Characterization of gas phase adsorption capacity of untreated and chemically treated activated carbon cloths (original) (raw)

Granular activated carbon (GAC) and powdered activated carbon (PAC) have long been used to effectively treat drinking water, waste water, and industrial gas streams. Undesired contaminants are removed by adsorption onto activated carbon. While activated carbon has been used extensively in industrial applications, little research has been performed to evaluate using activated carbon to remove low concentrations of volatile organic compounds (VOCs) from indoor air environments. In this research, activated carbon cloth (ACC) is examined for its equilibrium adsorption capacity for several VOCs of relevance to indoor air quality. If the technology proves viable, filters made from ACC could be placed in new or existing air circulation systems of buildings and residences to effectively remove VOCs from indoor air. Adsorption isotherms were measured for acetaldehyde, acetone, benzene, methyl-ethyl ketone, and water vapor and three ACC types. For the 10 to 1000 ppmv concentration range examined, benzene exhibited the highest adsorption capacity on ACC, followed by MEK, acetone, and acetaldehyde. Water vapor adsorption was not significant on ACC until relative humidities above about 50% (P/P o > 0.5), when capillary condensation of H 2 O (g) occurred within ACC pores. Equilibrium adsorption experiments were not performed for VOCs in the sub-ppmv concentration range, due to the long times (estimated at weeks to months) to reach equilibrium and the high cost of compressed gases. The Freundlich and Dubinin-Radushkevich equations were used to model the adsorption capacities into the sub-ppmv range for the four adsorbates and three ACC types examined in this research. The sub-ppmv concentration range is a more realistic concentration range for VOCs present in indoor air environments. It has been suggested that when using the DR equation to predict adsorption capacities of organic compounds using a reference adsorbate, reference adsorbates of similar polarity should be used. This hypothesis was examined by using acetone as a reference for polar compounds (e.g., acetaldehyde, MEK, and 1,1,1-trichloroethane). Using acetone as a reference adsorbate, predictions showed average errors of 9% for acetaldehyde and 5% for MEK (the improvement in prediction of adsorption capacity was not measured for non-polar compounds). ACC-20 was chemically modified, producing oxidized, chlorinated, and nitrated samples. Adsorption capacities for VOCs in the 10 to 1000 ppmv concentration and water vapor from 0 to 95% RH were measured. Oxidized ACC-20 showed an enhanced physical adsorption for acetaldehyde, acetone, and water vapor, probably due to increased dipole-dipole interactions and hydrogen bonding. Oxidation of ACC-20 changed the shape of the water vapor adsorption isotherm, so that it no longer resembles a Brunauer type V. Benzene showed a decreased adsorption capacity on oxidized ACC-20, which may be due to an increase in hydrophilicity of ACC-20 or a change in pore size distribution. iv Chlorination had little effect on VOC adsorption capacity, except in the case of acetone, where a decrease in adsorption capacity occurred. This may be due to pore blocking by chlorine molecules, or a decrease in hydrogen bonding between the ACC functional groups and acetone. Nitridation of ACC showed little effect on organic adsorption capacity, but increased the saturation adsorption capacity for water vapor on ACC-20 and increased the breadth of its hysteresis loop. These changes were the result of changes in the pore size distribution of the nitrided ACC-20. DR parameters were determined for VOC adsorption on ACC-20. The effects of relative humidity (RH) on the adsorption of soluble (acetone) and insoluble (benzene) volatile organic compounds (VOCs) on activated carbon cloths (ACC) were measured. A gravimetric balance was used in conjunction with a gas chromatograph/mass spectrophotometer to determine the individual amounts of water and VOC adsorbed on an ACC sample. RH values from 0 to 90% and organic concentrations from 350 to 1000 ppmv were examined. The presence of water vapor in the gas-stream along with acetone (350 and 500 ppmv) had little effect on the adsorption capacity of acetone even at 90% RH. Water vapor in the gas-stream had little effect on the adsorption capacity of benzene (500 ppmv) until about 65% RH, when a rapid decrease in the adsorption capacity of benzene resulted with increasing RH. This RH was also about where capillary condensation of water vapor occurs within ACC pores. At this point water vapor condenses within the ACC pores, making them unavailable for benzene adsorption. Increasing benzene concentration, however, can have a significant effect on the amount of water vapor adsorbed. At 86% RH and 500 ppmv, 284 mg/g water was adsorbed, while at 86% RH and 1000 ppmv, only 165 mg/g water was adsorbed. Thus, water vapor was more inhibitory for benzene adsorption as benzene concentration in the gas stream decreased. v Acknowledgements There are many people I would like to acknowledge during my graduate studies at the University of Illinois at Urbana-Champaign: