HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (17) Citing Articles via Google Scholar Google Scholar Articles by Stewart, M. A. Articles by McKay, L. D. Search for Related Content PubMed PubMed Citation Articles by Stewart, M. A. Articles by McKay, L. D. GeoRef GeoRef Citation Agricola Articles by Stewart, M. A. Articles by McKay, L. D. Related Collections Bioremediation and Biodegradation Heavy Metals Soil Pollution Soil Chemistry

TECHNICAL REPORTS
Heavy Metals in the Environment

Influence of Soil Geochemical and Physical Properties on the Sorption and Bioaccessibility of Chromium(III)

^a Environ. Sci. Div., Oak Ridge, TN 37831-6038
^b Dep. of Civil Engineering, 208 Harbert Engineering Center, Auburn Univ., AL 36849-5337
^c Dep. of Geological Sciences, Univ. of Tennessee, Knoxville, TN 37996-1410

* Corresponding author (jardinepm{at}ornl.gov)

Received for publication November 7, 2001.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION METHODS RESULTS AND DISCUSSION ENVIRONMENTAL SIGNIFICANCE REFERENCES

 
There are numerous Cr(III)-contaminated sites on Department of Defense (DoD) and Department of Energy (DOE) lands that are awaiting possible clean up and closure. Ingestion of contaminated soil by children is the risk driver that generally motivates the likelihood of site remediation. The purpose of this study was to develop a simple statistical model based on common soil properties to estimate the bioaccessibility of Cr(III)-contaminated soil upon ingestion. Thirty-five uncontaminated soils from seven major soil orders, whose properties were similar to numerous U.S. DoD contaminated sites, were treated with Cr(III) and aged. Statistical analysis revealed that Cr(III) sorption (e.g., adsorption and surface precipitation) by the soils was strongly correlated with the clay content, total inorganic C, pH, and the cation exchange capacity of the soils. Soils with higher quantities of clay, inorganic C (i.e., carbonates), higher pH, and higher cation exchange capacity generally sequestered more Cr(III). The amount of Cr(III) bioaccessible from the treated soils was determined with a physiologically based extraction test (PBET) that was designed to simulate the digestive process of the stomach. The bioaccessibility of Cr(III) varied widely as a function of soil type with most soils limiting bioaccessibility to <45 and <30% after 1 and 100 d soil–Cr aging, respectively. Statistical analysis showed the bioaccessibility of Cr(III) on soil was again related to the clay and total inorganic carbon (TIC) content of the soil. Bioaccessibility decreased as the soil TIC content increased and as the clay content decreased. The model yielded an equation based on common soil properties that could be used to predict the Cr(III) bioaccessibility in soils with a reasonable level of confidence.

Abbreviations: DoD, Department of Defense • DOE, Department of Energy • PBET, physiologically based extraction test • NRCS, National Resource Conservation Service • CEC, cation exchange capacity • DDI, double deionized • TOC, total organic carbon • TIC, total inorganic carbon • VIF, Variance Inflation Factor • XAS, x-ray adsorption spectroscopy

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION METHODS RESULTS AND DISCUSSION ENVIRONMENTAL SIGNIFICANCE REFERENCES

 
THE PRESENCE of chromium (Cr) in the environment is widespread due to its usage in many industrial processes. The metallurgic, tanning, and plating industries are just a few examples of very common applications, large and small, which use Cr on a daily basis (Nriagu and Nieboer, 1988). Chromium itself is thermodynamically stable in two oxidative states: cationic Cr with a valence of three, Cr(III), and anionic Cr with a valence of six, Cr(VI). Chromium(VI) is often considered to be mobile in the environment while the more environmentally stable Cr(III) is considered less mobile (Chung et al., 1994; Patterson et al., 1997). There are several factors that contribute to the decreased mobility of Cr(III) in soil: (i) strong adsorption onto the negatively charged soil surfaces, (ii) the ability to form complex molecules with organics found in the soil, and (iii) the formation of oxides and hydroxides and other insoluble minerals in soil (Fendorf and Zasoski, 1992; Losi et al., 1994; Dragun, 1998).

