Physical/Chemical Treatment of Organically Contaminated Soils and Sediments

Transcription

1 Journal of the Air & Waste Management Association ISSN: (Print) (Online) Journal homepage: Physical/Chemical Treatment of Organically Contaminated Soils and Sediments Robert D. Fox To cite this article: Robert D. Fox (1996) Physical/Chemical Treatment of Organically Contaminated Soils and Sediments, Journal of the Air & Waste Management Association, 46:5, , DOI: / To link to this article: Published online: 20 Nov Submit your article to this journal Article views: 907 View related articles Citing articles: 3 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 22 December 2017, At: 18:48

2 CRiTICAL REVIEW ISSN /. Air & Waste Manage. Assoc. 46: Copyright 1996 Air & Waste Management Association Physical/Chemical Treatment of Organically Contaminated Soils and Sediments Robert D. Fox International Technology Corporation, Knoxville, Tennessee INTRODUCTION Soils and sediments contaminated with industrial organic chemicals represent one of the largest of the environmental remediation challenges in the U.S. and globally, both in sheer number and in complexity. Cleanup of these sites to levels that are protective of human health and the environment has been and will continue to be a major expense for those responsible. This financial impact has led to the search for remediation systems that are "faster, better, cheaper, safer." For the various types of organic contaminants encountered, alternative treatments to remove or destroy these chemicals have become the object of large expenditures on research, development, and demonstration by government and industry. The result is that the user is now faced with a myriad of emerging technologies in varying stages of development from which to choose in remediating a site. The treatment of organically contaminated soils and sediments can be achieved by either physical separation or destruction of the organic chemical(s). Physical separation, or treatment technologies, transfer the organic contaminants from the soil/sediment matrix to another medium (e.g., soil to air or soil to water). The physical separation process requires a capture step for the contaminant, which can result in concentrating the contaminant for recovery and reuse or for more cost-effective ultimate disposal. This review includes as a physical separation treatment the isolation of the contaminant from the environment by immobilization via stabilization/solidification. These technologies, commonly used with metals and other inorganic contamination, involve the use of chemicals to render the contaminants immobile and resistant to leaching into the environment without changing the chemical structure of these contaminants. Toxic organic contaminants, unlike toxic metals or other inorganics, can be destroyed in treatment. They can be oxidized to carbon dioxide, or they can be reduced/transformed into simpler, more benign structures. These oxidations, reductions, or transformations are accomplished by thermal, chemical, or biological processes. Incineration by thermal oxidation has been the standard for destruction and removal efficiency against which all other technologies are compared. It is the conventional destruction technology for organic wastes in general, and it was the subject of a previous A&WMA critical review. 1 With the use of larger, high throughput transportable incinerators, the cost of remediation by incineration has been reduced to competitive levels. However, incineration has been saddled with a reputation as an undesirable source of toxicants, such that the "\" word is overwhelmingly avoided when discussing remediation options. As a result, the focus on remediation options has turned to physical/chemical and biological treatment technologies that are not only more acceptable, but can also be less costly. This critical review addresses the physical/chemical segment of these emerging technologies and their application areas, provides guidance on their evaluation/selection, and identifies key issues in the use of these technologies in remediation projects. Biological treatment is also an important segment of the remediation technology choices, and it is frequently combined with physical/chemical technologies to achieve enhanced remediation performance. However, it is beyond the scope of this review to include biological treatment processes. The examination of these treatment technologies will be presented in four classifications: physical treatment of soil in place (in situ) and after excavation (ex situ), and chemical treatment in situ and ex situ. This review relies heavily on EPA-generated data on the various technologies, especially data generated in the Superfund Innovative Technologies Program (SITE). These data are highly reliable in terms of objective evaluation, analytical thoroughness, and lack of vendor bias. An eightvolume series of monographs on remediation technologies that is less site-specific has been prepared under the auspices of the American Academy of Environmental Engineers (AAEE). No attempt will be made to discuss the costs of these technologies inasmuch as cost estimates are very site-specific and dependent on a variety of parameters. Generic cost data are available in the AAEE monograph series. The types of organic constituents that can be treated by these methods and the relevant properties of contaminated soils are first presented to aid the understanding of treatment methods. Types of Organic Contaminants For considering the applicability of various technologies available for treatment of organically contaminated soils, organic contaminants can be put in the following categories: Volume 46 May 1996 Journal of the Air & Waste Management Association 381

3 Volatile Organic Compounds (VOCs) -benzene, toluene, ethylbenzene, xylenes (BTEX) -chlorinated solvents (TCE, TCA, perchlorethylene) -some oxygenated VOCs (acetone, MEK) Semi-Volatile Organic Compounds (SVOCs) -total petroleum hydrocarbons (TPH) -polycyclic aromatic hydrocarbons (PAH) -phenols -phthalates -some oxygenated SVOCs (>C 3 alcohols, aldehydes) Pesticides, e.g., DDT, toxaphene Other chlorinated aromatics, e.g., PCBs, PCDD/ PCDFs Others -oils -tars The treatments examined below apply to one or more of these categories. Soil Characteristics Several properties of the various types of soil on the planet Earth significantly influence the applicability and effectiveness of technologies for soil remediation. Those that are relevant to physical/chemical treatment include: Heterogeneity the variability of the soil types at a site, such as sandy, clayey, gravels, the presence of sand or clay lenses or fractures. Permeability, the ability to move air and water through the soil, usually measured in cm/sec, with a lower limit of 10 4 cm/sec for in situ passage of fluids. Clay content -low permeability, typically 1O 6 cm/sec, -contaminants are usually associated with small particles, e.g., silts and clays at <60 microns, and are difficult to separate or react, -feed handling for ex situ treatment, such as drying, screening, feeding, become more difficult Humus content the natural organic matter present in soils. -affinity for contaminants, both organic and inorganic, is higher. -the humus or organic matter in soils represents an additional oxidation load when chemical oxidants are considered to destroy or transform organic contaminants. The physical/chemical treatments presented below are best evaluated for a particular site when knowledge of the soil properties above has been determined through soil surveys, soil sampling at depths, and chemical and geotechnical analysis of the soil samples. Another soil subsurface condition affecting evaluation of treatment alternatives is the groundwater level in relation to the zone of contamination. Soil above the groundwater level is the vadose or unsaturated zone, while the soil in contact with groundwater is the saturated zone. Most of the technologies reviewed in this paper are also discussed in Reference 2, prepared by the Federal Remediation Technologies Roundtable. 2 Similarly detailed reports from the U.S. Environmental Protection Agency's Superfund Innovative Technology Evaluation (SITE) program are available on most of the technologies and will be referenced in this review as appropriate. AAEE monographs have been published on several of the technologies in this review, namely vacuum vapor extraction, thermal desorption, soil flushing/ soil washing, solvent/chemical extraction, chemical treatment, and stabilization/solidification. PHYSICAL TREATMENT Remediation of soil by physical treatment encompasses technologies that separate the contaminants from the soil solids. The separation process is a volume reduction process that transfers the contaminant to another media, e.g., air or water, and collects it in a concentrated form. Depending on quantity and concentration, the new contaminated media may require further treatment. This additional treatment can either destroy the contaminant or concentrate it for recovery/reuse or ultimate disposal. Hence, physical separation treatment frequently leads to the need for a treatment train to complete the process. Physical treatment using separation technologies is done either in situ or ex situ (soil is excavated). The main advantages of in situ treatment are that it allows the soil to be remediated without having to excavate or transport it. It also avoids land disposal restrictions on redeposition of treated soil. In situ remediation generally involves longer treatment times. Because of the heterogeneity of the subsurface, it is also more difficult to assure uniformity of treatment. In situ treatment must also be concerned with avoiding the spread of contamination as a result of inducing the contaminants to move away from the zone of contaminated soil. In situ physical treatment of organically contaminated soil can be placed in three categories: 1) soil vapor extraction in the vadose zone, and numerous methods to enhance contaminant removal by vaporization; and for the saturated zone, 2) air sparging, (forcing air to bubble through groundwater to volatilize VOCs) and 3) soil flushing, which includes the flushing action of groundwater in pump-and-treat systems, as well as the use of chemicals such as surfactants to enhance the removal effectiveness of water passing through contaminated soil. In Situ Physical Treatment of the Vadose Zone- Soil Vapor Extraction Soil vapor extraction (SVE) relies on the volatility of the organic chemical contaminants to enable their separation from the vadose zone soil by vaporization. SVE is mostly applicable 392 Journal of the Air & Waste Management Association Volume 46 May 1996

4 to the VOC class of organics, although some enhancement technologies remove SVOCs, and other less volatile chemicals, by applying heat from various sources to the soil. SVE applies vacuum pressure to the contaminated unsaturated subsurface area to induce the controlled flow of air through the contamination zone and remove VOCs and some SVOCs. The vacuum is applied principally to vertical wells screened at the contamination zone. It is not uncommon to use monitoring wells for SVE systems. SVE can be used on contamination zones at depths from five feet to several hundred feet. Vacuums applied are those necessary to induce adequate air flow and can involve the full range of vacuum pump capabilities, depending on soil permeability. Additional wells for air inlet to the contamination zone are sometimes necessary to assure passage of air through the zone of contamination. SVE can also employ horizontal wells to extract VOCs from the vadose zone. This is especially useful for remediation of soils underlying buildings and active industrial facilities. Subsidence and subsurface obstructions become concerns when using horizontal wells in this type of situation. Other than the wells, the equipment required to carry out SVE is simple and consists of interconnective piping, a vacuum pump(s), vapor-liquid disengaging tank, appropriate valving, monitors, and controls, and most likely an air emission control system. The contaminated air and VOC vapors withdrawn by the vacuum system may require treatment before the air is discharged to the atmosphere. The cost of SVE is significantly impacted by the need to treat this air stream. The typical method of operating SVE systems is to periodically monitor the VOC concentration and flow of the air extracted from the subsurface. When the concentration of VOCs drops to a low level and remains low after a period of shutdown, soil sampling is used to verify that the contaminant concentrations in the zone of concern are at acceptable levels. SVE has been used for over 15 years to remediate VOCcontaminated soils. Only in the last five years has it found increased use and acceptability to the point where it is now a widely used remediation technology for VOCs. The latest EPA summary 3 of remediation shows that SVE was specified in 135 Records of Decision as of September 30, This usage is larger now, especially when state and private cleanups are included. Key SVE Application Considerations. The major factors to consider in evaluating and selecting SVE are: Soil permeability (at least 10 4 cm/sec), and soil heterogeneity or homogeneity. The cleanup criteria, usually judged by the absence of VOCs in the air removed from the subsurface, followed by actual samples of subsurface soil. Critical Review The estimated time to achieve cleanup, which ranges from months to years. The need for air emission control to remove VOCs from the extracted air; common control techniques are activated carbon adsorption, fume incineration, and catalytic fume incineration. The success of SVE in removing VOCs from subsurface soils is determined by analyzing the air extracted from the subsurface, and ceasing remediation when the mass removal becomes low. Soil samples are then collected and analyzed by procedures that rely on vaporizing residual VOCs. This method of cleanup assessment is not without questions. Travis 4 suggests that SVE may not be as effective as indicated by removal of available, vaporizable VOCs. For soils that have been contaminated with VOCS for several years, a significant fraction is trapped inside the soil matrix and is inaccessible to removal by either SVE or groundwater purnpand-treat systems. Not only are these interstitial VOC contaminants resistant to removal by SVE, they may also be undetected by the standard EPA method employing purgeand-trap techniques to measure VOCs in soil (EPA Methods 8010 and 8020). Thus, determining the cleanliness of treated soil by purge and trap analysis is not indicative that all of the VOCs have been removed. However, the residual levels, if truly inaccessible and immobile, may still be low enough to protect human health and the environment. These concerns raise additional questions about the reliability of the whole process of sampling, preserving, and analyzing soil samples for VOC contamination. Enhancements to SVE. A number of technologies are being developed to enhance the performance of SVE for in situ soil remediation. These are air injection, steam sparging, radio frequency heating, hot air heating, and electrical (joule) heating. These enhancements seek to speed up the vaporization of VOCs for collection by the SVE system and/or to vaporize SVOCs and other low volatility contaminants. All methods but ambient air injection involve heating the soil. A major advantage claimed for these thermal-based enhancements is the vaporization of soil moisture. As the water evaporates, soil permeability increases, facilitating improved contaminant removal. Ambient Air Injection. Ambient air injection enhances SVE by adding air injection wells to the vacuum extraction well network to provide additional air movement through the contamination zone to speed the removal of VOCs. A test of the ability of an air injection-vacuum extraction technology to remediate vadose zone soil was completed under the SITE program and reported in After one year of operation, the average mass reduction in VOC and SVOC contamination at the site was 80% from an initial average concentration of 340 mg/kg. The reductions over the area of the site, as determined from individual boreholes, ranged from 71% to 9%. Volume 46 May 1996 Journal of the Air & Waste Management Association 393

