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 aq uatic sediments was evaluated using freshwater sediments from the Poto mac River, Md, in which Fe(III) reduction was the predominant terminal electron accepting process. In laboratory incubations of sediments am ended 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 w ere recovered in extractable (1 M sodium acetate, pH 5) and soluble Fe (II) pools. There was proportionately more Fe-55(II) activity in sedim ents with higher Fe(II) concentrations. These results suggest that the re 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 tha t (SO42-)-S-35 is used to trace sulfate reduction. Although accumulati on of Fe-55 in Fe(II) pools paralleled unlabeled Fe(II) production, gi ven the relative ease with which Fe(II) pools in sediments can be meas ured there is no apparent advantage to using Fe-55 to follow Fe(III) r eduction.