| In recent years,with the rapid development of coal gasification industry in China,the treatment of coal gasification wastewater during the production process has become a restrictive factor for the development of gas production.The traditional wastewater treatment process is difficult to effectively remove the refractory organic compounds in coal gasification wastewater,resulting in the effluent of biological treatment is difficult to satisfy the "zero liquid discharge standard" in China.Therefore,the advanced oxidation technologies are applied to treat the effluent of biological treatment coal gasification wastewater,in order to effectively remove the refractory organic compounds and improve the biodegradability of the wastewater.The catalytic ozonation technology is widely applied in the advanced treatment of coal gasification wastewater because of its advantages of high treatment efficiency,low treatment cost and wide application range.Currently,many materials,involving activated carbon,alumina,cerium oxide and so on,are selected as catalysts in practical application.Though the laboratory synthesized metal oxides or carbon materials could effectively enhance the degradation of organic matters,the high production cost and the rigor preparation conditions limited the practical application of these catalysts.Therefore,the lava rock,as a kind of natural material,was used as the catalyst carrier,and a highly efficient and stable catalyst was prepared by using Mn and Ce composite metal oxides to modify the volcanic rocks.After optimizing the preparation of the catalyst,quinoline,a typical refractory organic compound in the secondary biochemical effluent of coal gasification,was selected as the target pollutant.Based on the removal performance for the target pollutant by catalytic ozonation,the catalytic mechanism of catalytic ozonation and the treat performance of actual wastewater were investigated.Through the above research,this paper mainly obtained the following conclusion:(1)Optimization of preparation conditions,surface characteristic and stability of MnOx-CeO2/lava:the catalyst of MnOx-CeO2/lava was prepared by the impregnation method,and quinoline was used as target pollutant to study the conditions of catalyst preparation,the surface morphology and properties of catalyst were analyzed,and the stability of catalyst and the change of catalyst surface properties before and after application were investigated.The results showed that when the concentration ratio of Mn and Ce was 1:1,the immersion time was 24 h,the activation time was 24 h,the roasting temperature was 2 h,and the calcination time was 400℃,the removal rate of quinoline reached 100%at 40 min,the removal rate of TOC reached 75.9%at 120 min in the catalytic ozonation.The characterization of MnOx-CeO2/lava showed that the surface of MnOx-CeO2/lava was wrinkled and dense,with a very small specific surface area and a large number of metal oxide particles.By the energy spectrum analysis,it was found that the metal elements of Mn and Ce were detected on the surface of MnOx-CeO2/lava compared with the original lava rocks,and the ratio was approximately equal to 1:1.The forms of Mn in the surface of catalyst were mainly Mn2+,Mn4+,Mn4+,while the Ce was mainly Ce4+ with a low content of Ce3+.The surface chemical adsorption oxygen and lattice oxygen content increased significantly in the surface of MnOx-CeO2/lava compared with the original volcanic rocks,the surface hydroxyl density increased from 3.56 mmol·g-1 to 4.21 mmol·g-1.After continuous used 10 times,the removal rate of quinoline and TOC were maintained at 90%and 70%,respectively,the concentration of Mn and Ce in the solution were only 0.0071 mg·L·1 and 0.0052 mg·L-1,respectively.Meanwhile,the surface functional group of the MnOx-CeO2/lava was not changed significantly,the surface hydroxyl component content decreased to 3.05 mmol·g-1.(2)Study on the catalytic performance of MnOx-CeO2/lava with quinoline as the target pollutant.The effects of different reaction conditions on the removal of quinoline by catalytic ozonation were studied,the mass transfer parameters of ozone during mass transfer were investigated,and the kinetic equations of quinoline degradation was explored to further evaluate the catalytic performance.The experimental results showed that when the dosage of ozone and catalyst was 3.6 mg·L-1 and 1 g·L-1,respectively,the reaction temperature and pH value was 20℃ and 7.14,respectively,the quinoline was completely removed at 40 min,the removal rate of TOC was more than 79%at 120 min.The adsorption of quinoline was very weak because the MnOx-CeO2/lava