EVALUATION OF FE-55 AS A TRACER OF FE(III) REDUCTION IN AQUATIC SEDIMENTS

The potential utility of Fe-55 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 Fe-55(III) oxide, the accumulation of Fe-55 in Fe(II) over time paralleled Fe(II) production. However, within 1 h of addition of Fe-55(III) oxide, significant quantities of Fe-55 were recovered in extractable (1 M sodium acetate, pH 5) and soluble Fe(II) pools. There was proportionately more Fe-55(II) activity in sediments with higher Fe(II) concentrations. These results suggest that there was rapid isotopic exchange between Fe-55 in poorly crystalline Fe(III) oxide and the Fe(II) pools. Because of this exchange, Fe-55 could not be used to trace Fe(III) reduction in sediments in the manner that (SO42-)-S-35 is used to trace sulfate reduction. Although accumulation of Fe-55 in Fe(II) pools paralleled unlabeled Fe(II) production, given the relative ease with which Fe(II) pools in sediments can be measured there is no apparent advantage to using Fe-55 to follow Fe(III) reduction.