Environ. Sci. Technol. 2000, 34, 4163-4168
Immobilization of Chromate from Coal Fly Ash Leachate Using an Attenuating Barrier Containing Zero-valent Iron T H O M A S A S T R U P , * ,† S . L . S . S T I P P , ‡ A N D THOMAS H. CHRISTENSEN† Department of Environmental Science and Engineering, Building 115, Technical University of Denmark, DK-2800 Lyngby, Denmark, and Geological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark
The purpose of this investigation was (i) to test the effectiveness of a barrier engineered to remove Cr(VI) from leachates of higher pH and salinity typical of coal burning ashes and (ii) to determine which geochemical processes control Cr immobilization. Laboratory column and batch desorption experiments show that a barrier composed of sand, Fe(0), and bentonite irreversibly immobilizes Cr. Concentrations fall from 25 mg Cr L-1 in the leachate to below detection limits (0.0025 mg Cr L-1) and solution pH increases by about two units. Solid-phase analytical techniques such as SEM, EDS, XPS, and TOFSIMS were used to characterize the barrier material prior to and after exposure to the Cr leachate. In the barrier material, Cr(III) was found associated with Fe(III)-oxides, as separate Cr oxides and as a Ca,Cr phase, probably Cachromite, CaCr2O4. The attenuating barrier can be an alternative to traditional liners and leachate collection systems at coal ash storage and disposal sites.
Introduction Energy production from coal produces ash that may generate Cr leachate to contaminate ground and surface waters. Temporary storage or permanent disposal of coal combustion residues over a permeable but chemically reactive barrier containing Fe(0) may offer a preferable strategy to traditional bottom sealing with an impermeable liner and long term collection and treatment of leachate. Cr(VI) is the most critical component in leachate from coal fly ash, and immobilization of this component in a reactive barrier makes the leachate much more environmentally compatible. The effectiveness of such a reactive barrier relies on (i) chemical conditions favoring immobilization processes, (ii) large surface area to serve as a source for reactants and as sites for sorption and precipitation of reaction products, and (iii) relatively low hydraulic conductivity to increase residence time thus increasing reaction potential. However, the hydraulic conductivity should be high enough for allowing all leachate to migrate through the barrier. The objectives of this study were to investigate the feasibility of using a permeable reactive barrier for removing * Corresponding author phone: (+45) 4525 1558; fax: (+45) 4593 2850; e-mail:
[email protected]. † Technical University of Denmark. ‡ University of Copenhagen. 10.1021/es0009424 CCC: $19.00 Published on Web 09/02/2000
2000 American Chemical Society
Cr(VI) from high pH and saline leachates that typically arise from storage or disposal of coal-burning ash and to determine which geochemical processes control Cr behavior. The attenuating barrier studied contains 5% Fe(0), which previously has proven effective in immobilizing Cr(VI) from groundwater (1, 2). Reactions are favored at low pH and low ionic strength (1, 3, 4), but we could find no reports of investigations at higher pH and salinity, typical for coal ash leachate.
