Study On Visible Light Catalytic Oxidation And Photo-Fenton Oxidation For The Degradation Of Organic Compounds In Water

The acceleration of industrialization has provided human beings with abundant materials,but it has also brought about some problems such as environmental pollution and ecological damage that should not be ignored.The discharge of industrial sewage causes a large amount of organics enter waters,resulting in increasingly serious water pollution.However,the traditional wastewater treatment technologies can not fully meet the needs,and thus the search for efficient and environmentally friendly wastewater treatment methods has become an important research hotspot in the environmental field.In recent years,photocatalytic oxidation and photo-Fenton oxidation have attracted an extensive attention due to their green properties and mild reaction conditions.For photocatalytic technology,photocatalyst with high catalytic performance is the key for the degradation of organics.Therefore,in this study:（i）a novel ternary photocatalyst Bi₂WO₆/BiOI/Ag was prepared,and its performance of degrading organic pollutants was investigated;（ii）a novel Na B-CN photocatalyst was prepared and a two-step oxidation system for organics degradation was constructed by combining photocatalytic oxidation and Fenton oxidation.Based on the characterization and photoelectric performance analysis of photocatalyst materials,besides the investigation of organics degradation performance,the related photocatalytic mechanisms were studied in depth.The main research contents and results are as follows:（1）A new Bi₂WO₆/BiOI/Ag heterojunction composite was prepared by depositing Ag nanoparticles on Bi₂WO₆/BiOI by a two-step hydrothermal method.The prepared photocatalysts were characterized and analyzed by various techniques including XRD、FTIR、XPS、SEM、HRTEM、DRS、EIS and PLS.The results show that the heterojunction structure and the effect of surface plasmon resonance（SPR）significantly enhance the visible light absorption and promote the separation and migration of photogenerated electron/hole（e^–/h⁺）pairs,enhancing the photocatalytic performance evidently.（2）Among the various ternary Bi₂WO₆/BiOI/Ag photocatalysts prepared in this work,Bi₂WO₆/BiOI/Ag-8 exhibited the highest photocatalytic activity.Under the conditions of catalyst dosage of 1.0 g L^–1 and optimum p H=5.0 or 7.0,the photocatalytic degradation of methyl orange and tetracycline(20 mg L^–1)presented the removal efficiencies of 94.3%and 96.2%,respectively in 120 min.The experimental results of tetracycline degradation under sunlight and cyclic degradation indicated that the prepared Bi₂WO₆/BiOI/Ag-8 also possessedhigh photocatalytic activity and stability under real sunlight.In addition,the comparative determination of the cumulative oxygen consumption of activated sludge in the tetracycline solution before and after photocatalytic degradation was carried out,and the obtained results showed that photocatalytic degradation reduced the toxicity of tetracycline solution and significantly improved its biodegradability.（3）The photocatalytic reaction mechanism was explored via the radical trapping experiments,and the results showed that?OH and?O₂^–were the main active species for the degradation of tetracycline.The organic intermediates generated during the degradation of tetracycline by Bi₂WO₆/BiOI/Ag photocatalyst were analyzed by LC-MS and GC-MS.The results show that the degradfation of tetracycline can form some organic intermediates through demethylation,hydroxylation,deamidation/hydroxyl and carbon chain cleavage.The further degradation will generated some organic alcohols and acids,and finally these small organic molecules can be degraded into CO₂and H₂O.（4）A novel sodium and boron co-doped graphitic carbon nitride（Na B-CN）was prepared by thermal polymerization.The crystal structure,surface morphology,elemental composition and photoelectric properties of the photocatalyst were investigated by various characterization techniques.It was found that Na and B co-doping significantly enhanced the light absorption properties,electron/hole（e^–/h⁺）pair separation and mobility of graphitic carbon nitride.（5）The performance of Na B-CN photocatalyst for reducing oxygen to H₂O₂ was investigated with ethanol as an electron donor.The results indicated that compared with un-doped carbon nitride（g-C₃N₄）,after Na and B co-doping,H₂O₂ generation performance was significantly improved.In the ethanol aqueous solution of 1.0 vol%,the photocatalytic reaction of 120 min with the optimized Na B-CN presented the H₂O₂yield of 22.3 mg L^–1 h^–1,which was about 15 times that of un-doped g-C₃N₄.（6）Na B-CN can not only effectively generate H₂O₂,but also exhibit high photocatalytic performance for tetracycline and 2,4-DCP degradation.Based on the results,the photocatalytic oxidation combined with Fenton oxidation process was developed in this study.The combined process presented efficient 2,4-DCP degradation without the addition of ethanol（electron donor）.After the photocatalytic degradation of 2,4-DCP solution(100 mg L^–1)for 240 min,the introduction of Fe²⁺(11.3 mg L^–1)led to the further Fenton oxidation degradation for 30 min,and finally the total degradation removal efficiency of 2,4-DCP reached 90.6%.（7）The mechanism of Na B-CN for the photocatalytic generation of free radicals and H₂O₂ was investigated by radical trapping experiments and electron spin resonance method.It was found that h⁺and?O₂^–were the two important factors for the degradation of 2,4-DCP by Na B-CN photocatalyst.Active species,especially?O₂^–played a most important role in the degradation.In addition,according to the analysis results,it is clarified that the Na B-CN photocatalyst can produce H₂O₂ through a two-electron oxygen reduction pathway:O₂ is reduced to?O₂^–,and?O₂^–is further reduced to H₂O₂.The organic intermediates formed during the degradation of 2,4-DCP were detected by LC-MS and GC-MS,and based on the identified organic intermediates,the degradation pathway of 2,4-DCP was proposed.The destruction of 2,4-DCP molecules may undergone dechlorination,hydroxylation,dehydroxylation,and central carbon cleavage,and finally organics can be degraded to CO₂ and H₂O.In addition,it was found that2,4-DCP and some degradation intermediates can act as electron donors to promote H₂O₂ generation during the photocatalytic degradation of 2,4-DCP.