Investigation On The Design,Preparation And Mechanism Of Mesoporous Zirconyl Fibers As Highly Efficient Phosphate Adsorbents

Fibers with various structures and components could be prepared by sol-gel method and electrospinning technology,regulated through preparation of precursor spinning sol,selection of electrostatic spinning equipment and parameters,and design of heat-treatment process.Mesoporous zirconyl fibers inheriting the advantages of mesoporous fibers such as large lengthdiameter ratio,good flexibility,self-supporting,large specific surface area,and combined with the strong affinity to phosphate ions of zirconyl materials,have great application potential in the fields of phosphorus adsorption in wastewater treatment.In this paper,the preparation,characterization and phosphate adsorption performance of mesoporous ZrO2 fibers were studied firstly,and then the properties of adsorbents were improved by regulating the fiber structure,compositing with lanthanum hydroxide and introducing magnetic materials.Hollow mesoporous ZrO2 fibers have been prepared by coaxial electrospinning technology,and mesoporous ZrO2 fibers with high specific surface area have also been prepared by a double surfactants complex system and steam heat-treatment process,and phosphate adsorption performance of these fibers are evaluated.Mesoporous Zr/La bimetallic hydroxides fibers were prepared by electrospinning and high-pressure steam heat-treatment process,which further improved the adsorption performance of fibers.The reaction mechanism was investigated and a new way of preparing bimetallic hydroxide fibers is proposed.Cobalt ferrite(CoFe2O4)fibers with large length-diameter ratio,good flexibility and high saturation magnetization have been prepared.The CoFe2O4 fiber adsorbents were modified with zirconia,zirconium hydroxide or mesoporous Zr/La bimetallic hydroxides,to improve the phosphate adsorption properties.The main contents are as follows:1.Hollow mesoporous ZrO2 fibers were prepared by electrospinning and sol-gel combined technique,and are used as phosphate adsorbents.The external sol of coaxial electrospinning process was prepared by polyacetylacetonatozirconium(PAZ),polyethylene oxide(PEO)and nonionic surfactant P-123,and the internal sol was liquid paraffin.Suitable injection rates and heat-treatment temperatures were selected in the preparation process.The uniform hollow structure could be obtained after heat-treatment at 500℃,when the injection rate of internal and external sol was 0.4 mL/h and 2.0 mL/h,respectively,and the specific surface area of fibers was 68.06 m2/g.The maximum adsorption capacity of phosphate ions by fibers was 77.32 mg/g.The phosphate adsorption process of fibers could reach equilibrium quickly,which was because that the hollow mesoporous structure enables the rapid transport and diffusion of phosphate ions on the fiber surface,and the large surface area provides abundant active sites.Preparation and phosphate adsorption studies of mesoporous ZrO2 fibers with high specific surface area.Double surfactants complex system and steam heat-treatment are used in the preparation process.Appropriate proportion of anionic surfactant ammonium dodecyl sulfate(ALS)and non-ionic surfactant P-123 were added into the zirconia precursor spinning sol.Mesoporous ZrO2 fibers with high specific surface area and highly concentrated pore size have been obtained after electrospinning and steam heat-treatment.When the molar ratio of surfactants n(ALS):n(P-123)was 4.65,and the pH value of spinning sol was 2.44,the specific surface area could reach 293.07 m2/g after steam heat-treatment at 300℃.The addition ALS could enhance the hydration degree of P-123 micelle,and then the micelle chain could be fully extended,so that the material would gain more uniform pore structure and larger specific surface area.The steam atmosphere heat-treatment process have made the organic groups inside fibers be removed by a softer way at a lower temperature,reduced the crystallization temperature of zirconia and protected the mesoporous structure.Mesoporous ZrO2 fibers has excellent adsorption performance of phosphate ions,with a maximum adsorption capacity of 222.79 mg/g.The main mechanisms of adsorption include hydrogen bond electrostatic attraction ion exchange and ligand exchange.Based on the first-principles calculation of density functional theory,the models of phosphate adsorption on t-ZrO2(111)were simulated.The adsorption models showed that the complexations consisted of monodentate and bidentate complexation forms,composed of hydrogen bond P-O-H…O-Zr and chemical bond P-O-Zr,and during the adsorption process,electron shuttle occurred between Zr atom and O atom of phosphate ions.2.Mesoporous Zr/La bimetallic hydroxides fibers were prepared by electrospinning and high-pressure steam heat-treatment.The heat-treatment process was conducted in a hydrothermal reactor.The effects of Zr/La mole ratio,heat-treatment temperature,holding time and steam pressure on pore structure of fibers were investigated.Under proper Zr/La mole ratio and reasonable heat-treatment parameters,the maximum specific surface area of the fibers was 233.31 m2/g.Some PAZ macromolecules were hydrolyzed to polyaceticzirconium(PEZ)and acetone molecules in high-pressure steam heat-treatment process,and then would be hydrolyzed to zirconium hydroxide and acetic acid.The other part of PAZ macromolecules could be directly hydrolyzed into zirconium hydroxide and acetyl acetone.This method of removing organic groups through hydrolysis was milder than that of combustion,and the water vapor molecules filled in the holes could provide some support to the pore structure,reduce the heat-treatment temperature and protect pore structure,and then increase the specific surface area of fibers.When the Zr/La molar ratio was 0.5:0.5,the fibers could gain maximum adsorption capacity as 453.63 mg/g.The adsorption process involved electron transfer between Zr/La and O atoms,and the lanthanum ions on the fiber surface were the main active center in phosphate adsorption process.In Zr/La hydroxide,the electronegativity difference caused the highest occupied molecular orbital electrons of La transfer to the lower energy orbital of Zr through O atoms.This caused the increasement of charges density on Zr surface,and reduced the energy level of the highest antibonding molecular orbital of La-O,and increased exposure to the positive charge of the lanthanum ions center,and then promoted the electron transfer between Zr/La and P-O.3.In order to achieve the magnetic separation of adsorbents,cobalt ferrite fibers,zirconyl composite cobalt ferrite fibers and mesoporous Zr/La hydroxide supported cobalt ferrite fiber were prepared.Three kinds of spinning sols were prepared to use in electrospinning,and cobalt ferrite fibers with good flexibility and large aspect ratio were obtained,whose saturation magnetization was 47.77 emu/g.To improve the adsorption performance of magnetic fibers,zirconyl composite cobalt ferrite fibers,i.e.the composite with zirconium oxide or zirconium hydroxide loaded on cobalt ferrite fibers,were prepared by sol-gel impregnation method,precipitation method and dopamine-assisted liquid deposition method respectively.Zirconia loaded cobalt ferrite fibers prepared by sol-gel impregnation method have the best adsorption cycling performance,which is because that the sintering process provide a strong bond between support materials and the base materials,and it is not easy to fall off in the cycle experiments.Mesoporous Zr/La hydroxide supported cobalt ferrite fiber adsorbents are prepared by sol-gel impregnation method and high-pressure steam heat-treatment technology,and the phosphate adsorption performance was systematically characterized.The specific surface area of fibers was 139.40 m2/g,the saturation magnetization was 17.78 emu/g,and the maximum adsorption capacity was 115.27 mg/g for phosphate ions.The mass transfer kinetics analysis of the adsorption process has showed that the pH value of the adsorption solution may affect the structure of the Stern double layer on the fiber surface,and then influence the mass transfer rate of phosphate ions,but the mesoporous structure of fibers increased the number of adsorption active sites,and thus weakened the effects of pH value on the mass transfer rate.