had a very small specific surface area.It is also indicated that the removal of quinoline by catalytic ozone oxidation system was mainly oxidized.The inorganic ions(HCO3-,CO32-,PO43-)in the reaction system inhibited quinoline degradation due to the free radical chain reaction was inhibited and the surface properties of the catalyst were changed.In the catalytic ozonation system,the decomposition of ozone was confirmed to the first-order kinetic,the pH of the reaction system had an important influence on the mass transfer of ozone according to the theory of liquid-membrane.The diffusion of ozone in the reaction system was increased with the increase of pH value,and the transfer of ozone in the gas-liquid phase was enhanced.The Hatta constant of ozone decomposition indicated that the decomposition process of ozone was a slow reaction process.The kinetic model of quinoline degradation in the liquid phase was established,and the effects of ozone dosage,pH value,temperature and catalyst dosage on the removal rate constants of quinoline was investigated to further evaluate the catalytic performance of MnOx-CeO2/lava.(3)Investigated on the catalytic mechanism of MnOx-CeO2/lava.The catalytic mechanism of quinoline degradation was obtained by detecting active free radicals in the catalytic ozonation and combine the characteristics of the surface of the catalyst.The degradation pathway of quinoline was proposed.The results showed that the ˉOH,H2O2 and O2·-were detected in the catalytic ozonation,it was indicated that the catalytic ozonation to remove organic compounds was follows OH reaction mechanism.Mn-Ce composite metal oxides have the synergistic effect:Mn atoms can enter the lattice structure of Ce,resulting in the high catalytic performance of Mn4+ formation.Meanwhile,the reduction of Ce3+ content could lead to increase the content of the lattice oxygen in the catalyst surface,the lattice oxygen could release to form chemical adsorption oxygen to enhance the catalytic performance because the chemical adsorption oxygen can improve the surface hydroxyl component content to promote OH chain reaction.In the quinoline degradation process,the hydroxyl appeared in the location which had high electron density,further oxidation open ring.According to the open-ring location,quinoline was degraded to form a large number of intermediate products by the benzene ring degradation and pyridine ring degradation.These intermediate products were oxidized by OH to generate formic acid,oxalic acid,acetic acid,acrylic acid and another small molecular acid.Particularly,the open ring of pyridine ring causes N elements in quinoline formation-NH2 in the reaction process,and releasing NH4+.NH4+could be further oxidized to produce NO3-.According to the toxicity test of quinoline degradation intermediate products,it was found that the toxicity of the intermediate products was lower than that of quinoline,and the toxicity of the solution was decreased gradually during the reaction.(4)Practical application of MnOx-CeO2/lava.Through studying on catalytic ozonation treatment of actual coal gasification wastewater,the catalytic performance of catalyst for actual wastewater was investigated,the effect of catalytic ozonation by continuous flow treatment was investigated,and the treatment effect of different processes on the actual wastewater was analyzed.The results showed that when the catalyst dosing was 2 g·L-1,pH value was 8,the ozone concentration of 0.636 g·h-1,the reaction temperature was 30℃,the COD removal rate of more than 60%and the biodegradability of wastewater increased to more than 0.35 at 120 min.By the determination of effluent water quality,catalytic ozonation could effectively remove COD and improve the biodegradability of wastewater,the rate of NO3-concentration which generated in the reaction process and the remaining amount of biodegradable organic compounds concentration reached 0.926 at 60 min,which provided a good reaction condition for the subsequent biological treatment of denitrification.For the continuous treatment of wastewater,the COD concentration of the effluent was 68-72 mg·L-1,the biodegradability of wastewater was greater than that of 0.31,it is indicated that MnOx-CeO2/lava had good stability in actual wastewater treatment.In summary,a new type of catalyst MnOx-CeO2/lava was prepared,the preparation condition of catalyst was optimized,the removal performance of catalyst for the target pollutant was investigated,the mechanism of catalytic ozonation was studied.The feasibility of industrial application of catalyst was verified. |
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