When assessing the risks posed by Cr(VI) and Cr(III), the exposure pathway of most concern is ingestion by children (Paustenbach, 1989; Davis et al., 1990; Sheehan et al., 1991; Skowronski et al., 2001). Chromium(VI) is considered the most harmful of the oxidative states since it is both a mutagen and a carcinogen at low sub-ppm levels (Levis and Bianchi, 1982). Although Cr(III) is generally considered less harmful to human health than its oxidized counterpart, it may be of concern due to its potential to oxidize to Cr(VI) and its ability to accumulate to very high solid phase concentrations in some soils (Fendorf et al., 1992). The bioaccessibility of organic contaminants in soils has been relatively well studied (Linz and Nakles, 1997); however, the bioaccessibility of soil-bound metals such as Cr has received less attention (Ruby et al., 1996; Rodriguez et al., 1999; Skowronski et al., 2001; Stewart et al., 2003), where the bioaccessibility is defined as that amount of contaminant that is soluble due to simulated in vitro gastric functions and has the potential to cross the intestinal wall (Hamel et al., 1998). Typically, calculated health risks are inappropriately based on a reference dose derived from studies that use soluble aqueous metal species. The ubiquitous metal-sequestering properties of soil may significantly lower the bioaccessibility of Cr upon digestion, which, in turn may influence the decision for remediation at contaminated sites. Thus, action levels set by state regulators concerning the bioaccessibility of Cr in soil may need to consider specific soil properties instead of using generic guidelines (Proctor et al., 1997).

The intent of this paper is to show that Cr(III) can be strongly sequestered by soil, which in turn influences its bioaccessibility. We developed a simple statistical model based on measured soil properties to estimate the bioaccessibility of Cr(III)-contaminated soils upon ingestion. We show that common soil properties, which are easily obtainable from the National Resource Conservation Service (NRCS) database, can be used to assess Cr(III) bioaccessibility at contaminated sites.

	METHODS

TOP NOTES ABSTRACT INTRODUCTION METHODS RESULTS AND DISCUSSION ENVIRONMENTAL SIGNIFICANCE REFERENCES

 
Soil Type and Characterization
A database of metal-contaminated Department of Defense (DoD) sites was obtained from the U.S. Army Environmental Center, Aberdeen Proving Ground, Maryland. Twenty (20) DoD Army facilities throughout the USA were chosen for consideration based on the high concentration of Cr in their soils and the possible need for remediation (Table 1). Because of the difficulty in obtaining actual contaminated soils from these sites, uncontaminated soils whose properties were similar to the contaminated soils were acquired and treated with Cr(III). The soil series present at the DoD sites of interest were identified using Soil Conservation Survey documents. The USDA-NRCS database was then utilized to locate pedon numbers associated with each soil series. The NRCS was contacted and in most cases 200 g of the A-horizon and the upper B-horizon soil were obtained for each soil series (Table 1). Two additional soils were obtained from the Oak Ridge Reservation in eastern Tennessee, which also had properties similar to DoD sites in the southeast USA. Thirty-five soils were acquired and these encompassed seven major soil orders (Table 1).

View this table: [in this window] [in a new window]   Table 2. Select soil chemical and physical properties.

Determination of Chromium on Soil
Total Cr on the soil was determined using a modification of EPA method 3052. The soil was digested in a CEM microwave, model MDS-81D, with hydrofluoric and nitric acid. Boric acid was added before sample analysis to facilitate the removal of hydrofluoric acid from solution through the formation of fluoroboric acid. Soils from the National Institute of Standards, with known concentrations of solid phase Cr, were also analyzed with each block of analysis. Samples were stored and analyzed for total Cr using inductively coupled plasma.

