Synthesis Of TiO₂/Natural Clay Minerals Nanocomposites And Their Adsorptive Properties

In recent years,the problem of water pollution has become very serious.Among them,the fluorine pollution and the discharge of dye wastewater are two prominent problems.Adsorption is an ideal method to treat sewage at present,which mainly depends on adsorbents with good performance and low cost.Natural clay minerals are believed to be cheap adsorbents for water treatment.However,they are generally small in specific surface area and low in activity,resulting in poor adsorption performance.The adsorption performance of natural clay minerals can be improved by modification.However,the commonly used modification methods have some shortcomings,such as high cost,complex experimental operation and high energy consumption.Therefore,the study of new and simple modification methods of natural clay minerals is very important to reduce the cost of adsorbents and improve their adsorption performance.In this study,natural coal-series kaolin was modified by TiO₂ nanoparticles surface modification method for the first time,which was accomplished by hydrothermal synthesis of TiO₂/natural coal-series kaolin nanocomposites.For this,the effects of hydrothermal temperature and hydrothermal time on the modification effect were studied.On this basis,ordinary natural kaolin and natural montmorillonite were also modified by the same method.The clay minerals before and after modification were used as adsorbents to study their adsorption properties for fluoride ion wastewater and congo red.The results are summarized as follows:TiO₂/natural coal-series kaolin nanocomposites were prepared by hydrothermal method using natural coal-series kaolin as raw material and titanium sulfate as titanium source.The nanocomposites were characterized by X-ray diffraction?XRD?,N₂ adsorption-desorption analysis,transmission electron microscopy?TEM?and inductively coupled plasma emission spectroscopy?ICP?.The results showed that the prepared nanocomposite was composed of natural coal-series kaolin and anatase-type TiO₂ nanoparticles loaded on its surface.The optimum condition was found at hydrothermal temperature=120?,hydrothermal reaction time=24 h,and natural coal-series kaolin/TiO₂=5/5?mass?.The performance of the nanocomposite was found to be the best at this condition,and its specific surface area reached118.0 m²/g,which is 7.3 times larger than that of original natural coal-series kaolin.The dissolution rate of Al³⁺ions reached 47.7%.Ordinary natural kaolin and natural montmorillonite were modified in the same way as mentioned above,and the nanocomposites of TiO₂/natural kaolin and TiO₂/natural montmorillonite were prepared under the above optimum conditions.The nanocomposites were characterized by various characterization methods.The results showed that the prepared nanocomposites were composed of natural kaolin,natural montmorillonite and anatase-type TiO₂ nanoparticles loaded on their surfaces.When the content of TiO₂ was 50%,the specific surface areas of TiO₂/natural kaolin nanocomposite and TiO₂/natural montmorillonite nanocomposite reached 88.1and 239.8 m²/g,respectively,which are 8.5 and 2.9 times larger than that of unmodified natural kaolin and natural montmorillonite.For TiO₂/natural kaolin nanocomposite and TiO₂/natural montmorillonite nanocomposite,the dissolution rates of Al³⁺ions were 54.2 and 55.0%,respectively.The above mentioned three kinds of natural clay minerals,their nanocomposites and pure TiO₂ were used as adsorbents and their adsorptivities for fluoride ion were investigated.The effects of adsorbent dosage,solution pH,adsorption time,initial concentration of solution and temperature on adsorption were studied in detail.When the concentration of fluoride ion solution was 10 mg/L,the volume of solution was 20 mL,the dosage of adsorbent was 100 mg,the adsorption time was 60 min,the solution pH=7,and the content of TiO₂ in nanocomposites was 50%,the adsorption rates of TiO₂/natural coal-series kaolin nanocomposites,TiO₂/natural kaolin nanocomposites and TiO₂/natural montmorillonite nanocomposites for fluoride ion wastewater reached the highest value of 86.7,89.2 and 89.7%,respectively,which are 71.4,79.4 and 89.7%higher than the corresponding unmodified natural clays.The adsorption of fluoride ion was well described by the quasi-second-order kinetic model and Langmuir adsorption isotherm model,indicating the monolayer adsorption.Moreover,the synthesized three kinds of nanocomposites exhibited excellent adsorption capacities for fluoride in groundwater?5.8 mg/L?and they could decrease fluoride concentration below national drinking water standard?<1.0 mg/L?.The above mentioned adsorbents were also used for the adsorption of Congo red dye and the effects of adsorbent dosage,solution pH,adsorption time and initial concentration of solution on adsorption were investigated.The results showed that the maximum adsorption of capacities of TiO₂/natural coal-series kaolin nanocomposites and TiO₂/natural kaolin nanocomposites for Congo red were 280.1 and 295.0 mg/g,respectively,which were much higher than that of natural coal-series kaolin and ordinary natural kaolin?34.2and 16.7 mg/g,respectively?.The adsorption of Congo red accorded with quasi-second-order kinetics model and Langmuir adsorption,which belongs to monolayer adsorption.On the other hand,the maximum adsorption capacity of TiO₂/natural montmorillonite nanocomposite for Congo red dye was 301.0 mg/g,which is similar to that of unmodified natural montmorillonite?304.0 mg/g?.The adsorption of Congo red on the nanocomposite fitted with quasi-second-order kinetics model and Freundlich adsorption isotherm model,indicating that the adsorption belongs to multi-molecular layer adsorption.