Study On The Preparation And The Photocatalytic Properties In Degrading Organic Pollutant Of BiOX (X=Cl, Br, I)

Photocatalysis represented by TiO₂is an effective way to remove toxic organiccontaminants which are degraded difficultly in natural environment. However, TiO₂haslow utilization of solar energy and low quantum yield which limit its practical applicationrange. Therefore it becomes a research hotspot to find effective ways to overcome theabove drawbacks. In addition to traditional doping, composite and modified, looking for anew type of photocatalyst, which can be excitated in sunlight or even in visible light, willsave cost and show good potential. BiOX（X=Cl, Br, I） is a ternary oxide semiconductorwhich has open-ended structure and the indirect transition mode so as to separate the hole-electron pair and transfer charges effectively, thus it shows good photocatalytic activity.In this paper, the connection between the preparation of BiOX（X=Cl, Br, I） and itsphysical and chemical properties will be studied systematacially. Their physicalmicrostructure are characterized by X-ray diffraction instrument （XRD）, transmissionelectron microscopy （TEM）, scanning electron microscopy （SEM）, surface area and poresize analyzer（BET） and UV-visible reflection absorption spectroscopy （UV-Vis）. Theadsorption characteristics of the three catalysts, including adsorption model and adsorptionkinetics, are discussed. Besides, the photocatalytic activity of BiOX（X=Cl, Br, I） iscompared, and their photocatalytic reaction mechanisms are explored, especially thecatalytic reaction of microcystins（MC-LR）, which is important for water pollutiontreatment and human health.1The sheet BiOX（X=Cl, Br, I） was prepared by introducing Hexadecyl sulfatepyridine which used as a template in the hydrothermal system. The adsorptive properties ofproducts were discussed by degrading Rhodamine-B. The results showed that theadsorption model of BiOX（X=Cl, Br, I） was Langumir. And the adsorption kinetics modelof BiOCl and BiOBr were second-order kinetics model, while the adsorption kineticsmodel of BiOI was intraparticle diffusion model. BiOX（X=Cl, Br, I） was also used ascatalyst to degrade the dye Rhodamine B（RhB）, the small molecule4-chlorophenol（4-Dichlorophenol,4-CP） and microcystin MC-LR. Three catalysts could rapidly degradedye under visible light, and BiOBr had the highest degradation efficiency of RhB.Nevertheless, only BiOBr and BiOI could degrade4-CP in visible light, and the formershowed higher catalytic efficiency. As for MC-LR, it was degraded quickly by BiOX under ultraviolet light and the order of reaction rate BiOBr BiOCl> BiOI; while under visiblelight, only BiOBr and BiOI degraded MC-LR and BiOBr had higher degradation rate.BiOCl had no absorption in the visible region, so it didn’t degrade MC-LR.2BiOCl with different morphology were synthetized by using different chlorinesources in hydrothermal method, and their photocatalytic activity were compared viadegrading SRB. The best chlorine source of preparing and the optimal reaction conditionof degrading SRB were screened out. The active species in the catalytic degradationprocess were determined. The results indicated that cetyl trimethyl ammonium chloride（CTAC） is the preferred chlorine source when BiOCl was prepared and the product had thehighest catalytic activity. The optimal reaction condition of degrading SRB was that pH=2and the dosage of catalyst was1.0g/L. According to the active species in the oxidationprocess, the mechanism of BiOCl degrading dye in visible light was photosensitizationmechanism.3Cetylpyridinium bromide （CPB） and NaBr were employed as bromine source toreact with Bi（NO3）3·5H2O, and two BiOBr were prepared. CPB not only can be used asbromine source, but also be used as a template to form a micro-reactor with special sizeand morphology in the preparation process which could control the particle size andmorphology of BiOBr. The catalytic activities of the two catalysts under visible light werecompared by degrading SRB and small molecule contaminants salicylic acid （SA）. Theresults showed that BiOBr synthesized by CPB had a higher catalytic activity under visiblelight than BiOBr synthesized by NaBr.4Hexadecyl sulfate pyridine was introduced in the hydrothermal system to synthesizenanosheet BiOI. Under visible light irradiation（>420nm）, the degradation of RhB and4-CP by BiOI were studied. The oxide species, infrared spectroscopy （IR） and the changeof the total organic carbon （TOC） in photocatalytic degradation process were determined.The results indicated the signal strength of O₂.-and.OH were weak. In addition, isopropylalcohol （isopropanol）, benzoquinone （BQ）, and KI were used as trapping agents of OH,O₂.-and h+respectively to infer the reaction mechanism of BiOI. The results demonstratedthat the photocatalytic reaction of BiOI was not through O₂.-and.OH oxidation, but thedirect oxidation of hole.5MC-LR was degraded by BiOX（X=Cl, Br, I） in the same condition under UVillumination. The concentration of MC-LR was detected by high performance liquid chromatography（HPLC）, and the initial intermediates of the oxidation reaction wereanalyzed by LC-MS, which in favour of inferring the initial oxidation reaction route ofBiOX degrading MC-LR. H218O isotope labeling experiments and oxygen consumptionexperiments were used to analyze the conversion and source of oxygen in the oxidationreaction, and compare the differences of their degradation mechanism, so the operatingparameters of degrading MC-LR were filter out. The LC-MS data showed that themechanism of BiOX degrading MC-LR under UV lightation was similar with that of TiO₂.The toxicity of MC-LR was reduced wia the attack of active sites（the double bond andbenzene ring in Adda amino acids） by OH. Isotope experiments and oxygen consumptionexperiments proved fully that the mechanisms of degrading MC-LR by BiOCl, BiOBr andBiOI were different, so did the source and conversion of oxygen in produc, which could beattributed to the different valence band structure of BiOX（X=Cl, Br, I）.