Environ. Sci. Technol. 1997, 31, 2954-2959
Comparison of XANES Analyses and Extractions To Determine Chromium Speciation in Contaminated Soils MELANIE D. SZULCZEWSKI,* PHILIP A. HELMKE, AND WILLIAM F. BLEAM Department of Soil Science, 1525 Observatory Drive, University of WisconsinsMadison, Madison, Wisconsin 53706
Chromium-contaminated soils from three sites in Wisconsin were analyzed for the total concentration of chromium with neutron activation analysis, extractable Cr(VI) with a typical phosphate buffer and exchangeable Cr(VI) with isotope dilution analysis. X-ray absorption near-edge structure (XANES) spectroscopy provided a physical method for determining the ratio of Cr(VI):Cr(III) in the contaminated soils. In most samples, less than 10% of the total Cr was present as Cr(VI). Of the total Cr(VI) measured with XANES analysis, only a fraction was isotopically exchangeable, and an even smaller fraction of the total Cr(VI) was extracted with a phosphate buffer. The speciation and extraction of chromium in actual contaminated soils are apparently more complex than traditional chemical methods can show.
Introduction Chromium, occurring as Cr(III) or Cr(VI) in natural environments, is an unusual element which has multiple roles as an important material resource, essential micronutrient, and toxic contaminant. The two oxidation states of chromium have very different chemical, biological, and environmental properties: Cr(VI) is both highly soluble and toxic to plants and animals, yet Cr(III) is relatively insoluble and an essential micronutrient. Environmentally relevant chromium analyses of soil and sediments must therefore determine the amount and species of Cr present in each oxidation state along with the total amount of Cr present. Since Cr(VI) is the toxic, mobile form, an accurate method for determining the total concentration of Cr(VI) in soilwater systems, both the soluble and insoluble forms, is needed. The United States Environmental Protection Agency (U.S. EPA) sanctions several methods for determining soluble Cr(VI) concentrations, including the use of diphenylcarbazide colorimetry, atomic absorption spectrophotometry, coprecipitation, and differential pulse polarography (1, 2). Methods used to measure the concentration of total Cr(VI) include coprecipitation, acid extractions, iron hydroxide scavengers, and the difference technique with oxidation state conversion (3-5). The drawbacks of these are obvious: incomplete conversion, introduction of contamination by the oxidation/ reduction reagents, and interferences from other metals. The most effective method for measuring total Cr(VI) in soil-water systems uses a carbonate/hydroxide solution at 90-95 °C to extract Cr(VI) (6). Not only do such vigorous treatments perturb the chemical and physical properties of the soil-water system, there is no way to verify that all of the Cr(VI) has been extracted. * Corresponding author fax:
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The amount of hexavalent Cr associated with the solid phase of a soil or sediment sample can be divided into two components: the unexchangeable or “fixed” quantity and the exchangeable or “available” quantity. Chemical extractions are designed to measure the amount of exchangeable as well as dissolved Cr(VI) present (5-8). Phosphate buffer extractions are the most common method for determining the concentration of exchangeable Cr(VI). Isotope dilution analysis is a potential alternative method to extraction for determining the concentration of exchangeable Cr(VI). This technique has been used for determining the exchangeable concentration of trace elements such as cobalt and zinc (9, 10). Unlike most other methods, isotope dilution analysis does not use strong acids or complexing agents to displace or solubilize the analyte. This eliminates the assumption that phosphate or other anions quantitatively and completely displace chromate. One of our objectives is to determine whether isotope dilution analysis measures the total Cr(VI) present in soil or sediment samples. X-ray absorption near-edge structure (XANES) spectroscopy can provide information about the oxidation state of an atom and the symmetry and bonding of its local environment (11-14). The core electronic states become more tightly bound as the oxidation state increases, producing a measurable shift in the X-ray absorption spectrum. A shift of the position of the edge can then be correlated to the differences in oxidation states of the element. The pre-edge region of the Cr XANES spectra is dramatically different for the Cr(VI) and Cr(III) oxidation states. A strong pre-edge peak due to a 3d-4p mixing in the four-coordinate Cr(VI) appears in the K-edge XANES spectrum whenever Cr(VI) is present (15, 16). A similar pre-edge feature appears in the K-edge XANES spectra of other four-coordinate transition metal ions, such as Ti4+, V5+, and Mn7+ (17, 18). The height of this pre-edge 3d-4p mixing peak in a chromium spectrum is proportional to the amount of Cr(VI) in the sample. The ratio of Cr(VI): Cr(III) in soils will be measured by analysis of the XANES spectrum. One of the challenges facing environmental scientists is development of contaminant speciation methods for complex samples such as contaminated soils. Although the Cr(VI) spikes in laboratory-contaminated soils in previous studies were effectively recovered, there has not been much work with actual contaminated soils. The primary goal of this research is to determine what fraction of the total Cr(VI) in contaminated soils detected with XANES measurements is exchangeable, i.e., chemically active, as determined with phosphate buffer extractions and isotope dilution analysis.
