Study On The Adsorption Performance Of Titanium/Cobalt Doped UiO-66 On Dyes In Wastewater

Dye wastewater pollution is one of the sources of water pollution at present.In addition to reducing the burden from the industrial source,efficient and convenient effluent treatment is also the key to the control of dye wastewater.Among many methods to treat dye wastewater,adsorption method is efficient,economical and environmentally friendly.As a widely concerned adsorption material,Metal-organic frameworks（MOFs）have significant advantages in the regulation of pore structure and specific surface area due to the diversity of self-assembled metal clusters and organic ligands.UiO is a three-dimensional porous nanostructure constructed by zirconium and dicarboxylic acid ligands.As a sub-MOFs material with excellent stability but poor dye adsorption capacity,it has been deeply studied in the field of material science in recent years.Based on the structure of UiO-66,metal-doped modified UiO-66 was synthesized to retain the advantages of stability and improve its adsorption performance for dyes.The main research contents and conclusions are as follows:（1）A kind of new Ac-UiO-66 nanoparticle was synthesized by using solvothermal method under the condition of glacial acetic acid modification.Glacial acetic acid and titanium/cobalt bidirectional coordination compete with zirconium and terephthalic acid to form a defect structure,and finally titanium-doped Ac-Ti/UiO-66 and cobalt-doped Ac-Co/UiO-66 were synthesized.（2）The XRD and SEM proved the successful synthesis of the main structure of the material and the reduction of the particle size after metal doping.The N₂ adsorption-desorption curves of the three materials are typical IV curves.The specific surface area of the cobalt-doped material reaches 869.72 m2/g,which is the maximum of the three.XPS analysis showed that the Zr/Co ratio on the surface of cobalt-doped material was about 12:1.The Zr/Ti ratio on the surface of Ti-doped material was about 0.75:1.Cobalt in UIO-66 is in the state of low doping and high dispersion while titanium replaced a large amount of zirconium in the coordination process.（3）The adsorption effects of Ac-UiO-66,Ac-Ti/UiO-66,and Ac-Co/UiO-66 on direct blue were studied.The three reached equilibrium after about 60 minutes of adsorption.The adsorption kinetics all fitted the pseudo-second-order kinetic model,and chemical adsorption was the main rate control step.The adsorption isotherms of the non-doped and cobalt-doped adsorbents fit the Langmuir model,and the adsorption process is monolayer adsorption.The fitting priority of Langmuir and Freundlich models for titanium-doped adsorbents is not obvious.Due to the lattice shrinkage in the system doped with heterogeneous and same group of metal,the uniformity of the distribution of active sites may be poor.From the thermodynamic model,the adsorption behavior of the three can be basically spontaneous.At 298 K,the Langmuir adsorption capacities of non-doped,Ti-doped and Co-doped materials for direct blue were 48.0 mg/g,158.4 mg/g and 378.3mg/g,respectively.（4）Ac-Co/UiO-66 performs better in adsorption capacity and reusability.In order to investigate its adsorption performance for other dyes,the cationic dye methylene blue and the other two anionic dyes Titan yellow and Congo red were selected as adsorption objects.The adsorption kinetics and adsorption isotherm of anionic dyes fit the second-order kinetic model and Langmuir isotherm model respectively.Therefore,it is judged that the adsorption process of anionic dyes is mainly chemisorption,and the adsorbed dye molecules are relatively independent and monolayer adsorption.The adsorption capacity of dyes fluctuated in the acid-neutral environment and decreased rapidly in alkaline environment.The adsorption capacity under the effect of different coexisting salt ions presented the following order:NaCl≈KCl≈NaNO₃<Na₂SO₄<Na HCO₃.It was found that the adsorption between anionic dyes appeared antagonistic effect in the binary dyes system,which reduces the adsorption capacity of each other by competing for adsorption sites.The increase of the concentration ratio in the binary dyes system will cause the competition effect of the dyes to evolve to the independent effect.（5）According to the effect of pH and characterization information,the adsorption mechanism of dye was studied.The study found that the adsorption force is mainly based on hydrogen bonding,supplemented by electrostatic attraction of positive and negative molecules in water environment,π-πstacking interaction and van der Waals force.The adsorption capacity of Ac-Co/UiO-66 remained at a high level after four cycles in the reuse experiment of anionic dyes.In counclusion,co-doped UiO-66 has the potential to remove anionic dye wastewater.