Environ. Sci. Techno/. 1995, 29, 2377-2381
Hexavalent Chromium Extraction from Soils: A Comparison of
soil matrix, but 63 and 78% recoveries were measured after spiking into the loam and LowHex COPR soils, respectively. Method-induced reduction of soluble Cr(VI) was not observed under the conditions of the heated alkaline extraction.
BRUCE R. JAMES*
Introduction
Soil Chemistry Laboratory, H.J. Patterson Hall, University of Maryland, College Park, Maryland 20742
The selective extraction and quantification of CrW)in soils is germane to establishing health-based regulations for Crcontaminated soils due to the greater toxicity and solubility of CrW) than Cr(II1) (1). An effective and reliable method for extracting both soluble and insoluble forms of Cr(VI) without inducing Cr(II1) oxidation or CrW) reduction is required (2). A hot (90-95 “C) 0.28 M Na2C03solution in 0.5 M NaOH has been used to define operationally total CrM)in soils enrichedwith chromite ore processingresidue (COPR)and in other soils spiked with Cr (VI). The accuracy and precision of this method have been evaluated in diverse soil materials, and new interpretations for low matrix spike recoveries in acidic or reducing soils have been proposed using redox-based parameters (2). However, this method has not been compared quantitatively to other extraction methods designed to speciate CrW) into soluble, exchangeable, and precipitated forms (3, 4). Therefore, the focus of the research reported here was to compare the carbonate-hydroxide extraction, with and without heating, to alternative methods using phosphate buffer (pH 7-01, distilled water (pH 5.7), and NaOH (pH 13)with sonication but without auxiliary heat. Distilled water and phosphate buffer extractions have been used to quantify and define operationally soluble and exchangeable forms of CrW) in numerous soils under diverse redox conditions, as established by pH, pE, and levels of oxidants and reductants for Cr (3). The fraction of total Cr(VI)added to the soil and not recovered by distilled water or phosphate buffer, termed nonexchangeable, includes Cr(II1) reduced from Cr(VI) by the soil and precipitated and chemisorbed CrW). In that previous work (3),an extraction method was not developed to measure directly total CrW) in a soil sample. An equilibration with 10 mM NaOH, however, recovered 100% of soluble Cr(VI) spiked into and sorbed by an iron(II1) (hydrloxide suspension, and the quantity of CrW) that was extracted by phosphate buffer was less than that extracted by the NaOH solution. The soluble and exchangeable fractions of Cr (VI) are useful parameters for estimating soil levels of Cr(VI) that may leach to groundwater, form a soluble “blush” on soil surfaces, or be absorbed by plants and microorganisms (5-7). Quantifymg insoluble forms of Cr(VI) is pertinent to environmental hazards associated with (a) airborne, respirable dust (8)and (b) colloid and solute movement in groundwater (9, 10). Therefore, the quantification of soluble, precipitated, and exchangeable forms of Cr (VI) [summed to equal total CrW) in a soil sample] has direct application to assessments of Cr hazards in the environment. Concerns have been expressed that native and spiked Cr(VI) might be reduced in soil samples subjected to the
J O H N C . PETURA Applied Environmental Management, Inc., Malvern, Pennsylvania 19355
ROCK J , VITALE AND GEORGE R . MUSSOLINE Environmental Standards, Inc., Valley Forge, Pennsylvania 19482-0911
