Imaging of Bioreduced Sediment: Tc(IV/VII) and Fe(II/III) redox kinetics and DTPA as a conservative tracer

Microbially mediated reduction of sediment bound iron from Fe(III) to Fe(II) following electron donor augmentation holds great promise for the sequestration of environmental contaminants including technetium, uranium, and chromium. We have used gamma camera imaging to measure soluble 99mTc concentrations in microcosm experiments; tracking the rate of reduction of 99mTc(IV)-pertechnetate, evaluating 99mTc-DTPA as a conservative hydrologic flow tracer, and examining the dynamics of redox cycling of both Fe(II)/Fe(III) and Tc(IV)/Tc(VII) in sediment from the DOE-IRFC site at Rifle, CO. Eleven experiments were performed on 4 water/sediment microcosms over 4 weeks, before and after microcosms were augmented with 6mM acetate to stimulate microbially mediated reduction of Fe(III) to Fe(II). A summary of the experiments' data is given in the Figure 1. Data analysis methods of fitting concentration curves of 99mTc(VII) in solution to a dual exponential function gives a quantitative index Tc(VII) to Tc(IV) reduction rate in the presence of Fe(II). Following acetate amendment, the rate of Tc(VII) reduction progressively increased with time. A reoxidation experiment showed that oxygenating the microcosms oxidized Fe(II) to Fe(III), subsequently preventing Tc(VII) reduction, but with a return to anoxic conditions, bioreduction of Fe(III) occurred faster than before oxygenation, an observation that is particularly relevant to field-scale work. Another reoxygenation experiment showed that when in reducing conditions, sediment-bound Tc(IV) is rapidly resolublized to Tc(VII) with an increase in dissolved oxygen. 99mTc-DTPA proves to be a conservative tracer, showing little interaction with sediment in both Fe(III)-reducing and non-reducing conditions. These experiments show the utility of nuclear medicine tools and readily available radiotracers for non-invasive monitoring of sediment geochemistry and hydrologic properties during reactive transport.