كليدواژه :
كيفيت هوا در محيط هاي داخلي (IAQ) , دي اكسيد تيتانيوم (TiO2) , كربن فعال , تركيبات آلي فرار , تولوئن , اكسيداسيون فوتوكاتاليتيكي , بستر جذبي- فوتوكاتاليتيكي
چكيده لاتين :
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 environmentfriendly
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.
Methodols: 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 noncombined
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 TiO2. 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.
