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Coupling of biogeochemical Fe(III) mineral reduction and pollutant transformation in anoxic environments

English title Coupling of biogeochemical Fe(III) mineral reduction and pollutant transformation in anoxic environments
Applicant Schwarzenbach René P.
Number 113871
Funding scheme Project funding
Research institution Institut für Gewässerschutz und Wassertechnologie ETH-Zentrum
Institution of higher education Swiss Federal Institute of Aquatic Science and Technology - EAWAG
Main discipline Other disciplines of Environmental Sciences
Start/End 01.10.2006 - 31.01.2007
Approved amount 18'048.00
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Keywords (14)

iron; biogeochemical cycles; redox reactions; microbial iron reduction; iron oxides; groundwater contamination; clay minerals; kinetic isotope ef-fects; isotope fractionation; dissimilatory iron reduction; microorganisms; groundwater contaminant toluene; nitroaromatic compound; isotope effect

Lay Summary (English)

Lead
Lay summary
In anoxic soils and aquifers, oxidation of organic contaminants such as fuel components by dissimilatory Fe(III) reducing microorganisms can generate mineral-bound Fe(II) species that reduce persistent compounds such as nitroaromatic pesticides and explosives or chlorinated solvents in an abiotic reaction. Using toluene and nitroaromatic compounds as model con-taminants, our preliminary results point out that the concurrent oxidative and reductive con-taminant transformation is feasible if Fe(III) oxides exhibiting different bioavailability and different reactivity - after Fe(II) adsorption - are present simultaneously. At contaminated sites, the quantitative assessment of oxidative and reductive pathways is of great interest to evaluate the efficiency of this potential remediation approach. However, an identification of the coupled processes exclusively based on species concentration measurements is difficult since various competing reactive and non-reactive processes influence the disappearance of contaminants.
In the proposed research we intend to use compound-specific isotope analysis to derive kinetic isotope effects (KIEs) and their variability pertinent to the transformation of organic contaminants in the coupled process. KIEs of carbon and nitrogen will be derived for oxida-tion of toluene by dissimilatory Fe(III) reducing microorganism Geobacter metallireducens as well as for the reduction of nitroaromatic compounds and chlorinated methanes by biogenic, mineral-bound Fe(II) in laboratory model systems. Since KIEs can be used for a qualitative identification of reaction pathways as well as for the quantification of the extent of transfor-mation, the principal goals of this project are (1) the determination of the kinetic isotope effects and their variability for the coupled oxidative and reductive contaminant transformation under anoxic conditions, and (2) an assessment of the efficiency of Fe cycling at mineral sur-faces and electron transfer between different contaminants based on the application of the ob-tained KIEs.
Direct link to Lay Summary Last update: 21.02.2013

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Associated projects

Number Title Start Funding scheme
101635 Coupling of biogeochemical Fe(III) mineral reduction and pollutant transformation in anoxic environments 01.10.2003 Project funding
116447 Using nitrogen isotope fractionation to assess redox reactions of organic contaminants 01.03.2008 Project funding

Abstract

In anoxic soils and aquifers, oxidation of organic contaminants such as fuel components by dissimilatory Fe(III) reducing microorganisms can generate mineral-bound Fe(II) species that reduce persistent compounds such as nitroaromatic pesticides and explosives or chlorinated solvents in an abiotic reaction. Using toluene and nitroaromatic compounds (NACs) as model contaminants, our results show that the concurrent oxidative and reductive contami-nant transformation is feasible if Fe(III) oxides exhibiting different bioavailability and differ-ent reactivity - after Fe(II) adsorption - are present simultaneously.

At contaminated sites, the quantitative assessment of oxidative and reductive pathways is of great interest to evaluate the efficiency of this potential remediation approach. However, an identification of the coupled processes exclusively based on species concentration measure-ments is difficult due to the inherent heterogeneity of aquifers and the limited availability of sampling points and since various competing reactive and non-reactive processes influence the disappearance of contaminants. Therefore, we evaluated the application of compound-specific isotope analysis (CSIA) for the qualitative and quantitative assessment of iron-mediated oxidative and reductive contaminant transformation. Owing to a normal kinetic isotope effect, isotopic fractionation leads to an enrichment of heavy isotopes in the substrate during a (bio)chemical reaction, which can be measured by CSIA. Isotope fractionation is characteristic for the underlying chemical reaction mechanism and allows one to assess the extent of contaminant transformation even in complex environments. For the purpose of process identification by CSIA, we derived isotopic enrichment factors and the correspond-ing apparent kinetic isotope effects (AKIEs) for microbial toluene oxidation by dissimilatory Fe(III) reducing bacteria and abiotic transformation of NACs by mineral-bound Fe(II) spe-cies. We found significant and very robust isotope effects for toluene oxidation (AKIE for C 1.007) and reduction of nitroaromatic compounds (AKIE for N 1.041), which strongly pro-mote the use of CSIA for the identification and quantification of iron-mediated oxidative and reductive pollutant transformation.
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