Physical Removal Processes

The phase (gas or particle) in which the PCBs, PCDDs and PCDFs occur in the atmosphere greatly affects their tropospheric removal processes and lifetimes. Analogous to other organic compounds, the PCDDs, PCDFs and PCBs in the atmosphere can be removed and/or transformed by a number of physical and chemical processes.71,72,76,77 While the present chapter focuses on the chemical transformations of the PCBs, PCDDs and PCDFs, the physical removal processes are also discussed for completeness and to assess the relative importance of the various tropospheric removal processes. 

Gas- and particle-phase PCBs, PCDDs and PCDFs can be removed from the troposphere by wet and dry deposition.71,72,76,77 Wet deposition refers to the removal of the chemical (or particle-associated chemical) from the atmosphere by precipitation events (through the precipitation of rain, fog or snow to the Earth s surface), while dry deposition refers to the removal of the chemical or particle- 

Atkinson, in Air Pollution, the Automobile, and Public Health, ed. A. Y. Watson, R. R. Bates and D. Kennedy, National Academy Press, Washington, 1988, pp. 99-132. 

Wet deposition of a semi-volatile organic compound is characterized in terms of the overall washout ratio, W, which is given by 

W = (concentration of chemical in aqueous phase)/(concentration of chemical in air) 

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In the air, both photolysis and physical removal processes such as gravitational settling of aerosols and wet deposition by rain and snow will probably determine the fate of 2-nitrophenol and 4-nitrophenol. The atmospheric half-lives of these compounds are not known. In water, both photolysis and biodegradation will be important fate processes. Photolysis will be more important in near-... 

Dry deposition refers to any physical removal process that does not involve precipitation. Three dry deposition mechanisms are discussed next gravitational settling, impaction, and absorption. 

This chapter is devoted to chemical removal processes in the troposphere. In this section, however, we briefly discuss physical removal processes. Gases and particles are physically removed from the atmosphere by deposition at the earth s surface (so-called dry deposition) and by absorption into droplets followed by transfer of the drops to the surface in the form of precipitation (so-called wet deposition). 

Despite the relative ineffectiveness for chemical disinfectants including iodine, in order to achieve the required 2-log inactivation of C. parvum, there are several applications that may incorporate its use in future designs. These designs are for personal water purification devices that incorporate a physical removal process (filtration) followed by the use of a chemical disinfectant. Iodine seems to be a popular choice, due to its relative stabihty and proven success against bacterial and viral contaminants. Iodine is used to impregnate the materials used in purification systems, such as a resin, or a physical removal system, such as a straw. The combination of physical and chemical processes does warrant further consideration to improve the achieved level of protection. 

The basic terms embodied in the differential equations of a model describe the transport properties of the troposphere, the rates of chemical reactions, and physical removal processes. Many models utilize anEulerian description of the troposphere by subdividing the airspace into an assembly of boxes that exchange air and trace constituents with adjacent boxes in accordance with the prevailing tropospheric flow field. Surface sources of trace constituents are prescribed by appropriate boundary conditions. The equations are solved numerically on fast computers. 

In the ocean, elements that form insoluble hydroxides have relatively short residence times (e.g., A1 and Fe have residence times in the ocean of 100 and 200 years, respectively). Cations, such as Na (aq) and K (aq), and anions, such as Cl (aq) and Br (aq), have longer residence times in the ocean ( 7 x 10 to 10 years). In the atmosphere, the very stable gas nitrogen has a residence time of a million years or so, while oxygen has a residence time of 5,000-10,000 years. Sulfur dioxide, water, and carbon dioxide, on the other hand, have residence times in the atmosphere of only a few days, 10 days, and 4 years, respectively. Of course, residence times may be determined by physical removal processes (e.g., scavenging by precipitation) as well as chemical. 

If decontamination caimot be left to natural processes, chemical neutralizers or means of physical removal must be employed. In general, the neutralizers are of two types chlorine-based oxidants or strong bases. Some neutralizers have been especially developed for the decontamination of chemical agents. 

Although it does not physically explain the nature of the removal process, deposition velocity has been used to account for removal due to impaction with vegetation near the surface or for chemical reactions with the surface. McMahon and Denison (12) gave many deposition velocities in their review paper. Examples (in cm s ) are sulfur dioxide, 0.5-1.2 ozone, 0.1-2.0 iodine, 0.7-2.8 and carbon dioxide, negligible. 

In its simplest form, a model requires two types of data inputs information on the source or sources including pollutant emission rate, and meteorological data such as wind velocity and turbulence. The model then simulates mathematically the pollutant s transport and dispersion, and perhaps its chemical and physical transformations and removal processes. The model output is air pollutant concentration for a particular time period, usually at specific receptor locations. 

There are two main schemes proposed for sequestration of carbon dioxide. The first (referred to as a chemical absorption process), suitable for use at low pressures and temperatures, is usually adopted where the CO2 is to be removed from exhaust flue gases. The second (usually referred to as a physical absorption process), for use at higher pressures, is recommended for separation of the CO2 in syngas obtained from conversion of fuel. 

Fritsch, J. and Moraru, C. 1. (2007). Development and optimization of a carbon dioxide-aided cold microfiltration process for the physical removal of microorganisms and somatic cells from skim milk. /. Dairy Sci. 91, 3744-3760. 

In the thermal desorption technique excavated soil is heated to around 200 to 1000°F (93 to 538°C). Volatile and some semivolatile contaminants are vaporized and carried off by air, combustion gas, or inert gas. Off-gas is typically processed to remove particulates. Volatiles in the off-gas may be burned in an afterburner, collected on activated carbon, or recovered in condensation equipment. Thermal desorption systems are physical separation processes that are not designed to provide high levels of organic destruction, although some systems will result in localized oxidation or pyrolysis. 

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