Abstract
AbstractIodine-129 is a high-yield fission product formed in nuclear reactors and is a risk-driving radionuclide in both contaminated land and radioactive waste disposal due to its high mobility and long half-life. Here, the bioreduction behaviour of iodate was investigated by tracking iodine speciation and concentration in solution during the development of progressive anoxia in sediment microcosm experiments incubated at neutral pH. Experiments with acetate added as an electron donor showed the expected cascade of terminal electron-accepting processes. Analysis of solution chemistry showed reduction of iodate to iodide during the early stages of metal (Mn(IV) and Fe(III)) reduction, but with no significant retention of iodine species on solids. There was, however, a net release of natural iodine associated with the sediments to solution when robust iron reduction / sulfate reduction had developed. In addition, over 210 days, the controls with no electron donor and the sterile controls showed no Mn(IV) or Fe(III) reduction but displayed modest sorption of iodate to the sediments in the absence of bioreduction. Overall these results show that under oxic conditions iodate may be partially sorbed to sediments over extended periods but that development of mildly reducing conditions leads to the reductive release of iodine to solution as iodide.
Highlights
IODINE-129 is of regulatory concern in radioactive waste disposal due to its long half-life (t 1⁄2 15.7 × 106 y), potential for bioaccumulation and its high environmental mobility as the iodide (I–) species (Fuge and Johnson, 1986)
The results of this study shows that in sterile and no-electron-donor control experiments, iodate can be sorbed to sediments over several months
Development of early metal-reducing conditions leads to complete reduction of iodate to iodide: all iodide that forms is released into solution
Summary
IODINE-129 is of regulatory concern in radioactive waste disposal due to its long half-life (t 1⁄2 15.7 × 106 y), potential for bioaccumulation and its high environmental mobility as the iodide (I–) species (Fuge and Johnson, 1986). In situ biostimulation through the addition of electron donors to sediments to promote the development of metal-reducing conditions has been proposed as a strategy for managing Tc and other redox-active radionuclides present as contaminants in groundwater (Lloyd and Renshaw, 2005; Newsome et al, 2014a). Such biostimulation regimes develop reducing conditions, under which the solubility of certain redox-active radionuclides including technetium and uranium is reduced It is possible, that biostimulation aimed at immobilizing Tc(VII) and U(VI) may increase the solubility of iodine if it is present in sediments by promoting iodide formation. Acetate was used as an electron donor to stimulate microbial redox processes, and the impact of elevated nitrate concentrations on iodine behaviour during bioreduction was tested
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