In Vitro Bioaccessibility
A physiologically based extraction test (PBET) was adapted from Ruby et al. (1996)(1999; Ruby, personal communication, 2000) to assess the in vitro bioaccessibility of Cr(III) from contaminated soils in humans. The method is designed to simulate the stomach digestive system in humans. The PBET method has been shown to agree with in vivo studies involving Pb-contaminated soils (Ruby et al., 1996) as well as As-contaminated soils (Rodriguez et al., 1999); however, limited data is currently available in the literature that evaluates Cr bioavailability in contaminated soils using in vivo methods (Witmer et al., 1989, 1991; Gargas et al., 1994), and this data does not appear useful for cross-correlating with the results of the current study. Nevertheless, the PBET method can serve as a useful approximation of Cr bioavailability until in vivo studies become available to validate the methods credibility with regard to Cr.

In the current study, triplicate 0.39 g moist samples (0.3 g dry wt) were placed in 50 mL polyethylene tubes to which 30 mL 0.4 M glycine at pH 1.5 and 37°C was added. The slurries were quickly placed in a rotating water bath of 37°C and agitated at 30 ± 2 rpm for 1 h. Supernatant was separated from the solid via centrifugation. The pH of the supernatant was measured to ensure that the final pH was within ±0.5 pH units of the initial pH. This scenario held for all cases. Thus, bioaccessibility was calculated as:

The PBET pH of 1.5 simulates the most aggressive stomach digestive scenario, which is a condition indicative of human fasting. Conditions of higher pH, as a result of food intake, would most likely decrease Cr bioaccessability even more profoundly than the results presented in the current study, thus offering a potential avenue for future research. Both Ruby et al. (1996) and Yang et al. (2002) found that soil Pb bioaccessibility was strongly pH dependent with soluble Pb decreasing profoundly over a pH range of 1.5 to 4.0.

Chromium Analysis
The PBET supernatant, soil spiking solution, and equilibrium solution were measured for Cr(VI) and total Cr (Cr_T). Chromium(VI) was measured using a modified s-diphenyl-carbohydrazide colormetric method (Bartlett and James, 1979) using a UV-VIS spectrophotometer at wavelength 540 µm (HP model 8453, Palo Alto, CA). Analysis of Cr(VI) was performed immediately on rapidly cooled PBET solutions to avoid possible reduction of Cr(VI) to Cr(III) by glycine (Jardine et al., 1999). Independent studies revealed that Cr(VI) reduction by glycine at 37°C and 1 h was insignificant. Total Cr was measured on a Perkin Elmer AAnalyst 800 atomic absorption spectrophotometer (Wellseley, PA). Standards were made using an atomic absorption Cr standard (EM Industries, Hawthorne, NY). Chromium(III) was calculated as the difference between Cr_T and Cr(VI).

Modeling
A multiple regression technique in the statistical software package SigmaStat 2.0 (Jandel Scientific) was used to derive an expression that related Cr(III) sorption and bioaccessibility to common soil properties. The model was run using forward stepwise regression to determine the most salient soil properties for calculating sorption or bioaccessibility. Multiple linear regression was then employed to determine the linear equation to use when computing the Cr(III) sorption or bioaccessibility based on the important soil properties previously ascertained.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION METHODS RESULTS AND DISCUSSION ENVIRONMENTAL SIGNIFICANCE REFERENCES

 
Influence of Soil Properties on Chromium Sorption
Chromium sorption (i.e., adsorption and surface precipitation) by the 35 soils varied markedly with values ranging from 736 mg/kg to 17 460 mg/kg (Table 3). Sorption of Cr(III) was independent of horizon type where no distinct trend between A- and B-horizons was evident. The majority of the soils adsorbed between approximately 3000 mg/kg to approximately 6000 mg/kg with four soils as high as approximately 18 000 mg/kg. These four soils were all Aridisols and are noted for their high soil pH and for their high TIC content. Observed Cr(III) loading levels on many of these different soil types were similar to those measured on actual contaminated soils from the DoD sites. For example, actual contaminated Kzin soil (Xeric Torriorthents) from the Desert Chemical Depot contained 27 000 mg Cr/kg soil. Artificially contaminated Kzin soils in this study contained approximately 18 000 mg Cr/kg soil.

