Section 2 Current Treatment Technologies Soil treatment technologies are often developed and evaluated in order to conform with regulatory demands, which may require or suggest that residual total petroleum hydrocarbon (TPH) concentrations in soil be reduced below 1000 mg/kg or, in some areas, below 100 mg/kg TPH. There are many technologies available for treating sites contaminated with petroleum hydrocarbons; however, the treatment selected depends upon contaminant and site characteristics, regulatory require- ments, costs, and time constraints (Ram, Bass, Falotico, and Leahy, 1993). These authors propose a decision framework that is structured and tiered for selecting remediation technologies appropriate for a given contamination incident. Commonly used technologies can be integrated to enhance performance. Variation in design and implementation of the technologies, with concurrent or sequential configurations, can help to optimize the effectiveness of the treatment. The American Petroleum Institute (API) developed a petroleum decision framework to facilitate decision making for investigation and cleanup of petroleum contamination of soils and groundwater (API, 1990). Kelly, Pennock, Bohn, and White (1992) of the U.S. Department of Energy Pacific Northwest Laboratories also produced a Remedial Action Assessment System (RAAS) for information on remedial action technologies. The EPA Risk Reduction Engineering Laboratory (RREL) provides a treatability database, which is accessible through the Office of Research and Development network retrieval system, the Alternative Treatment Technology Information Center (ATTIC), the EPA database for technical information on innovative treatment technologies for hazardous waste and other contami- nants ( Haztech News, 1992; Devine, 1994). An expert system for remediation cost information, Cost of Remediation Model (CORA), has been designed by EPA. EPA has also compiled descriptions of technologies for processes that treat contaminated soils and sludges (U.S. EPA, 1988). Emerging and developing technologies being studied in the EPA Superfund Innovative Treatment Evaluation (SITE) Program are also described (U.S. EPA, 1991). The EPA Soil Treatability Database organizes and analyzes treatment data from a variety of technologies, including innovative technologies (e.g., biotreatment, chemical extraction, and thermal desorption), for the applicability and performance in treating hazardous soil (Weisman, Falatko, Kuo, and Eby, 1994). The successful treatment of a contaminated site depends on designing and adjusting the system operations based on the properties of the contaminants and soils and the performance of the systems, and by making use of site conditions rather than force-fitting a solution (Norris, Dowd, and Maudlin, 1994). Integration of bioremediation with other technologies either simultaneously or sequentially can result in a synergistic effect among the techniques employed (National Research Council, 1993). Information regarding remediation systems is furnished by Katin (1995) to explain to the practicing plant engineer or small business person how to recognize a good design and the aspects of a good design that will allow ease of operation and maintenance. Remediation systems discussed include air strippers, oil/water separators, vacuum extraction systems, thermal and catalytic incinerators, carbon beds, sparging systems, and biological treatment systems. Table 2.1 lists a number of unit operations and the waste types for which they are effective (Canter and Knox, 1985). Table 2.2 compares various features and the applicability of a variety of remediation technologies (Ram, Bass, Falotico, and Leahy, 1993). 2.1 ON-SITE OR EX SITU PROCESSES Excavation is a common approach to dealing with contaminated soil (Lyman, Noonan, and Reidy, 1990). The excavated soil may be treated on site, treated off site, or disposed of in landfills without treatment. If treated, it may then be returned to the excavation site. Excavation is easy to perform, and it rapidly removes the contamination from the site in a matter of hours, as opposed to other remediation methods, which may require several months. It is often used when urgent and immediate action is needed. There are problems associated with excavation (U.S. EPA, 1989). It allows uncontrolled release of contaminant vapors to the atmosphere. Nearby buildings, buried utility lines, sewers, and water mains © 1998 by CRC Press LLC could be in the way, and aboveground treatment approaches tend to be more expensive than in situ methods. Contaminated soil may be considered a hazardous waste, and disposal is becoming increasingly restricted by regulation. In addition, the excavation site must be filled. The following physical, chemical, and biological processes are some of the techniques that might be employed to treat the contaminated soil, once it has been excavated and transported to an on-site or off- site location. 