Geochemistry

Looking Beneath the Surface

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Science  08 Oct 2004:
Vol. 306, Issue 5694, pp. 201
DOI: 10.1126/science.306.5694.201b

Perhaps the most important and familiar environmental redox couple is that between ferrous and ferric iron. We see this reaction daily in common rust, but its influence goes well beyond: The reduction or oxidation of iron are often coupled to that of other environmental contaminants, affecting their speciation and thus their chemistry and mobility. It is now recognized that the reduction and oxidation of iron are influenced by the actions of bacteria as well as the local chemistry and nature of mineral surfaces in rocks and soils, and by the presence of water, so detailed investigations are needed at this scale to probe reaction pathways.

Williams and Scherer use Mossbauer spectrometry to identify absorbed ferrous iron species on the surfaces of several representative iron oxides. Their data show that a separate ferric iron layer, with the same structure as that of the underlying mineral, is generated by electron transfer through an aqueous layer. Reactions driven by this film require the presence of aqueous ferrous iron, which implies that these mineral surfaces are not merely passive supports. — BH

Environ. Sci. Tech. 10.1021/es049373g (2004).

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