Reduction of organic contaminants by ferrous iron species in clay minerals
Thomas B. Hofstetter*, Anke Neumann, Marita Skarpeli-Liati, Maja Lüssi, and René P. Schwarzenbach
Institute of Biogeochemistry and Pollutant Dynamics (IBP), ETH Zurich, Switzerland, *thomas.hofstetter@env.ethz.ch
Reductive transformations involving clay mineral bound Fe2+ species are of great relevance for the fate of organic soil and groundwater contaminants. Clay minerals are ubiquitously present in soils and sediments and most of them contain some Fe, which is available for microbial reduction. Thus, clay-bound Fe species are important mediators for the transfer of electrons from the oxidation of organic material by Fe(III) reducing microorganism to organic contaminants. Because Fe2+ species responsible for contaminant reduction are predominantly bound within the mineral structure, Fe in clay minerals is less prone to reductive dissolution than Fe oxides. Thus, clay mineral bound Fe2+ represents a renewable source of reduction equivalents for contaminant degradation.
While the transformation of a variety of priority contaminants such as pesticides, solvents, and explosives by clay mineral bound Fe2+ has been demonstrated, little is known about the mineralogical factors determining Fe2+-reactivity. Using structure reactivity relationships of nitroaromatic and polychlorinated contaminants we investigated the influence of total iron content, Fe2+/Fe3+-ratio, Fe-clustering, and location of excess charge of three different smectites on the reactivity of structural Fe2+. We found that contaminant reduction in the presence of chemically reduced smectites containing little structural Fe (<5% wt) is generally slow. In Fe-rich smectites, however, neighboring Fe2+ or Fe3+ atoms give rise to octahedral Fe2+-species of very different reactivity. Our kinetic studies not only reveal the presence of these Fe2+-O-Fe2+- or Fe2+-O-Fe3+-entities but also suggest dynamic equilibrium processes for their regeneration. As a consequence, rates organic contaminant reduction are determined by both, contaminant interactions with clay mineral surfaces and the dynamics of Fe2+-distribution and regeneration within the mineral structure.
