.Using metatitanic acid as the source of titanium and potassium carbonate as the source of potassium,potassium hexatitanate nano whiskers with an average diameter of 440 nm and an average length of 6.64?m were prepared by the sintering method(1150?)at a potassium cabonate/titanium mole ratio of 1:3.5.The adsorption of Cu(?),Pb(?)and Cr(?)metal ions on potassium hexatitanate whiskers conforms to the pseudo-second-order adsorption kinetic model.The Langmuir adsorption isotherm fits the equilibrium adsorption data well,indicating the adsorption of such metal ions belongs to monolayer adsorption.The adsorption capacities of Cu(?),Pb(?)and Cr(?)metal ions on potassium hexatitanate are 9.9,19.4 and 8.5 mg·g^-1,respectively.