Section 6 Volatile Organic Compounds in Petroleum Products 6.1 EMISSIONS PRODUCED FROM SOIL CONTAMINATION Management of hazardous wastes involves many operations that can result in air emissions (Allen and Blaney, 1985). For example, disposal of wastes in landfills may release volatile organic compounds (VOCs). Waste transfer and handling operations may also be a significant source of VOC emissions. Emissions from hazardous waste treatment storage and disposal facilities (TSDFs) have been estimated to be in excess of 1.5 Mt/year, and possibly over 5 Mt/year (Breton et al., 1983). It has been reported that at least one third of the total emissions of over 50 volatile, hazardous chemicals are from TSDFs (Springer, Valsaraj, and Thibodeaux, 1986). VOC emissions from industry, transportation, and other sources have been estimated to be 10.7, 7.7, and 3.9 Mt/year, respectively (Breton et al., 1983). Table 6.1 gives the estimated relative emissions from various types of TSDFs. Landfarming can be used if volatilization of VOCs into the air is permitted; however, there is growing concern over VOC emissions and resulting air pollution (IT Corporation, 1987). Emissions from landtreatment facilities arise by volatilization from the wastes that have been spread on the soil prior to being incorporated within the top layers (Ehrenfeld, Ong, Farino, Spawn, Jasinski, Murphy, Dixon, and Rissmann, 1986). They later arise after the wastes have been mixed into the soil, as the materials volatilize and diffuse upward through the soil. The process of spreading and tilling of the contaminated soil results in volatilization of a significant fraction (up to 40 to 60%) of the volatile organics. By using the emission isolation flex chamber method to measure VOC emission rates, it was found that tilling caused a two- to tenfold increase in the emission rates, with peak emissions occurring within the first 4 h after tilling (Blaney, Eklund, Thorneloe, and Wetherold, 1986). Results from a landtreatment facility demonstrated that more than 90% of the organic compounds in hazardous oily wastes from a refinery were being biologically degraded, transformed, and volatilized in the soil (Fuller, Hinzel, Olsen, and Smith, 1986). Gases may be generated by reactions in the subsurface (Ehrenfeld, Ong, Farino, Spawn, Jasinski, Murphy, Dixon, and Rissmann, 1986). Aerobic or anaerobic biological activity may decompose organics to produce methane, hydrogen sulfide, carbon dioxide, or other gases, which bubble up through the impoundment, carrying volatile materials to the surface. The addition of microorganisms or aeration of the soil for stimulation of biodegradation would also increase volatile emissions. Chemical reactions may also increase emissions, if gases are produced. As VOCs pass through the soil, they can undergo a variety of transformations, such as biodegradation, adsorption onto the soil, dissolution in the soil water, and leaching into the groundwater (Valsaraj and Thibodeaux, 1988). Volatilization includes the loss of chemicals from surfaces in the vapor phase, indicating that it requires the vaporization and movement of chemicals from a surface into the atmosphere above the surface (Dupont and Reineman, 1986). The abiotic process of evaporation can contribute significantly to the overall removal process of contaminants from soil (Kang and Oulman, 1996). The evaporation rate of VOCs can be predicted by a model, which indicates that the rate of evaporation for a particular volatile liquid is proportional to the square root of the product of diffusivity and partial pressure divided by the molecular weight of the liquid. This partially explains why evaporative losses from sand are so much higher for gasoline than for diesel fuel. Volatile compounds are components in the soil and groundwater contamination at many, if not most, Superfund sites (Devitt, Evans, Jury, Starks, Eklund, and Gholson, 1987). Table 6.2 lists the 25 com- pounds most frequently reported at Superfund sites; 15 of these are volatile organic solvents. Table 6.3 shows more of the VOCs on the Hazardous Substance List (McDevitt, Noland, and Marks, 1987). The predominant hazardous volatile organics in hazardous wastes from the petroleum refining industry are benzene and toluene (Overcash, Brown, and Evans, 1987). The total estimated amounts of benzene © 1998 by CRC Press LLC and toluene in wastes treated by land annually by the U.S. petroleum industry are 150,000 and 950 lb, respectively. However, landtreatment of hazardous wastes is a minor source of VOCs, compared with other emissions sources. The main focus of Federal and state regulations of TSDFs in the past has been to minimize