5 Critical Review Air injection is also used in conjunction with SVE in removing VOC contaminants from the soil in the saturated zone. This is air sparging and will be described later. Another advantage of adding air to subsurface contamination zones is that it stimulates aerobic biological degradation activity by the naturally occurring organisms present in the soil. This technology is called bioventing; it speeds the removal of contamination, but is only effective on biodegradable organics. Steam Sparging. The injection of steam into subsurface contamination is designed to enhance VOC removal and promote SVOC removal by heating. It is useful both for soils and for groundwater. Steam is forced through the soil by injection wells (screened both above and below the water table) to thermally enhance the removal process. Extraction wells collect liquids, steam, air, and vaporized organics and transport them to the surface for further collection and treatment. This technology was tested in full-scale demonstrations in the SITE program during at a site in Huntington Beach, CA 6 and in 1993 at a site at Lawrence Livermore National Laboratory. 7 Both involved hydrocarbon fuels as contaminants. The Huntington Beach project achieved less than 50% removal of the spilled diesel fuel. An indication of the performance of steam injection coupled with soil vapor extraction is found in a full-scale remediation over a two-year period in the early 1990s. 8 Over 7,000 lbs of VOCs were extracted from a 1-acre, 20-ft.-deep contamination area. A total of 2,800,000 lbs of injected steam was used during 10,000 hours of vacuum pumping operation. The reported results on steam-enhanced SVE suggest that, even though significant amounts of hydrocarbons may have been removed, the success in removing SVOCs was much less than 90%, as determined by soil sampling before and after treatment. Radio-Frequency Heating. Radio-frequency (RF) heating is an in situ treatment process that uses electromagnetic energy to heat soil and enhance SVE. RF heating is done by placing exciter electrode or antenna arrays in the soil either vertically or horizontally. Horizontal insertion is through a horizontal well borehole. Application of energy causes heating to begin near the antenna and proceed outward. Soil can be heated to 250 C to 300 C. Water and contaminants are vaporized. Vapor collection devices are built into the antenna arrays on the surface or are provided as standard SVE systems to collect vapors generated for further treatment and recovery/disposal. The two major suppliers of RF heating technology to enhance SVE have participated in the SITE program, and Innovative Technology Evaluation Reports are available. 9 ' 10 Removal of organics in these SITE tests was measured using Total Recoverable Petroleum Hydrocarbons (TRPH) using EPA Method Removal efficiencies of TRPH were in the range of 30% to 95%. An increase in ketone-like VOCs in the soil was also recorded, which was postulated to result from degradation of organics in the regions of higher temperatures near the energy-producing arrays. Hot Air Heating. Heated air is injected into the soil below the unsaturated contamination zone in this process, evaporating soil moisture and vaporizing VOCs and SVOCs. Injection wells are drilled in distribution patterns to depths below the contaminated zone and screened at the bottom. Heated air at temperatures up to 650 C is used. An impermeable cover connected to a vacuum system is placed over the area being treated to capture vapors as they reach the soil surface. One developer of this technology demonstrated the in situ process in the SITE program on a JP-4 jet fuel spill site at Kelly AFB in u Other projects in Canada and Texas showed TPH removals to levels below regulatory requirements. 12 Electric Heating. In situ electrical heating of contaminated soil uses common AC electricity to heat soils to enhance SVE removal of organics. Two versions have been under development. The first uses electrodes inserted in the contaminated area with a technique employing six-phase heating of the soil. 13 This technique splits conventional three-phase electricity into six separate electrical phases. Each phase is connected to six separate electrodes placed in a circle, as shown in Figure 1. Because each electrode is at a separate phase, each one conducts to all the others. Voltage gradients in the six-phase array are relatively uniform, thereby facilitating uniform heating throughout the soil bounded by the electrodes. A seventh electrically neutral pipe is inserted in the center of the hexagon and connected to a vacuum system to capture vapors generated during heating. Condensed Wate Storage Tank Catalytic Oxidation Off-Gas Treatment System OW=Observation wel Figure 1. Surface equipment for in situ six-phase heating of soil to vaporize organic contaminants (electrode array and other wells drawn to scale; other equipment not drawn to scale). 314 Journal of the Air & Waste Management Association Volume 46 May 1998

6 As part of the DOE Integrated Demonstration Program for VOCs in Non-Arid Soils, a full-scale demonstration was conducted at the Savannah River Site on clayey soils contaminated with PCE and TCE. The electrodes were positioned in a 30-foot diameter circle and had electrical contact with the soil between 23 and 44 feet below the surface. The clay zone was heated up to -100 C in 8 days and held there for 17 days. Soil sampling results showed contaminant removal efficiency from the clay zone was 99.7% within the electrode array. Soil moisture was also removed, and this improved soil permeability. The second method of heating soil to enhance the removal rate of organics is a technology developed to remediate shallow (0-2 ft.) PCB contamination. It involves in situ heating of the surface and near-surface soils with an electrically heated thermal blanket. The blanket design includes a permeable electric heater and an impermeable.cover that facilitates capture of the volatilized contaminants by an SVE system. This technology was tested on a pilot scale at a site in New York state with surface contamination by PCBs at concentrations over 500 mg/kg. 14 Three 10-ft-by-10-ft grids were heated over several days; the PCB concentration in the top 3 inches of the 3-6 inch zone was reduced to below the target level of 2 mg/kg. This type of heating is capable of raising the temperature of surface and near-surface soils to several hundred degrees centigrade, enabling improved vaporization of non-volatile as well as volatile contaminants. Key Issues in Thermally Enhanced SVE. As the foregoing indicates, the use of various soil heating techniques to enhance the in situ SVE removal of volatile organic contaminants and to extend SVE to removal of SVOCs and low volatility organics such as PCBs has received much development activity. The key issues identified in the variety of tests conducted to date are: Efficiency the purpose of heating soil in situ by steam, hot air, radio, or electrical energy is to speed VOC removal and/or volatilize SVOCs. The results to date indicate that direct electrical heating is more efficient than other methods. Collection/control of contaminants all methods rely on a vacuum system to collect and control the vaporized organics. Containment and prevention of the spread of contaminants are key design considerations. Vaporization of soil moisture and the accompanying increase in soil porosity both aid in collection and efficiency, but they complicate design. High temperatures heating soil to temperatures above 100 C to remove SVOCs raises the possibility of adverse thermal degradation reactions that can create undesirable byproducts or compounds of reduced volatility Equipment life the expected life of heating devices, particularly electrical ones, is affected by the number of times they must be moved to remediate a site. Hence the equipment cost component is uncertain. Energy heating soil in the saturated zone requires much energy, and heating vadose zone soil with high moisture content can also carry high energy costs. Pneumatic Fracturing. In order to extend the applicability of SVE in vadose zone remediation to soil formations with low permeabilities, e.g., clay, the technique of pneumatic fracturing has been developed. In pneumatic fracturing, a well(s) is placed in the contaminated soil and screened at a depth below the contaminated zone. Packers below and above the zone are emplaced. Air at several hundred psi pressure is rapidly forced into the fracturing well, causing radial fractures in the soil. Improvements in soil vapor flow of several hundred percent 15 have been reported for high clay soils. It is also claimed that the fractured formation remains open after fracturing, and that the soils do not reconsolidate. Measured improvements in vapor flow support this claim. Pneumatic fracturing is also reported to be applicable to bedrock. The main issue for pneumatic fracturing is assurance that fracturing does not create pathways to spread contamination. In Situ Physical Treatment of the Saturated Zone In situ remediation of organic contamination in the saturated zone by physical treatment involves technologies that facilitate the transfer of the contaminant to the surface. The conventional technology is the pump-and-treat method, in which groundwater is pumped to the surface for treatment and discharged. It relies on the diffusive transport of the contaminant from the soil particles to the groundwater. This type of transport is generally very slow and results in pumping large volumes (e.g., 100s of gal/min) of water with very low contaminant concentrations. As an alternative to conventional groundwater pumpand-treat, several technologies are being developed that offer improved removal performance. Air Sparging. In situ volatilization of VOCs can be accomplished in saturated soils by sparging air through the soils in the contaminated zone. This technology removes VOCs sorbed to the soil particles and dissolved in the groundwater phase, thereby treating both soils and groundwater in the saturated zone. The vaporized VOCs pass to the vadose zone where they are collected by a SVE system. Air sparging aerates the soil and enhances the aerobic biodegradation of contaminants, especially SVOCs and nonvolatile organics. This type of remediation is called bioventing. Air sparging is seldom found as a stand-alone treatment, but is combined with SVE and biodegradation. In air sparging systems, the injected air flows through the water column past soil particles, transferring VOCs to the air bubbles, which rise into the vadose zone where they Volume 46 May 19i6 Journal of the Air < Waste Management Association 395

