Aaron J. Slowey

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

We demonstrate the utility of nuclear medical imaging technologies and a readily available radiotracer, [(99m)Tc]TcO(4)(-), 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 [(99m)Tc]TcO(4)(-) 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 (99m)Tc 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 (99m)Tc association with the sediment. The rate and extent of (99m)Tc association with the biostimulated sediment correlated well with the production of Fe(II), and a mechanism of [(99m)Tc]TcO(4)(-) 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 [(99m)Tc]TcO(4)(-) 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 [(99m)Tc]TcO(4)(-), a second commercially available tracer, (99m)Tc-DTPA (diethylenetriaminepentaacetic acid), had minimal association with sediment in both controls and biostimulated sediments. These experiments show the promise of [(99m)Tc]TcO(4)(-) and (99m)Tc-DTPA as noninvasive imaging probes for a redox-sensitive radiotracer and a conservative flow tracer, respectively.

Performance Evaluation of SPECT Imaging System for Sediment Column Imaging

The performance of a SPECT imaging system for imaging vertically oriented glass columns (10 cm diameter x 30 cm long) has been evaluated in terms of spatial resolution, image uniformity, and linearity. This imaging system was developed to support research in environmental geosciences by determining flow patterns in heterogeneous porous media (e.g., sediment) through simultaneous monitoring of sediment chemistry and flow parameters. Using a Derenzo-like hot-rod phantom, rods down to 7.9 mm diameter were resolved. Flood images had an integral uniformity of 8.5% and differential uniformity of 6.2% in the center of the field of view. The ratio of total detected counts to the mean image reconstructed value was within 2% of linear over a range of greater than two orders of magnitude of counts.

Chemical stability of (99m)Tc-DTPA under aerobic and microbially mediated Fe(III)-reducing conditions in porous media.

(99m)Tc-DTPA has been used as a conservative tracer to quantify water transport through porous media. However, more information on the reactivity of this (99m)Tc compound under varying geochemical conditions is desirable to better understand its potential uses. We measured the speciation of Tc following amendment of (99m)Tc-DTPA to batch systems spanning a range of controlled biogeochemical conditions. Our results suggest that (99m)Tc-DTPA is stable under the reducing conditions tested. However, freshly precipitated Al-ferrihydrite may displace Tc(IV) from DTPA in the absence of Fe(III)-reducing conditions.