contam- ination of surface and groundwater, to prevent air contamination by incineration, and to prevent accidental exposure (Springer, Valsaraj, and Thibodeaux, 1986). More recently, the emphasis has shifted to the emissions themselves and is focusing on specific hazardous constituents. The transfer of VOCs to ambient air is a concern of Michigan, for example (Love, Ruggiero, Feige, Carswell, Miltner, Clark, and Fronk, Table 6.1 Emissions from Hazardous Waste Treatment, Storage, and Disposal Facilities Facility Type Estimated Annual Emissions a (10 3 metric t/year) Treatment tanks 530 Nonaerated surface impoundment — storage 420 Nonaerated surface impoundment — treatment 310 Landfill 190 Nonaerated surface impoundment — disposal 66 Aerated surface impoundment 66 Land applications 43 Storage tanks 10 Total 1635 a For 54 selected chemicals. Source: Breton, M. et al. GCA Report No. GCA-TR-83-70-G, U.S. EPA. August 1983. Table 6.2 Most Frequently Reported Substances at 546 MPL Sites Substance Percent of Sites Trichloroethylene 33 Lead 30 Toluene 28 Benzene 26 Polychlorinated biphenyls (PCBs) 22 Chloroform 20 Tetrachloroethylene 16 Phenol 15 Arsenic 15 Cadmium 15 Chromium 15 1,1,1-Trichloroethane 14 Zinc and compounds 14 Ethylbenzene 13 Xylene 13 Methylene chloride 12 trans -1,2-Dichloroethylene 11 Mercury 10 Copper and compounds 9 Cyanides (soluble salts) 8 Vinyl chloride 8 1.2-Dichloroethane 8 Chlorobenzene 8 1,1-Dichloroethane 8 Carbon tetrachloride 7 Source: Devitt, D.A. et al. Report No. EPA-600/8-87/036, 1987. © 1998 by CRC Press LLC 1983). Here, the “best available technology” must be applied to treatment trains discharging to the air, and carcinogens must not be transferred to ambient air. The release rates of VOCs in a landtreatment system are characterized by a peak that occurs imme- diately upon waste application, followed by a rapid, exponential decline approaching steady conditions within minutes to hours (Figure 6.1; Overcash, Brown, and Evans, 1987). Additional tillage of the soil briefly releases another peak of volatiles of lesser magnitude, but the emission rates quickly return to those before the tillage. Bolick and Wilson (1994) report that the extent to which the contaminant VOC has spread in the subsurface has a significant effect on the cleanup time required, indicating that very substantial savings in cleanup costs can result from rapid response after a spill has occurred. It is suggested that if preliminary pumping is started as soon after the incident as possible, even before negotiations are completed, Table 6.3 VOCs Included on the Hazardous Substance List (HSL) Detection Limits a Low Water Low Soil/Sediment VOCs (µg/L) (µg/kg) Chloromethane 10 10 Bromomethane 10 10 Vinyl chloride 10 10 Chloroethane 10 10 Methylene chloride 5 5 Acetone 10 10 Carbon disulfide 5 5 1,1-Dichloroethene 5 5 1,1-Dichloroethane 5 5 trans -1,2-Dichloroethene 5 5 Chloroform 5 5 1,2-Dichloroethane 5 5 2-Butanone 10 10 1,1,1-Trichloroethane 5 5 Carbon tetrachloride 5 5 Vinyl acetate 10 10 Bromodichloromethane 5 5 1,1,2,2-Tetrachloroethane 5 5 1,2-Dichloropropane 5 5 trans -1,3-Dichloropropene 5 5 Trichloroethene 5 5 Dibromochloromethane 5 5 1,1,2-Trichloroethane 5 5 Benzene 5 5 cis -1,3-Dichloropropene 5 5 2-Chloroethyl vinyl ether 10 10 Bromoform 5 5 2-Hexanone 10 10 4-Methyl-2-pentanone 10 10 Tetrachloroethene 5 5 Toluene 5 5 Chlorobenzene 5 5 Ethyl benzene 5 5 Styrene 5 5 Total xylenes 5 5 a Medium water contract required detection limits (CRDL) for volatile HSL com- pounds are 100 times the individual low-water CRDL. Medium soil/sediment CRDL for volatile HSL compounds are 100 times the individual low soil/sediment CRDL. Detection limits listed for soil/sediment are based upon wet weight. Source: McDevitt, N.P. et al. Report No. AMXTH-TE-CR-86092. ADA 178261. U.S. Army Toxic and Hazardous Materials Agency, Aberdeen, MD, 1987. © 1998 by CRC Press LLC groundwater contamination may be reduced or avoided, and it would take considerably less time to remediate the site (Bolick and Wilson, 1994). As volatile materials move through the hazardous waste management process, they must be destroyed, accumulated, emitted, or recycled to a prior step at each stage (Ehrenfeld, Ong, Farino, Spawn, Jasinski, Murphy, Dixon, and Rissmann, 1986). Design and operating practices, in situ treatment techniques, and pre- and posttreatment methods can help reduce emissions. It has been proposed that the choice of control method should be dictated by facility and environmental setting, rather than by waste properties, but waste properties can influence the effectiveness of the treatment. Permeability of the contaminated soil greatly affects cleanup