Solvent extraction and soil washing treatment of contaminated soils from wood preserving sites: Bench-scale studies

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  • Solvent Extraction and Soil Washing Treatment of Contaminated Soils from Wood Preserving Sites: Bench-Scale Studies

    Endalkachew Sahle-Demessie Do.Jtg-las W Grosse Edward R. Bates

    Endalkachew Sahle- Demessie is a research engineer at the US. Environmental Protec- tion Agency (EPA), National Risk Manage- ment Research Labora- toty, Cincinnati, Ohio. He has been working on the application of emerging treatment technologies for remediating contami- nated soils. Doughs W Grosse has worked as an environmental engineer at the US. EPA in Cincin- nati, Ohio, f o r the past 21 years. Currently, he is working in Technology Transfer by serving as a specialist in site remediation and indus- trial wastewater treat- ment. Edward R. Bates has been with the US. EPA in Cincinnati, Ohio, as a physical scientist since 197% Since 1989 his principal duties have been to provide expert technical assistance on all aspects of Superfund site remediation, includ- ing characterization, remedy selection, remedy design, and field imple- mentation.

    Beizch-scalesol2iel.zt extmction and soil uashing studies wereperformed on soilsamples obtnzned from three abandoned woodpreservingsites zncluded in the Nationnl Priority List. %e soilsamples from thesesites were contumi- nated zuith high levels of po4yuromatic h-ydrocarboizs (PAHs), pentachlo- mphenol (PCP), dioxiizs, and heaty metals. The efectiveness of the solzient extraction process was assessed iwing liquefiedpropane or dimethyl ether as soheyits ozw a raiige of operating conditions. Tl2ese studies hatie demonstrated that a two-stage solvent extraction process using dimethyl ether as a solvent at a ratio of 1.6lperkg ofsoil could decrease dioxin leuels ipi thesoil by9-3 0t098.9percent9 andPCPlevels b y 9 5 lpercent. Reductioiz percentages. for benzo(a)py-ene (BaPl potenc-y estimnte and total detected PAHs wwe 82 4 mid 98.6percent, respectively. Metals coizcentratioiis were not reduced by the solvetit extraction treatment. i%ese remorial levels could be signiJicaiat(iI improved usiizg a multistage extraction system. Coin vier- cia1 scale solrwit extraction usiiig liquefied gases costs about $220per topi of contami?iatec/ soil. However, ,field application of this technology at the United Creosote site, Conroe, Te.xas, failed to perform to the leuel obsemJed at beiich scale due to the excessitie foaming and air emissiori problem

    Soil washijig using sqfactant solution and wet screening treatability studies were alsoperformed on thesoilsamples iiz order to assess remediation stra tegies jo I sites. A lth ot g h aqz 1 eo us phase solubility of co rz ta I n i 1. z ci nts seemed to be the most iinportaizt factor affecting removnl of contaminants @on% soil, sir ifiictant solutions (3 percent by weight) bavirzg iaonionic surfactmts with hydrophile-lipophile balance (HLB) of about 14 (Mukon- 12 artd Igepul CA 720) reduced the PAHlevels by an average of 71percent,

    Citation of product. company, or trade names do not constitute endorsement by the US. Environmental Protcction Agency and are provided only for the purpose of better describing inforniation in this article. Opinions expressed are those of the authors and should not be construed as representing positions or policy of the U.S. Environmental Protection Agency.

    0 2000 John Wiley & Sons, Inc. 85

  • ENDALKACHEW SAHLE-DEMESSIE DOUGLAS W. GROSSE EDWARD R BATES

    Many wood preserving facilities date back to the rapid development of railroads in the nineteenth century where creosote treated wooden ties were used.

    compared to no measurable change when pure deionized water was used. Large fractioza of clay and silt (

  • SOLVENT EXTRACTION AND SOIL WASHING TREATMENT OF CONTAMINATED SOILS FROM WOOD PRESERVING SITES

    Solvent extraction is a method of reducing the volume of the contaminated material that requires further treatment or disposal by concentrating the contaminants into a smaller volume of material.

    on the rate and effectiveness of extraction. Previous bench- and pilot-scale treatability studies have shown that solvent extraction may be an effective method for remediation of soils contaminated with organic wastes (Assink, 1985). However, a full-scale field application at the United Creosote site in Conroe, Texas, did not work as expected and was abandoned in favor of removing the wastes for off-site disposal.

    This paper presents the results of bench-scale solvent extraction studies performed on surface soils contaminated with PAHs, PCP, and dioxins obtained from the McCormick and Baxter Wood Preserving site in Stockton, California. The objectives of these studies were to select appropriate solvent or solvent mixtures; identify factors that affect the rate and efficiency of contaminant removal such as method of contacting, solvent/soil ratios, pretreatments, soil characteristics, and the number of contacting stages; and optimize the design and operation of the process to achieve remediation goals.

