DuPont soil washing technology program and treatment of arsenic contaminated soils

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  • DuPont Soil Washing Technology Program and Treatment of Arsenic Contaminated Soils 1. A. Legiec DuPont CR&D, Experimental Station, P.0, Box 80304, Wilmington, DE I9880

    1. P. Griffin, P. D. Walling, Jr., T. C. Breske DuPont Environmental Remediation Services (DERS), Wilmington, DE, and Bellevue, WA

    M. 5. Angelo DuPont Specialty Chemicals, Deepwater, NJ 08023


    R. S. lsaacson and M. 8. Lanza Weyerhaeuser Analytical & Testing Services, Analyhcal Chemistry Laboratories, Tacoma, WA 98477

    Thispaper summarizes a case study involving a site with inolga n ic a nen ic-conta m inated soils. Detailed soil characterization studies involving sequential extraction testing indicated that the arsenic was primarily associ- ated with the organic fraction and the iron oxide/manganese oxide fraction of the soil. A pwlimi- n a y treatability study demonstrated that adequate leaching of arsenic was possible. Alkaline leaching was spcf lc for arsenic. Future plans include a program of bench-scale treatability and field pilot studies focusing on soil washing technology application.


    Comprehensive and multifaceted technology research and development programs can guide the site remediation process with fundamentally good science, engineering and technology. Thorough characterization of complex soil ma- trices can provide information on chemical speciation, mo- bility, and distribution through the soil. This enables the project team to develop the most technically feasible and cost effective technologies which are protective of human health and the environment. Soil characterization studies can provide the fundamental information necessary to con- sider if technologies such as soil washing are feasible op- tions for a site. Soil washing is defined here as the class of ex-situ remediation treatment technologies that utilize wet classification, mechanical separation and chemical extrac- tion processes to clean soil. These ex-situ treatment tech- nologies can play a significant role in site remediation ef- forts.

    Soil washing is best applied when remediation goals are based on total concentration and when removal of contam- inants is desired or required. Also, it is more effective when the common tendency of contaminants to be distributed t o the finer soil fractions is observed. The overall objective in the application of soil washing technology is t o recover as much clean soil product as possible and to concentrate the soil contaminants into a small volume/mass of res- idue. To this end, physical treatment processes are often successfully and cost-effectively employed, although com- bination with more complex chemical extraction systems may be necessary depending on the established treatment goals. Recovered clean soil is ideally returned to the site of origin or at least beneficially reused/recycled in some other way. If economically viable, and/or required, the Small volume of concentrated residue can be further treated to reduce toxicity/mobility, recycled (if practical) or disposed of. In the case of disposal, the substantial volume reduc- tion and attendant saving in landfill space achieved through use of soil washing represents a significant benefit of the technology.

    This paper outlines a structured approach to the screen- ing, evaluation and development process for soil washing, an ex-situ soil remediation technology. A case study in- volves a site containing soils contaminated with inorganic arsenic. Selected soil characterization results for the site and their implications for technology selection are discussed, together with information obtained through a literature re- view of arsenic speciation and extraction. A subsequent treatability study performed to investigate the chemical ex- traction of arsenic from the site soils is also described, and

    Environmental Progress (Vol. 16, No. 1) Spring 1997 29

  • the associated results highlighted. Conclusions and recom- mendations from the initial studies are summarized and, fi- nally, plans for future bench-scale treatability and field pi- loting activities are presented.


    Soil Chamderhtion

    In environmental remediation, EPA analytical techniques and protocols (such as TCLP and SW-846 methods) are pri- marily employed to regulate the management, treatment and disposal of a specific waste. However, to help ensure good clean-up and treatment technology decision making, additional engineering and scientific information are needed when dealing with a complex matrix like soil. Chemical speciation and contaminant distribution through the soil, contaminant mobility/leachability and soil compo- sition (chemical, mineral, particle size distribution) are all key issues that should be addressed with a thorough soil characterization program. Soil characterization tests are based on soil science and mineral/mining technology (ex: sequential extraction from soil science, heavy media sink/float tests from mining) [ I ] . All the tests are commer- cially available and routinely used. Gathering this type of data early on in the remediation project results in im- proved understanding and provides consequently better insight into remediation technology selection. As an exam- ple, knowing that contaminants are distributed to fine soil fractions or are easily leached indicates that soil washing technologies are applicable. Ultimately, the need and scope of any soil characterization program should be addressed by the remediation site project team, in cooperation with regulatory oversight personnel.