Chromium(III) sorption by the soils was strongly correlated with these soil properties (r² = 0.794) suggesting that nearly 80% of the variability in Cr(III) sorption could be described by pH, TIC, clay, and CEC (Table 4). Incorporating the other measured soil properties from Table 2 (e.g., Fe-oxide content, TOC, etc.) did not improve the model fit. In fact, TIC could have been removed from the model if necessary, since the other three independent soil variables could describe approximately 77% of the variability in Cr(III) sorption. The four-parameter model above was statistically rigorous at the 95% confidence level since P values for the independent variables were all <0.05 (Table 4). Thus, it can be concluded that the independent variables, the soil properties, significantly contribute to predicting the dependent variable, Cr sorption. The Variance inflation factor (VIF) also suggested that collinearity between independent variables was not significant (Table 4). Values for VIF that are 1.0 or slightly larger suggest that the variables do not show multicollinearity and that the parameter estimates are reliable. Collinearity becomes an issue when values of VIF exceed 4.0. This model also passed the Normality Test (indicating that the data was normally distributed) and the Constant Variance Test (suggesting that the variance of the dependent variables was constant). One of the most important criteria of a successful model, however, is the true physical significance of the model parameters. Our model suggests that Cr(III) sorption is enhanced by higher soil pH, more TIC (i.e., carbonates), more clay, and higher CEC. For a sparingly soluble cation, such as Cr(III), these soil conditions should enhance sequestration as the model suggests.

View this table: [in this window] [in a new window]   Table 4. Parameter estimates, standard errors, and statistics obtained from a multiple linear regression analysis that related soil properties to Cr(III) sorption.

As with pH, TIC or carbonate content in soils enhanced Cr(III) sorption. The mechanism of increased sequestration is most likely a localized pH effect at the carbonate surface, which promotes the formation of Cr(OH)₃ species. The localized pH effect is the most plausible scenario since there was no correlation between soil pH and soil TIC, thus explaining why collinearity was not a problem for these parameters when the model was fit to the Cr(III) sorption data. Several acidic Inceptisols derived from interbedded limey shales and limestone have relatively large residual carbonate contents (Table 2), due to the slow dissolution of local scale dolomite, and this may serve to enhance Cr(III) sequestration in these systems, even though the overall bulk soil pH is acidic.

The model also shows a positive correlation between the amount of Cr adsorbed and the soil clay content and CEC. This was expected since clay minerals tend to be dominated by negatively charged sites on the surface due to isomorphic substitution (Klein and Hurlbut, 1993). These negatively charged sites attract the cation Cr³⁺ and a weak, electrostatic bond is formed. The more negatively charged sites that are available (i.e., larger CEC), the greater propensity for Cr(III) sorption. Further, clay minerals typically have a large surface area that is capable of accommodating large quantities of Cr³⁺ and Cr(OH)₃ precipitated phases. The more surface area a soil has, the more reactive sites the soil has, and consequently the more Cr that will adsorb to the soil.