2.1.1 PHYSICAL/CHEMICAL PROCESSES 2.1.1.1 Soil Treatment Systems 2.1.1.1.1 Thermal Treatment Thermal desorption is an innovative, nonincineration technology for treating soil contaminated with organic compounds (Fox et al., 1991). It is a proven method in the field of nonhazardous waste treatment and can be used for treating petroleum-contaminated soils (Molleron, 1994). Contaminated soil is heated under an inert atmosphere to increase the vapor pressure of the organic contaminants, transferring them from the solid to the gaseous phase (Wilbourn, Newburn, and Schofield, 1994). This separates the organics from the soil matrix. Boehm (1992) describes an on-site/off-site method to treat polluted soil, which is based on a thermal process to remove oxidizable, organic pollutants with low boiling points. The thermal treatment plant consists of a mechanical pretreatment of soil material, a thermal treatment in a rotary kiln, and an outlet- gas treatment. Since 1987, a mobile pilot plant has been in operation and has demonstrated remarkable success by cleaning up more than 70 different kinds of soil. Low-temperature thermal treatment (low-temperature thermal stripping or soil roasting) can be used on excavated, contaminated soil (Ram, Bass, Falotico, and Leahy, 1993). A mobile thermal processor, which uses low-temperature thermal treatment of soils contaminated by volatile organic compounds (VOCs) is described by Velazquez and Noland (1993). With this method, the soil is heated to 450°C in an indirect heat exchanger. Jensen and Miller (1994) cite the requirement of heating the soil to >600°C for successful thermal treatment of petroleum-contaminated soil. The effect of thermal treatment by means of a natural gas-fired, batch, rotary kiln; by a single particle reactor (SPR); and by a rotary reactor (BSRR) on toluene, naphthalene, and hexadecane was studied at 300 to 650°C (Larsen, Silcox, and Keyes, 1994). The ease at which the hydrocarbons were removed were toluene > naphthalene > n -hexadecane, and increasing the temperature increased their desorption rates. Moisture had a large effect on the desorption rate, which was first order with respect to individual and total hydrocarbon concentrations. Chern and Bozzelli (1994) showed that a continuous-feed rotary kiln is highly effective in removing volatile and semivolatile organic contaminants from sand and soils. Temperature, residence time vola- tility, and purge gas velocity are the main parameters affecting the desorption, with higher temperatures and longer residence times resulting in higher removal efficiency. For complete removal (98%) of the organics at 20 min residence time, the temperature should be 100°C for 1-dodecene, 200°C for 1-hexa- decene, 150°C for naphthalene, and 250°C for anthracene. Table 2.1 Summary of Suitability of Treatment Processes Process Volatile Organics Nonvolatile Organics Inorganics Air stripping Suitable for most cases Not suitable Not suitable Steam stripping Effective concentrated technique Not suitable Not suitable Carbon adsorption Inadequate removal Effective removal technique Not suitable Biological Effective removal technique Effective removal technique Not suitable — metals toxic pH adjustment precipitation Not applicable Not applicable Effective removal technology Electrodialysis Not applicable Not applicable Inefficient operation/inadequate removal Ion exchange Not applicable Not applicable Inappropriate technology — difficult operation Source: From Canter, L.W. and Knox, R.C., Ground Water Pollution Control, Lewis Publishers, Boca Raton, FL, 1985. © 1998 by CRC Press LLC Table 2.2 Technology Applicability Technology Applicability Soil Type and Saturated Zone Characteristics Variations Cost Permits LPH recovery LPH withdrawal All lighter-than-water petrochemicals except for the most viscous fuel and lube oils Works better with more- permeable soils Total fluid extraction, passive bailers, dual pump recovery, recovery wells, thermally assisted LPH recovery, mop and disk skimmers Variable Groundwater discharge, product storage, and possibly, groundwater withdrawal Vadose zone Soil vapor extraction LPH less than about 0.5 ft, contaminants with Vp > 1 mmHg (BTEX, gasoline, MTBE, PCE, TCE, TCA, mineral spirits, MeOH, acetone, MEK, etc.) Permeable soils, ROI > 10 ft, depth-to-water greater than 3 ft Thermally assisted venting, horizontal venting, surface sealing, passive vent points, closed loop venting, concurrent groundwater pumping for VOCs in capillary fringe Low Air discharge permit may be required In situ percolation (bioremediation) Any aerobically biodegradable chemical in the vadose zone Works better in permeable soils; depth-to-water greater than 3 ft Oxygen and nutrients need to be supplied to the subsurface Low to moderate Air discharge permit may be required when soil venting used to provide oxygen Excavation All soils and contaminants All soil types Dewatering may be used to expose soils in capillary fringe High On-site treatment of excavated soil may require permitting Saturated zone Sparging Contaminants in saturated zone with K H > 0.1 and Vp > 1 mmHg; contaminants: BTEX, gasoline, PCE, TCE, TCA, mineral spirits Hydraulic conductivity > 10 –5 cm/s (silty sand or better); at least 5 ft of saturated thickness Hot air, steam, and cyclic sparging, concurrent groundwater pumping Low Air discharge permit; water discharge if concurrent groundwater pumping In situ bioremediation Any biodegradable chemical in the saturated zone; inhibited by pH