7 Injection point for flushing gas Extraction of contaminated gas PRESSURIZED BLOWER MOTOR OR COMPRESSOR.i O 1 * 1 GHAVEu PACK Figure 2. Schematic diagram of in situ air stripping using horizontal wells, a flushing gas, and vacuum recovery. GHOUI "iftai \ GRAVLLHACK / can be captured by an SVE system. The air injection and SVE collection points can be either vertical or horizontal wells. Monitoring wells can be used in vertical well systems. The ease and affordability of installing multiple small diameter air injection points allows flexibility in design and construction of a remediation system to assure complete coverage of the contaminated area. Depth of air injection and injection pressure, and hence air bubble size, can have a significant impact on the degree of contact of the air bubbles with the soil formation and the route the air bubbles take to the SVE system. Several applications that demonstrate the effectiveness of air sparging technology and elucidate important design considerations have been reported. 16 A demonstration in the SITE program produced results that exceeded expectations, achieving reductions for seven target VOCs in the vadose zone that ranged from 71% to 99%. 17 Stimulation of biodegradation processes was also observed. Horizontal wells can be the only method available to remediate VOC-contaminated soil under buildings or tanks. The effectiveness of horizontal wells was tested in a fullscale demonstration on soils contaminated with chlorinated solvents at the DOE's Savannah River Plant. 18 This in situ air sparging concept uses two parallel horizontal wells: one below the water table and one in the unsaturated zone, as shown in Figure 2. The deeper well is used as a delivery system for air injection. VOCs are stripped from the groundwater into the vapor phase, and are collected and removed from the subsurface by vacuum extraction through the shallower well in the unsaturated zone. The reported case studies demonstrate that the use of air sparging in conjunction with vapor extraction is effective for cleanup of gasoline- and solvent-contaminated sites when the right site-specific conditions exist, particularly soil permeability and homogeneity. WELL SCRfEN i Figure 3. Simplified schematic of well-bore air stripping system, with air vented to vadose zone collected by soil vapor extraction. Well-Bore Air Stripping. This technology involves the use of air stripping in a specially constructed well to remove VOCs from groundwater that is drawn into the bottom of the well through a lower screened area. The air-stripped groundwater rises in the well and is then allowed to gravity flow back into the formation at another screened point above the contaminated zone. In this process, shown schematically in Figure 3, groundwater is treated in situ and returned to the formation to displace contaminated groundwater. This creates a circulation system in the formation around the well. The stripping air is either collected at the top of the well for surface treatment of the VOCs or allowed to vent into the vadose zone, where biodegradation can destroy the VOCs. Several variations of this type of well-bore air stripping system have been reported ' 21 These systems are similar to groundwater pump-and-treat systems in using water flow through the contaminated saturated soil to the water phase. Their main advantage is that the water is not removed from the well, and can be returned to the formation. The key issues with these systems are where the groundwater goes when it re-enters the groundwater zone and how long it takes to complete the recirculation route. Short-circuiting lessens the sphere of influence. High-permeability soils are required to minimize time. The technology relies on the flow of groundwater to remove contaminants from the soil in the same manner as a groundwater pump-and-treat system. ^r 398 Journal of the Air & Waste Management Association Volume 46 May 1996

8 Dual-Phase Vacuum Extraction. This process involves the use of vacuum dewatering techniques to remove both vapor and liquid from wells screened at the bottom of the contaminated saturated zone (see Figure 4). 22 An inner drop tube is inserted in the well and a vacuum is applied to the drop tube. Water is initially removed until the water inflow into the well does not sustain the original level and the water level drops below the drop tube. The vacuum drop tube then withdraws both vapor and water from the well. The depressed water table around the well converts the contamination zone from a saturated zone to a vadose zone, where removal of VOCs by vacuum extraction is faster than removal by water dissolution and stripping. This dual-phase vacuum extraction thereby remediates the soil around the well by a combination of SVE and water flushing (as in groundwater pumping). The net result of the use of this technology is faster remediation of the saturated zone than can be achieved with conventional pump-andtreat systems. Vacuum-Enhanced Pumping. Vacuum-enhanced pumping applies vacuum lift technology to remove groundwater from depths down to feet without the need for expensive downhole pumps. This is in contrast to the limit of 32 feet in depth for vacuum withdrawal of groundwater. This pumpless groundwater removal from depth, shown schematically in Figure 5, is achieved through the use of a vacuum applied to a drop tube. Holes are strategically placed in the drop tube to allow air to enter it. Groundwater rises in the SUCTION LINE VACUUM GAUGE I I \f\..threaded COUPLING - CASING VACUUM GAUGE -DROP TUBE Critical Review drop tube in response to the vacuum applied. As air mixes with the rising groundwater, the density of the groundwater-air mixture is lower, enabling the applied vacuum to pull groundwater from greater depths. The source of air is from the vadose zone; air enters the main well at an interval screened in the vadose zone. Air and water are treated at the surface to remove contaminants before discharge. Vacuum-enhanced pumping remediates subsurface contamination by the following actions: pumping contaminated groundwater, and flushing contaminants from the soil in contact with the groundwater; creating a cone of depression around the well by withdrawing water faster than it is replenished from the surrounding soils, thereby enlarging the vadose zone around the well; and extracting soil vapor to more rapidly remove VOCs from the enlarged vadose zone. This combination of removal actions achieves in situ remediation of saturated soils faster than the conventional pump-and-treat approach. 23 These new techniques of remediating contaminated soil in the saturated zone using air sparging and vacuum extraction techniques are finding increased use as more cost-effective alternatives to conventional pump-and-treat systems. Soil Flushing In order to speed the rate at which organic contaminants can be dissolved or entrained into a flowing water phase, surfactants have been extensively investigated. In soil flushing, water with added surfactants is injected into or applied to the contaminated zone. By withdrawal at recovery wells, the flow of the surfactant-water injectate is directed to the recovery wells. In situ surfactant flushing has been tested STATIC FLUID LEVEL WEL LSEAL-^^ W f V J * - VACUUM REUEF SOURCE TAPERATUHI (LOC* TION VARIES) SLOTTED PIPE - {INTERVAL VARIES) ER LEVEL ^ \ o GLOBE VALVE B Figure 4. Single pump vacuum well schematic screened at the bottom of saturated zone for extraction of liquid and vapors simultaneously. Figure 5. Vacuum enhanced pumping schematic shows the use of vacuum lift technology to remove groundwater from depths down to 90 to 100 feet without downhole pumps. Volume 46 May 199 Journal of the Air & Waste Management Association 3S7

9 Critical Review for enhanced washing of organic contaminants from the surfaces of soil particles and for the dissolution of nonaqueous phase liquids (NAPLS) that exist as pools in the subsurface or as globules entrapped in soil matrices. Extensive research has been done on surfactants and contaminants. Commercial surfactants are generally used at concentrations ranging from 0.5% to 2.0%. Surfactants with both hydrophilic and hydrophobic functional groups are most effective. Tests to screen surfactants for enhancement of organic solubility in water have been done both with actual soil from an aged contaminated site (weathered soil) and with soil that was contaminated with organics in the laboratory (spiked soil). The results from weathered soil tests have not been as good as those obtained with spiked soil samples. Any surfactant system proposed for soil flushing should be evaluated on actual weathered site soils. Field tests were conducted on a 36 cubic meter confined cell that was intentionally contaminated for testing purposes with 230 liters perchlorethylene (PCE). 24 At the beginning of the flushing test, the concentration of PCE in the recovered surfactant-water flushing solution was 50 times the aqueous solubility of PCE at 25 C. This simulated situation showed the effectiveness of surfactant flushing in enhancing the dissolution of NAPLs. A second field test was conducted at an actual industrial field site where carbon tetrachloride had been spilled. 25 Analysis of core samples indicated the presence of free phase substances in a contaminated zone that ranged from 12 feet to 24 feet below the surface over a clay aquitard. Pumping tests at two different levels in the contaminated zone with two different surfactants showed good removal of dense nonaqueous phase liquids (DNAPL) and complete cleansing of a significant portion of the test area. Use of a surfactant-water solution to remove PCBs and oil from actual contaminated soil was evaluated in a test plot of approximately 400 cubic feet. 25 The test plot was five feet deep, and the surfactant washing solution was applied to the surface and allowed to percolate down to a leachate collection point. The soil was contaminated with up to 0.62% PCBs and 6.7% oil. During 70 days of the test, 1.8 bed volumes of surfactant solution were applied, and 10% of the PCB and oil was recovered in the leachate. Other field tests and applications for proprietary processes are also reported The key issues for use of surfactant flushing for in situ treatment of soil are: Soil permeability must be high enough (e.g., 10 2 cm/ sec) to allow water to be pumped through the soil formation. Clean-up efficiency must be achievable. Soil homogeneity must be uniform so that the entire contamination zone is treated without short-circuiting. Containment of the injected solutions must be assured to prevent the spread of contamination by making use of existing impermeable barriers (e.g., clay formations) and/or by installing artificial barriers such as slurry walls or sheet piling. Fate of the residual surfactant on soil determines whether the surfactant must be removed by further flushing with water or can be left on the soil to be destroyed by natural soil processes such as biodegradation. Biological oxidation demand in soil caused by surfactant bio-fouling will affect the satisfactory performance of the process over a long period of time. Separation of contaminants from recovered wash solution is necessary to reduce the amount of waste residuals that require further treatment or disposal, and to enable recycling of the surfactant. Electroosmosis. The inducement of chemical transport in soils by imposition of an electromotive force (EMF) is termed electrokinetics and is under development for soil remediation. The main focus of electrokinetic remediation is on transport of ionic species, especially soluble metals. Electrodes are inserted in the contamination zone and a DC voltage is applied, causing migration of ions. A subset of electrokinetic remediation is electroosmosis, which achieves transport of soil moisture by EMF. As water migrates toward the electrode, organic contaminants in the soil are carried along, effecting their transport to and collection at the electrodes. Most studies on the use of electrokinetics for in situ removal of contaminants from soil have focused on metals. 28 Where metals and organics coexist at a site, data are also needed on the removal of organics. The main issues to address in the use of electroosmosis for removal of organic contaminants are: Degree of removal achievable for the organic contaminants by electroosmotic action. Time required to reach cleanup goals, which may involve months or years. Energy requirements are affected by the time required, the soil chemistry, and the amount and type of contaminants. Electrode reactions must be considered because of the organic and inorganic chemical transformations that result in decomposition. Hydraulic Fracturing. The effectiveness of soil flushing technologies (pump-and-treat, water circulation, surfactant washing) can be improved by increasing the soil permeability to liquid flow. Hydraulic fracturing is being developed to this end, using techniques similar to those used in oil production and mining operations. Water is injected under pressure into the formation at the zone of contamination. The hydraulic pressure causes horizontal fissures to open, thereby increasing the permeability of the soil. In order to keep the fissures open, a sand slurry must be pumped into the 398 Journal of the Air & Waste Management Association Volume 46 May 1996

