Soil washing treatability tests for pesticide-contaminated soil

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  • Soil Washing Treatzibility Tests for Pesticide-Contaminated Soil

    Raymond M. Frederick S. Krishnamurthy

    R a y d M. Frederick is a pbysical scientist at EPA's Risk Reduction Engineering Laboratory, Releases Control Brancb in Edison, New Jersey. S. Krisbnamurtby is a senior cbemistJhm tbe Anrerlcan Association of Retired Persons wbo i s working at tbe Releases Control Brancb under tbe Senior Environmental Employment Program Tbey are members of a researcb group investigating new and innovative extraction tecbnologies for tbe remediation of contaminated soiL

    llje autbors wisb to acknowledge tbe contributions of tbe late Patrick Augustin, wbo was a cbemical engineer witb tbe Soil & Material Engineering Section and was responsible for tbe pilot-scale activities conducted witb tbe VRU Mr. Augustin served as EPA'S on-site project "anager for the VRU during tbe Sand Creek treatabiuty tests.

    or commercial products does not constitute endorsement or recommendation for use by tbe U.S. Environmental Protection Agemy.

    Mention of trade names

    m e 1987 Sand Creek Operable Unit 5 record of decision (ROD) identified soil washing as the selected technology to remediate soils con- taminated with high levels of organochlorine pesticides, herbicides, and metals. Initial treatability tests conducted to mess the applicability of soil washing technology did not effectively evaluate the removal of the elevated contaminant concentrations that were found. To further evaluate the applicability of soil washing at this industrialsite, a second more compre- hensive pilot-scale treatability test was conducted.

    Twenty-three test runs were conducted over a two-week period in late September 1992, using a pilot-scale soil washing device called the volume reduction unit ( W U ) . The experimental design evaluated the effects of two wash temperatures, two p H levels, three su$actants, four su factant con- centrations, and two liquid-to-soil ratios on the contaminant removal efficiency of the soil washing process. Site soils from layers at three different depths were used in the study.

    Results from thepilot-scale treatability test indicated that the WUcould achieve contaminant reduction giciencies of 97percent for heptachlor and greater than 91 percent for dieldrin in the uppermost contaminated soils (surface to l-ft. depth). Residual concentrations of heptachlor and dieldrin in the treated soil ranged from 50ppm to less than 1 .6ppm, and 6.8ppm to less than 1 .Gppm, respectively. However, the analytical method detection limit of 1.6 ppm was not low enough to provide residual concentration data at the risk-based action levels of 0.55ppm for hep- tachlor and 0.15ppm for dieldrin.

    The Sand Creek Superfund Site is the former site of the Colorado Organic Chemical Company, which manufactured pesticides during the 1960s. The 300-acre site is located in an industrial park five miles northeast of downtown Denver and was listed on the National Priorities List (NPL) in 1982. The Sand Creek site is slated for cleanup by the end of 1994.

    According to the 1990 record of decision (ROD), soil washing was the selected technology for the site. However, bench-scale studies did not effectively evaluate the removal of elevated contaminant concentrations of pesticides found at the site. Also, the remedial design (RD) for this site is complicated by three factors:

    1. The action levels identified in the ROD are very low. 2. The volume of contaminated soil reported in the ROD is based on

    CCC 1051 -565W94IO40443-12 Q 1094 John Wiley & Sons, Inc 443


    sUCCe88fil application of soil washing technology to aparticular site depenrie on the phy8ical characteristic8 of the mil and the chemical nature of the contaminant.

    only 17 samples. 3. Cost figures provided in the ROD were not accurate because of

    uncertainty about the actual volume of soil to be remediated and the limited knowledge of soil washing costs at the time the ROD was prepared.

    To obtain additional information on the potential effectiveness of soil washing and to better predict the cost of remediation, EPA Region VIII personnel conducted a more comprehensive pilot-scale treatability test at the Sand Creek Superfund site. EPAs Office of Research and Development provided assistance to Region VIII by making available its pilot-scale soil washing unit from the Risk Reduction Engineering Laboratorys Edison, New Jersey, facility.