    Soil characterization is cost effective, often paying for it- self by focusing subsequent technology screening and treatability studies. The many possible remediation tech- nologies are narrowed down to those that are most appli- cable. Fewer technologies are then carried through subse- quent treatability and piloting phases.

    Treatability Studies and Pilot Testing

    The screening and final selection of remediation tech- nology is impacted by many factors such as regulations,

    clean-up goals, final site end-use and other project strate- gies/goals. Scientific and engineering data generated through soil characterization can be very useful in guiding the technology screening and development process. Based on a solid foundation of information, carefully designed treatability and/or field pilot testing should be used to evaluate promising remediation technologies. Treatability studies should be used to examine the technology-specific design and operating conditions and develop preliminary cost/performance data. Computer modeling of soil charac- terization or treatability data can assist in either guiding studies or validating results Generating a database from actual treatability study results will also help focus future treatability work by providing a basis for technology screening and predicting technology performance.

    Soil washing treatability study and pilot programs are designed to select and optimize those types of soil wdsh- ing equipment or unit operations (ex: screening to enable particle size separation, mineral jigs to enable density sep- aration) needed for a site-specific remediation. Field pilot testing will verify the suitability of a technology and pro- vide insight into site-specific issues such 21s soil hetero- geneity, site preparation needs and accessibility, and avail- able utilities; all of which affect full-scale deployment. Ad- ditional design information for system scale-up or opti- mization will also be obtained from pilot studies.


    Soil Characterization Physical Characterization

    The site soil is aerobic and classified as a USDA sand or sandy loam. It contains an average 2% organic matter, contains 10-20% ambient moisture, and exhibits a slightly acidic pH of 6.2. Hulk soil and soil fractions from 3 differ- ent locations at the remediation site were characterized. Wet screening (particle size distribution) analysis and asso- ciated chemical analysis of bulk soil and soil fractions re- vealed that the soil on average is composed of 60% gravel (> 1/4 inch), 30% sands (1/4 inch to 75 mm) and 10%0 silt and clay fines (< 75 mm). Tables 1 and 2 present the soil characterization results. Approximately j0-70% of the total arsenic in soil is concentrated in the silt and clay fines frac- tion. The arsenic distribution by particle size (Table 1) indi-

    TABLE 1. Arenic Distribution on Composite Site Soil Sample (Total As of Bulk Soil 149 mR/kg)

    Weight Distribution Total As in %) of AS

    Mesh Size Particle Size (wt %) Fraction (mg/kg) in Bulk > 1 > lrr 18.0 18 2.2

    - + 1/2 0.05 - 1 23.3 24 3.7 - 1/4 + 8m 2.36 mm - 0.25 10.6 57 4.0 - 1/2 + 1/4 0.25 - 0.5 17.2 36 4.1 -8m + l6m 1.18mm - 2.36mm 5.0 80 2.7 - 16m + 30m 600pm - 1.18mm 6.5 116 5.1 - 30m + 50m 300pm - 600pm 7.7 175 9.1 - 50m + lOOm 150pm - 300p.m 2.8 508 9.6 - l0Om + 200m 75p.m - 150p.m 2.1 602 8.6

    - 200m x < 75p.m 6.7 1136 51.1

    30 Spring 1997 Environmental Progress (Vol. 16, No. 1)

  • TABLE 2. Soil Characterization Summary of Results-QSD, Regulatory, and Mobility Bulk Soil Procedures Sample A Sample B Sample C

    Particle Size Distribution (PSD)

    PSD in % gravel/sand/silt 66/26/8 60/30/10 52/39/9 Arsenic distribution within PSD 12/28/60 18/30/S2 7/22/71 in o/o gravel/sand/silt

    pH (at 10% solids slurry) 6.0 6.2 6.2


    TCLP As in m d L < 0.03 < 0.07 < 0.03 RCRA Hazardous, DO04 No No N o

    Soil Chemistry

    Total Arsenic in mg/kg 41 29 353 Acetic acid to reach pH < 4 0.5 1 0.5 (in equivalents/kg)

    Sodium hydroxide to reach pH > 10 0.5 1 0.5 (in equivalents/kg)

    Leacbing and Mobility 96 As Leached under Acetic Acid 3 4 2 % As Leached under NaOH 75 80 74 (L/S = 20, 1.5 eq/kg acid/base conc.)

    cated that the total arsenic content of the soil is concen- trated on the smaller particle size fractions. Therefore, uti- lization of particle separation techniques could be used to separate clean coarse soil fractions from the contaminated fine soil fractions, depending on final negotiated clean-up goals. Typical particle size separation equipment found in soil washing processes are wet/dry screening, hydrocy- cloning and column separators.