10 formation. The sand prevents the saturated soil from re-consolidating to a low permeability condition. Two field tests of hydraulic fracturing were performed under the SITE Program. 29 A vapor extraction site was hydraulically fractured in two wells at depths of 6, 10, and 15 feet below the ground surface. Over a one-year period, vapor yield from the fractured wells was over ten times that of unfractured wells. A site where in situ bioremediation was underway was hydraulically fractured. The water flow into the fractured well was 25 to 40 times greater than the unfractured well and the bioremediation rates near the fractured well were 75% higher for BTEX and 77% higher for TPH, compared to those near the unfractured well. Summary of In Situ Physical Treatment The success of in situ remediation of organically contaminated soils by physical treatment methods is currently limited to the removal of VOCs by Soil Vapor Extraction (SVE). SVE is applicable to soils with good permeability; several techniques offer promise of extending its applicability to soils with lower permeability. Electrical heating of soils shows promise for extending physical treatment to SVOCs and nonvolatiles such as polychlorinated biphenyls (PCBs), and to clay soils via the drying action of the heat applied. Fracturing techniques have potential to extend the applicability of in situ treatment to soils of low permeability. Vacuum-induced withdrawal of vapor and liquid from the contaminated saturated zone shows evidence of faster remediation than conventional pump-and-treat methods. Ex Situ Physical Treatment of Soils In the physical treatment of excavated soils, the same technological principles are used to remove organic contaminants from the soil namely, volatilization and washing. Ex situ treatment of excavated soil has additional considerations in handling and preparing the soil for processing. These include control of emissions, dewatering/drying, screening, homogenization, and feed rate control and measurement. This discussion addresses only the treatment steps available for excavated soil. Volatilization Technologies. The full range of organic contaminants in soil, including SVOCs and non-volatiles, can be physically removed from the soil by heating it to temperatures sufficient to vaporize them, and then collecting them as a concentrate for further treatment or disposal. This vaporization step can be accomplished by aeration, vacuum, steam, or heating, using moderate energy input to heating to several hundred degrees centigrade, and combinations thereof. Aeration. This physical treatment process involves the placement of VOC-contaminated soil in containers and aerating the soil, similar to in situ SVE. The process is simple and low in cost, and uses inexpensive equipment such as roll-off boxes or Baker tanks. The usual practice is to accelerate the process by either heating the air or the soil in the container and/or applying a vacuum to the container. Commercial units are available that utilize one or more of these technologies ' 32 Not only does the use of heat and vacuum speed the volatilization of VOCs, but it also increases the probability of achieving the desired cleanup levels. It can also aid in removal of some SVOCs. Systems that heat soil require more energy. Energy requirements increase as the treatment temperature approaches 100 C, where soil moisture evaporation occurs. The accompanying large increase in steam vapors complicates the vapor collection and disposal system and the air emission control systems. The key issues in volatilization treatment of VOC-contaminated, excavated soils are: Control of air emissions during soil excavation and handling. The simplicity of the soil aeration process minimizes volatile losses once the soil is loaded in the container and sealed. The ability of the process to achieve the soil cleanup levels that are imposed on excavated soils. Compared to in situ volatilization treatment by SVE, where a significant fraction of the VOCs 4 is left on the soil, ex situ treatment will have much more stringent cleanup requirements, such as <2 ppm for PCBs or <50 ppb for TCE. Steam-induced Volatilization. The use of steam to heat excavated soils is limited by the initial condensation of steam at temperatures below 100 C, which adds to soil moisture. Collecting contaminant-laden steam vapors at temperatures above 100 C is also a technology challenge. The batch steam distillation process takes advantage of the use of steam to remove VOCs from solid matrices and is especially useful for sludges, sediments, and wet clays in which large amounts of water are present with the VOCs. The wet solid or slurry, as from a dredging operation, is fed to a batch steam distillation unit, where the solids are slurried with water and live steam is sparged into the slurry to raise the temperature. The VOCs are volatilized and collected along with the steam as a condensate for disposal. Removal of BTEX and chlorinated solvent VOCs in the range of 97% to 99% was achieved in SITE-supported pilot tests on three soils. 33 Besides the advantage of being able to process solids with high water content without dewatering or drying, the treated solids slurry is a ready-made feed for additional treatment by soil washing to remove metals or for bio-slurry remediation for SVOCs and non-volatile organics. This volatilization approach to VOC removal is a technology that fits special situations where it has advantages, namely low-volume, wet soils and sludges contaminated with VOCs. Thermal Desorption. Thermal desorption separates organic contaminants from soils by heating the soil to a sufficient Volume 46 May 1996 Journal of the Air & Waste Management Association 3S9

11 temperature to vaporize the contaminant. When the heating device is an indirectly heated chamber and the vaporized organics are condensed and/or collected as a concentrate for further treatment or disposal, thermal desorption is a physical treatment technology. A variety of devices have been developed for use as primary heating chambers for thermal desorption. Variations of the rotary calciner, an indirectly heated rotating metal alloy cylinder, are the most prevalent. The organic contaminants are volatilized at temperatures in the range of 250 "C to 550 C in a low oxygen purge gas and conveyed as gases to a condensation/scrubbing system that removes the contaminants from the purge gas. The condensed contaminants are collected as concentrates for recovery or further treatment. Further treatment by thermal destruction or chemical destruction can be carried out either on-site or off-site. A schematic diagram of low temperature thermal desorption with condensate collection is shown in Figure 6. The commercial literature frequently includes references to low temperature thermal desorption systems that utilize an afterburner to destroy the volatilized organics, rather than a condensing system to collect them as a condensate. This type of system is in reality an incinerator, and in the future such systems may be required to meet Resource Conservation and Recovery Act (RCRA) incinerator standards when used to treat sediments or soils contaminated with hazardous waste constituents. The performance of a low temperature thermal desorber in treating sediments or soils depends on the time-temperature conditions achieved in the primary chamber. In general, the higher the temperature, the shorter the time that the solids have to be at that temperature to achieve cleanup standards. FEED PREPARATION DESORBER OFF GAS FURNACE FLUi GAS COMBUSTION AIR PARTICULATE CONTROL SYSTEM Table 1. Thermal desorption treatment performance data summary. Contaminant TCD Dioxin PCB PCB PAH Pesticides Chlorinated Solvents Matrix Soil, Coral Soil Soil Soil Soil Soil Figure 6. Thermal desorber with condenser/scrubber system used to treat soils and solids to remove SVOCs, pesticides, PCBs, and PCDD/PCDFs. Scale Pilot Full Pilot Pilot Full Full Treated Soil Analysis Results < 1 ppb <2ppm < 2ppm < 1 ppm < 1 ppm < 50 ppb Lower temperatures require longer residence times in the primary chamber and therefore reduce throughput. For contaminants such as polychlorinated aromatics (e.g., PCBs) and polycyclic aromatic hydrocarbons (PAHs), processing temperatures at or below the normal boiling point of the contaminant will not meet stringent cleanup standards with practical, useful residence times of one hour or less. The key factors affecting the cost of thermal desorption are throughput requirements, quantity of sediment/soil to be treated, water content of feed, on-stream reliability, and disposal of residuals. The remediation of contaminated soils by low temperature thermal desorption has not been widely utilized. F,xtensive pilot plant testing with engineering-scale equipment has shown the ability of thermal desorption to provide cleanup performance equivalent to incineration. Representative pilot- and full-scale performance data are presented in Table I. 34 Full-scale results were obtained in actual site remediation situations using production size equipment. To date, full-scale applications of thermal desorption/condensation have been few. The Resolve, Inc. Superfund site in Massachusetts was successfully remediated using a thermal desorption/condensation system. The primary thermal chamber was an indirectly fired voc CONTROL SYSTEM ATMOSPHERE DISCHARGE OR RE-MOISTURE SOIL rotary calciner. Thirty-five thousand tons of PCB-contaminated soil were treated to <2 ppm PCBs at an average rate of 6.6 tons/hr. Prior to full-scale operation, the desorber system was fully tested in the SITE program. The results of that test 35 enabled proceeding with full-scale remediation. A hybrid thermal desorption system was used to remediate 12,700 tons of PCB-contaminated sediment from the Waukegan Harbor. This hybrid unit used a transportable anaerobic thermal processor. Prior to this application, this unit had 400 Journal of the Air & Waste Management Association Volume 46 May 1996

12 also remediated approximately 25,000 tons of PCB-contaminated sandy soil at Wide Beach, NY. The PCB concentrations in the dredged sediments at Waukegan Harbor averaged 10,400 ppm. Treated solids were <2 ppm. This unit was also tested in the SITE program. 36 The central element of this technology is the processor itself, which resembles a rotary kiln. However, inside the processor are three physically distinct zones and four zones characterized by different physical processes. 37 These include zones for preheat, retort (anaerobic desorption chamber), combustion, and cooling. In the combustion chamber, the desorbed solids are exposed to hot combustion gases to burn out residual organics. Because of this feature, the processor does not qualify strictly as a thermal desorber, even though it is equipped with a condensation system for the desorbed organics. Examples of other thermal desorption processes that have been evaluated in full-scale are: Direct fired rotary dryer with water quench, tested in the SITE program in Arizona on pesticide-contaminated soil and at five other sites. 38 Indirectly heated thermal screw processor with condensation, tested in the SITE program in Michigan on soil contaminated with volatile and semi-volatile organics. 39 Indirectly heated thermal screw processor with condensation, tested on refinery sludge solids. 40 Thermal desorption is a proven and effective remediation technology to achieve stringent cleanup standards in treating organically contaminated sediments and soils. When large quantities of material are involved, the cost of using these thermal technologies in on-site remediation is competitive with other alternatives. A volume-reduced feed of organic concentrate from soil treatment can be further treated in a cost-effective manner. The key issues in application of thermal desorption are: Indirect versus direct heat an indirectly heated thermal desorber vaporizes the contaminants into a much smaller volume of vent gas, whereas a desorber heated by direct firing of hydrocarbon fuel produces a vent gas in which the contaminants are mixed with the large volume of combustion gases. Capture of the contaminants by condensation and/ or cleanup of the gases is much less costly for the indirect units. Condensation versus afterburner condensation of the vaporized organic contaminants coming from the desorber collects them for subsequent recovery/ reuse or ultimate off-site disposal; an afterburner or other thermal destruction device converts the vaporized organic contaminants coming from the thermal desorber to carbon dioxide by high temperature oxidation, not unlike an incinerator. Soil remediation by a thermal desorption system with an afterburner continues to enjoy acceptability as an alternative to incineration. Formation of byproducts any process that treats organically contaminated soils by heating them to temperatures of 300 C to 500 C will generate some degree of decomposition byproducts. Examples are coke, char, noncondensibles such as methane, hydrogen, alkanes and alkenes, and carbon monoxide. The decomposition products of the highest concern are PCDDs and PCDFs, formed at ultra-low trace levels from treating chlorinated aromatic hydrocarbons. Feed handling involves feed preparation steps such as partial drying, homogenization, and screening and the consistent transfer of the prepared soil into the thermal desorber unit. Washing Technologies. Two methods of removing organic contaminants from excavated soil are listed in the washing technologies category. The first is washing the soil with water, or with water to which mass transfer enhancing chemicals, such as surfactants, have been added. The second is the use of organic solvents to extract organic contaminants into a separate solvent phase. Water Washing. Washing excavated soil with water to remove organic contaminants is a volume-reduction step that utilizes methods and equipment common to the mineral processing industry. As such, soil washing processes can be assembled that have very high throughput, leading to low costs. The separation processes physically separate the soil into size fractions based on particle size. Additives are also used to enhance transfer of the organics from the surface of the soil particle to the liquid phase. The processing equipment features proven unit operations such as screening (dry, wet, trommels), solid-liquid contractors (mixers, attrition mixer), and solid-liquid separators (wet screens, hydroclones, froth flotators, jigs, filter presses, centrifuges). Additional equipment is needed to recover organics from the wash waters. Soil washing cleans gravels and sands; the fines fraction generally contains the organic contaminants and requires further treatment or disposal. Because of this, soils with high fines content, e.g., clays, are not cost effectively treated by soil washing. A typical soil washing schematic is shown in Figure 7. The usual sequence of processing involves: Wet screening and water washing to separate coarse fractions (gravel). Separation of humic materials (roots, grass, twigs). Separation of sand and fines. Scrubbing of sand with additives, such as surfactants and detergents, to remove organics. Further washing of fines to increase recovery of clean particles and minimize contaminated fines for disposal. Volume 46 May 1996 Journal of the Air & Waste Management Association 401

