Preparation Of Coupled Photocatalyst And Its Application In Treatment Of Dye Wastewater

Nano-semiconductor photocatalytic technique has received considerable attentionfor treatment of environment pollutants. As an important photocatalyst, nano zincoxide (ZnO) was one of several semiconductors which could realize quantum sizeeffect and sometimes has better photocatalytic activities than TiO₂. In recent years, ithave attracted comprehensive attentions and showed a good prospect of applicationfor photocatalytic degradation of pollutants. But the photocatalytic efficiency ofsingle photocatalyst is relatively low, so it is of great importance to exploit novelphotocatalysts. At present, there are fewer studies on modification of nano ZnO thanTiO₂ so that how to improve its activities still need to be researched. And thephotocatalytic degradation mechanism still needs further study and perfection.The aim of this paper is to improve the photocatalytic activities of ZnO and prepareZnO-based composite catalysts coupled with ZrO₂ with excellent property by asimple and cheap synthesis route. The novel photocatalyst not only enlarge the typeof coupled oxide conductors with ZnO, but also perfect the photocatalyticmechanisms by kinetics of adsorption and degradation, and analysis of the reactionpathway. Moreover, it is of practical significance to realize its large-scale applicationin treatment of organic pollutants.The composite photocatalyst ZrO₂/ZnO was prepared by co-precipitation andthermal decomposition method, was characterized by X-ray diffraction (XRD),scanning electron microscopy (SEM), transmission electron microscopy (TEM),differential thermal analysis-thermogravimetry (TG-DTA) and Fourier TransformInfrared (FT-IR), and the effect of zinc source, calcinations temperature, calcinationstime and coupling ratio of Zr on the phase and catalytic property of ZrO₂/ZnO wasinvestigated. The results showed that owing to coupling with ZrO₂, the averagecrystallite size of 2.5ï¼…ZrO₂/ZnO was decreased, and its photocatalytic efficiency onacid red B was increased 11ï¼…, compared to pure ZnO, and slightly better thanDegussa P25 TiO₂, which showed photocatalytic activities of ZnO was improved.The influence of operational variables, such as initial dye concentration, catalyst dosage, pH, solution temperature, hydrogen peroxide concentration, light intensityand ethanol on UV photodegradation of acid red B was investigated. As thephotocatalytic reaction is not sensitive to solution temperature, and the decrease ofsolution temperature has limited impact on the reaction. Other factors have asignificant impact on the reaction. There is an optimal concentration of catalyst andhydrogen peroxide, and over it the reaction efficiency will be reduced. The effect ofpH on the reaction efficiency was affected by the change of electric adsorptionspecies on the catalyst surface and the worst effect was observed at isoelectric pointof 2.5ï¼…ZrO₂/ZnO. Except Ag⁺, Fe³⁺, Fe²⁺ and Mn²⁺ can improve the adsorption ofAcid Red B on the catalyst surface. Ag⁺ and Fe³⁺ favored the photocatalyticdegradation of acid red B, which was caused by the reduction of electron/holerecombination, the negative effect of Fe²⁺ and Mn²⁺ is enhanced with the increase oftheir concentration. NO₃^- has little effect on the reaction rate and adsorption of acidred B on catalyst surface. The inhibiting effect of SO₄^2-å’ŒCl^- was caused by theircompetitive adsorption and capturing hydroxyl radical. IO₄^- can produce·OH topromote the reaction under UV. Under sunlight, 2.5ï¼…ZrO₂/ZnO still possesses higherphotocatalytic property.The effect of catalyst dosage, initial dye concentration and solution pH on theequilibrium adsorption amount of acid red B on catalyst in the adsorption isothermexperiment was investigated. The Langmuir and Freundlich isotherm adsorptionmodel was used to simulate the adsorption kinetics of acid red B on pure ZnO and2.5ï¼…ZrO₂/ZnO. Then their model parameters and free adsorption energyâ–³_adG°were calculated and compared to study the differences of their adsorption properties.The results showed that 2.5ï¼…ZrO₂/ZnO not only have excellent adsorption property,but also was increased compared to pure ZnO.The photocatalytic degradation of acid red B follows L-H kinetic model. Themodel was modified for considering the competitive adsorption of intermediates. Thekinetic parameters before and after modification, the apparent and simulated half-lifevalues were calculated. It was showed 2.5ï¼…ZrO₂/ZnO has better photocatalyticproperty than pure ZnO and the intermediates have the inhibiting action.The decolorization and degradation of acid red B was analyzed according to the TOC decrease, pH variation, inorganic ion production of the dye solutions, such asSO₄^2-, NO₃^- and NH₄⁺, and change of UV-Vis spectra. With the reaction proceeded,decolorization rate was increased, but TOC removal was slower. The transformationof -SO₃^- and -N=N- in the degradation of acid red B was inferred from differencesbetween the theoretical value and the detected value of total S and total N. The -SO₃^-release and decolorization was the initial step, with the nitrogen of azo grouptransformed predominantly to N₂.To sum up, according to the experimental results of isothermal adsorption andphotocatalytic degradation of acid red B, if was proven that 2.5ï¼…ZrO₂/ZnOcomposite photocatalyst have higher adsorption and catalytic property and nano ZnOwas improved through coupling with ZrO₂, which achieve the research objective.Studies on the influence factors and degradation mechanism further enriched thephotocatalytic theory and provide the necessary information for the actual wastewatertreatment.