چكيده لاتين :
Background and aims: Clean air is one of the basic needs for health and well-being. However, along
with economic growth and development, transportation, urbanization and energy consumption have also
risen and provide many concerns such as air pollution, which require urgent and wide attention. Air
pollution in the worldwide is considered as a risk factor for human health and one of the main challenges
of modern countries. In this regard, the organizations responsible in different countries, determine the
rules of the threshold limit values of pollutants such as: carbon monoxide, nitrogen oxides, sulfur
compounds, heavy metals, volatile organic compounds and propose solutions for their control. Air
pollution sources are mainly composed of: suspended particles, volatile organic compounds, carbon
monoxide, ammonia, sulfur oxides, carbon dioxide. Meanwhile, volatile organic compounds are one of
the most important pollutants in communities due to their significant and irreparable effects on human
health and high production. These compounds have been rejected due to the destruction of the
stratospheric ozone, photochemical oxidant precursors, acid rain, climate change and global warming,
effects on the nervous system, cancer, and so on. In order to eliminate and control these emissions,
several methods have been identified such as catalytic and thermal oxidation, condensation, biological,
membrane separation, absorption and adsorption. The methods mentioned each for different reasons and
functional characteristics have their strengths and weaknesses in utilizing air purification technologies.
However, adsorption is one of the most effective control methods and activated carbon as a porous and
non-polar adsorbent is one of the most widely used adsorbents due to its hydrophobicity, high surface
area, high adsorption capacity and relatively cheap price in this field. In spite of the proper efficiency of
activated carbon in the adsorption of pollutants, especially for volatile organic compounds, attempts to
improve the adsorption capacity of activated carbon by various methods are being carried out with
researchers. Conventional modification methods for increasing the adsorption capacity of activated
carbon, includes: chemical methods (acid treatment, base treatment), modification by impregnation,
physical methods (thermal methods, oxidation), biological methods, ozone, plasma (dielectric barrier
discharge plasma), microwave, and so on. In recent years, the use of plasma has increased significantly
in order to modification of surface of various types of materials and compared with other conventional
technologies in the field of modification of surfaces, as a promising method, in a shorter and easier time.
In addition, there are no secondary pollutants in it. Plasma is ionized gas that all or a significant portion
of its atoms have lost one or more electrons and have become positive ions. Types of non-thermal
plasma includes: corona discharge, DBD (dielectric barrier discharge), glow discharge, microwave
discharge, gilding arc discharge etc. In fact, this method is very efficient and easy to modification of
surfaces, and by making physical and chemical changes in the surface structure of materials, the surfaces
are modified. Physical changes are mainly caused by UV radiation and other radiation emitted from the
discharge to surface, production of active particles such as ions, free radicals and ozone gas and usually
affect the porous structure of the adsorbent in order to increase the adsorption capacity.
Proper physicochemical background and widespread use of activated carbon in the removal of air
pollutants (especially volatile organic compounds) have led to special attention being paid to altering the
structural and chemical properties of these adsorbents using other existing techniques and emerging
techniques of air pollution control knowledge by various scientists and researchers. Therefore, the
purpose of this study was to make significant changes in the structural properties of the adsorbent by
using the plasma method as the newest techniques of air pollution control knowledge in order to increase
the adsorption capacity and efficiency of adsorbent.
Methods: Merck's activated carbon granule with mesh 12 as an adsorbent and Merck's toluene with
99.9% purity were used as pollutant. This study was conducted in two separate parts. The first part is
related to the plasma modification process, which were affected by four variables: temperature (40, 70,
100, 130 ℃), flow rate (0.12, 0.25, 0.50, 0.75 "Lpm"), exposure time with plasma (1, 2, 3, 4 min) and
voltage (0.6, 0.8, 1, 1.2 "kV"). Modification setup consists of high voltage power supply (alternating current), cylindrical DBD reactor as 1 mm thickness, anode and cathode respectively of foil cooper and
stainless steel, respectively plus two multimeters separately (for simultaneous reading of ampere and
voltage). In the second section, the modified activated carbon granule samples were adsorbed with
toluene vapors at a concentration of 200 ppm. The measurement of the toluene vapor concentration was
also performed by direct reading using a Phocheck based on photo ionization detector. The breakthrough
time and adsorption capacity of each activated carbon granule beds were determined and calculated
separately. Activated carbon granule beds with the highest breakthrough time and adsorption capacity
were investigated with SEM and BET analysis for the specific surface area, porosity diameter and
morphology of activated carbon granules as the most important adsorbent properties. Analysis of
variance of Minitab software was also used to determine the correlation between variables of the
modification process (temperature, flow rate, exposure time with plasma and voltage) with breakthrough
time and adsorption capacity.
Results: The results show that, the maximum breakthrough time and adsorption capacity of modified
activated carbon granules are in 130℃ temperature, 0.75 Lpm flow rate, 1 min exposure time with
plasma and 1 kV voltage. In these conditions of modification, 56% increase in adsorption capacity was
observed in comparison with the unmodified activated carbon granule. However, there was no
significant effect on the results of BET tests (in order to study of the specific surface area, total pore
volume and mean pore diameter) and the reason for the slight changes observed is the effect of the
plasma process on the adsorbent surface, which has resulted in the destruction or blockage of some
pores. Meanwhile, Fe-SEM images (with a magnification of 30, 5000, and 150,000) indicate the slight
change in the micro and nano scales on the modified activated carbon surfaces in comparison with the
unmodified activated carbon surface. In fact, the surfaces of activated carbon granules exposed to
dielectric barrier discharge plasma is better in terms of the presence of waste and pollutants on the
surface than the unmodified activated carbon. The reason for the decrease in adsorption capacity in some
beds can also be due to the high voltage during prolonged exposure which results in degradation of pores
and active molecules on the adsorbent surface. However, sometimes increasing the functional groups on
the surface of the adsorbent can lead to clogging of pores and a decrease in the specific surface area and
ultimately decrease the adsorption capacity. Among the variables of the modification process, except for
the temperature, no significant correlation was found between the variables (flow rete, exposure time
with plasma, voltage) and the adsorption capacity of the modified activated carbon samples, and only the
temperature variable showed a significant level of P-value. Some studies have similar results in this
regard. In one study, after survey of the orange acid adsorption in aqueous solution by plasma-modified
activated carbon fibers, they reported that the modification process resulted in a decrease in the specific
activated carbon level and the increased adsorption capacity of orange acid was attributed to the increase
in functional groups. In another study of mercury removal through adsorption on activated carbon
modified with plasma, a slight decrease in size and total volume of pores, a slight increase in the mean
diameter and specific surface area of meso and macro pores, the increase of the oxygen-containing
functional groups, increases in the active sites related to chemical adsorption on the adsorbent surface
and finally increase in adsorption capacity were reported.
Conclusion: Based on the results obtained and as well the results of other studies, the reason for
increasing the adsorption capacity of toluene vapors on the activated carbon granule, despite the
reduction of the structural properties of the activated carbon after modification, can be attributed to
changes in the chemical properties of the adsorbent surface (functional groups), which requires further
studies in this regard to confirm its accuracy. Generally, plasma as a novel and eco-friendly method, by
making changes in the chemical and physical properties of activated carbon granule, leads to an increase
in the adsorption capacity of toluene vapors. The most important reason for increasing adsorption
capacity attributed to chemical changes on the adsorbent surface affected by the modification process.