    TECHNOLOGY DESCRIPTION Soil washing is an aqueous process for removing contaminants by

    either solubilizing and suspending them in the wash solution or by concentrating them into a smaller volume of soil through particle size classification. Soil washers generally employ a combination of these techniques and auxiliary systems to perform wastewater treatment. The concept of soil washing is based on the finding that most organic and some inorganic contaminants tend to bind or adsorb, either physically or chemically, to the clay, silt, and humic components (fines) of the soil matrix. Washing processes scrub and separate the fines from the larger sand and gravel-size particles, effectively concentrating the contaminants into a smaller volume of soil. The fines are suspended in the wash slurry along with dissolved or solubilized contaminants. The highly contami- nated soil fines are recovered by gravity separation and flocculation units and are usually processed further by other technologies such as incinera- tion or solidification. The cleaned soil fraction (sand and gravel-size particles) are returned to the site as backfill material. In some cases, topsoil may be added to the backfill to promote revegetation of the site.

    Successful application of soil washing technology to a particular site depends on the physical characteristics of the soil and the chemical nature of the contaminant. Soil washing is most economical on sandy loam soil containing less than about 30 percent fines-i.e., particles less than 200 mesh (75 microns). The technology is applicable to a wide range of contaminant classes inchding volatile and semivolatile organics and metals. Complex mixtures of organics and metals usually require sequen- tial washing steps with surfactants and chelating agents or acid leaching.

    The Sand Creek soil characteristics provided a challenge to soil washing technology. Particle size distribution analysis determined the soil to be a clayey silt matrix with 33 percent less than 200 mesh (see Exhibit 1). Contaminants in the soils contained a complex mixture of organochlo- rine pesticides, herbicides, and metals.

    VOLUME REDUCTION UNIT PROCESS DESCRIPTION The volume reduction unit (VRU) is a mobile pilot-scale system for




    Exhibit 1. Particle Size Distribution by Soil Depth.

    Weight 010 Particle Size

    Mesh Size (-1 0 - 1' 1 - 3' 3 - 5' 0 - 3/8" > 9.5 0.00 0.00 0.00 3/8 - 4" 9.5 - 4.75 0.74 0.66 0.55 4 - 10" 4.75 - 1.70 1.94 1.88 1.74 10 - 40" 1.70 - 0.425 15.96 17.39 16.19 40 - 200" 0.425 - 0.075 46.58 48.87 46.39 200 - PAN < 0.075 34.78 31.20 35.13

    conducting soil washing research and treatability studies. The system uses commercial washing equipment and support utilities mounted on two trailers. Nominal design capacity of the system is 100 pounds of dry soil feed per hour. The VRU is small enough to economically conduct a wide range of experiments, yet large enough to obtain cost and performance data of sufficient quality for scale-up. The VRU has a flexible design that allows the addition of new extraction units with minimal modifications and permits individual process units to be isolated or bypassed if necessary. The main or "process" trailer contains equipment for soil feeding, volatiles recovery, soil washing and screening, gravity separation, flocculation, and water clarification. An oil-fired boiler on the main process trailer supplies steam for the thermal desorption or steam stripping of VOCs. The utility trailer contains water storage tanks, pumps, a water heater, water filters, and carbon adsorption drums for recycling the process water. An air compressor, small electric generator, and equipment storage compart- ments are also on this trailer (see Exhibit 2). (The VRU is described in greater detail by Masters et al. [19911.)

    "REATABILMY STUDY VARIABLES The contamination levels at the site are highest in the surface soils.

    During remedial action (RA), the surface soils will be excavated along with some of the deeper soils to depths of 5 feet in some areas. Therefore, surface soils mixed with deeper soils will be somewhat diluted and will have lower contaminant concentrations than the soils that were ultimately used in this treatability study. The surface to 5-ft. depth soils are more representative of concentrations to be encountered during the actual remediation effort. However, only two soil samples from this depth range were used during the treatability test. Most of the soils selected for this study are representative of the uppermost contaminated areas (surface to 1-ft. depth) and were selected to provide information on a worst-case scenario.

    The treatability test was designed to evaluate the effects of the




    Exhibit 2. Typical VRU Field Setup.

    following set of process variables in remediating the Sand Creek soils:

    1. Surfactant type: (a) Witcolate WAC-LA (this is a sodium dodecylsulfate compound

    (b) A mixture of 50/50 Adsee 799 and Witconol NP-100. (c) Tergitol NP-10. SDS is an anionic surfactant. Adsee 799, Witconol NP-100, and Tergitol NP-10 are nonionic.


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