times (Bolick and Wilson, 1994). The extent of the VOC contamination should be determined. However, greater emphasis should be placed on permeability measurements than on measurement of soil VOC concentrations, which have less effect, for calculating cleanup times by soil vapor extraction (SVE). Effectiveness of a treatment is measured, primarily, by the degree of reduction of the rate of emissions, not by the reduction in total emissions over long periods of time (Ehrenfeld, Ong, Farino, Spawn, Jasinski, Murphy, Dixon, and Rissmann, 1986). Retarding the rate of loss by volatilization keeps the materials in the facility for longer periods. If other loss mechanisms, such as biodegradation, are also involved, then reducing the emission rate may change the absolute quantity emitted over time. An overall system for the treatment, storage, and disposal of VOCs is given in Figure 6.2. Two methods of applying waste oily sludge were tested at a landtreatment site to compare the effects of application on the emissions (Eklund, Nelson, and Wetherold, 1987). Waste was applied either on the surface or injected 6 to 11 in. (0.15 to 0.28 m) below the surface and immediately disked. The plots were tilled two to three times a week for 5 weeks. The annual oil loading was about 8 to 27 kg/m 2 (300 to 1000 tons of wastes per acre at 12% oil). The volatile organics constituted about 0.8% of the sludge. The average emission rate of volatile organics over the 5 weeks from surface application was 47.1 µg/m 2 -s (RSD or relative standard deviation = ±14.9%), with a background of 6.16 µg/m 2 -s (RSD = ±10.6%) (Eklund, Nelson, and Wetherold, 1987). The average emission from subsurface application was 53.9 µg/m 2 -s (RSD = ±16.3%). The instantaneous emissions from the three were as high as 370.7, 38.5, and 324.9 µg/m 2 -s, respectively. The emission rate decreased exponentially over time. The ratio of volatile organics released over 5 weeks to purgeable organics in the waste was 0.30 for the surface application and 0.36 for the subsurface. The ratios of volatile organics emitted over 5 weeks to the mass of applied oil were around 0.012 and 0.014 for the two plots, respectively. Table 6.4 shows the cumulative emissions for individual compounds and the percentage of the applied quantity for each. Individual compounds behaved in the same manner as the total volatile organics, with diurnal fluctuations. Of 2896 tons of sludge applied to the landtreatment site over a year, approximately 43%, or 8900 kg/year, of the VOCs were emitted; 6,400 kg was released during the first 5 weeks after application. If emissions controls were 50% effective, a 4500 kg/year reduction in VOC emissions might be obtained. An emissions control of 90% efficiency would result in an 8000 kg/year reduction in emissions. Figure 6.1 Typical chart trace from a total hydrocarbon monitor. (From Overcash, M. et al. Report No. ANL/EES- TM-340. DE88005571. Argonne National Laboratory, Argonne, IL, 1987.) © 1998 by CRC Press LLC Figure 6.2 Overall system for treatment, storage, and disposal of VOCs. (With permission. Ehrenfeld, J.R. Surface Impoundments. Noyes, Park Ridge, NJ, 1986.) Table 6.4 Cumulative Measured Emissions of Selected Individual Compounds Cumulative Emissions (g) As Percent of Applied Control Plot A Plot B Plot C Plot A Plot B Plot C Compound (Surface) (Background) (Subsurface) (Surface) (Background) (Subsurface) n -Heptane a 152 1.06 243 9.36 0.07 15.0 Methylcyclohexane 183 2.12 272 9.40 0.11 14.0 3-Methylheptane 161 3.05 249 8.04 0.15 12.4 n -Nonane b 131 4.20 190 8.83 0.28 12.8 1-Methylcyclohexane 32.0 0.802 56.2 6.97 0.18 12.2 1-Octene a 35.9 1.18 53.5 7.77 0.26 11.6 β -Pinene 58.5 3.43 75.5 2.61 0.15 3.37 Limonene b 39.3 2.73 45.8 3.47 0.24 4.04 Toluene a 271 2.18 403 6.24 0.05 9.29 p -, m -Xylene 219 3.22 311 5.59 0.08 7.93 1,3,5-Trimethylbenzene 74.1 3.85 97.8 3.23 0.17 4.27 o -Ethyltoluene b 88.9 4.1 122 4.89 0.23 6.70 a Relatively light compounds in each class. b Heaviest compound in each class. Source: Eklund, B.M. et al. EPA-600/2-87/086a, Cincinnati, OH, 1987. © 1998 by CRC Press LLC A monoclonal antibody immunoassay for the rapid, on-site screening of gasoline- and diesel fuel–contaminated soil has been developed to detect volatile and semivolatile refined petroleum products (gasoline, diesel fuel, kerosene, and jet fuel) in the field (Mapes, McKenzie, Arrowood, Studabaker, Allen, Manning, and Friedman, 1993). The test involves use of PETRO RISc soil and ELISA (enzyme- linked immunosorbent assay), which detect these compounds at 100 and 75 ppm, respectively. It is simple to conduct, requires <20 min to perform, and is applicable to field testing. 