The Fine XAD Sorbent Coating—2000 R&D 100 Award Winner
A new technology developed by EETD's Lara Gundel, fine XAD sorbent coating used in air-sampling devices called diffusion denuders (such as EETD's Integrated Organic Gas and Particle Sampler), has improved the accuracy of sampling of airborne particles and won an R&D 100 award.
Establishing air-quality standards requires accurate measurement of pollutants in the air. But accurate measurement is difficult because of the varying sizes and kinds of particulates—for example, metals from incinerators, toxic compounds from gasoline and diesel fuels, and soot from unburned fuel. Many pollutants also exist in both particle and gas phases, including pesticides and polycyclic aromatic hydrocarbons, which are widely present in the environment from all combustion sources.
Historically, investigators have searched for ways to measure for the partitioning (distribution) of these chemicals between the gas phase and airborne particulate phase. Because of the U.S. Environmental Protection Agency's recent efforts to characterize airborne particles, gas/particle partitioning is receiving an enormous amount of attention in the research and monitoring communities. This is also a public health issue, since gas and particles are deposited differently in the lungs and have different physiological effects.
Accurate, direct methods of measurement have not been possible until recently, when Lara Gundel developed the fine XAD sorbent coating. This coating, applied to the inside surfaces of diffusion denuders (gas strippers), enables such measurement by trapping gas while allowing particles to be collected separately, using sticky resin beads whose pores are small enough to trap molecules of organic gases.
Most phase measurements have been made by trapping solid particles on a filter and then collecting gases from underlying adsorbent beds. However, vaporization and condensation effects from the particles still present on the filter can lead to errors. In diffusion denuders with the fine XAD sorbent coating, the relatively massive solid particles travel straight through. Gas molecules move around inside the tube, but eventually they hit the inner surface and stick. The tube is short enough so the particles stay airborne but long enough for the gas to get trapped. After an air sample is sucked through the denuder, the particle filter is removed. The gas trapped on the resin beads is analyzed, without including any extra vaporization or condensation effects. The particles are also analyzed so that accurate gas/particle distributions result.
The coating has two essential characteristics that significantly improve on the performance of existing technology. First, because of its submicrometer size distribution, the coating has 1,000 times the available surface (surface-area-to-volume ratio) of conventional macroreticular (highly porous) sorbent polymers. This imparts the advantage of superior mass-transfer speed in trapping (sorbing) organic species from air or water. Second, the tiny particles of the coating are small enough to adhere by themselves to materials such as glass, plastic, metal, and fabric. This ability to adhere results from the molecular attractive (van der Waals) forces between the particles and the molecules at the surfaces of the materials. At this microscopic scale, van der Waals forces are greater than gravity. Absolutely no adhesive substance other than the coating itself is necessary. Even the force of air or liquid flow cannot dislodge the coating. It can therefore be applied to many shapes and varieties of surfaces to trap and concentrate semivolatile organic gases for measuring and/or controlling air pollution.
The availability of this technology has revolutionized monitoring strategies for characterizing ambient particulate matter. Research groups at Environment Canada, including one led by Douglas Lane, and others throughout the United States have employed this sampling technology in studies of semi-volatile organic compounds. Among these are investigations of the atmospheric behavior of dioxins and the contribution of diesel, other vehicle exhausts, and wood smoke to smog formation—all under various weather conditions, in various parts of the country, and at different times of the day or night.
Diffusion denuders with the fine XAD sorbent coating are currently being tested by the U.S. Environmental Protection Agency as part of the effort to characterize pollution particles throughout the country. In addition, this coating will be instrumental in creating new, "active" surfaces with potential applications not only in pollution monitoring, but also in indoor climate control, ventilation, auto-control of buildings, and real-time instruments and sensors. Lane's Environment Canada group collaborated with Berkeley Lab to incorporate the FXADS into air-sampling tests scaled for ambient air sampling.
For more information, contact:
- Lara Gundel
- (510) 486-7276; fax (510) 486-6658
More information on the Integrated Organic Gas and Particle Sampler.
For more information on the R&D 100 Awards, visit the R&D 100 Awards web site.
This research was supported by the National Heart Lung and Blood Institute, the Department of Energy's ER-LTR Program, the U.S. Environmental Protection Agency, and Environment Canada.