13 85% CLEAN SAND RECYCLED WATER Figure 7. Schematic of typical soil washing process (courtesy ART, Inc.). The current status of soil washing for organic removal consists of a limited number of recent applications in the United States, with a longer history of use in Europe, particularly the Netherlands and Germany. Demonstration tests were completed on contaminated soil at a Toronto Harbor site 41 in 1992 and on PCB-contaminated dredge spoils from Saginaw Bay in Lake Huron in The feed to the Toronto harbor soil washing process contained low concentrations of oil and grease (0.8 mg/kg), naphthalene (11 mg/kg), and benzo(a)pyrene (2 mg/kg). The cleaned coarse fractions were 80% of the feed mass, and the process achieved 75% to 80% removal of these organic contaminants. The Saginaw Bay tests showed separation of a clean coarse fraction whose mass was over 90% of the initial mass treated. The organic contaminants were concentrated in the fines streams. PCB concentration in the clean coarse fraction was <0.2 mg/ kg, a reduction of 86%. Over 14,000 tons of TPH-contaminated sandy soils at the Gustavus, Alaska, airport were remediated by soil washing in TPH cleanup levels of <200 mg/kg were achieved in a surfactant-based soil washing system. The treatment of pentachlorophenol (PCP) and PAH-contaminated soil was demonstrated on a pilot scale in The washed clean coarse (sand) fraction that was recovered was 83% of the feed mass, and it retained about 10% of the feed soil contamination. Ninety percent of the organic contaminants were in the fines. The concentrations of PCP and PAH on the washed, clean soil were approximately 70 ppm and 50 ppm, respectively, representing 89% and 88% removals. Extensive bench-scale evaluations of soil washing have shown the potential for this process to treat a variety of organic contaminants. Examples from several reported tests are shown in Table 2. The key issues in the application of soil washing to remediation of organically contaminated soils are: Fines content; soil washing is a volume reduction process, and organic contaminants are generally associated with fines that require further treatment or disposal. Water used requires treatment prior to reuse or disposal. Use of chemical additives to promote mass transfer complicates water treatment/reuse and options for use of treated soil. Cost effectiveness depends on high throughput; commercial units typically have capacities >20 tons/ hour. Recovery and reuse of chemical additives may be essential to cost-effectiveness. Solvent Washing (Extraction). Because many of the hazardous organic substances in contaminated soil are hydrophobic, their removal using water-based washing processes lacks the mass transfer driving force that can be attained by washing with a hydrophobic organic solvent. Solvent extraction is used in the analytical methods that quantitate organic contaminants in soil, so use of the same physical-chemical principles to treat soil is attractive. Considerable effort has been expended to develop and demonstrate the washing of soil by solvent extraction. A variety of solvents and processes have been advanced to treat contaminated soils, sediments, and sludges. Examples of solvents tested include acetone, hexane, chlorinated solvents, triethylamine, liquefied propane, and supercritical carbon dioxide. Proprietary solvents and mixtures have also been used. Table 2. Bench-scale evaluations of soil washing. Contaminant PCB PAH TPH TNT Concentration on treated soil, mg/kg <1 <10 80 <1 Percent Removal >99 >90 >90 > Journal of the Air & Waste Management Association Volume 46 May 1996

14 Some developers have approached the mass transfer problem of removing hydrophobic organics from moist soils either by using a hydrophilic organic solvent or by removing the soil moisture as a first step in the solvent extraction process. The unit process steps in solvent extraction from soil are: Mixing of soil with solvent and subsequent separation of the two phases; in most applications, this step must be repeated a number of times to achieve the desired cleanup levels. Recovery of solvent from the treated soil. Recovery of solvent from the extraction liquid for reuse and to separate it from the contaminant concentrate. Recovery of solvent from the soil moisture; with solvent extraction, soil moisture will be removed from the soil as a separate liquid phase that must be treated. These steps require a complex chemical plant involving several processing steps. Bench-scale tests have shown that solvent extraction can achieve the required cleanup levels with a variety of soils and sediments. Achievement of these cleanup levels in engineering scale equipment has been more difficult. This difficulty is exemplified by three demonstrations of solvent extraction processes that have been conducted on contaminated sediments. Each involved proprietary processes employing unconventional solvents. The first process tested was on PCB-contaminated sediment from the Massachusetts New Bedford Harbor Superfund site. 45 The process utilized liquefied gases such as propane and butane as the extraction solvents. The advantages of liquefied gases over conventional liquid organic solvents are easier separation of the extractant from the solids and water, and easier recovery of the liquefied gases from the organic contaminants. The demonstration unit was rated at 20 barrels/day and operated at 240 psig and 70 F. Three test runs were made Table 3. New Bedford test results with the liquified gas extraction process. on PCB-contaminated sediment. At a starting PCB concentration of 350 ppm, 10 extraction cycles were necessary to reduce the PCB concentration to 10 ppm. This and other test results on the sediments are shown in Table 3. These data strongly indicate the difficulties in extracting PCBs from contaminated sediments to very low levels. The use of water-immiscible hydrophobic solvents, such as propane, exacerbates solvent extraction of sediments. A second demonstration of solvent extraction of contaminated sediments involved PCBs, PAHs, and oil and grease, and was performed on sediments obtained from two separate areas of the Grand Calumet River in Gary, Indiana. 46 The process tested employs multiple extraction cycles using both hot and cold triethylamine (TEA) as the extraction solvent. The main advantage of TEA is that at low temperatures (<60 F) it is miscible with water, while at higher temperatures, it is immiscible. These properties enable the process to overcome mass transfer barriers during extraction, and simplify water/solvent separations in the subsequent process steps. Two different river sediment samples were treated in the pilot unit at an average treatment rate of approximately 90 pounds per day. It was determined in laboratory benchscale tests that each sediment would require seven extraction sequences to achieve the desired contaminant reductions. The feed and treated sediment data are summarized in Table 4. These data show >99% removal of PCBs to low levels. For PAHs and oil and grease, the removals were >90%, but appreciable concentrations remained in the treated solids. Based on these data, one can conclude that solvent extraction technology can be used to remove gross amounts of organic contamination, but achieving stringent cleanup levels will be difficult. The third demonstration involved three soils contaminated with PCBs. 47 The technology is a batch solvent extraction process at ambient temperature and pressure, wherein the Test Number Number of Contact Stages Initial PCB Concentration * (ppm) Final PCB Percent Reduction , *Mean PCB concentrations, ppm dry weight basis. Tabl@ 4. Grand Calumet River test results with the TEA solvent extraction process. Sediment A Untreated Sediment Sediment B Sediment A Treated Solids Sediment B PCB mg/kg PAH mg/kg Oil/Grease mg/kg Moisture ,900 41% , ,000 64% ,530 Volume 46 May 1996 Journal of the Air & Waste Management Association 403

15 Table 5. PCB-contaminated soil extraction results. Number of extraction cycles Site 1 12 Site 2 24 Site 3 57 PCB Concentration; mg/kg Untreated Soil Treated Soil soils are loaded in extraction tanks, are repeatedly soaked with a proprietary solvent mixture, and drained and soaked as many times as required to meet treatment criteria. The extraction solvent is distilled to recover it, generating a concentrate of contaminants for disposal. Residual solvent is removed from the soil batch by aeration and vacuum extraction. Demonstration results for processing contaminated soil from three different sites in 12 cubic yard batches are presented in Table 5. These data show the large number of extraction cycles required to reduce PCB levels in the soil to below or near cleanup requirements. The key issues in cost effective treatment of excavated soils and sediments by solvent extraction are: Number of stages of solvent contact with the soil to reach the required cleanup levels, e.g., <2 mg/kg PCBs. Fines content of soil, because fines complicate solvent handling and recycling, and as with water washing, fines end up in the organic concentrate that requires further treatment or disposal. The amount of residual solvent left on treated soil; it affects the ultimate use of the soil. Solvent losses due to evaporation during processing steps, residual on treated soil, and solubility in water. Solvent flammability and toxicity involve safety concerns and special design considerations for spark-producing equipment. Air emissions due to solvent evaporation during processing cause odor and toxicity concerns. Summary of Ex Situ Physical Treatment The ex situ remediation of organically contaminated soils by physical treatment methods can be reliably performed by properly designed and applied volatilization technologies. The use of aeration and/or vacuum combined with moderate heating (up to 100 C) has been successful in treating VOC-contaminated soils. Volatilization of SVOCs and non-volatile organics from soil by thermal desorption treatment in indirectly heated process units equipped with vapor condensation collection systems is a reliable alternative to incineration that provides equivalent soil cleanup levels. Additional development/demonstration is needed on the collection, treatment, and disposal of the thermal desorber condensate. Proven, conventional equipment and processes are available to reduce the volume of contaminated soil by water washing. Further demonstration of these processes on a variety of organically contaminated soils and sediments is needed. The use of solvents to separate organics from soil in other than simple batch processes may have insurmountable obstacles to cost-effective use. CHEMICAL TREATMENT Remediation of soil by chemical treatment encompasses technologies that destroy or chemically transform the organic contaminants. The chemical destruction process involves oxidation to carbon dioxide using chemical oxidants such as hydrogen peroxide. The transformation reactions include chemical or ultraviolet (UV) reduction and dechlorination with alkaline reagents. Also included in chemical treatment technologies is stabilization of contaminated soil, wherein the chemical contaminants are immobilized and made more inert in the environment in a matrix formed by chemical reactions. Like physical treatments, chemical treatments can be done either in situ or ex situ. In Situ Chemical Treatment In Situ Chemical Oxidation. In situ chemical oxidation involves injecting a chemical oxidant, especially hydrogen peroxide, into a contaminated subsurface zone to effect oxidation of the organics. Hydrogen peroxide is preferred because of its ease of handling and innocuous decomposition products. However, hydrogen peroxide's reactivity to organics at practical rates and dosages is limited to compounds like aldehydes, ketones, and phenols. Addition of ferrous ion (or its presence in the soil matrix) can expand the reactivity of hydrogen peroxide to a greater number of organics. The in situ oxidation of organics was used to clean up two formaldehyde spill sites in the 1980s. 48 Both applications were motivated by the need to remediate soil without excavation. One site involved the gravel ballast under the tracks of the main line of a railroad that required cleanup in an emergency response situation. The formaldehyde contamination was too high for biological treatment, so a direct application of hydrogen peroxide solution was applied to the ballast to reduce contamination levels from several thousand mg/kg to below 500 mg/kg, so biological treatment could complete the destruction. The second site was contaminated under storage tanks and buried piping. The formaldehyde concentration was as high as 2%, and was treated in situ to <1 mg/kg, again by injecting hydrogen peroxide through four injection wells into the zone of contamination. Another field test of in situ chemical oxidation using Fenton's reagent, an oxidizing reagent comprised of hydrogen peroxide and ferrous ion, was reported for a site in New Jersey. 49 BTEX levels in groundwater were reduced from 10 ppm to 10 ppb over a treatment time of 200 days. The success in utilizing Fenton's reagent in treating PAHcontaminated soils in slurry reactors has spawned additional 404 Journal of the Air & Waste Management Association Volume 46 May 1996

