LEACHING OF TRACE ELEMENTS FROM SOILS STABLIZED WITH COAL FLY ASH

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1 LEACHING OF TRACE ELEMENTS FROM SOILS STABLIZED WITH COAL FLY ASH By Craig H. Benson 1, Sazzad Bin-Shafique 2, and Tuncer B. Edil 1 ABSTRACT: Batch water leaching tests (WLTs) and column leaching tests (CLTs) were conducted on coal-combustion fly ashes, soil, and soil-fly ash mixtures to characterize leaching of Cd, Cr, Se, and Ag. Concentrations in leachate from the WLTs on soil-fly ash mixtures are different from those on fly ash alone and cannot be estimated accurately based on linear dilution calculations using concentrations from WLTs on fly ash alone. The concentration varies nonlinearly with fly ash content due to the variation in with fly ash content. Initial concentrations from CLTs are higher than concentrations from WLTs. However, both WLT concentrations and initial concentrations from CLTs exhibit similar trends as a function of fly ash content, leachate, and soil properties. Scaling factors can be applied to WLT concentrations (50 for Ag and Cd, 10 for Cr and Se) to estimate initial concentrations for CLTs. Keywords: Leaching, trace elements, cadmium, chromium, selenium, silver, stabilized soil, fly ash, industrial byproduct, column test, batch test INTRODUCTION A variety of laboratory and field studies have shown that cementitious fly ashes are very effective in improving the mechanical properties of soft fine-grained soils encountered during highway construction (Ferguson 1993, Turner 1997, Edil et al. 2002, 2006). Far less attention has been placed on the potential environmental impacts of stabilizing soils with fly ash. Issues to be addressed include dust control, contamination of runoff, and ground water impacts. Of these, the effect on ground water is a primary concern because fly ash contains toxic trace elements that may leach into infiltrating rain water passing through a pavement system (Goh and Tay 1993, Erbe et al. 2003). This paper describes a study conducted to evaluate leaching of trace elements from soft fine-grained soils stabilized with fly ashes. Batch water leaching tests (WLTs) and column leaching tests (CLTs) were conducted on six soils, five fly ashes, and mixtures of the soils and fly ashes. 1 Professor, Department of Civil and Environmental Engineering, University of Wisconsin-Madison, Madison, Wisconsin, USA, benson@engr.wisc.edu, edil@engr.wisc.edu 2 Assistant Professor, Department of Civil and Environmental Engineering, The University of Texas-San Antonio, San Antonio, TX USA, sshafique@utsa.edu

2 MATERIALS Soils Six fine-grained soils and one coarse-grained soil (sand) were used in the study. Four of the fine-grained soils (, Plano silt loam,, and ) are from geographically diverse regions of Wisconsin. One fine-grained soil is from Montana (Clark sandy clay) and another is from Nebraska (Peorian loess). At their natural water content, all of the fine-grained soils are considered to be problematic soft soils in terms of highway construction. The four fine-grained soils from Wisconsin were used for most of the tests. Basic physical and chemical properties of the fine-grained soils are summarized in Table 1. These soils classify as low plasticity, high plasticity, or organic high plasticity clays (CL, CH, or OH) and have a 2- m clay fraction ranging between 17 and 65%. The natural gravimetric water contents are high (25-35%), which is why the soils are soft in their in-situ state. The paste of the soils ranges between 6.9 and 10.6 and the cation exchange capacity (CEC) ranges between 9.9 and 35.3 cmol + /kg. The highest CEC is associated with the highly plastic, which also has the largest clay content. Soil Liquid Limit Table 1. Properties and classifications of soils. Plasticity Specific USCS w Index Gravity Classification N CEC (cmol + /kg) Joy silt loam CL Plano silt loam CL Superior clay CH Theresa silt loam OH Clark sandy clay CH NA Peorian loess CL NA Note: w N = natural water content, CEC = Cation exchange capacity, NA = Not available. The sand was used as a control where adsorption on the soil solids would be negligible. The sand that was used is uniformly graded, has a median particle size of 0.24 mm, and a fines content (percent finer than 75 m) of 6.5%. The sand was washed in DI water to remove soluble salts and other contaminants prior to use in the testing program.