Influences of Soil Properties on Chromium Bioaccessibility
The bioaccessibility of Cr(III), as measured by the PBET method, varied widely as a function of soil type with most soils limiting bioaccessibility to <45% and <30% after 1 and 100 d soil-Cr aging, respectively (Table 3, Fig. 1a–e) . Bioaccessibility values were consistently higher for 1 d aging vs. 100 d aging. For all soils the percent bioaccessibility ranged from 3.0 to 54.7% at Day 1 and 1.5 to 35.5% at 100 d (Table 3, Fig. 1a–e). The aging effect is related to the enhanced stability of Cr on the soil surface with time. Structural reorientation of Cr surface bonds or slow precipitation reactions can account for the stronger sorption of Cr at longer times (Karthein et al., 1991). Previous studies by Stewart et al. (2003) have shown that aging effects are insignificant after 100 d and that the 100 d data are most relevant to actual DoD-contaminated soils. In general, the A-horizon soils had the lowest percent bioaccessible values, even when they adsorb more Cr(III) on the soil vs. the B horizons (Table 3). Bioaccessibility did not appear to be a function of soil order, suggesting that detailed soil series data, as is used in the current study, was necessary for predictive purposes (Table 3, Fig. 1a–e). Chromium(VI) was also measured in the PBET extractant to monitor for oxidation of Cr(III) to Cr(VI). The proportion of bioaccessible Cr that was Cr(VI) was always <1%, suggesting that oxidation reactions were minimal or that any oxidation products of Cr(VI) were tightly held by the soil. These results are consistent with the data presented by Stewart et al. (2003), which showed limited bioaccessibility of Cr(VI) in several soils.

View larger version (50K): [in this window] [in a new window]   Fig. 1. Percentage Cr(III) bioaccessibility after 1 and 100 d Cr-soil aging for (a) Ulitisols; (b) Alfisols; (c) Inceptisols; (d) Spodosols, Mollisols, Entisols; and (e) Aridisols.

with an r² value of 0.722, which indicated that as much as 72% of the variability in Cr bioaccessibility was explained by the model (Fig. 2) . The model was statistically rigorous at the 99% confidence level since P values for the independent variables were well below 0.01, indicating that they all contributed to predicting the % bioaccessible Cr(III) (Table 5). Values for VIF were all nearly 1.000, indicating that there was no redundant information in the other independent variables, i.e., soil properties, and that collinearity between independent variables was not of concern. This indicated that parameter estimates in the model were reliable, which is in agreement with the low standard errors on the estimated values (Table 5). The model also passed the Normality Test and the Constant Variance Test, suggesting that the data was normally distributed around the regression line and that the variance present in the dependent variable is constant. Most important, however, is the true physical significance of the model parameters. The model suggests that Cr(III) bioaccessibility decreases as the TIC content increases and as the clay content decreases. As shown with the Cr sorption data, Cr(III) sequestration is enhanced by soils with high levels of TIC. The presence of TIC promotes the formation of solid phase Cr(III)– hydroxides that are sparingly soluble, even under acidic conditions. These hydroxides [i.e., Cr(OH)₃] precipitate and cover the surface of the soil and are not easily bioaccessible even in the presence of the low pH in the simulated stomach fluid of the PBET. Consequently as the TIC content increases the bioaccessibility of Cr(III) in soil decreases. As shown with the Cr sorption data the clay content on the soil was also correlated with the amount of Cr sequestration and thus should be important in determining bioaccessibility. The bioaccessibility model suggested that, as the clay content of the soils increased, the percent of Cr on the soil that is bioaccessible also increased. Since the mechanism of Cr retardation on clay minerals is primarily weak electrostatic bonds, these bonds are easily broken under the conditions of the PBET, allowing Cr to desorb from the soil and be released into solution during the simulated digestion.

View this table: [in this window] [in a new window]   Table 5. Parameter estimates, standard errors, and statistics obtained from a multiple linear regression analysis that related soil properties (clay and TIC) to percent Cr(III) bioaccessibility.