16 attempts to apply in situ treatment of organics with Fenton's reagent. 50 The use of chemical oxidants such as hydrogen peroxide or Fenton's reagent to treat organic contaminants in situ offers an attractive alternative to soil excavation when the subsurface conditions are favorable. Key site characteristics are high soil permeability and the presence of a confining soil layer(s) to control migration away from the contamination zone. The emergence of hydraulic fracturing techniques offers the potential to expand the use of in situ chemical oxidation to less permeable soils. "Lasagna" Process. Another approach under extensive RD&D is the in situ remediation of contaminated subsurface zones by application of the Lasagna process. 51 This process has the basic configuration that resembles the pasta dish from which its name was derived. A subsurface reactor of high permeability is established using techniques such as hydraulic fracturing to create horizontal zones above and below (but in close proximity to) the contamination zone. The fissures created when hydraulic fracturing is used are injected with a granular solid to prop them open. By using graphite particles during hydraulic fracturing, in-place electrodes can be formed. The Lasagna reactor can then use electrokinetics to move metals and/or electroosmosis to move organics through the contamination zone. Horizontal treatment zones are also established in close proximity to the contamination. In the treatment zone, appropriate materials such as sorbents, catalysts, microbes, and oxidants can be placed to treat the organics and metals, if present. Liquid flow can also be periodically reversed to enable multiple passes of the contaminants through the treatment zone for more complete removal or treatment. The Lasagna configuration of establishing in-the-ground electrodes and treatment zones around a contamination zone can also be done vertically using construction techniques such as sheet piling, trenches, etc. Schematic diagrams of typical horizontal and vertical configurations are shown in Figure 8. Reported results to date on the Lasagna process 51 have been limited to bench-scale testing. One series of tests showed that electroosmosis could flush an organic contaminant (para nitrophenol) uniformly from clay soil into adjacent permeable layers. The technology continues to be attractive for treatment of clays and low volatility organics. In Situ Chemical Reductions. In situ chemical reduction involves the transformation of organics to less toxic or innocuous compounds by a reducing chemical reaction. The most noteworthy chemical reduction reaction for organics is dehalogenation, or more specifically, dechlorination. Critical Review break chemical bonds is well established. Common targets are toxic chlorinated compounds. Chlorinated aromatics are labile at the upper end of the range, while chlorinated aliphatics are labile at the lower end. UV in conjunction with hydrogen peroxide or ozone is an established technology for treating groundwaters and wastewaters. The challenge has been to utilize UV to dechlorinate contamination in sludges or soil. Separation of the chlorinated toxic chemicals by solvent extraction or thermal desorption has enabled their destruction by UV photolysis These references treated dioxin-contaminated sludge and soils. The direct UV photolysis of soil to destroy PCBs and dioxins was evaluated in the SITE Emerging Technology Program. 54 Initial tests showed that use of surfactants enhanced the rate of UV destruction during direct irradiation of sandy soils. Subsequent tests on soils with a high clay content were not successful, and further development was ended. Metal Reductions. The reductive dechlorination of halogenated solvents in groundwater by reaction with free metals such as iron, has opened the door to in situ destruction of these contaminants in soils and groundwater in the saturated zone. A permeable wall containing the reactive metal is placed in the groundwater flow. As it passes through the reactive wall, the halogenated organics are destroyed by reduction reactions. The permeable wall is usually made of a mixture of sand and iron particles. borehole ground surface ground surface Degradation contaminated Degradation Zone soil Zone Note: electroosmotlc flow is reversed upon switching electrical polarity. Granular Electrode Degradation Zone contaminated soil Degradation Zone Granular Electrode Dechlorination. The ability of ultraviolet (UV) radiation especially the wavelengths from around 180 to 300 nm to Figure 8. Schematic of Lasagna process, horizontal and vertical configurations. Volume 46 May1i96 Journal of the Air & Waste Management Association 40S

17 Theflowof contaminated groundwater is directed through the permeable wall by inserting impermeable barriers on the sides of the permeable wall. This "funnels" the contaminated groundwater and directs it through the permeable wall "gate," hence the term "runnel and gate" for this technology. 57 Field tests at a controlled site in Canada in 1991 showed that 90% of TCE and PCE was removed from groundwater passing through the reactive wall of iron grindings and sand placed in the 13 ft deep groundwater plume. S8 Residence time of the groundwater in the 3.5 ft thick wall was 10 days. An additional field test in the SITE Demonstration Program in an aboveground reactor tank at a site in New Jersey in 1994 and 1995 showed removal efficiencies of >99.98% for TCE and PCE during the 13 weeks of testing. 59 As an attractive in situ treatment method for chlorinated solvents in groundwater, this technology is the subject of additional R&D to further elucidate mechanisms, kinetics, and metal composition. The reduction of the chlorinated methanes has been shown to have significantly different rates. 60 In Situ Solidification/Stabilization of Organic Contaminants. Solidification/stabilization (S/S) reduces the mobility of hazardous contaminants in the environment through physical and chemical means. S/S traps or immobilizes contaminants in the soil or sludge instead of removing them or destroying them. S/S techniques can be used alone or in conjunction with other treatment methods to produce a material that is suitable for land disposal. In situ S/S can be implemented using a variety of devices to mix the S/S reagents and additives with the contaminated soils. S/S reagents produce chemical reactions to form a solidified mass that resists leaching. These reagents are typically cement or pozzolanic materials. In addition, S/S additives may also be used to absorb nonaqueous liquids to enable the reagents to react. Proprietary additives are also used to achieve a reduction in the teachability of certain organics (e.g., PCBs) by formation of chemically related bonds. In general, the performance requirements for in situ S/S include the following specifications: leachate concentration, no free water release in the paint filter test, unconfined compressive strength sufficient to support a cap, and low permeability. The full-scale use of in situ S/S for organically contaminated soils and sludges is illustrated in the following examples. A PCB-contaminated soil in Florida was treated in 1990 by S/S in which proprietary additives were used with silicates to react with the PCBs and generate a complex network of inorganic polymers. A vertical auger deep soil mixing system was used to mix the additives and silicates with the soil. An earlier SITE demonstration test at the site had shown inconclusive results in reducing the teachability of PCBs. 61 The 1990 leachate tests on treated and untreated soil samples were near or below detectable levels. The stabilized soil was still found to be intact and resistant to teaching one year later. A former manufactured gas plant site in Georgia (MGP) contaminated with PAHs was treated by S/S using the vertical auger deep soil mixing system. 62 The site is now a public riverfront park. An example of successfully stabilizing oily sludges in situ is a refinery site with waste sludge lagoons with high oil content (1.5% to 20%). Six refinery sludge lagoons were solidified using a mixture of fly ash and fluidized bed combustor ash. Conventional back-hoe type earthmoving equipment was used to mix these additives into the sludge, producing a dry solid surface that supported equipment and which met the compressive strength requirement of 20 psi and permeability requirements of <lxl0" 5 cm/sec. 63 Summary of In Situ Chemical Treatment Other than solidification/stabilization applications, in situ chemical treatment techniques for organics have been used sparingly. Achieving acceptable soil treatment levels by oxidation/reduction reactions on the soil is hindered by mass transfer limitations. Direct injection of hydrogen peroxide solutions has shown the possibility of treating readily oxidized hydrophilic organics in situ. Two promising new approaches to in situ treatment are emerging: 1) the use of electrokinetic technology, specifically electroosmosis, to mobilize organics into subsurface treatment zones, and 2) the use of zero valence metals, specifically iron, to destroy soluble halogenated organics in groundwater. Both methods require extensive development and demonstration. S/S remains as the low-cost choice for treating organically contaminated soils and sludges in situ, despite the fact that the contaminants are not removed, but only immobilized and made more resistant to contact with water and leaching. Ex Situ Chemical Treatment Chemical Oxidation. The field use of chemical oxidants to remediate excavated soils by destroying organic contaminants has seen very little application as judged by published data. Considerable bench-scale testing in soil-slurry reactors on a variety of organics has defined the treatment parameters and effectiveness, especially with Fenton's reagent. Recent attention has focused on the use of Fenton's reagent in conjunction with biological processes to treat PAH-contaminated soils Chemical oxidation with Fenton's reagent is used to break down and partially oxidize organics that are recalcitrant to biological treatment. The chemical oxidation step can be applied either before the biological treatment, after biological treatment, or between two biological treatment sequences. Successful treatment of PAHs with Fenton's reagent and biological oxidation has been reported in pilot-scale slurry reactors and in land treatment type treatment units. 66 The technology was especially effective in improving the treatment of 4- to 6-ring compounds. Over 30 different PAH-contaminated 406 Journal of the Air & Waste Management Association Volume 46 May 1996

