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Kazemi S, Safari Variani A, Khoshnavaz H, Nikpey A. Photo catalytic removal of Toluene vapor from air in the Adsorption-Photo catalytic bed. ioh 2019; 16 (5) :75-87
URL: http://ioh.iums.ac.ir/article-1-2313-en.html

Background and aims: Clean air is one of the most important components of health and sustainable development. Every person breathes about 10 kg of air per day and if it contains pollutants, it will have a serious impact on their health. Indoor air quality (IAQ) is one of the major health issues that have been addressed in recent years with changes in lifestyle patterns. Usually, due to the increased time of presence and activity in these environments and reduced air exchange with the outdoor environment, indoor air quality is poorer than outdoor environments. Toluene is a Volatile organic compound with widespread applications. VOCs has a high vapor pressure and high emission rate to environment. Due to its adverse effects on human and environment health, they must be controlled before discharging to the environment. Photo catalytic oxidation process is one of the environment-friendly and effective methods for the remove the organic compounds from the air which likely to be better in combination with other methods such as adsorption. Through the process of PCO, UV radiation adsorption on TiO­₂ is associated with forming electron and holes from electron escape. The resulted electrons have got high levels of oxidation power and act as a strong oxidant producing superoxide ion. The resulted holes have good oxidation potential; with superoxide ions, they make good conditions for oxidation of most organic compounds to less hazardous compounds such as carbon dioxide and aqueous vapor.  The most important limitation of Photo catalytic oxidation process is the dependence of the contaminant removal on the surface chemistry and the residence time of the contaminant on the photo catalyst surface. The most important limitation of the adsorption method is the decrease in adsorption removal efficiency and elimination capacity due to the filling of the adsorption sites.
According to this, by combining adsorption and photo catalytic oxidation, it is possible to increase the time of contaminant presence at photo catalytic oxidation sites and to enhance the surface chemistry and on the other hand, to restore the adsorption sites. This study is conducted with the aim at examining the effects of combination Activated carbon and Titanium dioxide (TiO2) on the toluene removal efficiency.
Methodology: In order to prepare samples, 5g of TiO2 and 5g of TiO2 and 1g of activated carbon dissolve in separate 100 ml distilled water under vigorous stirring. The surface modification was done by dip-coating method. The efficiency of the photocatalytic oxidation of toluene is evaluated in two separate reactors exposed to ultraviolet light. Additionally, to investigate the effect of initial concentration of toluene and airflow rate on the photocatalytic removal efficiency in photocatalytic and photocatalytic-adsorption beds, the RSM method was used to design experiments.
Also, Scanning Electron Microscopy was used to determine catalysts surface morphology. First, to obtain the adsorption capacity in both reactors, with the UV lamp being off, the considered concentrations were added to the reactors in 2-5 L/m airflows. Then, adsorption capacity of adsorption beds were evaluated according to the time needed for the outlet concentration to reach 10% of the inlet amount, as the fraction point of the adsorbent and saturated capacity.
Next, to compare the removal efficiency of toluene in the two reactors, the lamps were immediately turned on; concentrations were gradually decreased and when the outlet concentration was balanced, the data was collected.
Results: Images from an electron microscope of surfaces of the two catalysts showed that the distribution of nanoparticles on glass wool was similar and the particle size in the non-combined catalyst were smaller than 95 nm, and smaller than 87 nm in the adsorbent catalyst. In other words, the size of nanoparticles led to more contact area of the pollutant with the catalyst, increased reaction as well as removal efficiency. SEM photography confirmed that, combining Ti­­O₂ with activated carbon, the pores in the activated carbon were occupied and it made a good place for Ti­­O_2. Controlling the process of photocatalytic elimination in photocatalysts indicated that in TiO2-AC reactor, removal efficiency and elimination capacity of toluene were higher than TiO2 reactor and combining adsorbent with photocatalyst may lead to enhanced photocatalytic oxidation efficiency of organic compounds
The results illustrate that the removal efficiency and elimination capacity of toluene in photocatalytic and photocatalytic-adsorption beds are Influenced by airflow rate and inlet concentration of toluene. In optimized operational conditions, the removal efficiency in both combined and non- combined reactors in inlet loadings of 84.5 and 1.65 mg/m3.s and retention time of 2 and 8.5 s, was 99.98% and 95.14%, respectively. Also, elimination capacity in optimized operational points in the two absorbent and non-absorbent photocatalysts was 5008.71 and 1204.85 mg/m3.min, respectively.
 As the statistical analysis by Minitab indicated, in the concentration range of 10-40 ppm and the airflow of 2-5 L/min (in 2-8.5 s retention time) in the combined reactor, the removal efficiencies were 90% (min.) and 99% (max.); however, in the non-combined reactor, the minimum of removal efficiency was 10% and the maximum was 90%.
The results of this study also indicated that the retention time had a significant effect on the removal efficiency and the elimination capacity of toluene, that is, at the constant inlet concentration (25 ppm) increased with increase in retention time of the non-composite reactor. However, in the adsorbent reactor, lower retention time led to higher removal efficiency. According to the results, toluene removal efficiency and elimination capacity levels in the combined reactor in time retentions of 2, 3.3 and 8.5 s increased compared to the non-combined reactor. Also, evaluation of the effects of initial toluene concentration on removal efficiency and elimination capacity showed that they were higher in the combined vs the non-combined reactor. Removal efficiency of both photocatalysts was influenced by the initial concentration of toluene, so that, in TiO2 higher inlet concentration led to lower removal efficiency. Nevertheless, in the adsorbent photocatalyst, there was an increase in removal efficiency with higher concentrations.
According to the findings, the production of CO2 was dependent on toluene inlet concentration and the airflow. In the combined photocatalyst, the minimum and maximum of the produced carbon dioxide were 40 ppm and 80 ppm, respectively. And it was 84.82 in the optimum operational point (46.2 ppm; 5.62 L/min). However, in the non-combined reactor, the produced CO2 was 29.2 ppm in the optimum operational point and its minimum and maximum were 10 ppm and 25 ppm, respectively. The results also reported that in the given concentration and airflow, production of CO2 in TiO2-AC reactor was higher than TiO2.
Conclusion: Results of the present study indicated that combining titanium dioxide with activated carbon adsorbent is a useful method in removing toluene gas from air under UV and combining photocatalytic elimination with adsorption process by activated carbon led to increased removal efficiency. Self-cleaning photocatalyst had high efficiency in the present study and the use of photocatalytic-adsorption bed can as a suitable method with high removal efficiency causing adsorption and treatment of the pollutants. In other word, combination Activated carbon with Titanium dioxide improved the functionality of activity of photocatalytic oxidation through promoting adsorption sites and increasing residence time of the pollutant in the bed.
Results also indicated that removal efficiency was influenced by initial concentration of toluene and retention time of pollutants on the bed and optimizing these parameters may lead to maximum efficiency in photocatalytic setups. The combined catalysts with surface adsorbents led to improved decomposition efficiency based on photocatalytic decomposition and it is likely that the improvement is mostly the result of increasing adsorption sites compared to limited adsorption sites found in the photocatalyst. On the other hand, with more adsorption site, the pollutant had much more time to have contact with photocatalysts and consequently improved circumstances for surface oxidation reactions
 
Keywords: Indoor Air Quality (IAQ), Titanium Dioxide (TiO2), Activated Carbon, Volatile Organic Compounds, Toluene, Photocatalytic Oxidation, Adsorption-Photocatalytic bed.