    The other remediation technology presented in this paper is soil washing. Although aqueous solubility of the contaminant seems to be the most important factor affecting removal of contaminants from soil, surfac- tants have expanded the application of soil washing for low solubility contaminants. Bench-scale soil washing studies were conducted on soils contaminated with PAHs, PCP, and dioxins obtained from the McCormick and Baxter Wood Preserving site, the RAB Valley site in Panama, Oklahoma, and the American Creosote Works site in Jackson, Tennessee.

    Solvent Extraction Overview Solvent extraction is a method of reducing the volume of the

    contaminated material that requires further treatment or disposal by concentrating the contaminants into a smaller volume of material. It is a method of removing contaminants from the solid phase by contacting the solids with a nonaqueous fluid that selectively dissolves and mobilizes the contaminants. The fluids used are usually organic solvents, liquefied gases, or supercritical fluids that have high affinity for the contaminants. Solvent extraction has been demonstrated to be an effective method for reducing contaminants below the cleanup goals for sediments and soils contaminated with PCBs, oil refinery wastes, and pesticides (Hall & Sandrin, 1990).

    A flow diagram for a typical solvent extraction system is shown in Exhibit 1. The process consists of soil extraction and solvent recovery,' recycle. Several solvent extraction systems have been proposed and tested in the US EPA's Superfund Innovative Technology Evaluation (SITE) program. These systems were investigated for degree of contaminant reduction achieved, treatment cost, and emission problems. Most of the extraction processes have been tested on polychlorinated biphenyls (PCB) contaminated soils and sludges, and have been shown to reduce concentrations of PCB from the contaminated sediment by more than 99 percent (Meckes et al., 1992). Each extraction system has a unique contacting method, solvent type, and requirement for pre- or post- treatments (Meckes et al., 1993; US EPA, 1993a). Selection of the best

    REMEDIATION/SUMMER 2000 87

  • ENDALKACHEW SAHLE-DEMESSIE DOUGLAS W. GROSSE EDWARD R BATES

    Exhibit 1. Flow Diagram for a Solvent Extraction System

    Soil Feed

    Make-up Solvent

    I Solvent I

    A Non-condensable Gases

    Solvent

    Solvent Regeneration and Recovery

    Solid-Liquid Soh Separation

    Removal

    v Clean soil

    Concentrated Contaminant to Destruction process or Hazardous Waste

    extraction solvent for a particular contaminant is not obvious, and extraction efficiencies vary for different types of soils, levels of contami- nant, and site-specific parameters. Properties of solvents that are impor- tant are dissolving power, cost, volatility, flammability, surface tension, and heat of vaporization. Additionally, the solvent used must not be a US EPA listed hazardous waste.

    The effectiveness of solvent extraction for remediating soils contami- nated by PCBs, pesticides, and other hydrocarbons has been demonstrated (Sahle-Demessie et al., 1996; US EPA, 1995b; 1992; 1993b). However, the application of solvent extraction technology for wood preserving sites and measuring its efficacy in treating soils with high levels of dioxins, PAHs, and PCP has not been systematically investigated. The objective of this treatability study was to answer some of these questions.

    EXPERIMENTAL DESIGN Soil samples for this study were obtained from the McCormick and

    Baxter (MCB) Wood Preserving site in Stockton, California. This site is a

    88 REMEDIATION/SUMMER 2000

  • SOLVENT EXTRACTION AND SOIL WASHING TREATMENT OF CONTAMINATED SOILS FROM WOOD PRESERVING SITES

    29-acre foriner wood treatment facility. Preservative solutions used at the site contained creosote oil, PCP, and water-based compounds containing chromated coppei- arsenate (CCA) and aniiiioniacal copper zinc ai-senate (ACZA) (US EPA, 1978). PCP and PAHs have been found in the ground- water at the depth of 175 feet below the ground surface. Soil samples were collected from the surface to about 3 feet deep from the retort area, screened with a 0.5 inch screen and then homogenized using drum rollers. The soil has 2.8 percent total organic carbon, and is composed of brown silt or clay with 3 percent gravel and 38 percent sand. Sixty percent of particles are finer than 0.081 iiini.

    Extractions were perfornied by CF Systems Inc. using liquefied dimethyl ether (DME') as a solvent at moderate pressures of 160 to 250 pounds per square inch (psi) and a temperature of48 "C. A typical pressure and temperature history of a two-stage extraction process is shown in Exhibit 2. A total of seven batch extractions tests were made with each batch of soil t