Environ. Sci. Technol. 2001, 35, 4054-4059
Hydrogen Peroxide Effects on Chromium Oxidation State and Solubility in Four Diverse, Chromium-Enriched Soils MELANIE L. ROCK,† BRUCE R. JAMES,‡ AND G E O R G E R . H E L Z * ,† Department of Chemistry and Biochemistry and Water Resources Research Center and Soil Chemistry Program, Department of Natural Resources Science and Landscape Architecture, University of Maryland, College Park, Maryland 20742
High concentrations of H2O2 are being tested for in situ oxidation and remediation of buried organic contaminants in soils and groundwater. Peroxide is being considered as a direct chemical oxidant in Fenton-type reactions or as a source of oxidizing equivalents in bioremediation schemes. How H2O2 affects the oxidation state and solubility of Cr(III) and Cr(VI), common co-contaminants with organic chemicals, is explored here in four chemically diverse soils containing elevated levels of Cr. Soil contaminated with soluble Cr(VI) from chromite ore processing residue and soil containing high levels of recently reduced Cr (III) from electroplating waste both released dissolved Cr(VI) after single applications of up to 24 mM H2O2. In no case was there evidence that H2O2 reduced preexisting Cr(VI) to Cr(III), even though this would be allowed thermodynamically. Chromate in the leachates exceeded the U.S. EPA drinking water standard for total dissolved Cr (2 µM) by a factor of 10-1000. Anaerobic conditions in an organic-rich, tannery waste-contaminated soil protected Cr(III) from oxidation and mobilization. Mineral forms of Cr in serpentinitic soil near a former chromite mine also resisted oxidation on the time scale of days. Mobilization of Cr(VI) could be a hazardous consequence of using H2O2 for in situ remediation of chemically complex wastes, but H2O2 could prove attractive for ex situ treatment (i.e., soil washing). This paper demonstrates marked differences among Cr-contaminated soils in their capacity to release Cr(VI) upon chemical treatment with H2O2.
Introduction Hydrogen peroxide is a potentially useful reagent for in situ waste remediation. It can be used with high Fe(II) concentrations to effect Fenton-type oxidation of refractory organic contaminants in soils and groundwater. In one field test, 436 m3 of 50% H2O2 was injected into an aquifer to oxidize an * Corresponding author e-mail:
[email protected]; phone: (301)405-1797; fax: (301)314-9121. † Department of Chemistry and Biochemistry and Water Resources Research Center. ‡ Soil Chemistry Program, Department of Natural Resources Science and Landscape Architecture. 4054
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 20, 2001
estimated 33 000 kg of trichloroethylene (1). In contrast, H2O2 can be used at lower levels (3-300 mM) to deliver oxidizing equivalents to support in situ bioremediation (2). Such uses of H2O2 raise concern about possible side reactions with co-contaminants. A common co-contaminant in hazardous wastes is Cr, which is reported at 43% of the hazardous waste sites on the National Priorities List in the United States (3). The ability of H2O2 to act both as an oxidizing agent and as a reducing agent is well-known. Under appropriate conditions H2O2 can oxidize Cr(III) to Cr(VI) or reduce Cr(VI) to Cr(III) (4, 5). What its effect, if any, might be on Cr at hazardous waste sites is an open question. Chromium(VI) is a human carcinogen that predominates in soils and natural waters as mobile anions, HCrO4- or CrO42-. In contrast, Cr(III) is an essential trace element for humans and is relatively immobile in the same environments because of low solubility and propensity to sorb to natural solids (6). Cr contamination in soils has resulted from the disposal of industrial wastes produced by several different processes (e.g., electroplating, leather tanning, chromite ore mining and processing). Cr-contaminated sites are therefore chemically diverse with respect to forms, oxidation state, and solubility of this element. We explore in this paper the potential for solubilization of Cr by aqueous H2O2 in selected soils that incorporate some of the chemical and pedologic diversity found at contaminated sites. Differences include chromium deposition processes, total Cr levels, soil pH, redox conditions, chromate/ bichromatespeciation,andorganic/inorganicCr(III)speciation. Low millimolar H2O2 levels were chosen for the experimental treatments based on the concentration range being considered for enhanced bioremediation treatments (2).
Site Descriptions Sampling. A similar sampling protocol was followed at each site: an undisturbed area about 1 m2 was cleared of leaf and plant covering, a pit was dug, soil horizons were marked and identified, and samples were taken from each horizon. Samples were kept intact as large blocks (∼2 L volume), sealed in 0.004 in thick plastic bags, transported to the laboratory in coolers, and stored in a refrigerator at 4° C in the fieldmoist condition (0 to ∼0.1 kPa water potential). Intact blocks of soil from individual horizons were prepared for the experiments by passing them through a polyethylene sieve using gentle hand pressure to obtain a relatively homogeneous subsample from each horizon. Soil from the chromite ore processing site was prepared using a 0.40 cm sieve; a 0.25 cm sieve was used on all other soils. Electroplating Waste Site. National Chromium, Inc. (a small electroplating facility near Putnam, CT) discharged chromic acid-containing wastewater directly into an adjacent mixed hardwood wetland and flood plain from 1939 to 1975 (7). The peat-like uppermost horizon contained the highest total Cr levels of any soil in this study (Table 1). Green chromium(III) hydroxide coatings were evident on fallen branches and plant debris surrounding the site, and some soil samples contained as much as 60 g of Cr/kg of soil. The soil is very poorly drained, and its pH is low: 4-5. Even though its high levels of organic matter (200 g of C/kg of soil) and low pH might be expected to reduce Cr(VI) to Cr(III), it nonetheless contains leachable Cr(VI) (60-90 µM) in the uppermost horizon. The X-ray diffraction spectrum indicated the presence of quartz and Cr(III)-rich chlorite. Soluble Cr(VI) was not detected in the middle horizon at this site but did appear in the glacial till below 100 cm. Mattuck (8) 10.1021/es010597y CCC: $20.00
2001 American Chemical Society Published on Web 08/30/2001
TABLE 1. Soil Properties soil designation electroplating waste ore processing residue tannery chromite mine
horizon (cm)
soil pH
0-14 14-40 >100 surface 0-20 20-40 53-75
5.4 5.0 4.9 8.8 6.7 5.7 6.5
organic carbon (g/kg of soil) 280d 118d 5.2d 66 154d 202d 4.2d
total Cra (g/kg of soil)
total Cr(VI)b (g/kg of soil)
soluble Cr(VI)c (µM)
61 ( 5 21 ( 1 0.40 ( 0.03 8.6 ( 0.5 1.3 ( 0.1 4.7 ( 0.2 2.5 ( 0.2
0.071 ( 0.007 0.079 ( 0.005 0.079 ( 0.003 0.914 ( 0.022