18 soils have been tested. The same chemical-biological approach has been used to treat PCB-contaminated soils to <2 ppm in bench-scale tests. 67 The chemical-biological approach to remediation of PAHcontaminated soils has also been reported using a combination of persulfate and hydrogen peroxide as the chemical oxidant 68 Bench-scale tests showed enhanced treatment effectiveness on soil fines contaminated with phenanthrene. A chemical oxidation process using hydrogen peroxide and a proprietary catalyst was used to treat excavated TPHcontaminated soil at two sites in California in the late 1980s and early 1990s. The soil was treated in a pug mill-type mixing unit. The treatment at one site was evaluated by the state of California's Alternative Technology Division. 69 Their report concluded that the same degree of TPH removal was achieved by this catalytic process and by a control test where the soil was merely aerated. The excavated soil at the second site also was treated by the catalytic process and produced treated soil that met the cleanup criterion of < 100 ppm TPH. However, feed soil samples were not collected from the feed stockpile which had been excavated and stockpiled several years earlier, so the degree of removal by chemical oxidation versus handling losses could not be discerned. The use of chemical oxidants as a pre-oxidation step to biological treatment, or as one step in a multi-step sequence involving chemical and biological treatment, shows promise in being able to treat recalcitrant contaminants such as PAHs and PCBs to stringent cleanup levels. Chemical Reduction. The main reductive chemistry reactions employed for ex situ treatment of contaminated soils is dechlorination using alkaline reagents. PCBs and pentachlorophenol (PCP) have been the principal target contaminants. The alkaline reagents strip one or more chlorine atoms from the PCBs and replace it with a variety of organic moieties, depending on the reagent used. The typical substitutions are hydrogen, hydroxyl, and polyglycols. Hydrogen results in generation of successively lower chlorinated biphenyls, Critical Review and if the reaction goes far enough, will produce biphenyl. The others produce substituted biphenyl molecules with fewer chlorine atoms that do not analyze as PCBs. The processes employing chemical reagents that have been tested for dechlorination treatment of soil contaminated with PCBs are summarized in Table 6. The only process currently being operated at full-scale is base catalyzed dechlorination (BCD) technology. 70 A schematic of the BCD process is shown in Figure 9. The Ca/NH 3 technology is being used to treat CFCs. The APEG technology is no longer available, and the others listed above have not progressed to commercial status. BCD is a two-step process. The first step is more akin to thermal desorption than to dechlorination. PCB-contaminated soil is heated to approximately 300 C in an indirectly heated rotary reactor. Sodium bicarbonate and a "catalyst" are added as dechlorination reactants. Some PCB dechlorination of PCBs occurs in this first stage reactor, but much of the PCBs vaporizes and is condensed and collected along with the volatilized soil moisture in a condensation system. The condensed PCBs are then separated from the water and dissolved in a hydrocarbon solvent. Additional BCD reagent is added to the PCB-solvent solution and heated to effect dechlorination of the PCBs. The process is currently processing several thousand tons of PCB-contaminated soil on Guam. Besides development work on PCBs, data have been generated on the applicability of dechlorination reagents to other chlorinated aromatics, such as the DDT family. Appearance and disappearance of dioxins and furans during dechlorination treatment is regularly monitored. The dechlorination reaction destroys polychlorinated dioxins and furans at the reaction residence times usually employed. The search for the "silver bullet" to easily destroy polyhalogenated aromatics in soil continues. Virtually all dechlorination reactions have been explored, and a few survive for continued development. Table 8. Dechlorination treatment results. Site Contaminant Reagent Scale Temp, C Dechlorination Result Wide Beach, NY Buffalo, NY Guam Morrisville, NC Several soils Sandy and clay soils Foundry sludge Soil PCB PCB PCB PCB PCB PCB PCB PCB APEG APEG BCD BCD Ca/NH 3 DCR/CaO Alkasol Sulfur pilot pilot pilot full pilot bench bench pilot pilot >100 <2 ppm PCB <27 ppm PCB <1 ppm PCB <10ppmPCP <1 ppm PCB, biphenyl byproduct <9 ppm total PCBs <5 ppm PCB.3-7 ppm PCB APEG= Alkali hydroxide polyglycol with dimethyl sulfoxide BCD= Base-catalyzed dechlorination (bicarbonate) Alkasol= Complex mixture of alkaline dechlorination reagents and organic additives Ca/NH 3= Calcium metal dissolved in liquid ammonia DCR/CaO- Hydrophobized quicklime Sulfur= molten sulfur Volume 46 May 1996 Journal of the Air & Waste Management Association 407

19 r OIL VAPOR RECOVERY SUBSYSTEM SOIL PRETREATMENT (IF NECESSARY) VAPORS OIL SCRUBBER CONDENSEI VAPOR- WATER SCRUBBERS * DEMISTER PHASE CARBON ADSORBER CLEANED VAPORS TO VENT REAGENTS NITROGEN-^J (AS NEEDED) SOIL SAREX THERM-O-DETOX" THERMAL SOLIDS COOLER 1 TREATED SOIL OIL HEATER OIL OIL FILTER SPENT OIL SLOWDOWN OIL STORAGE TANK OIL BCD REAGENTS OIL i OIL/WATER f OIL/WATER SEPARATOR LIQUID BCD REACTOR kwater WATER CHILLER VAPORS CONDENSE! LIQUID- PHASE CARBON ADSORBER WATER TO REUSE WATER TO DISPOSAL TREATED SOIL TO DISPOSAL SOIL TREATMENT SUBSYSTEM Source: Adapted from B.H. Miller et al. Figure 9. Flow diagram of the BCD soil/sludge dehalogenation system. The key issues associated with direct dechlorination of these compounds on soils are: Byproducts produced their toxicity, mobility, biodegradability, and acceptability as residuals on treated soil are of concern. Residual reagent on soil its toxicity, mobility, biodegradability, and acceptability as a residual on treated soil are of concern. Disposal of any residuals, such as spent reagent, byproducts strong alkaline wastes are not readily disposed of. Temperature of operation temperatures utilized have ranged from -30 C to 350 C. Analysis of residuals analysis of chemically treated soil matrices for congeners such as PCBs is fraught with opportunities for misleading results. Solidification/Stabilization. In ex situ solidification/stabilization (S/S), like in situ S/S, contaminants are physically bound to proprietary reagents to reduce their mobility (stabilization). However, when contaminated soil is excavated to treat by S/ S, it typically requires replacement on the site or off-site disposal in accord with more stringent regulations. Hence the ex situ S/S treatment process must be highly effective. OIL TO OIL SCRUBBER OR DISPOSAL LIQUID TREATMENT SUBSYSTEM - " SOLID/LIQUID A measure of the effectiveness of the application of S/S to the treatment of organically contaminated soils is found in the processes that have been tested in the U.S. EPA's SITE program Proprietary additives were generally used in conjunction with cementitious and pozzolanic materials to restrict the mobility of the organics and encase them in a solidified mass. One example of the types of difficult wastes that can be adequately treated by S/S is a refinery site in Oklahoma. 73 Over 200,000 cubic yards of very acidic tars in several earthen pits were successfully stabilized using a proprietary hydrophobized quicklime as the solidifying agent. The TPH content in the pits averaged 16% to 24%. Hydrophobized quicklime delays reaction of the quicklime with water in the waste, so it can be thoroughly mixed before hydration begins. The key issues for ex situ S/S of organically contaminated soils are: Regulatory criteria, especially leachability of organics, e.g., <1 ppm for most chlorinated organics. Long term (e.g., 5-10 years) physical and chemical stability of the mass. Engineering controls on the repository, such as liners, caps, groundwater diversion barriers, and leachate collection and treatment. 408 Journal of the Air & Waste Management Association Volume 46 May 1996

20 Summary of Ex Situ Chemical Treatment The chemical treatment of organically contaminated soil to transform the contaminants by oxidation or reduction reactions has been the object of a large research and development effort. The results to date in dechlorination best exemplify an adage adapted from the chemical industry but now very apropos to the remediation industry: "a reaction is not a process." The success in tearing apart organic molecules has been hampered by the additional steps required to deal with the residuals produced. The current efforts in use of Fenton's reagent as one part of a sequence of treatments shows promise in treating the more recalcitrant contaminants such as polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs). Similarly, recent new dechlorination reactants are simpler and may lessen residual handling and disposal problems. As with in situ treatment, ex situ solidification/stabilization (S/S) continues to provide a low-cost alternative to treatments that remove or destroy the contaminants. Users accept the fact that the contaminants are still present, but immobilized at least for the near term. CONCLUSIONS For both in situ and ex situ treatment technologies, the U.S. EPA SITE Program has provided valuable performance information on a wide variety of emerging developmental techniques. The results of these evaluations have provided high quality, unbiased analytical data and have shown the difficulties in treating contaminated soils. Unfortunately the SITE demonstrations are one-time tests at specific sites and leave future users to translate the results to their particular situations. This shortcoming can be greatly alleviated by widespread and timely publication of test results from the numerous DOD and DOE programs now in progress. The following conclusions are derived from this review of in situ physical treatment technologies: Removal of volatile organic chemicals (VOCs) from soil and associated groundwater by soil vapor extraction (SVE) and/or air sparging has become an accepted and widely practiced remediation strategy for soils of adequate permeability. It is also useful as a subsurface containment strategy. The long-term fate of residual VOCs left in the soil after SVE/air sparging has been completed is not well documented. Use of specially constructed vertical wells to air-strip groundwater and recycle it through the contaminated area outside the well has not yet shown advantages that justify the high cost of the wells. VOC removal can be accelerated by soil heating using hot air, steam, radio frequency, or electrical techniques. Enhancement of volatilization of semi-volatile organic chemicals (SVOCs) and non-volatile organics by heating has not yet shown a clear advantage that warrants the selection of an enhancement remedy or the extra cost involved. Soil heating can also be beneficial in improving soil permeability. Fracturing techniques (both pneumatic and hydraulic) adapted from the oil production industry, show promise in being able to extend in situ technologies to clay soils. Technologies for drilling wells horizontally, though expensive and difficult to design, are likely to be the only way to access contamination under buildings and equipment. Soil flushing and the use of electrokinetics to mobilize subsurface contaminants for removal require much additional demonstration before they can be extensively applied for site 1 remediation. The following conclusions derive from this review for in situ chemical treatment: No proven method of direct, in situ chemical treatment of organics has been developed. Direct injection of a chemical oxidant (e.g., hydrogen peroxide) can be used to treat easily oxidized hydrophilic organics in situ, but injection of more aggressive oxidants such as Fenton's reagent to attack recalcitrants such as polycyclic aromatic hydrocarbons (PAHs) requires development and demonstration. Use of zero valence metals such as iron to treat groundwater contaminated with chlorinated solvents in situ is a promising new approach with the potential to be an important tool for in situ remediation. For physical ex situ treatment technologies, the conclusions of this review are: Thermal desorption systems provide the highest degree of removal of organics from excavated soil, successfully treating VOCs, SVOCs, PCBs, PAHs, polychlorinated dibenzo dioxins/polychlorinated dibenzo furans (PCDD/PCDFs), and pesticides to required cleanup levels, at costs competitive with incineration and soil washing/soil extraction processes. Aeration of heated soil is a low-cost method of removing VOCs and some SVOCs, if the temperature is raised high enough. Soil washing with water or water plus additives is a cost-effective method to reduce the volume of contaminated sandy soils. Clayey soils with high fines content do not achieve a volume reduction sufficient to justify the cost. Use of the solvent extraction of soils has been hampered by the large number of extractions required to achieve cleanup levels and the complexity of the solvent handling circuits. Volume 46 May 1996 Journal of the Air & Waste Management Association 400