6.1.1 GASOLINE VAPOR COMPOSITION Gasoline is a clear, volatile liquid that is a complex mixture of paraffinic, olefinic, and aromatic hydrocarbons (Phillips and Jones, 1978). The liquid gasoline contains up to 250 constituents and the vapor phase, from 15 to 70 components. The composition of gasoline varies greatly as a result of crude oil characteristics, processing tech- niques, and climate; therefore, there is no single threshold limit value (TLV) for all types of these materials (McDermott and Killiany, 1978). Light hydrocarbon compounds, such as butanes and pentanes, are blended into the gasoline to achieve the right volatility. To improve antiknock performance, branched chain aliphatic hydrocarbons or aromatic compounds resistant to detonation are blended into the gasoline. A gas chromatographic analytical technique can separate gasoline vapor into about 142 different com- ponents (Phillips and Jones, 1978). The aromatic hydrocarbon content generally determines the particular TLV. Consequently, the content of benzene, other aromatics, and additives should be determined to arrive at an appropriate TLV (Runion, 1975). The vapor contains more lighter hydrocarbons when bulk liquid gasoline evaporates during loading or dispensing activities (McDermott and Killiany, 1978). About 92% of the total gasoline vapor by volume is represented by 21 hydrocarbon compounds (Table 6.5). About 43% of the vapor in an average sample consists of butanes. When benzene constitutes about 1% of the liquid gasoline, it contributes about 0.7% to the total gasoline vapor. There is no Federal OSHA permissible exposure limit for gasoline, although there are separate limits for 14 individual hydrocarbon constituents and some additives (Phillips and Jones, 1978). The gasoline vapor component data were used to estimate a TLV for mixtures. Based on the toxicity of hydrocarbon compounds in gasoline vapor, 240 ppm TWA (time weighted average) exposure over 8 h and a 1000-ppm peak over 15 min have been suggested as reasonable criteria. 6.1.2 HUMAN HEALTH CRITERIA The U.S. EPA is developing information to set standards, as necessary, to control emissions from hazardous waste TSDFs (Eklund, Nelson, and Wetherold, 1987). These regulations are intended to protect human health and the environment from emissions of volatile compounds and particulate matter. The human health criteria provide estimates of ambient water concentrations, which, in the case of noncarcinogens, prevent adverse health effects in humans, and, in the case of suspected or proven carcinogens, represent various levels of incremental cancer risk (Bove, Lambert, Lin, Sullivan, and Marks, 1984). There is no method to establish the presence of a threshold for carcinogenic effects. The EPA policy is that there is no scientific basis for estimating “safe” levels for carcinogens. Therefore, the criteria for carcinogens state that the recommended concentration for maximum protection of human health is zero. The EPA Water Quality Criteria for protection of human health are presented in Table 6.6 for a 10 –5 risk level (i.e., one additional case of cancer in a population of 100,000). The TLV listing, published by the American Conference of Governmental Industrial Hygienists (ACGIH), is a major source of guidelines for safe exposure to toxic compounds (American Conference of Governmental Industrial Hygienists, 1976). Threshold limit values refer to airborne concentrations of substances and represent conditions under which it is believed nearly all workers may be repeatedly exposed day after day without adverse effect (American Conference of Governmental Industrial Hygienists, 1982). Threshold limits are based upon the best available information from industrial experience, from experimental human and animal studies, and, when possible, from a combination of the three. The TLVs, as issued by ACGIH, are recommen- dations to be used as guidelines for good practices. They do not have the force and effects of law. When two or more hazardous substances, which act upon the same organ system, are present, their combined © 1998 by CRC Press LLC effect, rather than that of either individually, should be given primary consideration. If such information is not available, the effects of the different hazards should be considered as additive. 