3 es Five fly ashes were used. Two of the fly ashes are from Wisconsin (Columbia and Dewey), one is from Minnesota (King), and two are from North Dakota (Coal Creek and Stanton). The fly ashes from Wisconsin and Minnesota were used for most of the testing program. Physical properties and chemical composition of the fly ashes are shown in Table 2 along with classifications per ASTM C 618. Three of the ashes (Columbia, Coal Creek, and Stanton) are Class C fly ashes. All others are considered off-specification ashes because they do not meet the criteria for Class C or Class F. Oxide contents were not available for the Coal Creek ash and CEC was not available for the Coal Creek and Stanton ashes. All of the fly ashes are effective for mechanical stabilization of soft fine-grained soils (Edil et al. 2006). Class. (ASTM 618) Table 2. Properties and composition of fly ashes. G s w LOI CaO Other Oxides Sulfur Trioxide Content CEC (cmol + /kg) Columbia C Dewey Off-spec King Off-spec Stanton C Coal Creek C NA Note: G s = specific gravity of solid, w = moisture content, LOI = loss on ignition, other oxides = SiO 2 + Al 2 O 3 +Fe 2 O 3, CEC = cation exchange capacity, NA = not available. METHODS Water Leach Tests Water leach tests (WLTs) were conducted on the fly ashes, the soils, and soil-fly ash mixtures in accordance with ASTM D 3987 using ASTM Type II deionized water. The soil-fly ash mixtures were prepared with fly ash contents of 10% and 20% to bracket the typical range of fly ash contents used for soil stabilization (Edil et al. 2006). A 2-kg sample of soil and fly ash was mixed homogeneously on a tray and tap water was sprayed onto the mixture to achieve a molding water content 2% dry of optimum water content. The mixture was blended until it appeared homogeneous. A portion was cured in a sealed plastic bag for 7 d before conducting the WLT. The remainder was used to prepare a specimen for column testing.

4 Column Tests Column leaching tests (CLTs) were conducted on the soil-fly ash mixtures to provide a more realistic assessment of leaching under flow-through conditions (Bin Shafique et al. 2006). Mixtures were compacted using standard Proctor effort (ASTM D 698) approximately one hour after mixing to simulate the delay between mixing and compaction that was observed in the field construction. The compaction water content was 2% dry of optimum. After compaction, the specimens were cured for 7 d at 100% humidity and constant temperature (21 o C). The CLTs were conducted immediately after curing. Specimens were placed in flexible-wall permeameters and permeated with 0.1 M LiBr solution using a hydraulic gradient between 7 and 10. Leachate (effluent) from the CLTs was collected in Teflon sampling bags. Additional details on the column tests can be found in Bin Shafique et al. (2006). Chemical Analysis Leachate from the WLTs and CLTs was analyzed for concentrations of Cd, Cr, Se, and Ag using furnace atomic absorption (AA) spectrometry following US EPA Methods 213.2, 218.2, 270.2, and All analyses were conducted using a Varian SpectrAA-800 Atomic Absorption (AA) Spectrometer controlled by a computerized Varian SpectrAA-880 Data Station. The SpectrAA- 800 was equipped with a graphite tube atomizer (GTA 100). RESULTS AND DISCUSSION Water Leach Tests Concentrations of Cd, Cr, Se, and Ag in the leachate from the WLTs are summarized in Tables 3 (soils and fly ashes) and 4 (soil-fly ash mixtures). Maximum concentrations cited in the Wisconsin Administrative Code for subgrade stabilization beneath highway pavements are also shown in Tables 3 and 4. All of the concentrations from the WLTs are lower than the maximum concentrations. Concentrations of Cd, Cr, Se, and Ag from the WLTs exhibit two types of behavior (Fig. 1). For Cd and Ag, the concentration is essentially independent of fly ash content, decreases slightly with fly ash content, or increases slightly with fly ash content for the mixtures prepared with fine-grained soil (Fig. 1a, Tables 3 and 4). In contrast, the concentration of Cr or Se increases appreciably as the fly ash content increases (Fig. 1b, Tables 3 and 4). Thus, for mixtures of finegrained soil and fly ash, WLTs on bulk fly ash provide a reasonable estimate of concentrations of Cd and Ag for WLTs on mixtures (perhaps fortuitously), whereas WLTs on bulk fly ash overestimate concentrations of Cr and Se from WLTs on mixtures.

