Synergistic Effect Of Photocatalytic And Ozonation On Molasses Alcohol Wastewater Treatment

Molasses alcohol wastewater (MAWW) came form molasses alcoholfermentation industries with high organic load, dark brown color and stronglyacidic (pH 4-5). The brown color resisted heat and light, and did not fade for along time. Meanwhile, the environmental impact of molasses alcoholicwastewater was very high. The pigments were hardly degraded by theconventional treatments. Because O₃ could strengthen photocatalytic oxidation,a combination of O₃ with photocatalysis was able to increase the color removalefficiency. Recently, the technique of photocatalysis combined with O₃ hadbecome wide prospect in treatment of organic wastewater by advancedoxidation processes. However, high activated photocatalysts were vital insynergistic effect ofphotocatalysis and ozonation.In this paper, a series of photocatalysts were synthesized. Thedecolorization rate of MAWW was investigated by a combination of them-basedphotocatalysis and ozonation. It was observed the activity of these catalysts inthis process. Experiment showed the solid-state reaction Bi₂O₃-WO₃ catalyst andthe impregnation Fe₂O₃/SnO₂ catalyst-based photocatalysis and ozonation processes exhibited excellent activity in decolorization to MAWW. Thestructure of these catalysts was characterized by XRD, UV-vis DRS, TG-DSCand IR. According to catalyst activity, the relationship between the structure ofcatalysts and their catalytic properties were also discussed. A series of researchresults were obtained.The Bi₂O₃-WO₃ catalysts synthesized by the solid-state reaction werecarefully studied. The results showed that the solid-state reaction Bi₂O₃-WO₃catalyst had the best catalytic activity with the atomic molar ratio of Bi: W to 1,calcined at 1073K for 3 hours. Decolorization rate of MAWW rapidly reached90.2% under reaction conditions which pH value was about 4.2, volume ofMAWW was 200ml, catalyst amount was 0.25g·L^-1, velocity of gas flow was48L·h^-1, the power of high voltage mercury-vapor lamp was 300 W and reactiontime was 120min. The characterization results showed that both Bi₂WO₆ andBi₂W₂O₉ with active sites could be observed in the solid-state Bi₂O₃-WO₃catalyst, and the concentration of them was in direct proportion to thephotocatalytic activity and was significantly enhanced activity of catalyst.Band-gap energy of Bi₂O₃-WO₃ photocatalysts was 2.92eV, which was far lowerthan that of TiO₂ (Eg=3.2eV). So the solid-state Bi₂O₃-WO₃ photocatalystexhibited excellent catalytic activity in decolorization rate of MAWW due toabsorb(ing) more energy of the visible light. The impregnated Fe₂O₃/SnO₂catalysts were also carefully investigated. The optimal preparation conditions ofthe supported Fe in the impregnation Fe₂O₃/SnO₂ catalyst were as follows: the loading of the Fe³⁺ was 0.08(mol%), the calcination temperature was 773K, andthe calcination time was 3h, and color removal ratio of MAWW was 90.1%under Bi₂O₃-WO₃ photocatalysts reaction conditions. The X-ray diffractionanalysis showed that only SnO₂ could be observed in the supported catalysts;Fe₂O₃ did not observe due to uniformly dispersing on the surface layer of theSnO₂. Transition metal oxide Fe₂O₃ doped SnO₂ could enhance photoactivity ofcatalyst because structural defects in the SnO₂ crystal lattice could attractphotogenerated electrons effectively and inhibit the recombination ofelectron-hole pairs. The IR spectra analysis showed that OH group appearedover the surface of SnO₂ and the IR band positions of Sn and O had red-shift.The DRS spectra analysis showed that the band gap decreased from 3.70eV to3.68eV. So the impregnation Fe₂O₃/SnO₂ photocatalyst exhibited high catalyticactivity in decolorization rate of MAWW due to the higher adsorption, red shiftsto a longer wavelength.The decolorization rates of MAWW were investigated by a combinationof the solid-state Bi₂O₃-WO₃-based photocatalysis and ozonation. Semi-batchexperiments had been performed in order to analyze the influences of catalystsamount, reaction time, ozone dosage, pH and scavenger on color and organicmatter removals. The experimental results showed that the combinedphotocatalyst with UV irradiation and ozonation (catalyst/UV/O₃) processconsiderably increasing decolorization rate compared with photocatalysis inUV irradiation and ozonation (O₃/catalyst) and photocatalytic ozonation (UV/O₃), due to synergistic effect in this process. Direct ozonation waspredominant to indirect hydroxyl radical oxidation reaction during thedegradation process of MAWW. Decolorization rate increased as time passing,this trend had grown steady after 120min. The optimum Catalyst amount was250mg·L^-1, and ozone dosage was 48L·h^-1. The effects of the pH value werevery important, at pH＜5 or pH＞12, the degradation rate of MAWW was fasterin strong alkaline than in strong acidity at initial stage. But decolorization rateof MAWW under these conditions rapidly reached more than 90% after120min.