6.2 PARAMETERS AFFECTING VOLATILIZATION For volatilization from soil to occur, organic compounds must move through a complex structure of solid particles and void spaces to the soil surface (Bell, Morrison, and Chonnard, 1987). At the surface, the pollutant must then traverse a relatively stagnant atmospheric film of air to escape into the atmosphere. An understanding of the mechanisms of transport of a pollutant through the soil is very important for predicting its volatilization from the soil. Several important mechanisms of pollutant transport are diffusion through the vapor and aqueous phases, flow of water-soluble pollutants to the surface due to capillary action, and evaporation of water from the soil surface. The rate of contaminant volatilization is a complex function of the properties of the contaminant and its surrounding environment (Dupont and Reineman, 1986). For organics in soil systems, the factors that affect volatilization include (Spencer and Cliath, 1977; Ehrenfeld, Ong, Farino, Spawn, Jasinski, Murphy, Dixon, and Rissmann, 1986): Contaminant vapor pressure Contaminant concentration The Henry’s law constant of the waste Soil/chemical adsorption reactions Contaminant solubility in soil water Contaminant solubility in soil organic matter Soil temperature, water content, organic content, porosity, and bulk density Table 6.5 Approximate Gasoline Vapor Components Airborne Gasoline Boiling Point Vapor Composition Compound (°C) mean vol% Alkanes Propane –42.1 0.8 n -Butane –0.5 38.1 Isobutane –11.7 5.2 Isopentane 27.9 22.9 n -Pentane 36.1 7.0 Cyclopentane 49.3 0.7 2,3-Dimethylbutane 58.0 0.7 2-Methylpentane 60.3 2.1 3-Methylpentane 63.3 1.6 n -Hexane 68.7 1.5 Methyl cyclopentane 71.8 1.3 2,4-Dimethylpentane 80.3 0.4 2,3-Dimethylpentane 89.8 0.7 2,2,4-Trimethylpentane 99.2 0.5 Alkenes Isobutylene –6.9 1.1 2-Methyl-1-butene 31.2 1.6 cis -2-Pentene 37.0 1.2 2-Methyl-2-butene 38.6 1.7 Aromatics Benzene 80.1 0.7 Toluene 110.6 1.8 Xylene ( p, m, o ) 142.0 0.5 Total Percent 92.1 Source: McDermott, H.J. and Killiany, S.E., Jr. Am. Ind. Hyg. Assoc. J. 39:110–117, 1978. With permission. © 1998 by CRC Press LLC Wind, humidity, and solar radiation Adsorption to soil (Valsaraj and Thibodeaux, 1988) The major contaminant property affecting volatilization is its vapor pressure, while the major envi- ronmental factors affecting contaminant mobility are the various soil/air, soil/water, and air/water par- tition coefficients for the various soil/water/air environments present within the soil system (Dupont and Reineman, 1986). It becomes more complex if the contaminant is added in a carrier fluid, such as oil in refinery wastes, where partitioning of the contaminant between the oil/soil, oil/water, and oil/air phases would also affect the volatilization of hazardous compounds in the waste. The chemical and physical properties of organic contaminants and the properties of the unsaturated zone that affect emissions are discussed below. 6.2.1 TEMPERATURE Soil temperature and the gradient that is established within the unsaturated zone can have an impact on the status of organic compounds (Devitt, Evans, Jury, Starks, Eklund, and Gholson, 1987). If there are large temperature gradients (surface layers), thermal diffusion will readily take place. Warming the soil lowers the suction and raises the vapor pressure of soil water (Hillel, 1971). Thus, a thermal gradient Table 6.6 EPA Water Quality Criteria a,b for Protection of Human Health (10 –5 Risk Level) Contaminant µg/L Contaminant µg/L Volatile Organics Acrolein 320 1,2-Dichloropropylene 87 Acrylonitrile 0.58 d Ethylbenzene 1,400 Benzene 6.6 d Methylene chloride (dichloromethylene) 1.9 d Bis (chloromethyl) ether 0.000038 d 1,1,2,2-Tetrachloroethane 1.7 d Carbon tetrachloride 4.0 d Tetrachloroethylene 8 d Chlorobenzene 488 (20) e Toluene 14,300 Chlorodibromomethane 1.9 d 1,1,1-Trichloroethane 18,400 Chloroform 1.9 d 1,1,2-Trichloroethane 6.0 d Dichlorobromomethane 1.9 d Trichloroethylene 27 d Dichlorodifluoromethane 1.9 d Trichlorofluoromethane 1.9 d 1,2-Dichloroethane 9.4 d Vinyl chloride 20 d 1,1-Dichloroethylene 0.33 d Acid Extractables Pentachlorophenol 1,010 (30) e Phenol 3,500 (300) e Base/Neutral Extractables Fluoranthene 42 Naphthalene NDA c Metals and Cyanide Antimony 146 Mercury 0.144 Arsenic 0.022 d Nickel 13.4 Beryllium 0.037 d Selenium 10 Cadmium 10 Silver 50 Chromium 50 Thallium 13 Copper NDA Zinc NDA Lead 50 Cyanide 200 a EPA water quality criteria documents (45 FR 79318, 28 November 1980). b Values in micrograms per liter (µg/L). c No definitive data available. d 10 –5 cancer risk criteria. e Taste and odor (organoleptic) criteria. Source: Bove, L.J. et al. Report to U.S. Army Toxic and Hazardous Materials Agency, Aberdeen Proving Ground, MD, on Contract No. DAAK11-82-C-0017. AD-A162 528/4. 