View larger version (55K): [in this window] [in a new window]   Fig. 2. The observed (data points) and model fitted (grid surface) relationship between the two most significant independent variables (% clay and TIC) and % Cr(III) bioaccessibility using the model: % Cr(III) bioaccessible = 16.02 + (0.426 x % clay) - (9.56 x % TIC) an r2 value of 0.722.

with an r² value of 0.674. This relationship was similar to the clay/TIC model where higher quantities of TIC and TOC resulted in decreased Cr(III) bioaccessibility. When clay, TIC, and TOC were used in the same model, the contribution of TOC was not significant at the 90% confidence level (P = 0.115). This scenario may be an artifact of our limited data set, where the most appropriate model, in fact, includes both TIC and TOC along with clay content. A more extensive data set will be necessary to test this hypothesis. Nevertheless, the model using clay and TOC was statistically rigorous at the 99% confidence level since P values for the estimated parameters were <0.01 and the VIF values are approximately 1.000, indicating that the variables all contribute significantly to the equation and that no multicollinearity was present among the independent variables. This model passed the Normality Test and the Constant Variance Test. The model suggested that as the clay content decreased and the TOC content increased, the % Cr(III) bioaccessible decreased (Fig. 3) . The trend regarding clay content is consistent with the previous model and the limited bioaccessibility of Cr in the presence of higher system organic C is conceptually correct. Organic matter found in soil is a major contributor to the overall negative charge in soils and thus is an important sorbent for heavy metal cations (Sparks, 1995). Organic matter has the ability to form strong bonds with the Cr(III) with the metal not readily released during the PBET process. As Cr(III) is considered a Lewis hard acid, it forms stable complexes with the carboxyl group of the organic matter (Sparks, 1995). These bonds are stable and not easily broken. The current model again explains more than 67% of the variability in Cr(III) bioaccessibility and should be useful for soils low in carbonate (TIC).

View this table: [in this window] [in a new window]   Table 6. Parameter estimates, standard errors, and statistics obtained from a multiple linear regression analysis that related soil properties (clay and TOC) to percent Cr(III) bioaccessibility.

View larger version (55K): [in this window] [in a new window]   Fig. 3. The observed (data points) and model fitted (grid surface) relationship between the two independent variables (% clay and TOC) and % Cr(III) bioaccessibility using the model: % Cr(III) bioaccessible = 15.54 + (0.408 x % clay) - (3.78 x % TOC) with an r2 value of 0.674.

	ENVIRONMENTAL SIGNIFICANCE

TOP NOTES ABSTRACT INTRODUCTION METHODS RESULTS AND DISCUSSION ENVIRONMENTAL SIGNIFICANCE REFERENCES

	ACKNOWLEDGMENTS

 
We appreciate the efforts of Ms. Cathy Vogel and Dr. Andrea Leeson, the contract officers for the U.S. DoD who supported this work and the efforts of Mr. Warren Lynn of the National Resource Conservation Service (NRCS) who provided us with most of the soils for this study.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION METHODS RESULTS AND DISCUSSION ENVIRONMENTAL SIGNIFICANCE REFERENCES

 
This research was sponsored by the U.S. Department of Defense (DoD) Strategic Environmental Research and Development Program. Oak Ridge National Lab is managed by the University of Tennessee– Battelle LLC, under contract DE– AC05– 00OR22725 with the U.S. Department of Energy.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION METHODS RESULTS AND DISCUSSION ENVIRONMENTAL SIGNIFICANCE REFERENCES

 

