FEATURE
The Challenge of Remediating Chromium-Contaminated Soil The complex chemistry of chromium compounds presents unique measurement and regulatory challenges. BRUCE R. JAMES
D
eveloping health-based cleanup standards and remediation strategies for chromium-contaminated soils based on the hexavalent forms of this heavy metal is a complex and controversial task. A major effort to address chromium contamination is currently under way in Hudson County, N.J. More than two million tons of chromate-bearing slag were used as fill from the early 20th century until the early 1970s (1) in what is now a populous urban and industrial area just west of New York City. This slag, known as chromite ore processing residue (COPR), was typically used as general fill material for voids or to create usable land from lowland areas Chromium in COPR-enriched soils is principally insoluble Cr(III) with variable concentrations of soluble and insoluble forms of Cr(VI) ranging from negligible to >20 grams per kilogram (g/kg) These disposal practices were routine and not considered to represent a health hazard for many years. But the brilliant yellow, soluble chromate crusts that appear on soil surfaces led to concerns about possible human health effects and prompted the New Jersey Department of Environmental Protection (NJDEP) to begin investigations in the mid-1980s. Since then, advances in understanding the chemistry and toxicity of chromium compounds have led to efforts to remediate these soils. Many Superfund sites are chromate-contaminated by diverse industrial wastes, and Cr(VI) and chromic acid rank 18th and 203rd, respectively, on the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) Priority List of Hazardous Substances (2). Indeed, EPA is considering for national approval new analytical methods developed by scientists working on Hudson County soils (3). And the New Jersey debate over appropriate toxic endpoints and methodologies upon which to base cleanup standards for total Cr(VI) (4) is being closely watched. Current research on innovative remediation methods is generating techniques to irreversibly reduce Cr(VI)
to Cr(III) using either in situ or ex situ treatments (5). Chromate-contaminated soils present a unique set of problems because the chemistry of chromium is so complex. One of the two common valence states, Cr(VI), is toxic and soluble; the other, Cr(III), is nontoxic and insoluble. In most soils as Cr(OH)3 or Cr203, Cr(III) is not readily absorbed by plants or translocated in the food chain (6). Hexavalent Cr is a Class A human carcinogen by inhalation and an acute irritant to living cells, whereas certain forms of Cr(III) are essential activators of insulin [7). Hexavalent Cr exists in neutral-to-alkaline, waste-amended soils principally as the soluble chromate anion (Cr042~), or as moderately-to-sparingly soluble chromate salts (e.g., CaCrO , BaCr0 , and PbCrO ). Chromium in COPR-enriched soils is principally insoluble Cr(III) with variable concentrations of soluble and insoluble forms of Cr(VI) [e.g., 10-20 g Cr(VI)/kg soil]. Most of these soils are strongly alkaline (pH 8-12) as a result of Na2C03 and CaC03 constituents added to chromite ore during a roasting process to produce Cr(VI). Oxidation and reduction reactions can convert Cr(III) to Cr(VI) and vice versa. The potential for this conversion from nontoxic Cr(III) to toxic Cr(VI) has complicated the task of determining whether a chromium-bearing waste or waste-contaminated soil is hazardous (8). In 1979, R. J. Bartlett and I (9) showed that a variable fraction (typically < 15%) of Cr(III) added to soils oxidized to Cr(VI) in laboratory tests. When soluble Cr(III) was added, the extent of oxidation was proportional to the level of easily reducible Mn(III,IV) hydroxides and oxides in the soil. But the extent of oxidation also depended on the form of added Cr(III) (soluble vs insoluble; organically complexed vs. inorganic forms). Smaller percentacres of Cr(III) (< 5%) oxidized if it was added to the soils as aged more crystalline precirjitates or as tan~ wastes (10) Reduction reactions of Cr(VI) may occur simultaneously with oxidation of Cr(III) in heteroge-
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neous soil materials containing organic matter, Fe(II), and Mn(III,IV) hydroxides and oxides. The relationship between Cr(III) and Cr(VI) depends on a balance between oxidation and reduction, as depicted in Figure 1 (11). The soil pH is a master variable (a rolling weight on the seesaw) that controls the balance between the two opposing redox reactions. Reduction of Cr(VI) by organic matter and other electron donors [e.g., Fe(II) and sulfides] is favored by pH values < 6, and both oxidation and reduction may be inhibited under more alkaline conditions (5). The coupling of oxidation and reduction reactions for Cr in soils also may be viewed as a cycle (Figure 1) in which Cr(III) and