5 Table 3. Concentration of Cd, Cr(T), Se, and Ag from WLTs on fly ashes and soils. or Soil Leachate Concentration ( g/l) Cd Cr (T) Se Ag (0.1) (2.0) (2.0) (0.2) Peorian loess Clark sandy clay Columbia Dewey King Coal Creek Stanton Wisconsin Criteria NA Notes: Detection limits shown in parentheses, T = total chromium, NA = not applicable. Table 4. Concentrations of Cd, Cr(T), Se, and Ag from WLTs on soil-fly ash mixtures. Columbia Dewey Soil Content Leachate Concentration ( g/l) Cd (0.1) Cr (T) (2.0) Se (2.0) Ag (0.2)

6 Table 4. Concentrations of Cd, Cr(T), Se, and Ag from WLTs on soil-fly ash mixtures (cont.) Soil Concentration ( g/l) Leachate Content Cd Cr (T) Se Ag (0.1) (2.0) (2.0) (0.2) King Coal Creek Peorian loess Clark sandy clay Stanton Peorian loess Clark sandy clay Wisconsin Criteria 100 NA The Cd and Ag concentrations are insensitive to fly ash content because these elements leach from the fine-grained soils and fly ashes alone at comparable concentrations (Table 3). The finegrained soil also tends to adsorb Cd and Ag. In contrast, when sand is used in the mixture, the Cd and Ag concentrations increase with fly ash content (Fig. 1a, Table 4). Cd and Ag are not present in the leachate from sand alone (Table 3) and the sand has essentially no sorptive capacity. In contrast, the concentrations of Cr and Se increase with fly ash content regardless of soil type because Cr and Se leach from fly ash at much higher concentrations than from finegrained soil or sand. The concentrations of Cr and Se increase non-linearly with fly ash content (Fig. 1b), even though the mass of both elements in the soil-fly ash mixture increases approximately linearly with increasing fly ash content. Thus, concentrations for soil-fly ash mixtures estimated based on linear dilution calculations using results from WLTs on fly ash alone are incorrect and may underestimate the concentration of the mixture. The non-linear behavior is believed to be due to the higher associated with higher fly ash content, which increases adsorption onto the solid surfaces and diminishes leaching. Thus, the potential of metal leaching from soil-fly ash mixtures cannot be estimated from the leaching potential of fly ash alone using a simple dilution calculation based on the relative masses of soil and fly ash in the mixtures.

7 Aqueous Phase Concentration ( g/l) Aqueous Phase Concentration ( g/l) (a) Cadmium - Columbia Content (b) Selenium - King Content Fig. 1. Concentrations from WLTs: (a) cadmium from soil-fly ash mixtures prepared with Columbia fly ash and (b) silver from soil-fly ash mixtures prepared with King fly ash. Column Tests Elution curves for Cr from CLTs conducted on mixtures of and King fly ash (10 and 20%) are shown in Fig. 2 along with fits of the analytical solution of the advectiondispersion-retardation equation (ADE) with instantaneous sorption. Similar elution curves were obtained for all of the tests, indicating that the initial concentration (C i ) from a CLT is a good indicator of the maximum leachate concentration from the mixtures for flow-through conditions. A summary of the average effluent s and the initial effluent concentrations obtained from the CLTs is in Tables 5 (soil alone) and 6 (soil-fly ash mixtures).