extremes, heavy metals, and toxic chemicals Nutrients are transported better in more-permeable soil Oxygen supplied by sparging or peroxide addition; nutrient addition with groundwater recovery and reinjection Moderate to high Water discharge for nutrient injection, air discharge if performed with sparging/venting Excavation All soils and contaminants All soil types Dewatering needed, groundwater containment may be used (slurry walls, sheet piles) Very high Permits for dewatering operations © 1998 by CRC Press LLC Groundwater recovery and treatment Groundwater recovery Uses: (1) LPH recovery, (2) provides hydraulic control of contaminant plume, (3) pump and treatment technologies Transmissivity, depth-to- water and saturated-zone thickness determine optimal strategy Recovery wells, well points, interceptor trenches Variable Well installation, groundwater withdrawal and groundwater discharge Liquid-phase carbon Removal of compounds with low solubility/high adsorptivity See groundwater recovery High pressure (75 to 150 psi) and low pressure (12 to 15 psi) Low to high depending on contaminant loading Water discharge permit Air stripping Compounds with K H > 0.1; contaminants with K H between 0.01 and 0.1 may require an air-water ratio > 100 See groundwater recovery Packed towers, low profile, heated and closed-loop air stripping; off-gas treatment may be required Low, if no off- gas treatment required Air and water discharge permits Advanced oxidation Most effective on sulfide cyanide, double- bonded organics (PCE, TCE), BTEX, phenols chlorophenols, PCBs, PAHs, some pesticides See groundwater recovery Hydroxy/radicals produced by combinations of UV, ozone, and peroxide Moderate to high Water discharge permit Bioreactors Any biodegradable compound See groundwater recovery Fixed-film and suspended growth reactors Moderate to high Water discharge permit Off-gas treatment Vapor-phase carbon Adsorptive capacity generally increases with increasing molecular weight NA Pretreatment dehumidification; on-site regeneration Moderate Air discharge permit Catalytic oxidation Conventional units can treat all compounds containing carbon, hydrogen, and oxygen; concentrations should not exceed about 20% of the LEL NA Some units can treat chlorinated compounds, exhaust gas scrubbing may be required Moderate to high Air discharge permit Thermal oxidation Compounds containing carbon, hydrogen, and oxygen; usually not amenable to halogen-containing compounds NA Exhaust gas scrubbing may be required Moderate to high Air discharge permit Abbreviations: NA, not applicable; LEL, lower explosion limit; ROI, radius-of-influence; LPH, liquid-phase hydrocarbon; MTBE, methyl tert -butyl ether; PCE, perchloroethylene; TCE, trichloroethylene; TCA, trichloroethane; MEOH, methanol; MEK, methyl ethyl ketone; BTEX, benzene, toluene, ethylbenzene, and xylenes; PCBs, polychlorinated biphenyls; PAHs, polyaromatic hydrocarbons. Source: Ram, N.M., Bass, D.H., Falotico, R., and Leahy, M. J. Soil Contam. 2(2):167–189. Lewis Publishers, Boca Raton, FL, 1993. Table 2.2 (continued) Technology Applicability Technology Applicability Soil Type and Saturated Zone Characteristics Variations Cost Permits © 1998 by CRC Press LLC Thermal desorption can be combined with the Thermatrix flameless oxidation process for an integrated waste-processing system offering operational simplicity, near zero emissions, heat recovery and reuse, and reduced costs (Wilbourn, Newburn, and Schofield, 1994). After the organic contaminants are separated from the soil, the Thermatrix unit (Figure 2.1) treats the vapors. The heat produced during operation of the unit can be used to facilitate desorption of organic contaminants from soil matrices. An integrated Thermatrix/thermal desorption system can treat soils contaminated with VOCs at a feed rate of 5 ton/h. Use of a laboratory-scale quartz furnace enabled researchers to remove BTEX (benzene, toluene, ethylene, and xylene) and BTEX with heavy metals from contaminated soil (Yang and Ku, 1994). The removal efficiency increased with increasing reaction temperature and reaction time. Thermal treatment of heavy metal-contaminated soil would stabilize the heavy metals within, resulting in a lower leaching toxicity. A bench-scale treatment of soil contaminated with polycyclic aromatic hydrocarbons (PAHs) employed the ReTeC screw auger process for thermal desorption (Weisman, Falatko, Kuo, and Eby, 1994). A pilot-scale treatment of soil contaminated with PAHs, heterocyclic compounds, and phenols utilized the IT Corporation process for thermal desorption. Another thermal desorption treatment for removal of PAHs on a pilot scale employed the WES screw auger-based process. The Chemical Waste Management, Inc., X TRAX process has also been used on a pilot scale for treatment of soil contaminated with solvents, chlorinated pesticides, and cyanide. A thermal desorption unit has been developed and patented for removing chemical contaminants from soil (Crosby, 1996). Contaminated soil is loaded and hydraulically sealed in a modified, sealable drum of a cement truck. A vacuum is drawn and the soil heated indirectly through a heat transfer plate from the natural gas of a propane-fired burner under the plate. The