Materials and Methods Column Studies. All glass and plastic equipment was soaked in 2 N HNO3 for at least 24 h and then rinsed several times in deionized water before use. Reagent grade chemicals (Merck) and deionized water were used for all solutions. Artificial leachate was made to match the composition of typical initial leachate from coal fly ash (5). It had pH of 9 ( 0.5 and was made with salts of sulfate and chloride. It contained 3000 mg‚L-1 SO42- (31 mM), 130 mg‚L-1 Cl(3.7 mM), 730 mg‚L-1 Na+ (32 mM), 900 mg‚L-1 K+ (23 mM), and 250 mg‚L-1 Ca2+ (6.3 mM). Ionic strength was 105 mM. Cr(VI) was added as K2CrO4 to produce a solution of 25 mg‚L-1 Cr(VI) (0.48 mM). The barrier material was prepared by mixing Fe(0), sand, and bentonite. The Fe(0) used in this study was commercially available iron powder (Merck) with a particle size of less than 150 µm and BET surface area of 0.1 m2‚g-1. The sand was obtained from an aquifer in Varde, Denmark. Petrographic analysis showed that it contained about 94% quartz, with about 5% feldspar, 1% horneblende, and other minerals. There was no visible iron oxide. Sand grain size ranged from about 100 to 300 µm, BET surface area was 1.7 m2‚g-1, and water content was about 20% by mass at the time of mixing. The clay additive was commercially available Na-bentonite, tawny in color, light, and powdery, with particle size of 3 µm or less and BET surface area of 49 m2‚g-1. Scanning electron microscopy (SEM) revealed that clay particles were in fact aggregates of many individual grains, which resulted in surface area measurements that were much lower than expected for this fine-grained powder. The barrier materials were mixed without any pretreatment or drying; iron filings, bentonite, and sand had mass ratio of 1:1:18. Reference material, without Fe(0), was mixed with mass ratio of bentonite to sand of 1:19. The materials were prepared the same day as the column experiments were initiated. The sand served as a permeable filling material, Fe(0) filings provided redox-capacity over a relatively high surface area, and bentonite, a swelling clay, served to reduce hydraulic conductivity as well as to increase surface area available for sorption and precipitation of Fe(III) and Cr(III) solids. Three columns were filled with barrier material and one with reference material; composition details appear in Table 1. Column diameter was 8 cm in all cases and length varied from 6 to 19 cm. The columns were loaded with artificial fly ash leachate flowing against gravity to minimize channelling. Experiments were conducted at room temperature over a period of about 5 months. The presence of bentonite in the barrier reduced hydraulic conductivity to about 10-7 m‚s-1. Flowrate was set at 10 m‚yr-1 in all columns. For about two months near the beginning of the experiment, the flowrate in columns I, II, and III was increased to 25 m‚yr-1 to enable breakthrough within the time allowed for the experiment. Effluent samples were collected to establish breakthrough curves. Column III was established with sample ports at intervals along the vertical axis, from which samples of pore water could be taken. After the column experiments were completed, the barrier and reference materials were removed VOL. 34, NO. 19, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Experimental Setup columna
materials
dry weight (g)
bulk density (g‚mL-1)
pore volume (mL)
flow velocity (mL‚d-1)
total no. of pore volumes
I II III IV
Fe(0), sand, bentonite Fe(0), sand, bentonite Fe(0), sand, bentonite sand, bentonite
530 860 1300 430
1.9 1.8 1.8 1.8
150 250 410 160
130-340 130-340 130-340 340
132 115 67 105
a
ID ) 8 cm, length: 6-19 cm.
from the columns under N2 atmosphere. Some samples were digested using 7 N HNO3 and analyzed for Cr content. Some samples were stored in airtight glass bottles under N2 for examination by surface-sensitive techniques. Reversibility of the immobilization process was tested in a series of batch release experiments. Samples of reacted barrier and reference material from the region closest to the inlet in each of the columns were tested, at liquid/solid ratio of 10:1. They were exposed to the same artificial fly ash leachate that was used in the column experiment; sample replicates were exposed to 10-3 M CaCl2. Both solutions were made of deoxygenated and deionized water without addition of Cr and experiments were conducted in an N2 atmosphere. pH was maintained at specified values in the range of 4 to 10 by daily addition of small amounts of HNO3 or NaOH. All samples were agitated throughout the experiment using a horizontal shaker. For determination of dissolved concentrations, solution samples were centrifuged, and the supernatant was acidified with concentrated HNO3, resulting in pH less than 1.5, and then analyzed by graphite furnace atomic absorption spectroscopy. Analytical Techniques. Several high-resolution techniques were used to examine bulk phases and solid surfaces. Although all of these techniques require