Preparation And Adsorption Properties Of TiO₂-WO₃ Pillared Attapulgite Clay Composites

In recent years,with the rapid development of modern mining,pigment and alloy industry,heavy metal ions and organic dyes pollution become more and more serious,which disturbs the ecological balance.At the same time,when the concentration of non-biodegradable heavy metal ions is very low,it will also harm to human health.Attapulgite clay is widely used in environmental protection,agriculture,medicine and other fields because of its porous structure,large specific surface area,cation exchange properties and low cost.The natural attapulgite clay has lower adsorption capacity and the adsorption effect is not ideal.In order to improve the adsorption performance of the attapulgite clay,the TiO₂-WO₃ pillared attapulgite composite material was prepared by purification,acid activation and pillaring modification.Taking methylene blue,Ni（Ⅱ）,and Pb（Ⅱ）as target pollutants to investigate the adsorption and desorption performance of composite materials,as well as the influence of various factors on the adsorption performance of composite materials.The mechanism of adsorption was explored by adsorption thermodynamics and kinetics,the results are as follows:Firstly,attapulgite clay was purified and acid activated to dredge its pores and remove impurities such as carbonate.In the process of acid activation,the effects of hydrochloric acid concentration,acid activation time and temperature on the adsorption performance of attapulgite clay were investigated,and orthogonal experiments were designed to determine the optimal acid activation conditions as hydrochloric acid concentration 5 mol/L and acid activation at 70°C for 60 min.A series of TiO₂-WO₃ pillared attapulgite clay composite adsorbents with different Ti/W molar ratios were prepared by sol-gel method using tetrabutyl titanate（TBT）and sodium tungstate as raw materials.The structure and morphology of attapulgite clay,acid activated attapulgite clay and TiO₂-WO₃ pillared attapulgite composites were characterized by scanning electron microscopy,infrared spectroscopy,X-ray diffraction and specific surface area analysis.It was found that the acid activation treatment gave the attapulgite clay a clear pore structure.It was also found that TiO₂-WO₃ pillarer was successfully pillared into the interlayer domain of attapulgite clay.After pillared,the pore structure of attapulgite clay became rich and the specific surface area became larger,and the pillared modification did not change the basic structural framework of attapulgite clay.The effects of adsorption time,adsorption temperature,pH of solution,initial concentration of adsorbate and amount of adsorbent on adsorption of Ni（II）,Pb（II）and Methylene blue of attapulgite clay,acid activated attapulgite clay and TiO₂-WO₃ pillared attapulgite composites were investigated by single factor experiments,it was found that the TiO₂-WO₃ pillared attapulgite clay composite adsorbent showed good adsorption performance for target pollutants,and the adsorption effect was significantly better than that of attapulgite clay and acid-activated attapulgite clay.The adsorption effect of TiO₂-WO₃ pillared attapulgite composites on the target pollutants is greatly affected by the pH value of the system,and the adsorption effect of methylene blue is much better in alkaline condition than in acidic condition.The adsorption of Ni（Ⅱ）and Pb（Ⅱ）should be carried out under acidic conditions,and the higher the pH value is,the better the adsorption is.The adsorption rate of the adsorbent to the target pollutant is proportional to the quantity of the adsorbent,and inversely proportional to the initial concentration of the target pollutant,while the variation trend of the adsorption capacity is opposite to the adsorption rate.Therefore,the adsorption rate and adsorption capacity should be considered in the selection of adsorption conditions.The recycling experiments were carried out to study the regeneration performance of TiO₂-WO₃ pillared attapulgite clay composite adsorbent.The results show that the composite adsorbent can cyclically adsorb methylene blue 5 times,Ni（Ⅱ）and Pb（II）can only cyclically adsorb 3 times.This is because the adsorption of methylene blue on the composite adsorbent is mainly physical adsorption,which force is relatively weak,and it is easy to occur desorption.The adsorption of Pb（Ⅱ）and Ni（Ⅱ）are dominated by chemical adsorption,which is relatively stable,and it is difficult to recycle the adsorbent.The quasi-first-order,quasi-second-order,Weber-Morris internal diffusion kinetic models,and the thermodynamic adsorption isotherms of Henry,Langmuir,Freundlich and Temkin were fitted.The thermodynamic function is analyzed.The results of fitting and analysis show that that the adsorption process of methylene blue,Ni（Ⅱ）,Pb（Ⅱ）on the TiO₂-WO₃ pillared attapulgite composite material conforms to the quasi-second-order kinetic model and the Temkin thermodynamic model.The adsorption rate was controlled by membrane diffusion and intra particle diffusion,and the heat of adsorption decreases linearly with the surface coverage during the adsorption process.The adsorption process of methylene blue on TiO₂-WO₃ pillared attapulgite composite is exothermic and spontaneous.The interaction between TiO₂-WO₃ pillared attapulgite composite and methylene blue is mainly van der Waals force.The adsorption is physical adsorption,and low temperature is conducive to the adsorption.The adsorption process of Ni（Ⅱ）on TiO₂-WO₃ pillared attapulgite composite is an endothermic and spontaneous process,which mainly through chemical precipitation,and accompanied by physical adsorption.The adsorption process of TiO₂-WO₃pillared attapulgite composite to Pb（Ⅱ）is also an endothermic and spontaneous process.Pb（Ⅱ）is removed from the aqueous solution through chemical precipitation,complexation reaction and physical adsorption.