contaminants are vaporized and flow through the vacuum discharge pipe toward the condenser unit, through a series of refrigerated condensing coils. The vapors are liquidized, collected, recycled, or sent to an appropriate facility. The treated material is then downloaded into a roll-off-type container for posttreatment analysis and cooldown prior to recycling or backfilling. Process time is about 45 min to 1 h for a 6 yd 3 batch. The system is self- contained, mobile, and operable by a two-person crew. 2.1.1.1.2 Incineration For complete destruction of the contaminants, incineration is one of the most effective treatments available. Greater than 99.99% destruction of carbon tetrachloride, chlorinated benzenes, and polychlo- rinated biphenyls (PCBs) was achieved by a trial burn with an EPA mobile incinerator (Yezzi, Brugger, Wilder, Freestone, Miller, Pfrommer, and Lovell, 1984). Aqueous waste streams are difficult to incinerate, Figure 2.1 Flameless thermal oxidizer (straightthrough with gas preheat). (From Wilbourn, R.G. et al. in Proc. 13th Int. Incineration Conf., University of California, Irvine, 1994. With permission.) © 1998 by CRC Press LLC but contaminated soils can be handled effectively (Absalon and Hockenbury, 1983). However, inciner- ation is a relatively expensive process. The most common types of incinerators in use are the rotary kiln, multiple hearth, fluidized bed, and liquid injection incinerators (Ehrenfeld and Bass, 1984). Rotary and multiple hearth incinerators can be used with most organic wastes, including solids, sludges, liquids, and gases, while liquid injection incinerators are limited to pumpable liquids and slurries. Fluidized-bed incinerators work well with liquids and can also be used with solids and gases. Incineration may generate incomplete combustion products and a residual ash that may need to be disposed of as a hazardous waste, but it offers one of the best methods for the destruction of organic compounds. Section 6.3.4.1 describes this technology in depth, although mainly in connection with treatment of gaseous emissions. High-temperature thermal treatment, such as incineration, pyrolysis, and vitrification technologies are generally not considered for treating petroleum hydrocarbon-contaminated soil because of their high costs (Ram, Bass, Falotico, and Leahy, 1993). 2.1.1.1.3 Soil Washing Soil washing is a variation of the soil flushing process, with similar requirements (Lyman, Noonan, and Reidy, 1990). It is performed above ground in a reactor and has been shown to be more effective than the in situ flushing system. This approach overcomes some of the problems that may be encountered with the in situ method — low hydraulic conductivity, channeling, and contamination of underlying aquifers. However, tightly bound contaminants are difficult to remove by flushing or washing. See Section 2.2.1.7 for a discussion of in situ soil flushing techniques. A Mobile Soils Washer was built for the U.S. EPA to remove hazardous and toxic materials from soils (Elias and Pfrommer, 1983). The unit includes A drum washer operating at rates up to 18 yd 3 /h, while separating and washing the stones and other large materials from the drier soils; A four-stage countercurrent extraction operation processing up to 4 yd 3 /h; A mobile flocculation/sedimentation trailer to remove soil fines and inorganic contaminants from water prior to recycle or discharge to additional water treatment equipment. There are several state-of-the-art soil-washing systems, including the EPA mobile system, two hot water systems for removing oil from sandy soils, and a flotation process (Assink and Rulkens, 1984). The quantity of residual sludge formed in the extraction process can be a problem and, generally, requires additional handling as a hazardous waste. A multiple-stage, continuous-flow, countercurrent washing system, each stage consisting of a com- plete mixing tank and clarifier, for soil remediation has been simulated to produce a mathematical model, which can be used to manage a treatability study and assist the operator in determination of the steady state in the system (Chao, Chang, Bricka, and Neale, 1995). A proprietary soil-washing process has been developed in Germany (Castaldi, 1994). It is a two-step mechanical separation using water, with no detergents, solvents, acids, or bases as an extracting agent. The process concentrates contaminants in a froth, which is discharged during flotation separation, thickened, and dewatered with gravity thickeners and plate-filter presses. There is another two-stage process for soils containing semivolatile and nonvolatile organic com- pounds, such as substituted phenols, PAHs, fuel oils, creosote, lubricating oils, and diesel fuel (McBean and Anderson, 1996). The contaminated soil is excavated, piled onto polymer linings, washed to extract the hydrocarbons into an aqueous phase (by slowly flooding and draining from the bottom), and returned to its original site. The next stage involves biological treatment of the leachate with conventional wastewater technologies. The advantage of separating these stages is that conditions for each can then be optimized, without negatively impacting the other. For example, surfactants may