Assessing abiotic reduction of nitroaromatic groundwater contaminants in using compound-specific stable isotope analysis

ENVR 141

Thomas B. Hofstetter, thomas.hofstetter@env.ethz.ch1, Akané E. Hartenbach1, Anke Neumann1, Michael Sander1, Michael Berg, michael.berg@eawag.ch2, William A. Arnold, arnol032@tc.umn.edu3, Christopher J. Cramer, cramer@pollux.chem.umn.edu4, Timothy J. Strathmann, strthmnn@uiuc.edu5, and René P. Schwarzenbach1. (1) Department of Environmental Sciences, Institute of Biogeochemistry and Pollutant Dynamics (IBP), Universitatsstr. 16, ETH Zurich, Zurich, CH-8092, Switzerland, (2) Department for Water Resources and Drinking Water, Eawag, Swiss Federal Institute of Aquatic Science and Technology, Ueberlandstrasse 133, CH-8600 Dubendorf, Switzerland, (3) Department of Civil Engineering, University of Minnesota, 500 Pillsbury Dr. SE, Minneapolis, MN 55455, (4) Department of Chemistry and Supercomputing Institute, University of Minnesota, 207 Pleasant St. SE, Minneapolis, MN 55455-0431, (5) Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, 205 North Mathews Avenue, Urbana, IL 61801
In anoxic subsurface environments, biogeochemical processes provide a variety of abiotic reductants such as iron oxide- or clay mineral-bound Fe(II)-species, Fe(II)-organic material complexes, as well as reduced, dissolved organic material for transformation of various classes of organic contaminants including nitroaromatic compounds (NACs). Depending on the number and position of nitro groups and other aromatic substituents as well as on the prevailing environmental conditions, abiotic transformation of NACs to aromatic amines can occur at very different rates and pathways might be difficult to identify due to competing biodegradation reactions. In addition, quantitative assessment of NAC degradation can be compromised by simultaneous (ad)sorption and dispersion processes that also lead to a decrease of contaminant concentration. Compound-specific stable isotope analysis (CSIA) can provide quantitative estimates of NAC transformation regardless of contaminant concentration trends on the basis of measured shifts in stable nitrogen isotope signatures and established isotopic enrichment factors for the elements in the affected bonds.

We found that abiotic reduction of NACs with mineral-bound Fe(II) species is accompanied by a substantial 15N-fractionation owing to a kinetic isotope effect (KIEN) for N-O bond cleavage. The corresponding 15N-enrichment factors hardly show substituent effects and are, thus, independent of reduction rates. Almost identical observations were made for NAC reduction by anthrahydroquinone-disulfonate (AH2QDS) and Fe(II)-catechol complexes, both used as surrogates for dissolved reductants in groundwaters. Our results suggest that δ15N-signatures can be used to estimate NAC degradation via abiotic reduction under anoxic conditions using an average KIEN of 1.037±0.004. For very fast reactions, for example with di- and trinitrotoluenes, however, observed 15N-fractionation was masked by commitment to catalysis affecting the electron transfer reactions preceding the isotopic H2O elimination from substituted N,N-dihydroxy anilines. This mechanistic interpretation of NAC reduction is consistent with density-functional theory (DFT) calculations of transition state structures and intrinsic KIEN for N-O bond cleavage.

 

Advances in Abiotic Transformation Processes for Micropollutants in Drinking Water and for Sourcewater Protection
1:30 PM-5:10 PM, Tuesday, April 8, 2008 Morial Convention Center -- Rm. 237, Oral

Division of Environmental Chemistry

The 235th ACS National Meeting, New Orleans, LA, April 6-10, 2008