Modification of activated carbon by cold plasma and its effect on the adsorption of toluene

Abstract Background and aims: Clean air is one of the basic needs for human 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 ), exposure time with plasma (1, 2, 3, 4 min) and voltage (0.6, 0.8, 1, 1.2 ). Modification setup includes 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  and  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  flow rate, 1 min exposure time with plasma and 1  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. Keywords: Plasma, volatile organic compounds, modification, adsorption, activated carbon granule