1984. © 1998 by CRC Press LLC induces flow and distillation from warmer to cooler regions. Organic vapors migrating from the ground- water to the soil surface during warm months and during the daytime will have to move against a temperature gradient (i.e., movement by concentration gradient). During colder months, if the soil surface freezes, vapors may not be able to escape and would concentrate or move laterally. Organic compounds with boiling points lower than soil temperatures, such as the gaseous alkanes propane and isobutane, which boil at –42.1 and –11.17°C, will be highly volatile (Mackay and Shiu, 1981). Increasing the temperature increases the vapor pressure of a compound (Ehrenfeld, Ong, Farino, Spawn, Jasinski, Murphy, Dixon, and Rissmann, 1986). There is a three- to fourfold increase in soil vapor pressure for every 10°C increase in temperature (Dibble and Bartha, 1979a). The temperature of water greatly affects the diffusivity, which increases approximately as the 1.8 power of the absolute temperature (Springer, Valsaraj, and Thibodeaux, 1986). Thus, a high liquid temperature favors more- rapid volatilization. In an evaluation of a vapor-phase carbon adsorption system for the removal of toluene from a contaminated airstream, it was found that by increasing the relative humidity from 30 to 90%, at constant temperature, the carbon loading could be cut by about 50% (Foster, 1985). Increasing the operating temperature from 20°F, with constant relative humidity, also reduces the carbon loading by 50%. There is no apparent correlation between soil bed temperature and VOC removal efficiency (McDevitt, Noland, and Marks, 1987). There does appear to be an inverse relationship between the inlet air temperature with air stripping and the VOC removal efficiency. Although decreasing inlet air temperature corresponds with increasing removal efficiency, it may not be the cause of it. Section 5.1.2 further discusses the role of soil temperature in bioremediation and how it can be modified. Management of this factor can affect VOC emissions. 6.2.2 OPERATING SURFACE AREA The rate of emissions is directly proportional to the operating surface area (Ehrenfeld, Ong, Farino, Spawn, Jasinski, Murphy, Dixon, and Rissmann, 1986). Minimizing surface area can help control emissions. This is essentially a linear relationship. The total quantity of materials volatilized would not be changed over the long run but would simply take longer to volatilize. 6.2.3 WIND/BAROMETRIC PRESSURE The rate of emission into still air is slower than evaporation into the wind (Ehrenfeld, Ong, Farino, Spawn, Jasinski, Murphy, Dixon, and Rissmann, 1986). Surface turbulence by wind or mechanical agitation increases the rate of volatilization. Wind erosion of wastes depends upon waste type, wind velocity, moisture content, and surface geometry. The effect of barometric pressure on soil gas transport is minor, being greatest at or near the soil surface (Buckingham, 1904). Periods of high wind and low barometric pressure may be optimal for maximum earth out-gassing (Reichmuth, 1984). However, this is a surface phenomenon. 6.2.4 SOIL MOISTURE/VOLUMETRIC WATER CONTENT Soil moisture content provides an indication of VOC removal efficiency and possibly processed soil VOC residuals (McDevitt, Noland, and Marks, 1987). Soil moisture is important in determining the extent of adsorption of neutral, nonpolar molecules like most VOCs onto soil surfaces (Poe, Valsaraj, Thibodeaux, and Springer, 1988). Polar compounds show a greater degree of adsorption than nonpolar and slightly polar adsorbates. VOCs are strongly adsorbed to soils at low moisture contents. They are displaced from their adsorption sites as soil moisture increases, as a result of competition for adsorption sites on the polar mineral surface from polar water molecules. The adsorption of VOCs by dry soils is considerable and is dominated by mineral adsorption (Poe, Valsaraj, Thibodeaux, and Springer, 1988). Most of the adsorption occurs on the external surface of the soils. The high degree of adsorption in dry soils retards the movement of volatile organics from hazardous waste landfills and from surface soil during land application of hazardous wastes. The volumetric water content is the ratio of the volume of water in a porous medium to the total volume (Devitt, Evans, Jury, Starks, Eklund, and Gholson, 1987). When water fills the entire pore volume, the medium is saturated. Coarse soils have lower volumetric water contents at saturation than do medium- textured soils, and medium-textured soils have less than clayey soils. As the volumetric water content increases, the air-filled porosity decreases and the path for vapor flow becomes restricted. © 1998 by CRC Press LLC It is common practice when wastes are being covered with soil to spray water over the soil as a dust control measure and to help compact the soil (Goring, 1962; Letey and Farmer, 