Experimental Section Soil Samples. The soil samples were collected from former electroplating plants undergoing evaluation or remediation by the Wisconsin Department of Natural Resources in Janesville, Chippewa Falls, and Milwaukee, Wisconsin. Contamination of the soil at these sites occurred over several decades from leakage of plating vats, spillage of plating solutions, aerosols carried by exhaust fans, and discharge of plating solutions into storm sewers. The soil samples were passed through a 2 mm sieve but were not ground except for XANES analysis. Total Chromium Analysis by Neutron Activation. Neutron activation is a sensitive and accurate analytical method which can be used to measure total element concentrations in soils and other complex materials. This method is relatively free of the possibilities of contamination and interferences and is not affected by the chemical state of the element of interest (19). Approximately 0.2 g of each soil and a standard
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[a chernozemic A horizon soil sample, SO-4, from the Canadian Certified Materials Project, with 72.4 µg of Cr per g of soil (20)] were irradiated at the University of Wisconsin research nuclear reactor for 30 min at a neutron flux of 1 × 1013 neutrons cm-2 s-1. This converts a fraction of the 50Cr isotope (4% natural abundance) into the radioactive 51Cr isotope which has a half-life of 27.70 days. The irradiated samples and the standard were radioassayed after activation with a high-resolution γ-ray spectrometer. The 320 keV 51Cr γ-ray was used for the Cr analyses (19, 20). Hexavalent Chromium Analysis by Isotope Dilution. Each soil suspension contained 3 g of the sieved soil, 5 mL of 0.005 M magnesium nitrate (ACS certified, Fisher Scientific), and 10 µL of chloroform (ACS certified, EM Science) as a biocide in a 50 mL polypropylene centrifuge tube. After shaking the tubes for approximately 1 h, 1.0 mL of the carrierfree radiotracer was added to each tube. The stock tracer solution (Dupont Chemicals) contained 3 × 104 Bq of 51Cr(VI) (t1/2 ) 27.7 days) in distilled, deionized water. The mass of Cr in the radioisotope added to the system as carrier-free tracer (4 × 10-9 mol) is negligible relative to the indigenous chromium. The rate of exchange between Cr(VI) and Cr(III) is very slow (21), so that the radioisotope should exchange only with Cr(VI) on the short time scale of these experiments. When isotopic equilibrium is reached, the tracer is considered to be distributed in the same proportion as the nonradioactive exchangeable Cr(VI). We prepared a series of samples from several contaminated soils and measured isotopically exchangeable chromium at varying times up to 170 h. When the activity of the 51Cr(VI) no longer changed, we assumed steady state had been reached. On the basis of these experiments, the equilibration time was 72 h. The tubes were shaken at room temperature on a shaker for 72 h. Samples were run in duplicate or triplicate. We measured the total solution concentration of Cr(VI) by the colorimetric diphenylcarbazide method, with a detection limit of 0.5 mg L-1 (1). Each sample was radioassayed with a Ge γ-ray detector, using the 320 keV 51Cr γ-ray for 1-8 h, depending on the Cr(VI) content. To determine the behavior of a 51Cr-labeled spike of Cr(VI), we added potassium dichromate to an uncontaminated soil under the same experimental conditions with 72 h equilibration time and measured the recovery of Cr(VI) by isotope dilution analysis. From 50 to 1000 mg of Cr(VI) was added with the radiotracer per kilogram of uncontaminated soil. Chromium(VI) Phosphate Buffer Extraction. We followed the phosphate buffer extraction procedure of James et al. (6) as an alternative method for determining exchangeable Cr(VI). Samples were run in duplicate. The phosphate buffer is a solution of 5 mM potassium phosphate dibasic (ACS certified, Mallinckrodt Chemicals) and 5 mM potassium phosphate monobasic (ACS certified, J. T. Baker Chemicals). We added 50 mL of this buffer to approximately 2.5 g of soil. After shaking for 30-45 min, the suspension was centrifuged and filtered through a 0.4 µm polycarbonate membrane. We then analyzed the solution for Cr(VI) concentration with the DPC method (1). Chromium K-Edge XANES Experiments. When dealing with XANES, sample uniformity is extremely important, and better signal-to-noise ratios are obtained with finely ground samples. We prepared the chromium XANES standards as physical mixtures of chromic oxide (>98% pure, Aldrich Chemicals) and potassium dichromate (ACS certified, Mallinckrodt Chemicals). These solids were ground and sieved sequentially through U.S. Standard testing sieves with 106, 63, and 38 µm screens (170, 230, and 400 mesh) to give a fine powder. The powders were then mixed with boron nitride (99% pure, Aldrich Chemicals) or lithium carbonate (ACS certified, Mallinckrodt Chemicals), neither of which absorb in the X-ray region, to form 5% Cr by weight standards with various proportions of Cr(VI):Cr(III). The standards and
FIGURE 1. Recovery of a 51Cr(VI) spike from an uncontaminated soil containing from 50-1000 mg of Cr(VI) (with radiotracer) per kilogram soil. The solid line represents the fit from a linear regression.
TABLE 1. Concentrations of Soluble, Isotopically Exchangeable, and Phosphate Buffer Extractable Cr(VI) in Contaminated Soils from Three Wisconsin Sitesa Concentration of Cr(VI)ex sample
[Cr(VI) soln] (mg kg-1 soil)
from ID (mg kg-1 soil)
from PBE (mg kg-1 soil)
J1 J2 J5 J7 J9 JF1 JF2 JF3 JF4 JF5 M1 M2 BB1
