Monitoring Tc Dynamics in a Bioreduced Sediment: An Investigation with Gamma Camera Imaging of 99m Tc-Pertechnetate and 99m Tc- DTPA Nicholas T. Vandehey,* James P. O’Neil, Aaron J. Slowey, Rostyslav Boutchko, Jennifer L. Druhan, William W. Moses, and Peter S. Nico Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California, USA * S Supporting Information ABSTRACT: 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. ■ INTRODUCTION The use of medical radiotracer imaging techniques for studying bioremediation of environmental toxins is gaining interest bridging the fields of environmental research and medical imaging. 1-3 Radiotracers have been used in environmental sciences for many years over a wide range of applications due to their ability to probe specific chemical processes. 4,5 The recent use of nuclear medical imaging tools (i.e., gamma camera, single photon emission computed tomography [SPECT], or positron emission tomography [PET]) is driven by their ability to dynamically measure the 3D distribution of radiotracers inside sediment systems, with better than 1 cm resolution and picomolar sensitivity. These tools hold the potential for noninvasive, real-time monitoring of both physical and chemical properties of porous sediment, which is important in understanding the feedback pathways that couple chemically and microbially induced physical changes in pore structure and flow paths within porous media, 6 a key question for many areas of subsurface sciences. Specifically, the ability to simultaneously measure hydrological properties using a conservative tracer while measuring the location and extent of reducing microenviron- ments using a redox-sensitive radiotracer represents a significant new tool for understanding of the relationship between chemical transformations and the associated porosity and permeability evolution during reactive transport. For example, the production of Fe(II) through microbial reduction of iron oxides for bioremediation of metal contaminants 7,8 can also lead to substantial changes in flow field permeability due to biomass growth, production of methane, and precipitation of iron, carbonates, and sulfides. 6 The relative contribution of Fe(II) to this flow field evolution and the coupled influence of permeability change on iron reduction in the system has been indirectly inferred through electrodic potentials 9 but has never been directly quantified. With the goal of developing methods for Fe(II) detection and evaluation of a potential conservative radiotracer, we used Received: August 25, 2011 Revised: October 9, 2012 Accepted: October 19, 2012 Published: October 19, 2012 Article pubs.acs.org/est © 2012 American Chemical Society 12583 dx.doi.org/10.1021/es302313h | Environ. Sci. Technol. 2012, 46, 12583-12590