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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

and

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 associated 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.

INTRODUCTION

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, mobility, 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 extraction processes to clean soil. These ex-situ treatment technologies 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 contaminants 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 “residue”. To this end, physical treatment processes are often successfully and cost-effectively employed, although combination 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 reduction 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 screening, 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 review of arsenic speciation and extraction. A subsequent treatability study performed to investigate the chemical extraction 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 piloting activities are presented.

APPROACH TO SOIL WASHING

Soil Chamderhtion

In environmental remediation, EPA analytical techniques and protocols (such as TCLP and SW-846 methods) are primarily 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 example, 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 technologies are narrowed down to those that are most applicable. Fewer technologies are then carried through subsequent treatability and piloting phases.

Treatability Studies and Pilot Testing

The screening and final selection of remediation technology 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 characterization 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 separation) needed for a site-specific remediation. Field pilot testing will verify the suitability of a technology and provide insight into site-specific issues such 21s soil hetero- geneity, site preparation needs and accessibility, and available utilities; all of which affect full-scale deployment. Ad- ditional design information for system scale-up or optimization will also be obtained from pilot studies.

SOIL WASHING: CASE STUDY OF ARSENIC CONTAMINATED SOIL

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 different locations at the remediation site were characterized. Wet screening (particle size distribution) analysis and associated chemical analysis of bulk soil and soil fractions revealed 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 fraction. 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

Regulatory

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 concentrated on the smaller particle size fractions. Therefore, utilization 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 equilibrium 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 arsenic concentrations less than 0.01 mg/L. Sequential extraction testing revealed that less than 1% of total arsenic in

soil was water soluble. Leaching and mobility studies indicated 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 arsenic 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) arsenic. 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 environment. For example, studies reported in the literature indi-


~~~~ ~ ~

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% arsenic.

cated that As(II1) would be mobilized in acidic reducing conditions and that As(V) would be most leachable in alkaline conditions [ 4, 51. The presence of phosphate in soils (from fertilizers) also was reported to increase arsenic mobility [ 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 contaminated with As has been carried out through iron precipita- tion [ 81,

The soil characterization study at the subject site had determined that arsenic was solubilized under alkaline conditions (Table 2). Speciation analyses (Table 3) also revealed that the arsenic was associated with organic and iron oxide fractions of the soil, and indicated that arsenic was present in the oxidized form as As(V). Therefore, based on the soil characterization results as well as literature findings, alkaline extraction of arsenic and/or enhancement of As leaching with phosphate addition was targeted for the subsequent leaching treatability study.

Arsenic Leaching Treatability Study

In order to minimize the volume of soil to be disposed of, arsenic leaching in combination with physical soil washing was chosen for treatability study evaluation pre- ceding field pilot studies. Arsenic leaching was targeted for the sands fraction of the soils, since this fraction is about 30 weight percent of the bulk soil, and is moderately contaminated with arsenic. The fines fraction represents less than 9 weight ?h of the bulk soil and contains more concentrated, tightly bound arsenic; for these reasons the fines would be disposed of instead of targeted for leaching. The sand fraction (75 mm to 0.25”) of a sample was utilized for the treatability study. Two aliquots were obtained from the sample and submitted for total As content; the total As concentrations were determined to be 103 and 106 mg/kg As. The As leaching treatability study was split into three phases. The first phase investigated the potential to extract arsenic using aqueous solutions of either NaOH or Na,CO,. Enhancement of As leaching due to phosphate addition (as monoammonium phosphate, o r NH,H,PO,) was also investigated. The second phase of the study investigated the preferred alkaline extraction conditions (or combinations there00 as determined from the previous phase, and were carried out at three different solid to liquid ratios. The results indicated that extraction efficiency was not constrained by solubility limits. The third phase of the feasibil- ity study simulated full scale field processes for alkaline extraction at the laboratory scale.

The first phase of the treatability study investigated alkaline extraction of arsenic under oxidizing conditions using aqueous solutions of either NaOH or Na,CO, as well as phosphate enhanced leaching. All extractions were carried out to equilibrium (assumed 24 hours), at room temperature, and at a liquid to solid ratio of 20. Acidic extraction under reducing conditions was also tested to observe any arsenic mobility from As(II1) species. In total, three concentration levels of each alkaline solution and two levels of acid were investigated. Target phosphate solutions were set at 1 X , 2 X , and 3 X the molar concentration of arsenic (as arsenate) in the soil. The phosphate leaching extract pH


I Phase 1 -%Arsenic Recovered by Leaching I

P

I 2 4 6 0 10 12 Equlllbrlum pn

FIGURE 2

values were all near neutral to mildly acidic (refer to Figure 2). Approximately 5 to 10% of the total arsenic leached from the soil with phosphate addition alone. The acidic extractions removed less than 5% of the total arsenic. The aqueous caustic solutions showed the most promise for removal of arsenic from the soil. Arsenic leaching was found to be highly dependent on pH; higher amounts of leachable arsenic were obtained with increasing alkalinity.

The second phase of the treatability study investigated aqueous alkaline extraction at the highest concentration level using either NaOH (0.015N) or Na,CO, (35,000 mg/L), since these leaching conditions mobilized the most arsenic. Both of the alkaline solutions were also tested in combination with phosphate (at the 3 X molar concentration level). All extractions were carried out at three different liquid to solid ratios. The data were normalized to percent removal of As (wt %) and are represented in Figure 3.

Arsenic recovery from the soil ranged from 5% up to 25% when phosphate was added (at 3 times the molar concentration of arsenate on soil) in the presence of 0.015M NaOH. The presence of phosphate buffered out the solution pH to near neutral pH. However, utilization of 0.33M Na,CO, with phosphate (instead of NaOH) increased the arsenic recovery of the soil to 20-25%. Future work in this area will evaluate alternative reagents to introduce phosphate to the system.

Up to 57% removal of the arsenic from the bulk sample was achieved with a single stage batch extraction at a pH of 11.5 using only 0.02N NaOH. In general, sodium carbonate solutions were found to perform as well as the sodium hydroxide solutions at similar pH. However, much higher reagent doses of carbonate (0.33M Na,C03 versus 0.02M NaOH) were necessary to achieve a pH of 11 or greater.

Information from the first and second phase of the treatability study was compared to evaluate consistency. The first phase of the treatability study revealed that ar-

Phase I-% arsenic recovered by leaching.

FIGURE 3 Phase 11-% arsenic recovery.

senic leaching (liquid to solid ratio 20 L per kg) was highly dependent on pH. The data from the first phase was utilized to determine the arsenic leaching dependency on pH as a non-linear, empirical, function. The calculated arsenic removal, based on the linear regression analysis from the first phase data, was compared to the results from the second phase (Figure 3). The percent arsenic removal for the NaOH and the Na,C03 extractions show a similar pH dependency.

The liquid to solid ratio, or L/S, for each of the Phase I1 data points is also detailed in Figure 3. Carrying out the extractions at a lowered liquid to solid ratio (less than 20 L per kg of soil) did not affect the arsenic recovery as the percent removal for a given pH is equal to or higher than attained under Phase I. Arsenic leaching does not appear to be constrained by solubility limitations for L/S from 5 through 20. Further optimization of the liquid to solid ratio must be carried out prior to field implementation.

In the third phase of the treatability study, full scale field processes for alkaline extraction were simulated in the laboratory. The first extraction process simulated was a single stage batch extraction. This extraction was carried out for 5 hours at a liquid to solid ratio of 20 and utilized 0.02N NaOH (aqueous) as the extractant solution. The soil for the single stage batch extraction was split into three size fractions and the extraction efficiency per particle size determined. The results are presented in Table 4. Approxi- mately 59 mg/kg dry weight basis (dw) of As was removed from the bulk soil sample at an extract pH 11.7. The percent removal of arsenic from the bulk soil was determined to be 52%. The extraction efficiency (as % arsenic removal) was higher for the larger particle sands fraction versus the finer sands due to differences in soil physical and chemical characteristics of each fraction. This indicated that

TABLE 4. Extraction Efficiency by Particle Size Fraction (Total As of Original Bulk Soil Sample-4 14 mg/kg dw)

Initial Post-Extraction Weight % Total As Total As As%

1.18mm < x < 0.25” 52.5 64 26 59.4 Soil Fraction of Bulk (mg/ kg dw) (mg/kg dw) Removal

300mm < x 34.7 < 1.18mm

75 mm < x < 300mm 12.8

118

197

73

130

38.1

34.0


..... . . .~ . . . . .... ..... ... . .. . .

FIGURE 4 Phase 111-Laboratory extractions: simulation of multistage countercurrent extraction.

leaching of c o m e sands would be preferred since higher arsenic extraction efficiencies would be expected.

A multiple stage countercurrent extraction was simulated in the laboratory through several sequential extraction? (L/S = 20, room temperature, 5 hours). Refer to Figure 4 for experimental design and data. The upper arm of the diagram depicts the same volume of extract solution sequentially being contacted with soil. This was carried out in order to simulate one end of a countercurrent extraction column. The lower arm of the diagram depicts the same soil sample being sequentially extracted with fresh 0.02N NaOH in order to simulate the opposite end of a countercurrent extraction column.

The countercurrent extraction simulation indicated that the soil buffered the NaOH and reduced the extractant pH, thereby reducing the amount of As leached. However, sequential extraction of the soil with 0.02N NaOH (the lower arm of the diagram) achieved approximately 60% arsenic removal upon four sequential batch extractions.

Treatability Studies: Conclusions

The treatability studies demonstrated that adequate leaching of arsenic was possible utilizing alkaline solutions. These results concurred with the literature findings that AdV) is leachable under oxidizing alkaline conditions. The soil organic matter needs to be considered in full scale operations since extracted humic matter may form col- loidal suspensions in solution, causing difficulty in a filtra- tion process.

PROJECT PATHFORWARD

Building on the information developed under the com- pleted soil characterization study and arsenic treatability studies, a focused pilot tredtdbility program was developed and carried out. The first phase of this program involved a more detailed bench-scale study to investigate the viability and effectiveness of specific treatment processes (including wet deck screening, attrition scrubbing, froth flotation and alkaline leaching) provisionally proposed for on-site pilot scale operations. The bench study addressed issues such as leaching kinetics and the treatment/recovery of arsenic-laden leachants. Data generated at the bench scale was used to select specific unit operation and develop ap- propriate design and operating parameters for piloting. The

field pilot study will be used to gain further insight into site specific treatability issues affecting full-scale deployment, verify the suitability of soil washing technoloby, and gener- ate full cost, scale-up and process optimization data.

ACKNOWLEDGMENTS

The authors would like to thank E. I. DuPont de Nemours & Co., DuPont Environmental Remediation Services, and Weyerhaeuser Corporation for supporting this work and enabling this publication. SEM analytical work was carried out by Hazen Kesearch, Inc., Golden, Colorado. Special acknowledgments are made to J. A. Debidso, Jr. (DERS) for sample preparation and management/project support. M. F. McDevitt and J. M. Whang (DuPont CKKrD) provided additional peer review of the document. The DuPont Cor- porate Remediation Technology Team and G. E. Quinton provided technical assistance. This work was initially presented at the I&EC Special Symposium. American Cheini- cal Society, in Atlanta, GA, September 19-21, 1994.

LITERATURE CITED

1. Igwe, G. J., P. D. Walling Jr., and D. M. Johnson, “Physical and Chemical Characterization of I.cad-Con- taminated Soils,” Presented at the 1994 Wufer und Environment Federation ConjLJrence, Miami, FL (1994).

2. Tessier, A, P. G. C. Campbell, and M. Bisson, “Sequential Extraction Procedure for the Speciation o f Particulate Tracc Metals,” Analyficul Chemistq: Vol. 5 1 , No. 7, p p . 844-853 (June, 1979).

3. Jordao, C. P., and G. Nickless, “Chemical Associa- tions of Zn, Cd, Pb, and Cu in Soils and Sediments De- termined by the Sequential Extraction Technique,” Eiz- uironmentuf Technology Letters, Vol. 1 0 . pp. 743-752

4. Masscheleyn, P. H., R. D. Deluane, and W. H. Patrick, Jr., “Heavy Metals in the Environment: Ar- senic and Selenium Chemistry as Affected by Sediment Kedox Potential and pH,” J. 13’nziiron. Qirul., Vol. 20, pp. 522-527 (July-September, 1991).

5. XU, H., B. Allard, and A. Grimvall, .‘Effects o f Acidi- fication and Natural Organic Materials on the Mobility of Arsenic in the Environment,” Wuter, Air, Kr Soil Pol- lutiorz, Vol. 57-58, pp. 269-278 (1991).

6. Davenport, J. R., and F. J. Peryea, ”Phosphate Pcr- tihzers Influence Leaching of Lead and Arsenic in a Soil Contaminated with Lead Arsenate,” Wufw, Air, & .Soil Pollution, Vol. 57-58, pp. 101-110 (1991).

7 . Schnitzer, M., “Chapter 30-Organic Matter Charac- terization,” Methods of Soil Analysis, Part 2. Chemical and Microbiolog icul Properties, AS A-SSSA, Madison. WI, pp. 581-594 (1982).

8. O h , C. A, and B. Wilson, “Removal of Arsenic and Hexavalent Chromium from Leachates o f an Industrial Landfill,” Presented at the 1992 ACS ICEC Special Symposium, Atlanta, GA, p . 200 (September, 1’3%).

(1 989).

I . A. Legiec is present& at UuPont, .I24 13triWing Chambers Works. Deepwater, IYJ 0802.3. Corre.vponderrce concerning this paper should hL. addressed to I . A . Legiec.


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