8 Effluent Concentration ( g/l) Chromium + King fly ash 10% 20% ADE with instantenous sorption 100?? Pore Volumes of Flow Fig. 2. Typical elution curves for total chromium from a CLT. Smooth lines are fits of analytical solution of the advection-dispersion equation (ADE) with instantaneous sorption. Table 5. and initial effluent concentration from column leaching tests on soils alone. Soil Content Average Effluent Initial Effluent Concentration ( g/l) Cd (0.1) Cr (T) (2.0) Se (2.0) Ag (0.2) Peorian loess Clark sandy clay Columbia Table 6. and initial effluent concentration from CLTs on soil-fly ash mixtures. Soil Content Avg. Effluent Initial Effluent Concentration ( g/l) Cd (0.1) Cr (T) (2.0) Se (2.0) Ag (0.2) NA NA NA NA

9 Table 6. and initial effluent concentration from CLTs on soil-fly ash mixtures (cont.). Avg. Initial Effluent Concentration ( g/l) Soil Content Effluent Cd Cr (T) Se Ag (0.1) (2.0) (2.0) (0.2) NA Dewey NA King Coal Creek Stanton Peorian loess Clark sandy clay Peorian loess Clark sandy clay NA NA NA NA The C i exhibit similar behavior as observed in the WLT concentrations; i.e., C i of Cd and Ag change modestly with fly ash content for mixtures prepared with fine-grained soil (Fig. 3a, Tables 5 and 6), whereas C i of Cr and Se increase appreciably for all mixtures as the fly ash content increases (Fig. 3b, Tables 5 and 6). The C i also varies non-linearly with fly ash content in response to the variation in, as shown by the examples in Fig. 3 for mixtures prepared with Dewey and King fly ashes. The largest changes in Ci are associated with the sand-fly ash mixtures because the sand has little adsorptive capacity.

10 Initial Effluent Concentration ( g/l) Initial Effluent Concentration ( g/l) 100 (a) Silver - Dewey Content 800 (b) Chromium - King ? Content Fig. 3. Initial effluent concentrations from CLTs for (a) silver from soil-fly ash mixtures prepared with Dewey fly ash and (b) total chromium from soil-fly ash mixtures prepared with King fly ash. Comparison of Initial Effluent Concentrations and WLT Concentrations Comparisons are shown in Fig. 4 between C i from the CLTs and concentrations from the WLTs (C w ) on soil-fly ash mixtures for Ag, Cd, Se, and Cr. Data are shown for the Wisconsin soils mixed with Wisconsin-Minnesota fly ashes along with the Nebraska and Montana soils mixed with fly ashes from North Dakota. The two data sets are similar in all cases, suggesting that the trends observed with the mixtures of Wisconsin soils and Wisconsin-Minnesota fly ashes apply generally to mixtures of fine-grained soil and fly ash. The graphs also show that C i for Ag and Cd is between 3-50 times higher than C w (Figs. 4a, b). Similarly, C i for Cr and Se are 1-10 times higher than C w (Figs. 4c, d). Thus, C i can be conservatively estimated as 50C w for Ag and Cd and 10C w for Cr and Se.

