Enhancement Of The Photocatalytic Activity Of TiO₂-supported Clinoptilolite For Removal Of Crystal Violet Or Methyl Orange Dyes From Aqueous Solutions

Water pollution is one of the major environmental concerns in this modern era.Effluents emerging from industrial,agricultural and domestic activities carry a variety of chemicals that are discharged into lakes,rivers and oceans,thereby affecting both aquatic and terrestrial life.Dyes are an important class of pollutants largely used in industries and their impact on the environment is a major concern due to the carcinogenic properties of these chemicals.Therefore,water containing dyes must be treated before it is being discharged into the aquatic life.A number of methods have been used for the wastewater treatment containing dyes,involving a variety of different environment friendly materials.Advanced Oxidation Technologies(AOTs)are the most priority technologies for treatment of wastewater that involves the use of highly reactive radicals e.g.^·OH for the degradation of waste organic matter.Heterogeneous photocatalysis is the most priority AOTs for treatment of wastewater containing toxic organic contaminants which involves the use of a semi-conductors photo-catalyst(TiO₂,ZnO,CuO)for the photo-catalytic degradation of organic wastes and leads to almost complete mineralization of hazardous organic matter.In a typical heterogeneous photocatalytic reaction,a semiconductor photocatalyst is activated by absorption of photon energy equal to or greater than the band gap of the semiconductor,generating electron-hole pairs,which causes oxidation of the organic compounds adsorbed at the surface of the photocatalyst.TiO₂ based heterogeneous photocatalysis has attracted worldwide attention as a potential efficient,low cost and environmentally benign AOTs for the treatment of wastewater and generation of renewable energy.However,some challenges relating to achieve high photocatalytic reaction efficiencies have been found due to the fast electron-hole recombinatin rate,complicated treatment of catalyst recovery step and electron-holes transport mechanism of TiO₂ which restrain the large scale practical application of the process.Development of heterogeneous photocatalysts via loading TiO₂ onto the surface of porous adsorbents as a supporting matrix could be a possible solution to overcome these limitations of bare titania.A variety of supporting matrixes e.g.zeolites,silica,graphite etc.,are used for immobilization of TiO₂ in order to enhance its photo-catalytic properties.Zeolites have been found to be the most frequently used materials in this regard thereby exploiting the adsorptive,ion exchanging and catalytic properties of these materials.Zeolites are crystalline three dimensional micro-porous hydrated aluminosilicates of alkali and alkaline earth metals.They have widely been used as catalysts,adsorbents,ion exchangers and have a number of beneficial properties like non-toxicity,exchangeable cations,high surface area,thermal and physical stability and good cost-benefit ratios that make them to be widely used.Although most of the zeolites occur abundantly in nature and are inexpensive,but because of the impurities and structural defects in natural zeolites,their applications becomes limited.So,in a recent few decades a number of attempts have been made to synthesize pure zeolites in laboratory and industrial scale that are proved to be more efficient than the natural ones.In this work,clinoptilolite(CP)type of zeolite was used as supporting matrix for TiO₂.Clinoptilolite is the most abundantly occurring zeolite in nature which belongs to the heulandites family with Si/Al?4.Clinoptilolite was also synthesized in the laboratory so as to use both natural and synthetic forms of the zeolite as support for TiO₂ with an aim to improve dispersion,reduce coagulation of TiO₂,suppress electron-hole recombination rate and enhance adsorption of organic molecules and consequently to increase the photo-catalytic efficiency.To draw new prospects in this domain,TiO₂-supported clinoptilolite(TiO₂/clinoptilolite)were successfully prepared via hydrothermal,in-situ hydrothermal and sol-gel methods.In-situ hydrothermal method is a newly developed method in our study in which TiO₂/CP composites are prepared via in-situ hydrothermal crystallization of clinoptilolite crystals in TiO₂ dispersion medium.The effect of various parameters including temperature,acidity,concentration of titania precursor and concentration of Fluoride ions(F^-)added during hydrothermal process,on the particle size,crystal phase,crystallinity and agglomeration of TiO₂ supported onto clinoptilolite were studied.It was found that increasing temperature and concentration of F^-ions,and strengthening acidity caused a net decrease in surface area and an increase in particle size and enhance crystallinity degree of loaded TiO₂ during hydrothermal process.Increase of acidity and concentartion of F-ions was found to cause more uniform distribution of TiO₂ on the surface of clinoptilolite.The TiO₂ loaded onto clinoptilolite exhibits rutile or anatase phase depending on the acidity or p H of the media.The influence of physico-chemical properties i.e.crystal phase,and particle size,surface areas,surface charge potential of the synthesized TiO₂/CP,on its photocatalytic efficiency was investigated.The structure,texture,morphology and composition of TiO₂/clinoptilolite were thoroughly characterized via X-ray diffraction(XRD),scanning electron microscope(SEM),high resolution transmission electron microscope(HRTEM),Fourier transform infrared(FTIR)spectroscopy,UV-visible spectrophotometer and BET-isotherm.The results showed that in-situ hydrothermal method leads to well dispersions of loaded-TiO₂ particles on the surface of leaf-like CP,while obvious aggregated TiO₂ on a relatively distorted structure of CP is obtained using sol-gel and hydrothermal methods.The TiO₂/CP obtained via in-situ hydrothermal method revealed highly negatively charged surface with low pH_PZC values of 2.3 like the parent clinoptilolite support while TiO₂/CP synthesized via sol-gel and hydrothermal method revealed a relatively high pH_pzc value of 4.1 and 5.9 respectively.The TiO₂/CP photocatalysts were used for the photo-catalytic degradation of crystal violet(CV)and methyl orange(MO)dyes as the model organic pollutants in wastewater.The results revealed that the photocatalytic activity of TiO₂/CPs is greatly dependent upon the crystal phase and particle size of loaded TiO₂.Anatase phase with relatively large particle size loaded onto CP is found to be more active as compare to the rutile phase.The addition of F^-ions during the hydrothermal process remarkably enhanced the photocatalytic activity of loaded TiO₂.The results also demonstrated that among the three methods used for the synthesis of TiO₂/CPs,the in-situ hydrothermally synthesized TiO₂/CPs were the best due to their moderate energy cost,highest adsorption capacities and removal efficiencies of CV dye.A comparative study of natural and synthetic forms of CP as supports for TiO₂ revealed that natural clinoptilolite exhibits a profound effect on the energy cost of TiO₂/CPs,while synthetic CP is proved to be more effective in terms of removal efficiencies.The effect of operational parameters such as dose of the photocatalyst,initial pH of the solution,and initial concentration of the dye on the photodegradation performance of TiO₂/CPs were studied.