21 Critical Review For chemical ex situ treatment the conclusions are: The use of chemical oxidants, such as Fenton's reagent, shows promise as a way to achieve cleanup levels for recalcitrants such as PAHs when combined with biological oxidation in a treatment sequence. The search for the "silver bullet" to dehalogenate polyhalogenated organics (e.g., PCBs) to innocuous byproducts has not yet yielded a proven technology. With respect to solidification/stabilization (S/S): S/S is a low-cost remedial strategy to immobilize a variety of organic chemicals in a solidified, stable mass that meets regulatory criteria for leaving contaminants in place or for re-deposition on land. The long term physical and environmental stability of soils treated by S/S has not been well documented. CURRENT TRENDS This review of physical/chemical treatment technologies for organically contaminated soils and sediments provides insight into current trends in in situ and ex situ treatment situations. The main focus of in situ treatment approaches is to use systems that combine soil vapor extraction, air sparging, and biodegradation. This is commonly referred to as bioventing. Soil vapor extraction and air sparging remove volatile organics; air sparging increases the available oxygen to the naturally occurring microorganisms in the subsurface, stimulating aerobic biodegradation of biodegradable organics. This combined treatment provides remediation of organically contaminated sites in a cost-effective manner. In situ treatment of chlorinated aliphatic organics, such as trichloroethylene, is being developed on two fronts the use of zero valence metals to destroy trace levels in groundwater and the initiation of their biodegradation by injecting the necessary chemicals to activate the specific degrading microorganisms. In situ treatment of soils with low permeability, such as clays, is the object of developmental efforts in using fracturing techniques and electrokinetics-related methods to move contaminants through soil. Horizontal wells are finding increased use, both for accessing contamination under buildings and processing facilities, and for removing discrete layers on top of subsurface strata or groundwater. Solidification/stabilization of organically contaminated soils continues to be a widely used remediation option, both in situ and ex situ. With the lowest cost of all remediation options, this method of isolating the contaminants from the environment has been acceptable to date, even though it does not eliminate them. For other ex situ treatment approaches for soils, the trends are: Use thermal treatment by thermal desorption systems to achieve treatment performance equivalent to incineration while avoiding its negative reputation. Use soil washing of organics as an additional separation step when the motivating driver for its selection is the treatment of metals contamination in soils and sediments. Use chemical oxidation to extend the applicability of biological oxidation treatment of soils and sediments to organic contaminants that are resistant to biodegradation. Continue to search for dechlorination reagents and processes that will compete with dig-and-haul methods for disposal of soils and sediments contaminated with chlorinated organics, such as polychlorinated biphenyls. RELEVANT ISSUES AND RESEARCH NEEDS Several issues have been identified relative to the treatment of organically contaminated soils. These issues suggest additional research needs that would be of benefit to both the user and the regulatory community in making decisions about the selection of soil remediation strategies. These issues and the research needs are summarized below. Developing a Remediation Process In making the transition from developing technologies to proven remediation processes, the adage "a reaction is not a process" is more appropriately rewritten as "a separation/ reaction is not a process." The history of the development of processes for the treatment of hazardous wastes has shown repeatedly that dealing with the residuals produced by a technology is a significant issue. Many innovative ideas for physical or chemical treatment languish or fail because of the residuals they produce. A simple separation, such as soil vapor extraction or air sparging, can be much more difficult and expensive when air emission controls are required. Complex, multi-step processes such as soil washing or solvent extraction have the majority of the equipment and cost associated with handling and disposal of the residuals. Anyone with a new idea for treating hazardous wastes is well advised to carefully and realistically examine, early in the development, the residuals that will be generated and how they will be handled. In Situ Treatment Effectiveness Almost all in situ treatment methods, whether proven or under development, leave a residual level of contamination after treatment is completed that is greater than the levels required in treating excavated soil. The example referenced earlier is the residual volatile organic chemicals left in soil after removal by soil vapor extraction and/or air sparging has been completed. Achieving anywhere near +90% removal is very difficult. The question arises whether the amount of organic contamination remaining after treatment presents a reasonable risk to human health and the 410 Journal of the Air & Waste Management Association Volume 46 May 1996

22 environment. New emphasis on risk assessment in cleanup decisions will require greater investigation of the fate of residuals in soil treated in situ. In situ treatment effectiveness is currently limited to soils of high permeability. New methods of extending in situ treatment to low-permeability soils require continued development and demonstration. Similarly, in situ treatment of non-volatile organics is limited to biological processes. For non-biodegradable, nonvolatile organics, only high temperature thermal processes can remove them to the desired cleanup levels. Chemical processes offer promise as an alternative, but more research and development is needed to demonstrate their practicality and cost effectiveness. For many in situ treatment processes that generate byproducts, either by thermal, chemical, or biological reaction, data on the toxicity and fate of these residuals are sparse. More research on their properties and their effects on the environment is needed to adequately assess risk. Chemical Analysis of Soil Treatment Processes There is growing concern that the current standard analytical methods for soils do not provide an accurate measure of the true chemical contamination. This is especially true for measurement of organic contamination, where the standard method involves either a purge-and-trap technique for volatile organic chemicals, or an extraction method for compounds like polycyclic aromatic hydrocarbon or polychlorinated biphenyls. It is generally acknowledged that aerating soil to remove volatile organic chemicals does not remove a high percentage of volatile organic chemicals. If so, how accurate are the volatile organic chemical analyses employing purge-and-trap techniques in quantitating the total mass of contaminant? A re-examination of the entire sample handling and analysis for volatile organic chemicals is recommended. The analysis of soil samples and treated soil matrices by extraction is another area where simple extraction procedures can produce false negatives. For complex matrices involving soils and treatment chemicals, it is difficult to overcome the effect of the matrix in interfering with efficient transfer of the organic contaminant to the solvent for concentration and measurement. Rigorous extraction procedures are needed to reduce matrix effects. How Clean Is Clean? Three concerns relative to cleanup levels have been expressed in this review: The degree of contaminant reduction achievable with in situ treatment. The higher degree of contaminant reduction required with ex situ treatment technologies. The accuracy of current analytical methods for soils. Critical Review Concerns such as these, coupled with greater emphasis on risk assessment in remediation decisions, have spawned a research initiative on "environmentally acceptable endpoints." This initiative involves collaboration among the Gas Research Institute, the Petroleum Environmental Research Forum, the EPA, and the U.S. Air Force. This collaboration will investigate and develop environmentally acceptable endpoints in soil. The initial focus is on hydrocarbons and biodegradation. This initiative will establish the protocols and decision-making frameworks for extending the research to other chemical contaminants and other treatment technologies. Establishing risk-based cleanup levels based on sound scientific information is a key issue in achieving the country's environmental protection objectives. Proprietary Technology and Patents This review has deliberately omitted any reference to patented technology in the foregoing presentation. However, it must be noted that many of the technologies discussed above are the subject of patents awarded by the U.S. Patent Office. Consequently, selection of environmental remediation strategies should include an assessment of applicable patents and the validity of their claims. Soil Preparation and Handling When soil is excavated for ex situ treatment, preparation and handling of the soil for processing are frequently major factors in successful implementation of the remediation process. Activities such as control of emissions during excavation, water removal, size reduction, and homogenization are all in need of further development and demonstration for different soil matrices and contaminants. Long Term Effectiveness of Solidification/Stabilization Information on the long-term (over years) environmental and physical stability of wastes treated by solidification/ stabilization is sparse. Solidification/stabilization has been used to treat organically contaminated soils in a number of applications. A program of ongoing sampling is needed to characterize the stability of these applications, to provide guidance on the continued use of solidification/stabilization as a remediation strategy. SUMMATION In physical/chemical treatment technologies for organically contaminated soils, the only proven separation technology is volatilization of the volatile organic chemicals using soil vapor extraction and/or forcing air through the soil. For in situ applications, soil of good permeability, e.g., 1CH cm/sec or higher, is required. The range of applicability of these technologies is being extended to other contaminants by combining them with biodegradation technology. Volume 46 May 1i9i Journal of the Air & Waste Management Association 411

23 Many sites are being remediated by solidification/stabilization, in which the organic contaminants are not removed or treated, but the contaminated soil is rendered inert in the environment by mixing it with cementitious additives. The solidified mass is left on site. Thermal desorption is an effective ex situ technology for separation of the full spectrum of organic types from soils to below regulatory limits. All other technologies, both in situ or ex situ, are unproven and in various stages of development, some with the promise of cost-effectiveness, some struggling for survival. Those with promise have the potential to be attractive options for site remediation. ACKNOWLEDGMENTS The author gratefully acknowledges the tireless effort of my administrative assistant, Mrs. Diana Hague, for her assistance in preparing this manuscript for publication. Without her word processing skills, we would never have met the deadlines. I would also like to express my thanks to the reviewers of this manuscript for their insightful comments and suggestions. GLOSSARY OF TERMS A&WMA Air & Waste Management Association AAEE American Academy of Environmental Engineers, 130 Holiday Court, Suite 100, Annapolis MD AC Alternating Current AFB Air Force Base APEG Alkali Hydroxide Polyethylene Glycol BCD Base Catalyzed Dechlorination BTEX Benzene, Toluene, Ethylbenzene, Xylenes CFC Chlorofluorocarbons DC Direct Current DDT Dichloro Diphenyl Trichioroethane DNAPL Dense Nonaqueous Phase Liquid DOD Department of Defense DOE Department of Energy EMF Electromotive Force EPA U.S. Environmental Protection Agency MEK Methyl Ethyl Ketone MGP Manufactured Gas Plant NAPL Nonaqueous Phase Liquid PAH Polycyclic Aromatic Hydrocarbon PCB Polychlorinated Biphenyl PCDD Polychlorinated Dibenzo Dioxin PCDF Polychlorinated Dibenzo Furan PCE Perchloroethylene PCP Pentachlorophenol R&D Research and Development RCRA Resource Conservation and Recovery Act RD&D Research, Development, and Demonstration RF Radio Frequency s/s SITE SVE svoc TCA TCDD TCE TEA TNT TPH TRPH UV Vadose Zone VOC REFERENCES Solidification/Stabilization Superfund Innovative Technology Program Soil Vapor Extraction Semi-Volatile Organic Chemical 1,1,1, -trichioroethane Tetrachlorodibenzo-p-Dioxin Trichloroethylene Triethylamine Trinitrotoluene Total Petroleum Hydrocarbon Total Recoverable Petroleum Hydrocarbon Ultraviolet Subsurface area above groundwater Volatile Organic Chemical 1. 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"Remediation of Acid Tar Sludge at a Superfund Site," Proceedings of Superfund XVI, Washington, DC, Nov. 6-8, About the Authors Robert D. Fox, retired Vice President of Technology Development for International Technology Corporation in Knoxville, Tennessee, is currently a consultant to IT Corporation in developing and evaluating technologies for the environmental industry. He has 29 years of experience in waste treatment and pollution control, including pollution prevention, technology review, engineering management, research and development, and management of intellectual property. He has successfully managed the development of technologies such as thermal desorption, ultraviolet photolysis of dioxins, soil washing, photocatalytic oxidation of volatile organic compounds, and biodegradation. Volume 46 May 1996 Journal of the Air & Waste Management Association 413