be necessary in the initial extraction stage, and they can be added at a concentration that would be inhibitory to microor- ganisms, if the two steps would not separate. A concentration of at least 1% surfactant is typically necessary, while concentrations greater than 2% reduce the hydraulic conductivity. The wash solution can then be treated on- or off-site by an acclimated mixed microbial culture. This process is especially useful for areas with a cold climate. Hydrocarbons are rapidly removed, and the leachate is treated under optimized conditions. Removal efficiencies of over 90% are possible with sandy soils. BioGenesis Enterprises, Inc. developed a soil- and sediment-washing process (BioGenesis SM ) for cleaning heavy hydrocarbon pollutants, such as crude oil, fuel oils, diesel fuel, and PAHs, from most © 1998 by CRC Press LLC matrices (Amiran and Wilde, 1994). Controlled temperature, pressure, friction, and duration are combined with proprietary chemical blends tailored to specific site requirements. Synthetic biosurfactants continue remediation after washing is completed. Washing of tar-contaminated soils (attrition of soil, separation of light particles and soil fines) can be significantly enhanced by using additives (Sobisch, Kuehnemund, Huebner, Reinisch, and Olesch, 1995). To reduce the amount of contaminated soil fractions for disposal, the fraction of soil fines can be cleaned by a subsequent extraction step using surfactant solutions. Ultrasound-enhanced soil washing with a surfactant (octyl-phenyl-ethoxylate) is being investigated as a means of improving the performance and economics of this method (Meegoda, Ho, Bhattacharjee, Wei, Cohen, Magee, and Frederick, 1995). Results of the preliminary studies indicate that ultrasound energy supplied by a 1500-W probe operating at 50% power rating, applied for 30 min to 20 g of coal tar–contaminated soil with 1% surfactant in 500 mL can enhance the soil-washing process by over 100%. For soil heavily contaminated with coal tar, the surfactant to contaminant ratio of >0.625 and a solvent ratio >10 is needed for near total removal efficiency. The solution pH does not contribute to removal efficiency, and the ultrasound energy increases soil temperatures. Soil washing can be enhanced by use of solid sorbents and additives (El-Shoubary and Woodmansee, 1996). Hydrocyclone, attrition scrubber, and froth flotation equipment can be used to remove motor oil from sea sand. Sorbants (e.g., granular activated carbon, powder activated carbon, or rubber tires) and additives (e.g., calcium hydroxide, sodium carbonate, Alconox, Triton X-100, or Triton X-114) are mixed with soils in the attrition scrubber prior to flotation. Addition of these nonhazardous additives or sorbents can enhance the soil-washing process, thereby saving on residence time and number of stages needed to reach the target cleanup levels. Soil washing has been used on a pilot scale to treat soil contaminated with cadmium, chromium, cyanide, and zinc, by use of the Chapman soil-washing process (Weisman, Falatko, Kuo, and Eby, 1994). 2.1.1.1.4 Chemical Treatment Peroxide spraying can be used to treat excavated, contaminated soil (Ram, Bass, Falotico, and Leahy, 1993). A new laboratory method for stagnant digestion studied oil release from oil–sand aggregates (Hupka and Wawrzacz, 1996). Oil is released when submerged in an alkaline solution of pH 10.5. The rate of oil release can be two to seven times greater at 50 than at 20°C, depending upon the kind of oil, surfactant concentration, and size of sand grains. The efficiency of oil liberation from sand is inversely proportional to oil–sand-conditioning time and is controlled by surfactant concentration (at least 1 wt%). Organic substances can be destroyed by indirect electro-oxidation (Leffrang, Ebert, Flory, Galla, and Schnieder, 1995). The oxidation agent, Co(III) is used because of the high redox potential of the Co(III)/Co(II) redox couple (EPV0PV = 1.808 V). Organic carbon is ultimately transformed to CO 2 and to small amounts of CO. 2.1.1.1.5 Chemical Extraction Chemical extraction, such as heap leaching and liquid/solid contactors, can also be used in the treatment of excavated, contaminated soil (Ram, Bass, Falotico, and Leahy, 1993). Chemical extraction has been employed on a pilot scale for remediating soil contaminated with PAHs, by applying the Resource Conservation Company solvent extraction process (Weisman, Falatko, Kuo, and Eby, 1994). Multiple regression analysis of solvent extractions of pyrene and benz(a)pyrene from sand, silt, and clay gave an equation for the optimal extraction efficiency and process parameters (Noordkamp, Gro- tenhuis, and Rulkens, 1995). Soil type and extraction time did not affect extraction efficiency. Acetone, methanol, and ethanol were similar in efficiency, although the optimal extraction efficiency was with 19% water and 81% (vol/vol) acetone, which was surprising because the compounds are more soluble in pure acetone. 