Article pubs.acs.org/est
Hexavalent Chromium Reduction by Tartaric Acid and Isopropyl Alcohol in Mid-Atlantic Soils and the Role of Mn(III,IV)(hydr)oxides Dominic A. Brose* and Bruce R. James Department of Environmental Science and Technology, University of Maryland, College Park, Maryland 20742 ABSTRACT: Chromium is a naturally occurring transition metal and a soil contaminant in the Cr(VI) oxidation state, but reduction of Cr(VI) to Cr(III) mitigates its toxicity. Tartaric acid reduces Cr(VI) via a termolecular complex with isopropyl alcohol and Cr(VI), but its efficacy in soils has not been demonstrated. Five Mid-Atlantic soils from Maryland, U.S. were examined for their potential to enhance the reduction of Cr(VI). A control treatment (no soil +12 mM tartaric acid + 0.29 M isopropyl alcohol) reduced 0.37 mM Cr(VI) (19%) in 99 h. Reduction was enhanced to 1.97 mM (99%) with addition of a Russett Ap soil horizon (fine-loamy, mixed, semiactive, mesic Typic Hapludult). With a half-life of 18.7 h, the rate of reduction of Cr(VI) with the Russett soil sample was 20 times faster than with no soil (371 h). Soil Mn was solubilized in this reaction and plays a role in the enhanced reduction of Cr(VI). Mn(III/IV)(hydr)oxide-coated quartz sand reduced 1.24 mM (62%) Cr(VI), with all of the Mn(III,IV)(hydr)oxides solubilized. The addition of isopropyl alcohol and tartaric acid to soils enhances the reduction of Cr(VI), and this reduction is further enhanced by the catalytic behavior of Mn(II) from easily reducible Mn(III,IV)(hydr)oxides in soil.
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INTRODUCTION Remediation-by-reduction can be used to mitigate the toxic effects of Cr(VI) and to clean up Cr(VI)-contaminated sites to meet regulatory standards and protect human health. 1 Chromium is a naturally occurring transition metal, but the potential adverse health effects of Cr(VI) have led to concerns about contaminated soils, groundwater, and drinking water supplies. Soils contain electron-rich and electron-poor compounds that can reduce Cr(VI) and oxidize Cr(III), respectively. Such redox-active soil constituents may be from abiotic sources or metabolites from microbial processes. For example, quinone and phenol functional groups within humic and fulvic acids of soil organic matter,2−5 α-hydroxy carboxylic acids,6−8 Fe(II),9−11 and H2S and HS−12,13 reduce Cr(VI) to Cr(III). The rates and
extents of the reactions vary among the reducing agents and environmental conditions of soil-water systems. Of particular interest is the reduction of Cr(VI) by α-hydroxy carboxylic acids, such as lactic, tartaric, or citric acid. Although not strong reducing agents at environmentally relevant pHs (4−6), in the presence of isopropyl alcohol, the ability of these organic acids to reduce Cr(VI) is enhanced.14,15 The mechanism is by formation of a termolecular complex with isopropyl alcohol, tartaric acid, and Cr(VI)15−17 (Figure 1). An instantaneous three-electron transfer to Cr(VI) results in the oxidation of isopropyl alcohol (CH3CHOHCH3) and tartaric acid (COOH(CHOH)2COOH). The reduction of Cr(VI) via a single decarboxylation step results in the partial oxidation of tartaric acid as presented in eq 1:
Received: Revised: Accepted: Published:
Figure 1. The complex formed by tartaric acid, hexavalent chromium, and isopropyl alcohol. © 2013 American Chemical Society
12985
May 1, 2013 September 18, 2013 October 9, 2013 October 9, 2013 dx.doi.org/10.1021/es401903s | Environ. Sci. Technol. 2013, 47, 12985−12991
Environmental Science & Technology
Article
Table 1. Characterization Data for the Five Maryland Soilsa soil
textureb (g/kg sand, silt, clay)
field pHc
Eh (mV)d
organic C (g/kg)e
CBDf Fe (g/kg)
CBDf Mn (g/kg)
exch Crh (mg/kg)
Russett Atsion Collington Jackland Downer
sandy loam (58, 37, 5.8) sandy (94, 5.9, 0.1) loamy sand (83, 14, 3) silt loam (35, 57, 8) loamy sand (75, 21, 4)
5.0 3.5−4.0 4.0 6.0 5.5
577 524 606 470 490
25 ± 0.1 20 ± 0.4 37 ± 0.0 9.0 ± 0.1 3.1 ± 0.1
4.1 ± 0.3 0.1 ± 0.0 3.0 ± 0.4 6.3 ± 0.3 1.5 ± 0.4
0.3 ± 0.0