A quantitative extraction method for total Cr(VI) (soluble and insoluble forms) in soils is needed to assess potential hazards from this heavy metal to humans and ecological systems. A laboratory study was conducted to measure native levels of Cr(VI) and recovery percentages of soluble and insoluble Cr042- matrix spikes in four diverse soil materials following equilibration with commonly used extractants: distilled water (pH 5.7), phosphate buffer (5 m M K2HP045 m M KH2P04, pH 7.0),carbonatehydroxide extractant (0.28M Na2C040.5 M NaOH; pH = 11.8)with and without heating, and hydroxide solution (0.1M NaOH; pH 13)with sonication. The heated carbonate-hydroxide solution was the most effective extractant for operationally defining total CrIVl) in soils that contained native Cr(VI) or in soils that had a sufficiently high redox status to maintain Cr as Cr(VI). More than 90% of a soluble K2Cr04 spike [1.0g of Cr(VI) kg-’1 was recovered using the heated carbonate-hydroxide extraction method on a redox-inert quartz sand, loamy-textured A horizon soil (Ultic Hapludalf), and chromite ore processing residue-enriched soil (HighHex COPR) containing 29% of its total Cr(VI) (1.4g kg-l) in insoluble forms. No Cr(VI) was recovered by any of the methods after spiking an anoxic sediment, as predicted by the reducing conditions of this soil for Cr(VI). Lead chromate (Ksp 1.8 x 10-14)was completely recovered by the heated carbonate-hydroxide extractant in the absence of a soil matrix, after being spiked into the loam soil containing no native Cr(VI), and after being spiked into another COPR soil (LowHex COPR) with 61% of its native Cr(VI) in insoluble forms. In contrast, BaCr04 (Ksp1.6 x 10-lo)was completely recovered by the carbonate-hydroxide extractant in the absence of a 0013-936X/95/0929-2377$09.00/0
1995 American Chemical Society
* Corresponding author.
VOL. 29, NO. 9, 1995 /ENVIRONMENTAL SCIENCE & TECHNOLOGY
2377
conditions of the hot carbonate-alkaline extractionmethod The fourth soil was anthropogenic material representa(method-induced reduction) and that Cr(II1) might be tive of chromite ore processing residue (HighHex COPR) oxidized to Cr(VI) (method-induced oxidation) (11). The mixed with other fill material that had been deposited before aerobic, alkaline condition of the extraction, however, 1974 to reclaim marshlands for commercial and industrial inhibits reduction of Cr(VI) during its dissolution (12),and uses. For this study, the COPR represented a matrix these conditions maintain a low activity of C ~ ( H Z O ) ~ ~ ' , containing a high level of CrW) (approximately 1.4g kg-'1. thereby minimizing oxidation to Cr (VI) (13). Nonetheless, The sample was collected from a site in the Hackensack River watershed of Hudson County, NJ. The soil complex these method-induced redox reactions must be considered to be separate from reactions with the soil itself that may in the region where the HighHex COPR was collected may be described as abandoned industrial land, and vegetation oxidize or reduce Cr. on the site includes early successional, weedy species. Despite the demonstrated efficacy of the hot carbonatehydroxide solution to dissolve insoluble forms of CrW) in Groundwater depth is typically2-3 m below the soil surface, and soils in the area have been disturbed by extensive filling soils while minimizing method-induced oxidation of Cr(II1) or reduction of Cr(VI), a method employing 0.1 M NaOH with construction debris, borrowed soils, and industrial residuals. Warehousing, transportation routes, and comand ultrasonic dispersion instead of heat has been tested mercial activity are the principal land uses. Soil mapping to quantify total Cr(VI) in soils ( 4 ) . Since this method does and series designations have not been done (5). not employ a high temperature to dissolve insoluble forms of Cr(VI), it was previously hypothesized that it would Spiking Procedures for Cr203 and K2Cr04. For each of minimize method-induced oxidation and reduction while the soil materials, the equivalent of 2.50 f 0.02 g of oveneffecting the same dissolution of Cr(VI) as does the dried soil material (air-dried loam, moist sand, saturated carbonate-hydroxide extraction using heat ( 4 ) . However, anoxic sediment, and field-moist HighHex COPR) was weighed into 45 250-mL beakers weighed empty to the sonication of colloidal suspensions can cause dissolution nearest 0.01 g. The beakers were divided into five groups and oxidation of some sorbed organic compounds (14) and thereby could cause reduction of CrW) in some soil of nine, and each group was further divided into three groups of three. To a group of nine beakers, the following samples. This raised the issue of the efficacy of heating vs sonication for complete dissolution of Cr (VI) under alkaline spiking treatments were applied directly to the soil samples, conditions. each in triplicate: On the basis of the need for a reliable method for Cr203. 