2.1.1.1.6 Supercritical Fluid (SCF) Oxidation Oxidation in supercritical water is fast and can lead to total oxidation of the organic compounds (Brunner, 1994). Supercritical water is an excellent solvent for extraction of mineral oil fractions from soil, even without oxygen, and the effluents are biologically degradable. A supercritical water oxidation system can clean PAH-contaminated soil by extracting hazardous material from the soil and completely destroying it by an oxidation reaction (Kocher, Azzam, and Lee, 1995). Since most organics dissolve readily in supercritical water, the oxidation reaction proceeds very © 1998 by CRC Press LLC rapidly, producing a clean soil with residual hydrocarbon contamination of <200 ppm and a top gas stream rich in CO 2 and water. The process can be an effective, ex situ remediation technology that can readily be implemented on a mobile unit. See Section 2.1.1.2.5 for a full description of this process. 2.1.1.1.7 Volatilization Enhanced volatilization refers to any process that removes contaminants from soil by increasing their rate of volatilization (Lyman, Noonan, and Reidy, 1990). This includes the processes of mechanical volatilization, enclosed mechanical aeration, pneumatic conveyer systems, and low-temperature thermal stripping, which is considered to be the most effective. Repeated rototilling with successively deeper levels of excavation results in volatilization of contaminants from greater depths. Enclosed mechanical aeration systems use pug mills or rotary drums to increase turbulence in the reactor, with greater aeration and volatilization. Low-temperature thermal stripping systems are similar but include heat to increase the volatilization rate. Pneumatic conveyers use both increased temperature and high velocity airflow to remove contaminants. Excavated contaminated soil can be treated by surface spreading, soil pile aeration, or soil shredding (Ram, Bass, Falotico, and Leahy, 1993). 2.1.1.1.8 Steam Extraction Laboratory-scale tests and a semi-industrial-scale plant equipped with vapor condensation and subsequent wastewater treatment capability demonstrated that steam extraction can be easily used to remove soil contamination caused by diesel fuel, solvents, and PAHs (Hudel, Forge, Klein, Schroeder, and Dohmann, 1995). The process is not limited by soil structure (grain size distribution). Treatment costs of about 300 Deutsch marks/Mg soil are expected for an industrial-scale plant with a 5 Mg/h capacity. There is interest in the U.S. and Germany for industrial-scale plants. 2.1.1.1.9 Solidification/Stabilization This approach incorporates chemical or biological stabilization processes to treat excavated, contami- nated soil (Ram, Bass, Falotico, and Leahy, 1993). Use of carbon-grade fly ash as the only binding agent is a simple, inexpensive method acceptable to the Toxicity Characteristic Leaching Procedure (TCLP) of stabilization/solidification of hazardous wastes (Parsa, Munson-McGee, and Steiner, 1996). Waste and fly ash are mixed and compacted for <3 s at 1.4 to 6.9 MPa to form a monolith. The optimum operating conditions are a waste pH of 9.2 and an applied pressure of 4.65 MPa. If the effectiveness of stabilization is to be mainly determined by the total constituent analysis rather than the previous TCLP, it will be more difficult to meet the standards by stabilization treatment (Conner, 1995). Thus, new stabilization additives and formulations are being developed. These include cement- based formulations with additives, such as activated carbon, organoclay, and proprietary rubber partic- ulates (KAX-50 and KAX-100). The rubber particulates were superior to the other additives. Stabilization of low-level organic constituents in soils is feasible, even for volatile organics. Bench-scale studies of soil contaminated with lead, cadmium, zinc, barium, chromium, and nickel have employed either the Risk Reduction Engineering Laboratory process, the TIDE process, or the WES process for stabilization (Weisman, Falatko, Kuo, and Eby, 1994). 2.1.1.1.10 Encapsulation Other than asphalt blending and other thermoplastic encapsulation methods, most stabilization techniques for fixing organic contaminants in a soil matrix use pozzolanic materials (portland cement, fly ash, kiln dust) as the main ingredient (McDowell, 1992). This process does not work with moderate to high levels of hydrocarbons. The increase in volume and need for pozzolanic materials can be avoided with the Siallon process for microencapsulation of hydrocarbons, which uses two water-based products, an emulsifier, which is specifically selected for different hydrocarbons and soil types, and a reactive silicate. The first stage desorbs and emulsifies the hydrocarbon; the second applies the reactive silicate, which reacts with the emulsifier to form a nonsoluble silica cell measuring <10 µm. The silica cell is essentially pure silica, is nonporous and relatively solid, has a honeycomb or mazelike interior, reduces the mobility and toxicity of hydrocarbons, and does not change the physical characteristics of the soil. It has been successfully applied by in situ or ex situ remediation of sites contaminated with gasoline, diesel, waste motor oil, crude oil, coal tars, and PCBs. © 1998 by CRC Press LLC 2.1.1.1.11 Supercritical Fluid Extraction Use of supercritical CO 2 is a novel technique to remediate contaminated soil, but there is limited information for costs and timing estimates (Zytner, Bhat, Rahme, Secker, and