Synthesis And Application Of New Photocatalyst For Wastewater Treatment

In recent years,water pollution has become an urgent and serious problem for human society.Among the water treatment technologies,the photocatalytic oxidation has drawn widely attention for its environment-friendly,high efficient and energy saving.TiO₂ is considered as a promising photocatalyst.However,its large band gap and fast recombination rate of photogenerated charge hinder its application.Therefore, it is necessary to develop new photocatalyst with high activity.The vanadate is a kind of photocatalyst with great potential,it has been reported that BiVO₄,InVO₄,Ag₃VO₄ have a very narrow band gap,enabling the lull utilization of solar to degrade pollutants.In this thesis,Bi(NO₃)₃·5H₂O and NH₄VO₃ were used as raw materials to obtain BiVO₄ precursor by adding ammonia solution.And then,well-crystallized BiVO₄ powders with different morphologies and particle sizes were synthesized via a low temperature molten salt method at different temperatures for 8h in a weight ratio of 1:8 of the precursor to the molten salt;Starting from Y(NO₃)₃·6H₂O and NH₄VO₃,YVO₄ precursor was formed by adding ammonia solution as a co-precipitant.Then, well-crystallized YVO₄ nanoparticles were synthesized via a low temperature molten salt method at different temperatures for 8h in a weight ratio of 1:8 of the precursor to the molten salt.The samples were characterized with X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM),UV-visible absorption spectra(UV-Vis) respectively.Finally,the photocatalytic degradation was carried out by using YVO₄ nanoparticles as photocatalyst and Rhodamine B as dye wastewater.The effects of the particle size,the pH value and the dosage of the YVO₄ photocatalyst on photodegradation were studied.In addition,the YVO₄ photocatalytic mechanism was also proposed,Results were as following:(1) The calcining temperature had a significant impact on morphology,particle size,crystalline phases and optical absorption of BiVO₄ particles.XRD patterns showed that:At 200℃,a pure tetragonal-phase BiVO₄ could be obtained,with the increasing of the calcining temperature from 270℃to 410℃,the tetragonal phase BiVO₄ gradually transformed to the monoclinic phase.SEM images revealed that: tetragonal-phase BiVO₄ particles were bigger than monoclinic-phase BiVO₄;The UV-Vis absorption spectra presented that:compared with tetragonal-phase BiVO₄ and TiO₂,monoclinic-phase BiVO₄ had a broader visible-light spectral response.(2) The molten salt and calcining temperature had great effects on the crystallinity, particle size and optical absorption of YVO₄ nanoparticles.XRD patterns showed that: the addition of the molten salt obviously improved the crystallization of YVO4;TEM images indicated that YVO₄ particles remarkably growed up with the increase of the calcining temperature;Their UV-Vis absorption spectra revealed that YVO₄ nanoparticles with 10nm possess the strongest UV absorption.(3) By adopting the orthogonal experimental method,the effects of the particle size, the pH value and the dosage of the YVO₄ photocatalyst on rhodamine B's photodegradation were studied.The order of the factors which influence the degradation was as follows:the pH value＞the dosage of photocatalyst＞the particle size.(4) For more detail,on the basis of the orthogonal experiment,the experiment of single-factor was carried out to fine the best conditions for the photodegradation of rhodamine B were:pH value for 4,the dosage of photocatalyst for 1g/L,YVO₄ nanoparticle size for 10nm.By using the unitary linear regression method,we could find out the relation of the degradation rates under different conditions.(5) It was predicted that the process of YVO₄ photodegradation was under the photocatalysis and photosensitization's association action.Moreover,the photosensitization played a major role.When the two factors reached a balance,the maximum rate was presented.