    Soil Characterization: Contaminant Mobility

    Several leaching and mobility tests were carried out on site soils (refer to Table 2 for results). Along with the Toxi- city Characteristic Leaching Procedure (TCLP), an equilib- rium leaching procedure with synthetic acid rain (SARI was carried out under conditions similar to the TCLP. SAR was substituted for acetic acid, and an extended leaching time was used (96 hours). Soil buffering capacity and arsenic leaching dependence on pH were also investigated. Ar- senic leaching with sodium hydroxide (2 eq/kg) resulted in 70 to 80% removal of arsenic. A sequential extraction procedure based on a modified Tessier methodology was used to infer arsenic-soil binding mechanisms for arsenic speciation analysis [ 21.

    The leaching and mobility tests indicated that the site soils do not exhibit the toxicity characteristic (DO04 waste code) for arsenic. The TCLP results for the site soils varied from 0.03-0.07 mg/L of arsenic in TCLP leachate, versus the 5.0 mg/L standard limit for As. Arsenic contained in site soils also exhibited very low solubility in aqueous and SAR leaching. Equilibrium leaching of the soils resulted in ar- senic concentrations less than 0.01 mg/L. Sequential ex- traction testing revealed that less than 1% of total arsenic in

    soil was water soluble. Leaching and mobility studies indi- cated that arsenic does not readily leach from site soils, and is not impacting surface or ground water (confirmed by site water analysis). The results determined that excavation and/or stockpiling soil for subsequent ex-situ remediation treatment should not impact mobility. Also, chemical addi- tives would be required to solubilize and recover arsenic from the soils.

    Soil Characterization: Contaminant Speciation

    Sequential extraction, scanning electron microscopy (SEM), x-ray fluorescence (XRF) analysis, and organic ar- senic analysis were carried out to further define the species of arsenic present in the site soils. These results indicated that the arsenic was inorganic and in the oxidized form of As (V). Refer to Table 3 for chemical speciation results. Ar- senic was also primarily associated with the organic matter and iron oxide/manganese oxide fractions of the soil. For example, Figure 1 depicts a SEM photo of an iron oxide particle containing approximately 3.8% (by weight) ar- senic. Sequential extractions using a modified Tessier method revealed that 50-8096 of the total arsenic in soil was leachable in both alkaline (sodium hydroxide) and alkaline phosphate (sodium pyro-phosphate adjusted to pH 10) aqueous leachants [ 2, 31.

    Soil Characterization Speciation and Leaching Literature

    Arsenic speciation and leaching is highly dependent on the pH and oxidation-reduction potential of the environ- ment. For example, studies reported in the literature indi-

    Environmental Progress (Vol. 16, No. 1) Spring 1997 31

  • ~~~~ ~ ~

    TABLE 3. Summary of Test Results for Speciation

    Soil Fraction Procedures Sequential Majority of arsenic associated Majority of arsenic associated Majority of arsenic associated Extraction with the organic fractions, with the organic and non- with the crystalline iron

    Sample A Sample R Sample C

    and to a lesser extent, with the non-crystalline iron oxide fractions. with the crystalline iron with the non-crystalline

    crystalline iron oxide fraction, and to a lesser extent,

    oxide fraction iron oxide fraction

    but associated mainly with iron oxide (including with iron oxide. Also suspected geothite); arsenic material. Also positive associated with silicates, and also detected in particles present as iron-arsenic- (containing Fe, Al, Cd, and arsenic-phosphate, lead-arsenic- phosphate. Si) attached to carbon charcoal sulFate, and potassium-

    oxide and organic fractions, and to a lesser extent,

    SEM Very little arsenic detected, Arsenic associated mainly Arsenic associated mainly with iron oxide and organic

    identification o f iron-

    particles and with lead-man- iron-sulfate. ganese-oxide.

    XRF Presence of silica-alumina- Presence of silica-alumina- Presence of silica-alumina- iron-matrix matrix iron-matrix

    FIGURE 1 Iron oxide particle containing about 3.8% ar- senic.

    cated that As(II1) would be mobilized in acidic reducing conditions and that As(V) would be most leachable in alka- line conditions [ 4, 51. The presence of phosphate in soils (from fertilizers) also was reported to increase arsenic mo- bility [ G]. Arsenic associated with organic matter in soil was also reported to leach under alkaline conditions, as humic matter is extracted off of soils with strong caustic [ 71. Treatment of aqueous extracts and groundwater contami- nated with As has been carried out through iron precipita- tion [ 81,

    The soil characterization study at the s...


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