Stiver, 1995). Partition results suggest a weak dependence on the vapor pressure of the contaminant and on soil type. The film mass transfer coefficient appears not to be a rate-limiting kinetic step. Key parameters are axial dispersion and internal aggregate diffusion. A pilot-plant experiment indicated that SCF extraction was effective for cleanup of hydrocarbon- contaminated soils (Schulz, Reiss, and Schleussinger, 1995). The residual concentration of benzo(a)pyrene after the extraction was <1 mg/kg in the soil at 140°F. Supercritical CO 2 can be used to extract anthracene and pyrene from soil at conditions ranging from 35 to 55°C and 7.79 to 24.13 MPa (Champagne and Bienkowski, 1995). Cleanup of soils contaminated with organics by extraction with supercritical carbon dioxide is influenced by additional substances (Schleussinger, Ohlmeier, Reiss, and Schulz, 1996). Both continuous and discontinuous addition of water elevates the extraction yield by altering the adsorption phenomena, which indicates the extraction is limited by adsorption and not by diffusion effects. The contaminant is more accessible and transported faster out of the soil with water. 2.1.1.1.12 Beneficial Reuse Soil that has been contaminated by petroleum products can be excavated and incorporated into asphalt or other construction applications (Ram, Bass, Falotico, and Leahy, 1993). Sometimes, the waste can be converted into a useful product, such as a compost for landscaping (Savage, Diaz, and Golueke, 1985). However, the toxic contaminant and toxic breakdown products must first be completely destroyed or reduced to an acceptable level. Also, the residue can be made quite small by using the compost product as a bulking agent and recycling it in the compost system. 2.1.1.2 Leachate/Wastewater Treatment Systems Contaminated leachate may be released during the process of remediating contaminated soil. It may be necessary to treat any leachate collected, or it may be desirable to prevent a leachate from occurring. Therefore, background information on leachate formation and a variety of leachate, wastewater, and groundwater treatment systems are discussed as possible options for dealing with this phase of the remediation program. Large concentrations of many organic compounds, both volatile and nonvolatile, can leach through landfill sites into the groundwater (Sawhney and Kozloski, 1984). Leachate is generated as a result of the movement of liquids by gravity through a disposal site (Shuckrow, Pajak, and Touhill, 1982b). The leachate percolating through a particular waste reflects the composition of all the materials through which that leachate has passed and depends upon site characteristics, such as annual rainfall volume and composition, evapotranspiration, biological activity, and the nature of the surrounding soil and wastes (Ham, Anderson, Stegmann, and Stanforth, 1979). It is possible that the liquid could be multiphase, e.g., water, oil, and solvents, with the various phases moving through the solid medium at different rates (Shuckrow, Pajak, and Touhill, 1982b). Soil batch leaching protocols based on the EPA TCLP for petroleum hydrocarbons were evaluated and refined by Daymani, Forster, Ahlfeld, Hoa, and Carley (1992) for the ability to predict the leaching potential of volatile organic compounds in gasoline-contaminated soils. They substituted deionized water as an extraction fluid, reduced the test time to 2 h, and found that the TCLP was most effective in assessing the leaching characteristics of gasoline constituents with relatively high solubilities and low vapor pres- sures. They also determined that the relationship calculated from the TCLP ratio study results, between the mass of soil and mass of contaminant leached from the soil, may be used to obtain an indication of the amount of contamination that leaches from an area of homogeneously contaminated soil. Under the new regulatory test methods and treatment standards used by the EPA in the Land Disposal Restrictions, the effectiveness of stabilization is judged primarily by the total constituent analysis rather than, as previously, by the TCLP (Conner, 1995). This approach will likely be extended to remedial actions in the future. A unique analytical method was developed by GTEL Environmental Laboratories in cooperation with the Shell Development Company Westhollow Research Center (Felten, Leahy, Bealer, and Kline, 1992). The analysis segregates hydrocarbons by their respective elution times, which correspond to molecular weights. Hydrocarbons are segregated into five fractions: © 1998 by CRC Press LLC Fraction 1 containing pentane and compounds eluting prior to pentane; Fraction 2 containing benzene and compounds eluting between benzene and pentane; Fraction 3 containing toluene and compounds eluting between toluene and benzene; Fraction 4 containing ethylbenzene and compounds eluting between ethylbenzene and toluene; and Fraction 5 containing compounds that elute after ethylbenzene. Fraction 1 contains the most-volatile compounds and Fraction 5, the least volatile. Leaching ability is related to the proton and electron environments (Lowenbach, 1978; Rai, Serne, and Swanson, 1980) and the presence of solubilizing