37 mL of a suspension containing 6.6 mmol of extracting Cr(VI) from soils and the knowledge base for Cr203L-l, equivalent to adding 10 g of Cr(II1) kg-' of soil. other selective methods, the following objectives were &Cr04. 1.25mLof a38.5 mM Cr(VI)solution, equivalent established for this research: (1) to compare the effectiveto 1.00 g of CrW) kg-I of soil. ness of the carbonate-hydroxide extraction method (with Control. No Cr spike added. and without heat) to other extraction methods using distilled Chromium(II1)oxide and K2Cr04were chosen as spiking water, phosphate buffer, and hydroxide solution with compounds to assess method-induced oxidation of Cr(II1) sonication; (2) to compare the effectivenessof the methods and method-induced reduction of CrW) since they represent forms of Cr that exist in contaminated soils. for diverse soil materials with a range of redox-related properties pertinent to Cr; and (3) to quantify recoveries Chromium(II1)oxide is a sparingly soluble form of Cr(II1) of soluble and insoluble spiking materials with respect to likely to exist in wastes, and K2Cr04was selected because of its high water solubility and its common use in standard completeness of dissolution. analytical test methods for Cr(VI). Materials and Methods Extraction Methods. To a group of nine beakers Soils. Four soils with diverse oxidation-reduction propercontaining soil and the two spiking materials (and no spike ties pertinent to Cr were chosen to compare initially the controls),50 mL of one of the following extracting solutions five extraction methods for Cr (VI). A quartz sand was used was added: as a soil material expected to be inert with respect to Distilled Water. (pH 5.7) glass-distilled and having an oxidation of Cr(III), reduction of Cr(VI), and sorption of electrical conductivity of %0.001dS m-I. Phosphate Buffer. (pH 7.0) 5.0 mM K2HP04in 5.0 mM cationic or anionic forms of either valence state. The commercially purchased sand was white, and it contained KH2PO4. negligible organic matter ( < l g kg-') and no detectable Carbonate-Hydroxide Solution. (pH 11.8- 12.3) 0.28 Cr(VI) ( 100 mg of sulfide of beakers containing hydroxide solution were transferred k g ' ; properties indicative of a strongly reducing soil to a sonicating bath containing distilled water without environment for Cr(VI). The highly odoriferous (organic auxiliary heat for 30-45 min. The beakers containing and H2S smells) sediment was black and represented distilled water and phosphate buffer solution remained at sediment found in estuarine marshlands of the Hackensack 25 "C for the 30-45-min period of heating and sonication River Basin in New Jersey. of the other treatments. All the beakers were swirled briefly 2378 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 9. 1995
every 10-15 min during the 30-45 min extraction period, and distilled water was added to the heated beakers to maintain the suspension volume at 30-40 mL. At the end of the heating and sonicating periods, the solutionvolumesofallthe beakers were brought to 120 mL by weight by adding distilled water (assuming a density of 1.0 kg L-'I. After centrifuging a 10-mL portion of the suspensions (25 "C, 10 min. 104g),CrM) was measured colorimetrically in the centrifugates by the diphenylcarbazide method after making appropriate dilutions in distilled water [1:1 to 1:40, depending on the CrMl level, soil, and extracting solution] (5). The centrifugates containing NazC03or NaOH required a dilution of at least 1:5 to attain the required pH < 2 following addition of the diphenylcarbazide reagent. Spiking Procedures for BaCrOl and PbCr04. Initial trials in which the above procedures were conducted with BaCr04 as a spiking material resulted in spike recoveries of
