Hexavalent Chromium Extraction from Soils: Evaluation of an Alkaline Digestion Method R. J. Vitale, G. R. Mussoline, J. C. Petura,* and B. R. James ABSTRACT The accurate quantification of total Cr(VI) in soils is relevant to human health concerns because Cr(VI) is significantly more toxic than Cr(HI). Hot alkaline solution has been shown to extract soluble and insoluble forms of Cr(VI) from soils, but incomplete recovery of Cr(VI) spikes andthe oxidation of soluble Cr(m) spikes in certain soils have been suggested as methoddeficiencies. A laboratory methodstudy was performed to (i) test the method’s accuracy, (ii) understand the soil chemical processes responsible for poor Cr(VI) spike recoveries, and(iii) develop definitive interpretations for Cr0fI) spike recovery data. Test results for >1500 field soil samples and the method study of eight diverse soil materials demonstrated dissolution of soluble and insoluble Cr(VI) spikes and the method’s reliability for Cr(VI) characterization. Complete dissolution of K2CrO4, BaCrO4, and PbCrO4 spikes confirmed the extraction of soluble and insoluble Cr(VI) forms. Ancillary soil chemical parameters,including oxidation-reduc- tion potential (ORP) (reported herein as E~), pH, ~-, and t otal organic C were quantified and interpreted to explain poor Cr(VI) spike recoveries. Highly reducing samples yielded 0% Cr(VI) spike recoveries, as predicted from Eh-pH relationships, and unspiked soil samples contained no detectable Cr(VI). In soils containing Cr(VI) and in most aerobic soils without native Cr(VI), acceptable Cr(VI) spike recoveries were obtained. Ancillary parameter characterization demonstrated that strongly reducing samples cannot maintain Cr(VI) laboratory matrixspikes. Correct interpretation of poor CrCv’I) spike recovery data should avoid labeling these data as unacceptable method results without ancillary parameter characterization of such samples. a~ ACCURATE AND PRECISE METHODfor extracting and .analyzing Cr(VI) from soils, sediments, and waste materials is needed because of human and ecological concerns related to Cr(VI) in the environment (Eisler, 1986; Nieboer and Jusys, 1988; Sheehan et al., 1991; WHO, 1988). A lack of regulatory agency-approved methods for Cr(VI) has prevailed since 1986, when U.S. Environmental Protection Agency (USEPA) re- search study did not achieve consistent results with Method3060, an alkaline digestion procedure for solid samples (USEPA, 1984a; USEPA, 1986). Subsequently, the method was removed from the USEPA manual Test Methods for Evaluating Solid Wastes, SW-846,3rd ed. (USEPA, 1990a). The research report concluded that "the stability of the chromium oxidation state once solubi- lized in either acid or base media is matrix dependentand cannot be predicted in environmental samples" (USEPA, 1986). The removal of Method3060 as an acceptable method is important because of the significant difference in toxic- ity between Cr(VI) and Cr(III): Cr(VI) is a human carcin- ogen (via inhalation), and Cr(III) is an essential dietary R.J. Vitale and G.R. Mussoline, Environ. Standards, Valley Forge, PA 19482; J.C. Petura, Applied Environmental Management,16 Chester County Commons, Malvem, PA 19355; and B.R. James, Soil Chemistry Lab., Agron. Dep., Univ. of Maryland, College Park, MD 20742. Re- ceived 5 Nov. 1993. *Corresponding author. Published in J. Environ. Qual. 23:1249-1256(1994). element for humans and other mammals (Eisler, 1986; Anderson, 1989; USEPA, 1993). Thus, a reliable method is needed to distinguish these valence states of Cr, and to quantify total Cr(VI) in soil matrices. Thereare several USEPA-approved methods to differentiate between the Cr(III) and Cr(VI) in solution (e.g., the diphenylcarbaz- ide colorimetric method [Method 7196A] and ion chro- matography [Method 7199]), and to analyze aqueous samplesand soil digests for total Cr using atomic absorp- tion or inductively coupled plasma (ICP) atomic emission spectroscopy (USEPA, 1983, 1990a). For the determina- tion of total Cr(VI) in solid media, however, there are only recently developedtechniques available that are not applicable to soils, such as ASTM Method D5281-92 (ASTM, 1992) for collecting airborne particulate matter in an alkaline impinger solution with analysis by ion chromatography/visible absorption spectroscopy. Method 3060 is a procedure for digesting solid samples in a hot, alkaline (pH 12) solution containing 0.28 Na2CO3 and 0.5 MNaOH that solubilizes both soluble and insoluble Cr(VI) compounds (James, 1994). Cr(VI) is in solution, the digest is analyzed by adding a diphenylcarbazide (DPC)solution in acetone, and ad- justing the solution to pH 2 using H2SO4. The Cr(VI) reacts with DPC, which is highly selective for Cr(VI), to produce a red-violet complex, and its absorbance is measured spectrophotometrically at 540 nm. This analyt- ical method is designated Method 7196A, an approved method in SW-846, 3rd ed. (USEPA, 1990a). The use of DPC for measuring Cr(VI) has been known and use for almost a century (Cazeneuve, 1900). Ion chroma- tography coupled with postcolumn DPC chemical reac- tion provides an acceptable alternate methodologyfor measuring Cr(VI) in the alkaline digest (SW-846,Method 7199) (USEPA, 1990a). Over the past several years, modifications of Method 3060 have been made to enhance the efficiency of the digestion process, both in terms of the time required and consistency needed for accurate and precise analyti- cal data for quality control purposes. Thesemodifications have included reducing the soil sample weight, and de- creasing the sample weight/digest volumeratio, as well as several other changes (Table 1). The achievement acceptable spike recoveries as specified in Table 1 in most nonreducingsoils has established the reliability of the method. Minor modifications, which do not alter the basic chemistry of Methods 3060 and 7196A, evolved from analyzing >1500 diverse, field soil samples for total Cr(VI), ranging from anoxic sediments to chromite ore processing residue (COPR), representing a wide Abbreviations: USEPA, U.S. Environmental Protection Agency; ORP, oxidation-reduction potential; ICP, inductively coupled plasma; DPC, diphenylcarbazide; COPR, chromite ore processing residue; TOC, total organic carbon; RSD, relative standard deviation. 1249 Published online November, 1994