agents (Means, Kucak, and Crerar, 1980). The proton and electron environments are determined for natural environments and landfill leachates by measuring the pH, redox potential, ionic strength, and buffering capacity (Baas Becking, Kaplan, and Moore, 1960; Chian and deWalle, 1977). Movement of organic pollutants through soil may be increased in the presence of organic solvents (Green, Lee, and Jones, 1981). Solubilizing agents include constituents, such as complexing and chelating agents (hydroxyl ion, ammonia, ethylene diamine tetracetic acid [EDTA]), colloidal constituents (unicelles or surfactants), and organic constituents (melanic materials, humic acids) (Baas Becking, Kaplan, and Moore, 1960; Chian and deWalle, 1977). Some of these agents can affect the mobility of inorganic and organic constituents of the waste, even at low concentrations of the agents. A number of factors affect the quality of a leachate (Shuckrow, Pajak, and Touhill, 1982b). Solubility is one of the most important factors. Chemical composition of the leachate determines dissolution and reaction rates. Dissolution is directly proportional to the surface contact area. Porosity influences the flow rate of liquid and, thus, the contact time between liquid and solids. Longer contact times permit more-complete chemical reactions until an equilibrium concentration is reached. The pH also has a significant effect on the leachate composition. Soil admixtures also influence solubility. For example, acid soils tend to promote solubilization of waste constituents, whereas the higher pH in alkaline soils likely will retard solubilization. Warmer temperatures increase reaction rates between liquid and solid and improve microbial catalysis. The main physical transformation expected in the leaching process is plugging of pore spaces and the resultant influence on chemical processes and leachate flow rates. On-site hazardous leachate treatment can be used to accomplish either pretreatment of the leachate with discharge to another facility for additional treatment before disposal or treatment complete enough to meet direct discharge limitations (Shuckrow, Pajak, and Touhill, 1982b). The major difference between complete on-site treatment and pretreatment is likely to be the extent of the treatment. Most leachate treatment processes result in production of by-products, such as sludges, air pollution control residues, spent adsorption or ion exchange materials, or fouled membranes, which also require disposal. Residue disposal considerations may determine selection of a leachate management technique. One possible approach to on-site leachate management is leachate recycling (Shuckrow, Pajak, and Touhill, 1982b). This technique involves the controlled collection and recirculation of leachate through a landfill to promote rapid landfill stabilization. Information on leachate composition is used in judging the adequacy of a leachate treatment system (Garrett, McKown, Miller, Riggin, and Warner, 1981). A leachate procedure provides a realistic leachate profile, showing the change in constituent concentration with amount of leaching. It can be site specific and applicable to a variety of solid wastes. A leaching procedure has been developed to estimate the total amount of leachable species to be released from a unit mass of solid waste (Garrett, McKown, Miller, Riggin, and Warner, 1981). In addition, the profile of the leachate will indicate the concentration or mass of that constituent likely to be present in the leachate and the time period, in terms of total volume of leachate produced, when that constituent will be present at any particular concentration or mass. This information will also indicate the composition of leachate that can be expected in the field under the duplicated conditions. Ideally, the leaching medium and test conditions used in a leaching test should reproduce the actual leachate and conditions to be encountered at the field disposal site (Garrett, McKown, Miller, Riggin, and Warner, 1981). While no single medium can duplicate field conditions, certain factors have been identified that influence leaching and, thus, determine the leaching medium composition (Table 2.3). Test Conditions Distilled, deionized water is used as the leaching medium with a monofilled solid waste (Garrett, McKown, Miller, Riggin, and Warner, 1981). Where environmental conditions warrant, alternate media, © 1998 by CRC Press LLC [...]... wastes Vegetable-canning wastes Potato-processing wastewater g/kg soil/year lb/ft3 soil/year 11 0.98 148 12. 25 87 7. 82 16 1.40 44 4.00 22 1.97 29 1.97 79 7.16 62 5. 62 67 6.00 17 1.54 — — 1 .2 0.11 . (98%) of the organics at 20 min residence time, the temperature should be 100°C for 1-dodecene, 20 0°C for 1-hexa- decene, 150°C for naphthalene, and 25 0°C for anthracene. Table 2. 1 Summary of. Hazardous Materials Agency on Contract No. DAAK1 1-8 2- C-0017, 1984. AD-A1 62 528 /4.) © 1998 by CRC Press LLC Touhill, 1982b). Inorganics can often be transferred to a less toxic or more easily. cite the requirement of heating the soil to >600°C for successful thermal treatment of petroleum- contaminated soil. The effect of thermal treatment by means of a natural gas-fired, batch, rotary