11 Initial Effluent Concentration - Column Tests ( g/l) Initial Effluent Concentration - Column Tests ( g/l) Initial Effluent Concentration - Column Tests ( g/l) Initial Effluent Concentration - Column Tests ( g/l) (a) Silver 50:1 10:1 1: (b) Cadmium 50:1 10:1 1: Wisconsin & Minnesota North Dakota Water Leach Test Concentration ( g/l) 1 Wisconsin & Minnesota North Dakota Water Leach Test Concentration ( g/l) 1000 (c) Selenium 1000 (d) Chromium (T) :1 10: :1 10:1 10 1:1 10 1:1 1 Wisconsin & Minnesota North Dakota Water Leach Test Concentration ( g/l) 1 Wisconsin & Minnesota North Dakota Water Leach Test Concentration ( g/l) Fig. 4. Comparison of initial effluent concentrations from CLTs and concentrations from WLTs for soil-fly ash mixtures: (a) Ag, (b) Cd, (c) Se, and (d) Cr. CONCLUSIONS Water leach tests and column leach tests were conducted on soils, fly ashes, and soil-fly ash mixtures to study leaching of Ag, Cd, Cr, and Se from soft fine-grained soils mechanically stabilized with fly ash for highway construction. Concentrations in leachate from the WLTs on soil-fly ash mixtures tend to be lower (1.5 to 2.5 times) than those from fly ash alone and vary non-linearly with fly ash content. Thus, concentrations for soil-fly ash mixtures estimated based on linear dilution calculations using results from WLTs on fly ash alone are incorrect and may underestimate the concentration of the mixture. The non-linearity in concentration is attributed to the non-linear relationship between and fly ash content, and the effect of on adsorption. The WLTs also showed that characteristics of both the fly ash and soil influence concentrations in the leachate.

12 Initial concentrations from the CLTs showed similar trends with fly ash content, leachate, and soil properties as the concentrations from the WLTs. In all cases, initial concentrations from the CLTs were higher than concentrations from WLTs on comparable soil-fly ash mixtures. Comparison of initial concentrations from the CLTs and concentrations from the WLTs on soilfly ash mixtures showed that scaling factors can be used to estimate initial concentrations for CLTs conservatively from WLT concentrations. A scaling factor of 50 can be used conservatively for Ag and Cd, whereas 10 is suitable for Cr and Se. ACKNOWLEDGEMENTS Financial support for this study was provided by the US Department of Energy s Combustion Byproducts Recycling Consortium and the University of Wisconsin-Madison Consortium for Fly Ash Use in Geotechnical Applications. The opinions and conclusions described in the paper are those solely of the authors and do not necessarily reflect the opinions or policies of the sponsors or others that assisted with the study. REFERENCES Bin-Shafique, S., Benson, C., Edil, T., and Hwang, K. (2006), Leachate concentrations from water leach and column leach tests on fly-ash stabilized soil, Environmental Engineering Science, 23(1), Edil, T., Acosta, H., and Benson, C. (2006), Stabilizing soft fine-grained soils with fly ash, J. Materials in Civil Engineering, 18(2), Edil, T., Benson, C., Bin-Shafique, S., Tanyu, B., Kim, W., and Senol, A. (2002). Field evaluation of construction alternatives for roadway over soft subgrade. Transportation Research Record, Erbe, M., Keating, R., and Hodges, W. (2003). Evaluation of water quality conditions associated with the use of coal combustion products for highway embankments, Case Study 09, Coal Combustion Products Consortium, US Environmental Protection Agency, Office of Solid Waste and Emergency Response, Washington, DC, 10. Ferguson, G. (1993). Use of self-cementing fly ash as a soil stabilizing agent. In Fly ash for soil improvement, GSP No. 36, ASCE, Reston, VA, 1. Goh, A. and Tay, J. (1993). Municipal solid waste incinerator fly ash for geotechnical applications. J. of Geotechnical Engineering. 119 (5), 811. Turner, J. (1997). Evaluation of western coal fly ashes for stabilization of low-volume roads. Testing Soil Mixed with Waste or Recycled Materials, STP 1275, American Society of Testing and Materials, West Conshohocken, PA, 157.

Stabilizing Soft Fine-Grained Soils with Fly Ash

Stabilizing Soft Fine-Grained Soils with Fly Ash Tuncer B. Edil, M.ASCE 1 ; Hector A. Acosta, M.ASCE 2 ; and Craig H. Benson, M.ASCE 3 Abstract: The objective of this study was to evaluate the effectiveness