analysis in a vacuum, meaning samples must be dried, previous experience has shown that surfaces retain a remarkable level of information about the sample’s history prior to exposure to vacuum (6, 7). Scanning electron microscopy (SEM) provides images of rough material with resolution at fractions of a micrometer while energy dispersive X-ray spectroscopy (EDS) offers chemical analyses of the bulk with sampling diameter and depth on the order of micrometers. Micrographs of reacted barrier material were acquired for this study using a Phillips SEM 505 with Phillips PV9900 EDS. For higher resolution, X-ray photoelectron spectroscopy (XPS) provides information about surface composition with spatial resolution of a millimeter or less, with information depth limited to 10 nanometers. This allows the possibility to investigate the average chemical nature of the solid that was actually in contact with solution. Chemical identity, bonding structure, and redox state can be determined. Spectra for this work were collected using a Perkin-Elmer 5400 with Mg KR radiation; charge referencing for these insulating samples was made by assuming adventitious carbon C(1s) binding energy at 285 eV. Charge shift ranged from 0.9 to 1.1 eV. Peaks were fit based on the theoretical calculations of Gupta and Sen (8, 9) and the experiments of McIntyre and Zetaruk (10). Even higher resolution chemical information is available with time-of-flight secondary ion mass spectrometry (TOFSIMS). With this technique, lateral resolution is about a micrometer, but only the top one or two atomic layers are sampled; mass differentiation to less than 0.1% allows high confidence in ability to differentiate elemental masses from hydrocarbon fragments. The chemical maps produced by TOF-SIMS represent the ions that reached the detector rather than the ions that were present on the surface, and each solid has its own characteristic ability to release ions, so 4164
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FIGURE 1. Breakthrough of Cr in columns with barrier and reference material. intensities cannot be used to derive absolute surface concentrations. However, chemical maps are very good at telling us relative surface concentration and its change as a function of sample treatment or time. Spectra and chemical maps for this study were collected using a 69Ga+ primary beam in a Charles-Evans TRIFT II system. Because the purpose of the SEM and EDS investigations was to examine the grains in cross-section to observe changes in composition of the reaction layer, polished sections of the powder were prepared and coated with carbon, which is necessary for this type of analysis. For XPS and TOF-SIMS, where the purpose was to investigate the composition of the solid surface and the influence of the solution on it, we analyzed the samples as they were, without further treatment. We hand-picked a few grains of the black Fe(0) and the clear quartz sand using tweezers under an optical microscope and analyzed them separately. All particles were pressed into sample holders of Indium foil, which is a very malleable metal with a spectral signal not easily confused with the components of the samples. No attempt was made to separate the clay from the iron or sand; it was too fine-grained to physically pick out the particles, and chemical separation methods would certainly alter the composition of the top few atomic layers, invalidating surface analysis results. Samples for both XPS and TOF-SIMS were prepared in a glovebox under N2. They were exposed to O2 for only about two minutes, while they were loaded into the fast entry lock of the analytical chamber. Data were treated with the commercial software provided with the instruments.
Results and Discussion Column Solutions - Cr Uptake. Figure 1 shows breakthrough curves for the reference column (column IV), which was packed with only sand and bentonite. Cr breakthrough occurred after about 25 pore volumes. In the composition range of the experimental solutions where pH > 8, thermodynamics predicts the predominant Cr(VI) species to be CrO42-. The shape of the breakthrough curve suggests Cr retardation results from anion adsorption, probably to the sand and the bentonite.
FIGURE 2. (a) Cr concentration in pore water, (b) pH in pore water, and (c) Cr concentrations in reacted barrier material from column III taken after 57 pore volumes had passed through the column. Comparison of the reference breakthrough curve with that of the barrier material (Figure 1) proves that Cr mobility is significantly retarded by the presence of Fe(0), even in basic solutions with high salinity; partial breakthrough occurs only after 140 pore volumes in the shortest of the columns (column I). After 57 pore volumes had passed through column III, a composition profile was made by taking samples of pore water from outlets along its length. Figure 2a shows that Cr concentration in the pore water decreases from the input value of 25 mg‚L-1 to below the detection limit (