• Bartlett, R., and B. James. 1979. Behavior of chromium in soils: III. Oxidation. J. Environ. Qual. 8:31–35.[Abstract/Free Full Text]
• Bartlett, R.J., and J.M. Kimble. 1976. Behavior of chromium in soils: I. Trivalent forms. J. Environ. Qual. 5:379–383.[Abstract/Free Full Text]
• Chung, J., R.J. Zasoski, and S. Lim. 1994. Kinetics of chromium(III) oxidation by various manganese oxides. Korean J. Agric. Chem. Biotechnol. 37:414–420.
• Davis, S., P. Waller, R. Buschbom, J. Ballow, and P. White. 1990. Quantitative estimates of soil ingestion in normal children between the ages of 2 and 7 years. Population-based estimates using aluminum, silicon, and titanium as soil tracer elements. Arch. Environ. Health 45:112–122.[Web of Science][Medline]
• Dragun, J. 1998. The soil chemistry of hazardous materials. Amherst Scientific Publ., Amherst, MA.
• Fendorf, S.E., M. Fendorf, D.L. Sparks, and R. Gronsky. 1992. Inhibitory mechanisms of Cr(III) oxidation by -MnO₂. J. Colloid Interface Sci. 153:37–54.
• Fendorf, S.E., G.M. Lamble, M.G. Stapleton, M.J. Kelly, and D.L. Sparks. 1994. Mechanisms of chromium (III) sorption on silica: I. Cr(III) surface structure derived by extended x-ray adsorption fine structure spectroscopy. Environ. Sci. Technol. 28:284–289.
• Fendorf, S.E., and D.L. Sparks. 1994. Mechanisms of chromium (III) sorption on silica: II. Effects of reaction conditions. Environ. Sci. Technol. 28:290–297.
• Fendorf, S.E., and R.J. Zasoski. 1992. Chromium(III) oxidation by -MnO₂: I. Characterization. Environ. Sci. Technol. 26:79–85.
• Gargas, M.L., R.L. Norton, M.A. Harris, D.J. Paustenbach, and B.L. Finley. 1994. Urinary excretion of chromium following ingestion of chromite-ore processing residues in humans: Implications for biomonitoring. Risk Anal. 14:1019–1024.[Medline]
• Hamel, S.C., B. Buckley, and P.J. Lioy. 1998. Bioaccessibility of metals in soils for different liquid to solid ratios in synthetic gastric fluid. Environ. Sci. Technol. 32:358–362.
• James, B.J., and R.J. Bartlett. 1983. Behavior of chromium in soils: VII. Adsorption and reduction of hexavalent forms. J. Environ. Qual. 12:177–181.
• Jardine, P.M., S.E. Fendorf, M.A. Mayes, S.C. Brooks, and W.B. Bailey. 1999. Fate and transport of hexavalent chromium in undisturbed heterogeneous soil. Environ. Sci. Technol. 33:2939–2944.
• Karthein, R., M. Motschi, A. Schweiger, S. Ibric, B. Sulzberger, and W. Stumm. 1991. Interactions of chromium(III) complexes with hydrous delta-Al₂O₃ rearrangements in the coordination sphere studied by electron-spin-resonance and electron spin-echo spectroscopies. Inorg. Chem. 30:1606–1611.
• Klein, C., and C. Hurlbut, Jr. 1993. Manual of mineralogy. 21st ed. Wiley, New York.
• Levis, A.G., and V. Bianchi. 1982. Mutagenic and cytogenic effects of chromium compounds. p. 171–208. In S. Langjard (ed.) Biological and environmental aspects of chromium. Elsevier Biomedical Press, New York.
• Linz, D.G., and D.V. Nakles (ed.) 1997. Environmentally acceptable endpoints in soil. Am. Academy of Environ. Eng., New York.
• Losi, M.E., C. Amrhein, and W.T. Frankenberger, Jr. 1994. Bioremediation of chromate-contaminated groundwater by reduction and precipitation in surface soils. J. Environ. Qual. 23:1141–1150.[Abstract/Free Full Text]
• Mehra, O.P., and M.L. Jackson. 1960. Iron oxide removed from soils and clays by a diothionite–citrate system buffered with sodium bicarbonate. Clays Clay Miner. 7:317–327.
• Nriagu, J.O., and E. Nieboer. 1988. Chromium in the natural and human environments. John Wiley & Sons, New York.
