Article pubs.acs.org/est
Modifications to EPA Method 3060A to Improve Extraction of Cr(VI) from Chromium Ore Processing Residue-Contaminated Soils Christopher T. Mills,*,† Carleton R. Bern,‡ Ruth E. Wolf,§ Andrea L. Foster,∥ Jean M. Morrison,† and William M. Benzel⊥ †
U.S. Geological Survey, Crustal Geophysics and Geochemistry Science Center, MS964D, Denver Federal Center, Denver, Colorado 80225, United States ‡ U.S. Geological Survey, Colorado Water Science Center, Denver Federal Center, Denver, Colorado 80225, United States § PerkinElmer, Incorporated, 75 Nicholson Lane, San Jose, California 95132, United States ∥ U.S. Geological Survey, Geology, Minerals, Energy, and Geophysics Science Center, 345 Middlefield Road, Menlo Park, California 94025, United States ⊥ U.S. Geological Survey, Central Mineral and Environmental Resources Science Center, Denver Federal Center, Denver, Colorado 80225, United States S Supporting Information *
ABSTRACT: It has been shown that EPA Method 3060A does not adequately extract Cr(VI) from chromium ore processing residue (COPR). We modified various parameters of EPA 3060A toward understanding the transformation of COPR minerals in the alkaline extraction and improving extraction of Cr(VI) from NIST SRM 2701, a standard COPR-contaminated soil. Aluminum and Si were the major elements dissolved from NIST 2701, and their concentrations in solution were correlated with Cr(VI). The extraction fluid leached additional Al and Si from the method-prescribed borosilicate glass vessels which appeared to suppress the release of Cr(VI). Use of polytetrafluoroethylene vessels and intensive grinding of NIST 2701 increased the amount of Cr(VI) extracted. These modifications, combined with an increased extraction fluid to sample ratio of ≥900 mL g−1 and 48-h extraction time resulted in a maximum release of 1274 ± 7 mg kg−1 Cr(VI). This is greater than the NIST 2701 certified value of 551 ± 35 mg kg−1 but less than 3050 mg kg−1 Cr(VI) previously estimated by X-ray absorption near edge structure spectroscopy. Some of the increased Cr(VI) may have resulted from oxidation of Cr(III) released from brownmillerite which rapidly transformed during the extractions. Layered-double hydroxides remained stable during extractions and represent a potential residence for unextracted Cr(VI).
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INTRODUCTION EPA Method 3060A was developed to quantitatively extract “soluble, adsorbed, and precipitated forms” of Cr(VI) from a broad range of solid samples without redox transformation from Cr(VI) to Cr(III) or vice versa.1,2 The high pH (>11.5) extraction solution prevents the reduction of Cr(VI) by organic matter or reduced inorganic compounds and also prevents the adsorption of Cr(VI) (which is present as the polyatomic ion CrO42−) onto mineral surfaces. The high pH and concentration of carbonate (0.28 M) aid in the dissolution of otherwise insoluble chromate salts (e.g., PbCrO4, BaCrO4).2−5 The alkaline conditions can promote the oxidation of Cr(III) to Cr(VI) by manganese oxides present in the sample or molecular oxygen. However, oxidation by O2 is limited to relatively soluble Cr(III) compounds such as CrCl3 or recently precipitated Cr(OH)3.2−4 Several studies have shown that extraction of residual Cr(VI) from chromium ore processing residue (COPR)-contaminated materials by EPA Method 3060A is not quantitative when This article not subject to U.S. Copyright. Published XXXX by the American Chemical Society
compared to Cr(VI) content estimated by X-ray absorption near edge structure (XANES) spectroscopy.6−9 Malherbe et al.8 reported that the certified value for Cr(VI) of 551 ± 35 mg kg−1 in NIST standard reference material 2701 as determined by Method 3060A is approximately 18% of the total Cr(VI) they estimated using XANES spectroscopy. Others found that the method extracted 48% to 60% of total Cr(VI) (as determined by XANES) in untreated COPR material7,9 and 0% to 68% of total Cr(VI) in COPR material treated with several different experimental remediation methods.6,7 This discrepancy violates the stated requirement of Method 3060A that the extracting solution solubilize all forms of Cr(VI). Despite this significant issue, the Cr(VI) contents of COPRcontaining samples and standards determined by Method Received: April 3, 2017 Revised: August 10, 2017 Accepted: August 23, 2017
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DOI: 10.1021/acs.est.7b01719 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Article
extraction time. In addition to assessing the Cr(VI) yield under different extraction conditions, we also monitored sample transformation with total element and mineralogical measurements.
3060A are being used to evaluate the performance of alternative Cr(VI) extraction methods as well as determine Cr(VI) levels at contaminated sites.10,11 The reasons for the lack of quantitative extraction of Cr(VI) from COPR-contaminated material by Method 3060A have not been thoroughly investigated. The processing residue has a gravelly to sandy texture and a large proportion of residual Cr(VI) in COPR can be encapsulated within granules.12 Thus, much of the Cr(VI) in a COPR-contaminated sample may be inaccessible to extraction fluid unless the granules are broken up. Method 3060A does not specify a particle size range for samples, only that the sample be field moist.1 The effect of COPR particle size on Cr(VI) reduction was studied for two different experimental remediation treatments: Ca polysulfide13 and Fe(II)SO4.14 These studies compared the efficiency of Method 3060A on four different particle sizes of the same COPR material from
