Monitoring Tc Dynamics in a Bioreduced Sediment: An Investigation with Gamma Camera Imaging of ^99mTc-Pertechnetate and ^99mTc-DTPA

Abstract

We demonstrate the utility of nuclear medical imaging technologies and a readily available radiotracer, [^99mTc]­TcO₄^–, for the noninvasive monitoring of Fe­(II) production in acetate-stimulated sediments from Old Rifle, CO, USA. Microcosms consisting of sediment in artificial groundwater media amended with acetate were probed by repeated injection of radiotracer over three weeks. Gamma camera imaging was used to noninvasively quantify the rate and extent of [^99mTc]­TcO₄^– partitioning from solution to sediment. Aqueous Fe­(II) and sediment-associated Fe­(II) were also measured and correlated with the observed tracer behavior. For each injection of tracer, curves of ^99mTc concentration in solution vs time were fitted to an analytic function that accounts for both the observed rate of sedimentation as well as the rate of ^99mTc association with the sediment. The rate and extent of ^99mTc association with the biostimulated sediment correlated well with the production of Fe­(II), and a mechanism of [^99mTc]­TcO₄^– reduction via reaction with surface-bound Fe­(II) to form an immobile Tc­(IV) species was inferred. After three weeks of bioreduction, a subset of microcosms was aerated in order to reoxidize the Fe­(II) to Fe­(III), which also destroyed the affinity of the [^99mTc]­TcO₄^– for the sediments. However, within 3 days postoxidation, the rate of Tc­(VII) reduction was faster than immediately before oxidation implying a rapid return to more extensive bioreduction. Furthermore, aeration soon after a tracer injection showed that sediment-bound Tc­(IV) is rapidly resolubilized to Tc­(VII). In contrast to the [^99mTc]­TcO₄^–, a second commercially available tracer, ^99mTc-DTPA (diethylenetriaminepentaacetic acid), had minimal association with sediment in both controls and biostimulated sediments. These experiments show the promise of [^99mTc]­TcO₄^– and ^99mTc-DTPA as noninvasive imaging probes for a redox-sensitive radiotracer and a conservative flow tracer, respectively