1974). The amount of water added decreases the air-filled pore space available for vapor diffusion and, thus, affects the volatilization through the soil cover. By virtue of the solubility of benzene in water, increasing the soil water content will increase the capacity of the soil to retain benzene in the solution phase, reducing the quantity of benzene available for vapor-phase diffusion. The rate of volatilization and wicking of gasoline in soil is also reduced by an increase in soil water content (Smith, Stiver, and Zytner, 1995). Gasoline flux toward the soil surface is dependent upon wicking. Organic contaminants in the unsaturated zone are susceptible to leaching, depending upon the frequency and amount of rainfall (Devitt, Evans, Jury, Starks, Eklund, and Gholson, 1987). Vertical concentration gradients are altered as vapors reaching the rainfall-saturated zone concentrate or move laterally or are resolubilized to some extent. An equilibrium partitioning model has been developed to predict the effect of soil moisture content on vapor-phase sorption (Unger, Lam, Schaefer, and Kosson, 1996). Soil moisture and how it can be controlled to improve biodegradation is further discussed in Section 5.1.1. 6.2.5 MASS TRANSFER COEFFICIENT/PARTITION COEFFICIENT The VOC removal efficiency is directly related to the total VOC concentration in the feed soils; as the feed concentration increases, the VOC removal efficiency also increases (McDevitt, Noland, and Marks, 1987). The driving force for mass transfer is the difference between the VOC concentration in the airstream and the VOC concentration in the soil. An increase in the driving force causes an increase in mass transfer, with a corresponding increase in VOC removal efficiency. The mass transfer process that governs the volatilization of almost all the chemicals of interest is the liquid-phase process (Springer, Valsaraj, and Thibodeaux, 1986). Therefore, differing rates from one chemical to another are largely a matter of liquid-phase diffusivity differences, and the rates do not vary greatly from one chemical to another, despite vapor pressure variations. It is assumed that the volatility is high enough that it is not limiting. If the volatile materials are present as a dilute aqueous solution, the basic mass flow equation shows that the rate of emission depends upon the overall coefficient, the exposed area, and the concentration or mole fraction in the liquid (Ehrenfeld, Ong, Farino, Spawn, Jasinski, Murphy, Dixon, and Rissmann, 1986). The concentration in the gas phase in uncovered soil is essentially zero, with fresh air continually sweeping over the system. Emissions can be controlled through the mass transfer coefficient (Ehrenfeld, Ong, Farino, Spawn, Jasinski, Murphy, Dixon, and Rissmann, 1986). The controllable parameters that determine the value of the mass transfer coefficient are the wind speed at the surface and the effective depth. Barriers and fences can reduce the wind speed. The dependence on depth is inverse. In theory, deeper impoundments have lower mass transfer coefficients, with reduced rates of volatilization. In the case of a layer of lighter-than-water, immiscible organic compounds floating on the surface, the controlling mechanism will be diffusion in the gas phase, not the liquid phase (Ehrenfeld, Ong, Farino, Spawn, Jasinski, Murphy, Dixon, and Rissmann, 1986). The mass transfer coefficient then depends upon the operating parameters. Reducing the wind speed should inhibit the rate of volatilization. The rate of mass transfer depends upon the Henry’s law constant, as well as the individual mass transfer coefficient. It is temperature dependent, increasing with increasing temperature. Controls that reduce the surface temperature would, therefore, inhibit the rate of volatilization. Benzene may move by molecular diffusion in soil, in both the vapor phase and the solution phase. The relative importance of each phase is determined by the relative magnitude of the concentration in air (vapor density) and the concentration in solution (Goring, 1962; Letey and Farmer, 1974). Chemicals with partition coefficients between the soil water/soil air Ӷ 10 4 will diffuse mainly in the vapor phase, and those with higher coefficients will diffuse primarily in the solution phase. Since benzene has a partition coefficient of 4.6 at 25°C, it should diffuse primarily in the vapor phase. 6.2.6 EFFECTIVE DEPTH Theoretically, the deeper the contaminant is in the subsurface, the lower the mass transfer coefficients and rates of volatilization (Ehrenfeld, Ong, Farino, Spawn, Jasinski, Murphy, Dixon, and Rissmann, 1986). © 1998 by CRC Press LLC [...]