Cr(VI) are interconverted by a manganese redox cycle and the oxidation of organic matter (11). Focusing on how soil conditions affect the revolution of this cycle for different forms of Cr(III) and Cr(VI) is likely to be necessary in regulating and cleaning up COPR-enriched soils in which Cr(III) and Cr(VI) coexist. For example, recent studies have shown that the forms of Cr(III) in COPR do not oxidize to Cr(VI) in the presence of manganese oxides and hydroxides (my unpublished data), and that soluble Cr(III) added to COPRenriched soils does not oxidize to Cr(VI) (5). If oxidation of Cr(III) to Cr(VI) and reduction of Cr(VI) to Cr(III) can occur simultaneously in soils, then how should the hazards of chromium-bearing wastes be evaluated, and how should chromiumenriched soils be regulated? These questions are a challenge for regulatory agencies charged with setting limits for Cr in soils and natural waters. In 1991, EPA refused to delist Cr 2 0 3 as a hazardous waste (or as a component of soils) because of its potential for oxidation to Cr(VI). However, James et al. have shown Cr 2 0 3 to be inert to oxidation in soils (12). Measuring hexavalent chromium in soils Although this complex chemistry makes it difficult to predict the speciation of Cr in soil, it is now possible to measure it direcdy A new analytical method developed by chemists and engineers working on the Hudson County project is gaining wide acceptance as a means of determining the total Cr(VI) level in soil samples. Quantifying total Cr(VI) in soil samples requires dissolution of all forms of Cr(VI) while minimizing oxidation of the Cr(III) and reduction of Cr(VI) in the sample (12). An alkaline extraction method previously approved by the EPA Office of Solid Waste (SW-846 Method 3060) was criticized for possibly inducing oxidation of Cr(III) and reduction of Cr(VI) during extraction (13) and was therefore considered unreliable. In 1986, it was removed from the list of acceptable methods approved by the Office of Solid Waste (SW-846, second edition) (14) for soils and wastes.
"Chromate blooms," shown in a controlled test plot in Hudson County, N.J., occur when brilliant yellow hexavalent chromium salts rise with soil water and precipitate on the soil surface. Photograph courtesy Bruce James.
Revised SW-846 Method 3060A uses a 0.28 M Na2CO3/0.5 M NaOH solution heated with the soil for 60 minutes to solubilize all forms of Cr(VI). The pH of extraction (11.8-12.4) minimizes Cr(III) solubilization and oxidation, and it inhibits reduction of Cr(VI). In addition, the revised method uses a wide solution-to-soil ratio (40:1) and a near-boiling temperature (90-95 °C) to dissolve sparingly soluble forms of Cr(VI), such as PbCr04 (12,15). Subsequent measurement of solubilized Cr(VI) by diphenylcarbazide colorimetry is the most common, selective method using either UV-VIS spectrophotometry or ion chromatography with postcolumn derivatization. Revised Method 3060A breaks new ground in the metal analysis of soils by providing quality assurance methods for difficult soils such as reducing soils containing high concentrations of organic matter, Fe(II), or sulfides (e.g., anoxic sediments). Field measurements of Eh and pH are compared with thermodynamically derived Eh-pH conditions for Cr(VI) reduction to understand low recoveries of Cr(VI) matrix spikes that may be obtained in difficult-toanalyze soils. The extraction method (15) has recently undergone an evaluation and review by industry groups and by New Jersey and federal regulators. The Office of Solid Waste has proposed its reinclusion in the third update of the SW-846 compendium of methods for solid waste analysis. Method 3060A includes a precaution that the method could produce a positive artifact if a soil contains soluble Cr(III), such as in a fresh spill, a phenomenon not observed with insoluble forms of Cr(III). Having a reliable, officially recognized method to determine total Cr(VI) in soils is an important new tool for investigating Cr contamination. But at most VOL. 30, NO. 6, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 2 4 9 A
FIGURE 1
Chromium chemistry in soils
proach: establishing the ratio of insoluble-tosoluble Cr(VI) in a soil sample. The idea is to use the results of the total Cr(VI) extraction test with a test that measures "water-soluble Cr(VI)" (ASTM Method D3987-85) {19) in an attempt to establish a partition coefficient as the ratio of insoluble-to-soluble Cr(VI), calculated from native levels of Cr(VI) in field samples. Obtaining this "field partition coefficient" does not require addition of Cr(VI) to the soil to quantify sorption and therefore represents the field situation better than do the laboratory-generated isotherms normally used to obtain Kd values {17, 18). Such an approach is novel because it considers only one valence state of Cr, requires the measurement of soluble and total concentrations of Cr(VT) and is foilsed on metal dissolution and desorption estimates in unamended field soil samples
