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Adsorption of gas phase contaminant mixtures.

NIOSH 2005 Feb; :1-22
Gas phase contaminant removal processes based on adsorption are used for: indoor air cleaning, cabin air purification, odor removal, personal protective equipment (respirators impregnated clothing), and various air sampling devices. In all these applications contaminated air is passed through a bed of adsorbent where the molecules of the contaminant are transferred to the solid phase. Despite the development of various new adsorption media used for the removal of some specific gases and vapors, activated carbon remains the choice adsorbent in many applications because of its good adsorption efficiency for a wide range of gases and vapors and because of its reduced cost. However, there is an ongoing effort to improve the understanding of the adsorption mechanism on activated carbon, taking into account the many parameters, which influence this process. This effort will eventually lead to the development of improved adsorbents and sampling devices more appropriate to be used in a complex environment. The performances of activated carbon based adsorbents have been evaluated under dynamic flow conditions generally using a single vapor or gas as the sole contaminant. However, the real atmosphere in many buildings and workplaces contains a variety of contaminants in a range of concentrations. The adsorption performances are some times modified drastically when a second component is adsorbed, interfering with the initial vapor or gas. Experimental studies on the adsorption of mixtures on activated carbon are limited because of the difficulties in obtaining data for different concentrations in various mixture/adsorbent systems. There are virtually no experimental data on adsorption of mixtures of gases and vapors having very different properties such as toluene and sulfur dioxide, despite the fact that some respirator cartridges are explicitly marketed for "organic vapors and acid gases The scope of the research conducted in this study was to investigate multi-component adsorption on activated carbon focusing on: 1. Gas and vapor binary mixtures with components of different chemical nature; 2. The influence of the humidity on the binary component adsorption of various components; 3. Adsorption of a trace gas component when another organic vapor component, or water vapor is already adsorbed Overall the results obtained from this research will help to answer the following two questions: For an activated carbon of known parameters: 1. if the adsorption capacity for the individual vapor or gas of a mixture at low concentration is known, what would be its adsorption capacity for the same component when present in a binary mixture of various proportion? 2. if the adsorption capacity of individual components of a mixture are determined, how can the breakthrough cure be predicted without measurements of the multiple-component system? Answering the first question will benefit particularly those involved in the design and use of air monitors. The models obtained from this study will allow determine the safe sampling period for mixtures of gases and vapors, including water vapors, from the adsorption isotherms of pure compounds. The answer to the second question can be of crucial importance for the determination of service life for respirator cartridges. Service lives are usually determined for a reference vapor from the breakthrough cure and are assumed to be quite the same for a large variety of vapors and gases. This assumption can be totally false when respirators are used in an environment containing multiple gases and vapors. The result obtained can be classified in three main categories: 1. Adsorption of single component gases and vapors under dry and/or normal conditions: toluene, ethanol, sulfur dioxide, dichlormethane. Activated carbon shows to be a good adsorbent for most of these compounds. It is highly efficient for toluene, and the least efficient for sulfur dioxide. Most adsorption models can be applied successfully to explain single component adsorption at low concentrations. For radioactive xenon and dichlormethane only a limited number of tests were performed due to equipment and time limitations. However the adsorption of these compounds on activated carbon does not behave differently than the other tested compounds. 2. The influence of relative humidity on the adsorption onto activated carbon was tested extensively for toluene and ethanol. A large number of breakthrough tests were also conducted for sulfur dioxide. However the interference of water vapors with the detection system (Infrared analyzer) made these results less reliable. A main finding of this par is that water vapors inhibit the adsorption of non-polar compounds like toluene at least at low concentration and enhances the adsorption of polar compounds like ethanol. 3. Adsorption of mixtures was analyzed for a number of binary mixtures: sulfur dioxide toluene, ethanol-toluene, toluene-dichlormethane mainly at low and or normal relative humidity. Although the adsorption of most mixtures resulted in complicated breakthrough cures, most can be explained using the common models found in the literature. A particular case is the sulfur dioxide - toluene mixture, which tend to be influenced a great deal by the presence of water vapors. A new model for the adsorption of this mixture is under construction, none of the known models explaining satisfactory the multi step process observed in some binary breakthrough cures. An important conclusion of this research is that in most cases activated carbon is an excellent adsorbent, and can be used successfully for respiratory protection. However, predicting the breakthrough, in particular for mixtures, and in conditions of high relative humidity is a very difficult task. Most predictive models work well for single compounds, but for mixtures a large amount of data is still necessary to be generated to develop a model able to be effective for a number of mixtures.
Samplers; Sampling-equipment; Sampling-methods; Adsorbents; Personal-protective-equipment; Respirators; Respiratory-protective-equipment; Organic-solvents; Organic-vapors; Organic-chemicals; Organic-acids; Organic-compounds; Synergism
Division of Environmental Health Sciences, School of Public Health, University of Minnesota, Box 807 Mayo Memorial Building, 420 Delaware St. S.E., Minneapolis, MN 55455
108-88-3; 7446-09-5; 7440-63-3
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Final Grant Report
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National Institute for Occupational Safety and Health
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University of Minnesota Twin Cities