... situ removal of spilled VOCs from soils to prevent contamination of groundwater (Bennedsen, 1987) In situ removal may be a cost-effective alternative to the more usual remedy of excavation and off-site disposal of the contaminated soil Off-gas treatment may be required from SVE systems of air-stripping units to limit hydrocarbon discharge to the atmosphere Figure 6. 4 is an illustration of a soil gas... 101 103 101 102 102 102 KH 2.2 × 10–1 6. 6 × 10–2 9.4 × 10–1 1.5 × 10–1 1.2 × 10–1 2.2 2 × 10–3 6. 7 × 10–4 5.9 × 10–4 3.5 × 10–2 3 .6 × 102 4 × 10–2 1.3 × 10–4 1.5 5 × 10–2 1 × 10–3 1.4 × 102 7 × 10 6 6.7 × 10–3 4.4 × 10–1 3 × 10–1 1.5 9.7 × 101 TD (day) 129 5100 45 292 121 16 2.9 × 4.2 × 1.1 × 500 9.3 343 2.4 × 17 6. 2 × 1.7 × 21 9.2 × 2 .6 × 77 143 29 10 107 1 06 105 1 06 103 104 105 105 With φ = 0.5, a =... volatilize This is discussed in Sections 2.1.1.2.1 and 2.1.2.2.1.2 The other method is to drive off the VOCs, such as by use of air stripping or evaporation, and capture the emissions on the activated carbon Vapor-phase adsorption is the accumulation of a chemical from an off-gas stream onto the surface of a solid (Ram, Bass, Falotico, and Leahy, 1993) The contaminated off-gas may be treated by passing... persist longer The effect of chemical structure on biodegradation is reviewed in Section 4.1.8 6. 2.21 AIR-FILLED POROSITY The air-filled porosity of a porous medium, such as soil, is defined as the ratio of the volume of air in the soil pores to the total volume (volume of air, water, and soil combined) (Devitt, Evans, Jury, Starks, Eklund, and Gholson, 1987) It is the portion of the total soil volume... Trichloroethylene Tetrachloroethylene 1,1,1-Trichloroethane Carbon tetrachloride cis-1,2-Dichloroethylene 1,2-Dichloroethane 1,1-Dichloroethylene 0.1 1 10 50 100 1000 100 10 1 1000 100 10 1 1000 100 10 1 1000 100 10 1 1000 100 10 1 1000 100 10 1 1000 100 10 1 76: 1 54:1 32:1 11:1 96: 1 72:1 45:1 18:1 198:1 90:1 35:1 8:1 19:1 15:1 10:1 6: 1 104:1 77:1 52:1 26: 1 56: 1 42:1 28:1 14:1 10:1 8:1 5:1 3:1 54:1 32:1... all air-to-water ratios tested (Singley and Williamson, 1982) The size and costs of operating the large compressors necessary for increased air-to-water ratios may negate any performance improvement of operating at the higher airto-water ratio Therefore, the most efficient performance possible at lower air-to-water ratios should be determined Although this may be the best approach with diffused-air aeration... over time A shortcoming in the design of redwood slat aerators is the lack of inspection ports, making routine examination difficult (Fronk-Leist, Love, Miltner, and Eilers, 1983) This has prevented detection of accumulation of as much as 15 to 20 cm (6 to 8 in.) of iron sludge Between 400 to 500 kg of iron could accumulate in such a unit, over the course of a year of continuous running The iron sludge... excess of 1 and often 10% (Mackay, Roberts, and Cherry, 1985) Organic compounds with specific gravities of less than 1.0 associated with solubilities of less than 1% are referred to as floaters (New York State Department of Environmental Conservation, 1983) Section 4.1.7 provides more information on the effect of density on biodegradation of organic compounds in soil 6. 2.17 VISCOSITY The viscosity of a... first carbon unit Air stripping with carbon treatment of off-gas is often more cost-effective than liquidphase carbon treatment alone, since vapor-phase carbon adsorbs 5 to 20 times more of a given contaminant per pound than liquid-phase carbon 6. 3.3.1.5.1 Gaseous Carbon Adsorption Activated carbon is the most commonly used adsorbent for removal of volatile compounds from a gas stream, and other materials... liquid in the subsurface (Mackay, Roberts, and Cherry, 1985) See Section 4.1 .6 for background on the effect of viscosity on biodegradation of organic compounds in soil 6. 2.18 DIELECTRIC CONSTANT The dielectric constant of a medium defines the relationship between two charges and the distance of separation of the two charges to the force of attraction (Devitt, Evans, Jury, Starks, Eklund, and Gholson, . 1.3 2,4-Dimethylpentane 80.3 0.4 2,3-Dimethylpentane 89.8 0.7 2,2,4-Trimethylpentane 99.2 0.5 Alkenes Isobutylene 6. 9 1.1 2-Methyl-1-butene 31.2 1 .6 cis -2 -Pentene 37.0 1.2 2-Methyl-2-butene 38 .6 1.7 Aromatics Benzene. 22.9 n -Pentane 36. 1 7.0 Cyclopentane 49.3 0.7 2,3-Dimethylbutane 58.0 0.7 2-Methylpentane 60 .3 2.1 3-Methylpentane 63 .3 1 .6 n -Hexane 68 .7 1.5 Methyl cyclopentane 71.8 1.3 2,4-Dimethylpentane. disposal 66 Aerated surface impoundment 66 Land applications 43 Storage tanks 10 Total 163 5 a For 54 selected chemicals. Source: Breton, M. et al. GCA Report No. GCA-TR-8 3-7 0-G, U.S.