Human health endpoints Most states and EPA base Cr(VI) cleanup standards or soil screening levels on either the risk of cancer The role of pH. The balance between Cr(lll) oxidation by Mn(lll,IV) hydroxides caused by inhalation or noncarcinogenic effects and oxides and Cr(VI) reduction by organic matter in soils is affected by pH. The dynamic shifts in valence state are more significant for soluble forms of caused by ingestion, but the NJDEP is proposing the Cr(lll) and Cr(VI) than for more inert and insoluble compounds. use of allergic contact dermatitis caused by repeated contact with Cr(VI) in water, dust, or fine particles (4). Allergic contact dermatitis can occur in occupational settings following repeated exposure to Cr(VI), which eventually creates an allergic reaction in sensitive individuals. To meet this goal, New Jersey proposed a generic standard in September 1995 for Cr(VI) in soil of 15 mg/kg as part of its proposals for regulating Cr(VI) and Cr (III) (4). Not only is New Jersey's choice of a human health endpoint controversial, but interested groups maintain strongly divergent views on the data and methods used to derive the standards. The methodology of skin test studies used by the state has been challenged recently as inadequate to estimate accuThe role of other soil components. The extent and rate of interconversion of rately the threshold dose of Cr(VI) {20-22). ControCr(lll) and Cr(VI) in soils are governed by an Mn redox cycle and reactions versy also surrounds the method used to convert the by reducing agents, such as organic compounds. Movement of the cycles will be governed by the forms of Cr2 and by soil conditions that affect the threshold dose to a corresponding Cr(VI) soil conquantities and reactivity of Mn, 02, and C. centration. The NJDEP proposes using studies based on a dermatitis test patch concentration [mg Cr(VI)/L] by COHtailllOcHcQ SllcS, COUGCFOS dOOut tOc lllUOlilly o t making the assumption of equivalence between a l_ir(.VlJ m U S t aQQrcSS i t s m o v e m e n t 11110 grOUnQW 0.5 Cr203 + 1.5 C6H402 + 2.5 H20
come of this process, however, remains difficult to predict. In particular, how specific cleanup standards will affect the regulated community and those living near contaminated soils remains unknown. But modified regulatory proposals due to be released by New Jersey later this year should point the way toward the next step.
References (1) (2) (3) (4)
(5) (6) (7) (8) (9) (10) (11) (12) (13)
(14)
(15) (16)
(17) (18) (19) (20) (21) (22)
Paustenbach, D. J. et al. Toxicol. Ind. Health 1991, 7,159. Hazardous Waste Consultant 1993, 11(3), 2.26-2.30. Fed. Regist. 1995, 60, 37974. Soil Cleanup Criteria for Trivalent and Hexavalent Chromium: Basis and Background; New Jersey Department of Environmental Protection. Site Remediation Program: Trenton, NJ; September 1995. James, B. R. /. Environ. Qual. 1994, 23, 227. Cary, E. E.; Allaway, W. H.; Olson, O. E. /. Agric. Food Chem. 1977, 25, 305. Yassi, A.; Nieboer, E. In Chromium in Natural and Human Environments; Nriagu, J. O.; Nieboer, E., Eds;; WileyInterscience: New York, 1988; pp. 443-95. Fed. Regist. 1991, 56, 58859. Bartlett, R. J.; James, B. R. /. Environ. Qual. 1979, 8, 31. James, B. R.; Bartlett, R. J./. Environ. Qual. 1983,12,173. Bartlett, R. J.; James, B. R. In Chromium in Natural and Human Environments; Nriagu, J. O.; Nieboer, E., Eds.; Wiley-Interscience: New York, 1988; pp. 267-304. James, B. R. et al. Environ. Sci. Technol. 1995, 29, 2377. Messman, J. D. et al. Determination of Stable Valence States of Chromium in Aqueous and Solid Waste Matrices— Experimental Verification of Chemical Behavior, U.S. Environmental Protection Agency: Washington, DC, 1986; EPA/600/4-86/039. Test Methods for Evaluaiing Solid Wastes: Physical/ chemical Method; SW-846, 2nd ed.; Office of Solid Waste and Emergency Response. U.S. Environmental Protection Agency: Washington, DC, 1984. Vitale, R. J. et al. /. Environ. Qual. 1994, 23, 1249. Soil Screening Guidance. U. S. Environmental Protection Agency: Washington, DC, 1994; NTIS document nos. 9355.4-1, EPA/540/R-94/105, PB95-965530; Risk Based Concentration Table 1995. Region III, Philadelphia, PA. Amacher, M.; Selim, H. M. Ecol. Modell. 1994, 74, 205. Kent, D. B. et al. Water Resour. Res. 1995, 31, 1041. Standard Test Method for Shake Extraction of Solid Waste with Water. Method D-3987-85; American Society for Testing and Materials: Philadelphia, PA, 1994. Nethercott, J. et al. Occup. Environ. Med. 1994, 51, 371. Horowitz, S. B.; Finley, B. L. Regul. Toxicol. Pharmacol. 1994, 19, 31. Horowitz, S. B.; Finley, B. L./. Toxicol. Environ. Health 1993, 40, 585.
Bruce R. James teaches and conducts research in soil chemistry at the University of Maryland—College Park. VOL. 30, NO. 6, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 2 5 1 A
