Evaluation of 55Fe as a tracer of Fe(III) reduction in aquatic sediments

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.