Article pubs.acs.org/est
Influence of Soil Geochemical and Physical Properties on Chromium(VI) Sorption and Bioaccessibility P. M. Jardine,*,† M. A. Stewart,† M. O. Barnett,‡ N. T. Basta,§ S. C. Brooks,∥ S. Fendorf,⊥ and T. L. Mehlhorn∥ †
Biosystems Engineering and Soil Science Department, Institute for a Sustained and Secure Environment, University of Tennessee, Knoxville, Tennessee 37996-4531, United States ‡ Department of Civil Engineering, Auburn University, Alabama 36849-5337, United States § School of Environment and Natural Resources, The Ohio State University, Columbus, Ohio 43210, United States ∥ Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6038, United States ⊥ Environmental Earth System Science, Stanford University, Stanford, California 94305-4015, United States S Supporting Information *
ABSTRACT: The Department of Defense (DoD) is faced with the daunting task of possible remediation of numerous soil-Cr(VI) contaminated sites throughout the continental U.S. The primary risk driver at these sites is hand-to-mouth ingestion of contaminated soil by children. In the following study we investigate the impact of soil geochemical and physical properties on the sorption and bioaccessibility of Cr(VI) in a vast array of soils relevant to neighboring DoD sites. For the 35 soils used in this study, A-horizon soils typically sorbed significantly more Cr(VI) relative to B-horizon soils. Multiple linear regression analysis suggested that Cr(VI) sorption increased with increasing soil total organic C (TOC) and decreasing soil pH. The bioaccessibility of total Cr (CrT) and Cr(VI) on the soils decreased with increasing soil TOC content. As the soil TOC content approached 0.4%, the bioaccessibility of soil bound Cr systematically decreased from approximately 65 to 10%. As the soil TOC content increased from 0.4 to 4%, the bioaccessibility of Cr(VI) and CrT remained relatively constant at approximately 4% and 10%, respectively. X-ray absorption near edge structure (XANES) spectroscopy suggested that Cr(VI) reduction to Cr(III) was prevalent and that the redox transformation of Cr(VI) increased with increasing soil TOC. XANES confirmed that nearly all bioaccessible soil Cr was the Cr(VI) moiety. Multiple linear regression analysis suggested that the bioaccessibility of Cr(VI) and its reduced counterpart Cr(III), decreased with increasing soil TOC and increasing soil pH. This is consistent with the observation that the reduction reaction and formation of Cr(III) increased with increasing soil TOC and that Cr(III) was significantly less bioaccessible relative to Cr(VI). The model was found to adequately describe CrT bioaccessibility in soils from DoD facilities where Cr(VI) contaminated sites were present. The results of this study illustrate the importance of soil properties on Cr(VI) sorption and bioassessability and help define what soil types have the greatest risk associated with Cr(VI) exposure.
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INTRODUCTION Hexavalent chromium (i.e., chromate or Cr(VI)) is a known carcinogen, with the inhalation of vapor or dust and ingestion being the major routes of exposure. The primary health risk from exposure to Cr(VI) is an increased likelihood of developing lung, kidney, and liver damage and cancer.1 On Department of Defense (DoD) Cr contaminated sites, the ingestion of soil by children is the primary risk factor that drives the need for remediation.2−5 Recently Cr(VI) has become one of the most stringently regulated substances used in manufacturing and maintenance operations. The DoD considers Cr as one of its four top priority soil−metal contaminants of concern, behind Pb and As, with regards to the human health risk and site remediation.6−8 Chromate has long been used as a corrosion © 2013 American Chemical Society
inhibitor for DoD aircraft fuselages and various weapon components. Soil contamination has resulted from releases of Cr(VI) during disposal of effluent from coating processes and during stripping of metal finishes and coatings. The risks associated with Cr(VI) exposure and soil contamination are such that the DoD has invested a large sum of funding for research in the search of suitable Cr(VI) substitutes (i.e., chromate-free conversion coatings, CCC).9−12 Received: Revised: Accepted: Published: 11241
April 16, 2013 August 4, 2013 August 13, 2013 August 13, 2013 dx.doi.org/10.1021/es401611h | Environ. Sci. Technol. 2013, 47, 11241−11248
Environmental Science & Technology
Article
Environmental Center, Aberdeen Proving Ground, Maryland. Nineteen DoD Army facilities throughout the U.S. were chosen for consideration based on the high concentration of chromium in their soils and the possible need for remediation (Table 1 in Stewart et al. 2003 a7). The USDA−NRCS database was than utilized to locate pedon numbers associated with each soil series. The soils were than obtained from the NRCS (see Table S1 in Supporting Information (SI)). Soils were also obtained from 21 contaminated Cr(VI) sites from 8 different DoD facilities located in six different States within the U.S. The facilities spanned from the east to the west coasts and had vastly different geochemical and physical properties. The names of the facilities and their corresponding soil properties cannot be released since their bioaccessibility data, as related to a particular site, is proprietary information. Soils were dried at room temperature, disaggregated with gentle grinding using a mortar and pestle, and sieved to provide a soil fraction
