Evaluation of55Fe as a tracer of Fe(III) reduction in aquatic sediments

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Abstract
The potential utility of 55Fe as a tracer for Fe(III) reduction in aquatic sediments was evaluated using freshwater sediments from the Potomac River, Md, in which Fe(III) reduction was the predominant terminal electron accepting process. In laboratory incubations of sediments amended with poorly crystalline 55Fe(III) oxide, the accumulation of 55Fe in Fe(II) over time paralleled Fe(II) production. However, within 1 h of addition of 55Fe(III) oxide, significant quantities of 55Fe were recovered in extractable (1 M sodium acetate, pH 5) and soluble Fe(II) pools. There was proportionately more 55Fe(II) activity in sediments with higher Fe(II) concentrations. These results suggest that there was rapid isotopic exchange between 55Fe in poorly crystalline Fe(III) oxide and the Fe(II) pools. Because of this exchange, 55Fe could not be used to trace Fe(III) reduction in sediments in the manner that 35SO4 2− is used to trace sulfate reduction. Although accumulation of 55Fe in Fe(II) pools paralleled unla‐beled Fe(II) production, given the relative ease with which Fe(II) pools in sediments can be measured there is no apparent advantage to using 55Fe to follow Fe(III) reduction.