Adsorption Of Humid Volatile Organic Compounds On Activated Carbon Deposited By Coating Modification

Volatile organic compounds (VOCs) are a kind of representative pollutants with high potential risk for the environment and existe in a large number of contaminated sites. VOCs are easy to evaporate into the atmosphere, directly threatening to various ecological environment. Recently, technologies for the control of VOCs have been therefore explored. As an important one of these technologies, adsorption methods via activated carbon become a cost-effective way to deal with VOCs, along with high purification efficiency and less secondary pollution. However, there is large percent of water vapor with VOCs during the adsorption process, either industrial waste gas or VOCs from soil extraction gas. Competitive adsorption between water vapor and VOCs on the surface of activated carbon seems to reduce the adsorption selectivity of VOCs on activated carbon. Therefore, it is a hotspot issue to investigate the possible modification of activated carbons, in order to enhance the adsorption selectivity of VOCs under humid conditions. Polydimethylsiloxane (PDMS), as a kind of super hydrophobic organic silicon polymer material with high chemical stability, was thus selected for the coating modification of ACs. Activated carbons with PDMS hydrophobic coating were obtained by the heating-condensing method. PDMS coating was loaded onto the surface of activated carbon with a series of temperatures. Then, PDMS-coated activated carbons were used to explore the adsorption process of aqueous VOCs under humid conditions.Physical-chemical properties of activated carbons before and after PDMS coating were characterized by BET analysis, Scanning Electron Microscope, Boehm titration and X-ray photoelectron spectroscopy method. Water vapor adsorption isotherms of them under different humidity were studied by a gravimetric method in 30℃. The adsorption of toluene, benzene and acetone on the involved activated carbons were determined by gas chromatography in relative humidity of 0%,50% and 90%. The Yoon-Neslon adsorption model was introduced to analysis the kinetics during adsorption process. Origin was used to calculate the adsorption capacity of VOCs on activated carbons. Water vapor adsorption capacity was calculated by gravimetric method combined with the results of VOCs adsorption capacity. The results showed that the BET area, micropore volume and surface functional groups decreased with the PDMS modification, and surface hydrophobicity of the modified AC was enhanced. Such changes lead to a less water adsorption capacity. Water vapor adsorption isotherms showed that PDMS coating reduced the activated carbon adsorption capacity of water vapor. Water vapor adsorption amount was reduced with the increasing of temperature, and the greatest reduced value was 59% when the temperature was 250℃. An obvious relationship was found between the water vapor adsorption capacity of activated carbon and the pore volume of activated carbon, along with the surface acidic functional groups and the surface water contact angle. VOCs adsorption breakthrough curves showed that PDMS-coated activated carbon was less affected by the water vapor in adsorbing toluene and benzene; what’s more, the adsorption capacity increased compared with original activated carbon. An increase in the adsorption capacity of acetone was not significant, may owing to the polarity of VOCs molecule.