| Photocatalytic oxidation shows excellent performance in cleaning gaseous organic pollutants. However, nano-TiO2 powder has some disadvantages in the practical applications, such as a narrow spectral response range (light absorption is only limited to UV region), lower optical quantum efficiency and difficult to recycling. So immobilizing and visible-modifying nano-TiO2 become two research hotspots in the photocatalysis fields. When TiO2 was loaded on the porous carriers, the synergies of adsorption and photocatalysis helped to realize the rapid degradation of the organic pollutants. In this paper, activated carbon fibers (ACFs) with narrow pore size distribution and high absorption capacity were selected as carrier, the preparation method of TiO2/ACFs composite photocatalyst material was optimized; toluene and formaldehyde were selected as model organic pollutant, the photodegradation activities and dynamic behaviories of gaseous flowing toluene and formaldehyde by TiO2/ACFs were studied; for improving the responsibility to visible-light of nano-TiO2, CdS/TiO2 composite photocatalysts was prepared by using semiconductor CdS with narrow enegy band modifying TiO2; CdS/TiO2/ACFs composite photocatalysts were prepared by loading CdS/TiO2 on the ACFs, then their photodegradation performance, mechanism and reaction dynamic process were studied under visible-light irradiation. The pollution of malodour and offensive odour from wastewater were common in the pharmaceutical companies, and the organic component in the odour were mainly VOCs, so TiO2/ACFs composites was used to clean the odour in the field experiment, the aim was to preliminarily estimate the application effect of TiO2/ACFs. The results were as follows.1. Considering the advantages of simplicity of the dip-coating method and firmness of the adhesive cohering method, using the sodium carboxymethyl cellulose (CMC) with good performances as the adhesive, a new preparation method of TiO2/ACFs-’Dip-cohering method’was proposed. The surface morphology, microstructure and crystal structure of TiO2/ACFs photocatalyst were characterized by SEM, EDS, BET and XRD. The results showed that the surface area and pore volume of TiO2/ACFs decreased slightly, and the original anatase crystal form of TiO2 could be kept. By orthogonal experiments the better preparation conditions were determined:TiO2 suspension concentration was 5 mg-mL-1, drying temperature was 100℃, drying time was 3 h, the amount of the added CMC was 0.25%. Compared with sol-gel method, the dip-cohering method was simple, moderate preparation temperature and low cost.2. With UV irradiation, the photodegradation rate curve of the gaseous flowing toluene by TiO2/ACFs composite showed that it firstly rapidly reduced, after 60min gradually increased to a maximum of points, then decreased and stabilized. The highest removal rate of toluene by TiO2/ACFs prepared with different amount of the added CMC (0.15%、0.25%、0.35%) were 66.8%、70.4% and 59.9%, respectively. Under experimental conditions, the photodegradation rate of toluene by TiO2/ACFs increased with the increasing of light intensity, while it decreased with the increasing of toluene initial concentration or gas flow, and increasing photocatalyst amount could improve the photodegradation performances of toluene, however if the amount was too much, the effect became unsignificant. The photocatalytic reaction kinetic process of the gaseous flowing low-concentration toluene by TiO2/ACFs fitted Langmuir-Hinshelwood pseudo-first-order kinetic equation.3. The experimental conditions of the phenol reagent spectrophotometric determining formaldehyde were optimized. When the concentration of formaldehyde was higher, phenol reagent or deionized water should be selected to dilute the absorption solution to appropriate volume, however, taking into account the economy, the deionized water was preferred. At the initial stage of UV irradiation, the photodegradation rate of the gaseous flowing formaldehyde by TiO2/ACFs prepared by the sol-gel method was higher than that of the dip-cohering method, while after 80min the degradation rate of the former was slightly lower than that of the latter. Under experimental conditions, the photodegradation rate of formaldehyde by TiO2/ACFs increased with the increasing of light intensity or photocatalyst amount, while it decreased with the increasing of formaldehyde initial concentration or gas flow. The photocatalytic reaction kinetic process of the gaseous flowing low-concentration formaldehyde by TiO2/ACFs fitted Langmuir-Hinshelwood pseudo-first-order kinetic equation.4. Degussa P25 TiO2 modified CdS (CdS/TiO2) composites were prepared by two methods, sol-gel method and precipitation method, then CdS/TiO2 loaded on ACFs composites (CdS/TiO2/ACFs) were prepared by dip-cohering method. The composites were characterized by XRD, UV-vis, SEM, EDS and BET. The results showed that in the CdS/TiO2 composites TiO2 was still main and only modified by a small amount of CdS on the surface, which caused a little impact on the crystal structure and average particle size of P25 TiO2; the absorption edge of CdS/TiO2 had a pronounced’red shift’. on the ACFs CdS/TiO2 were in the form of small clusters, but not very uniform; compared with the original ACFs, the surface area and pore volume of CdS/TiO2/ACFs decreased slightly, respectively, while the average pore diameter was not changed. The photodegradation kinetics of methylene blue by CdS/TiO2 or CdS/TiO2/ACFs composites under sunlight fitted with the Langmuir-Hinshelwood equation. The apparent first-order rate constant of CdS/TiO2 was about 1.3-1.4 times as large as that of P25 TiO2, and the apparent first-order rate constant of CdS/TiO2/ACFs was about 1.2-1.7 times as large as that of P25-TiO2/ACFs. The photocatalytic activities of the composites prepared by the sol-gel method were better than that of precipitation method.5. The photodegradation rates of toluene by the CdS/TiO2/ACFs composite was significantly higher than that of TiO2/ACFs both in a static system under sunlight irradiation and in a dynamic system under fluorescent light irradiation. It was obvious that CdS enhanced the utilization efficiency of TiO2 to the visible light. The adsorbed organic matter in the composite materials were qualitatively analysed by GC-MS, the results showed that besides unreacted toluene out, several straight-chain alkanes, such as tetradecane, pentadecane and hexadecane, were found, without benzyl alcohol, benzaldehyde, benzoic acid and other intermediates were detected. When CdS/TiO2 was irradiated by UV or visible-light with different energy, the charge-transfer mechanism was different. Based on visible-light, the photocatalytic reaction kinetics of gaseous toluene by CdS/TiO2/ACFs could be divided into seven consecutive steps.6. The malodour and offensive odour from wastewater treatment station of a pharmaceutical company in Hebei province was selected as the research object, gaseous samples were collected by vacuum stainless steel SUMMA canisters, then they were preconcentrated for three times, finally they were taken into GC-MS for qualitative and quantitative analysis. The results showed that the major VOCs in the odour were ester, mercaptan, sulfide, ketone and so on, in which the concentrations of butyl acetate was the highest, up to 890 mg-m-3. TiO2/ACFs composites were used to purify such waste gas. The results showed that the method could purify the malodour and offensive odour in varying degrees, in the initial operation stage the removal efficiency ranged between 47.4%-98.7%. During the experiment, in the initial 30min after turning on UV, the temperature in the photocatalytic reactor increased rapidly, while the relative humidity rapidly reduced, after 30min, the temperature and relative humidity gradually became stable. So taking into account of the economy, when TiO2/ACFs were used to adsorb and photodegrade VOCs, temperature and humidity controller may be not added in practical application. |
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