• Patterson, R.R., S. Fendorf, and M. Fendorf. 1997. Reduction of hexavalent chromium by amorphous iron sulfide. Environ. Sci. Technol. 31:2039–2044.
• Paustenbach, D.J. (ed.) 1989. The risk assessment of environmental and human health hazards: A textbook of case studies. John Wiley & Sons, New York.
• Proctor, D.M., E.C. Shay, and P.K. Scott. 1997. Health-based soil action levels for trivalent and hexavalent chromium: A comparison with state and federal standards. J. Soil Contam. 6:595–648.
• Rodriguez, R.R., N.T. Basta, S.W. Casteel, and L.W. Pace. 1999. An in vitro gastrointestinal method to estimate bioavailable arsenic in contaminated soils and solid media. Environ. Sci. Technol. 33:642–649.
• Ruby, M.V., A. Davis, R. Schoof, S. Eberle, and C.M. Sellstone. 1996. Estimation of lead and arsenic bioavailability using a physiologically based extraction test. Environ. Sci. Technol. 30:422–430.
• Ruby, M.V., R. Schoof, W. Brattin, M. Goldade, G. Post, M. Harnois, D.E. Mosby, S.W. Casteel, W. Berti, M. Carpenter, D. Edwards, D. Cragin, and W. Chappell. 1999. Advances in evaluating the oral bioavailability of inorganics in soil for use in human health risk assessment. Environ. Sci. Technol. 33:3697–3705.
• Sheehan, P.J., D.M. Meyer, M.M. Sauer, and D.J. Paustenbach. 1991. Assessment of the human health risks posed by exposure to chromium contaminated soils. J. Toxicol. Environ. Health 32:161–201.[Medline]
• Sheppard, S.C., W.G. Evanden, and W.J. Achwartz. 1995. Ingested soil: Bioavailability of sorbed lead, cadmium, iodine, and mercury. J. Environ. Qual. 24:498–505.[Abstract/Free Full Text]
• Skowronski, G.A., M. Seide, and M.S. Abdel-Rahman. 2001. Oral bioaccessibility of trivalent and hexavalent chromium in soil by simulated gastric fluid. J. Toxicol. Environ. Health Part A 63:351–362.[Medline]
• Sparks, D.L. 1995. Environmental soil chemistry. Academic Press, New York.
• Stewart, M.A., P.M. Jardine, M.O. Barnett, L.D. McKay, T.L. Mehlhorn, S.E. Fendorf, and K. Paul. 2003. Effects of contaminant concentration, aging, and soil properties on the bioaccessibility of Cr(III) and Cr(VI) contaminated soil. Soil Sediment Contam. (in press).
• U.S. Environmental Protection Agency. 1996. USEPA Method 3052. Microwave assisted acid digestion of siliceous and organically based matrices. [Online.] [20 p.] http://www.epa.gov/epaoswer/hazwaste/test/pdfs/3052.pdf. USEPA, Washington, DC.
• Witmer, C.M., R. Harris, and S.I. Shupack. 1991. Oral bioavailability of chromium from a specific site. Environ. Health Perspect. 92:105–110.[Medline]
• Witmer, C.M., H.S. Park, and S.I. Shupack. 1989. Mutagenicity and disposition of chromium. Sci. Total Environ. 86:131–138.[Medline]
• Yang, J.K., M.O. Barnett, P.M. Jardine, and S.C. Brooks. 2002. Factors controlling the bioaccessibility of arsenic(V) and lead(II) in soil. Soil Sediment Contam. (In press).

This article has been cited by other articles:

				R. A. Moseley, M. O. Barnett, M. A. Stewart, T. L. Mehlhorn, P. M. Jardine, M. Ginder-Vogel, and S. Fendorf Decreasing Lead Bioaccessibility in Industrial and Firing Range Soils with Phosphate-Based Amendments J. Environ. Qual., October 23, 2008; 37(6): 2116 - 2124. [Abstract] [Full Text] [PDF]

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (17) Citing Articles via Google Scholar Google Scholar Articles by Stewart, M. A. Articles by McKay, L. D. Search for Related Content PubMed PubMed Citation Articles by Stewart, M. A. Articles by McKay, L. D. GeoRef GeoRef Citation Agricola Articles by Stewart, M. A. Articles by McKay, L. D. Related Collections Bioremediation and Biodegradation Heavy Metals Soil Pollution Soil Chemistry