In situ technetium-99 ( 99Tc) immobilization by Fe(II) associated with clay minerals is a potential cost-effective method for Tc remediation at contaminated sites. The oxidation state of Fe is constantly cycled in sedimentary environments, however the effect of this cycle on Tc reduction and immobilization has not yet been investigated. The objective of this project was therefore to study how multiple cycles of reduction-reoxidation of Fe-rich clay mineral, nontronite, affected its reactivity toward Tc (VII) reduction. Iron-rich nontronite NAu-2 was used as a model clay mineral. NAu-2 suspension was first bioreduced by Shewanella putrefaciens CN32, which was subsequently re-oxidized by air. Three cycles of reduction-oxidation were conducted and bioreduced NAu-2 samples from all three cycles were collected and used for Tc(VII) reduction experiments. Each redox cycle resulted in a small fraction of dissolution of small size and/or poorly crystalline NAu-2. The released Fe(II) from the dissolution was likely adsorbed onto NAu-2 surface/edge sites with a high reactivity. Upon exposure to O 2, this reactive Fe(II) fraction was oxidized more rapidly than structural Fe(II) and may have accounted for a two-step reoxidation kinetics of NAu-2 associated Fe(II): rapid oxidation over first few hours followed by slow oxidation. Progressive increase of this reactive fraction of Fe(II), from increased dissolution, accounted for the successively higher rate of bioreduction and reoxidation with increased redox cycles. The same Fe redistribution accounted for two-step Tc(VII) reduction kinetics as well. Rapid Tc(VII) reduction in the first few hours may be attributed to a small fraction of highly reactive Fe(II) at the NAu-2 surface/edge sites, and more steady Tc(VII) reduction over longer time may be carried out by structural Fe(II). Similar to the increased rates of Fe(III) reduction and Fe(II) oxidation, the Tc(VII) reduction rate also increased with redox cycles and could be explained by progressive increase of the reactive Fe(II) on NAu-2 surface/edges. Iron-rich clay minerals undergo important changes after redox cycles, but eventually reach a steady state with continued reactivity toward heavy metals.
Bibliographical noteFunding Information:
The research was supported by grants from the Office of Science (BER), U.S. Department of Energy , Grant nos. DE-FG02-07ER64369 and DE-SC0005333 to HD and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education Science and Technology ( 20110013407 ) to JWK. We are grateful to Dr. John Morton from the Department of Geology of Miami University for the help on the DCP and XRD analyses. A portion of the research was performed using EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research located at Pacific Northwest National Laboratory. The authors are grateful to two anonymous reviews whose comments improved the quality of the manuscript.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology