Effects of Progressive Anoxia on the Solubility of Technetium in Sediments

Abstract

Technetium is a significant radioactive contaminant from nuclear fuel cycle operations. It is highly mobile in its oxic form (as Tc(VII)O4-) but is scavenged to sediments in its reduced forms (predominantly Tc(IV)). Here we examine the behavior of Tc at low concentrations and as microbial anoxia develops in sediment microcosms. A cascade of stable-element terminal-electron-accepting processes developed in microcosms due to indigenous microbial activity. TcO4- removal from solution occurred during active microbial Fe(III) reduction, which generated Fe(II) in the sediments and was complete before sulfate reduction began. Microbial community analysis revealed a similar and complex microbial population at all three sample sites. At the intermediate salinity site, PauII, a broad range of NO3-, Mn(IV), Fe(III), and SO4(2-) reducers were present in sediments including microbes with the potential to reduce Fe(III) to Fe(II), although no differences in the microbial population were discerned as anoxia developed. When sterilized sediments were incubated with pure cultures of NO3(-)-, Fe(III)-, and sulfate-reducing bacteria, TcO4- removal occurred during active Fe(III) reduction. X-ray absorption spectroscopy confirmed that TcO4- removal was due to reduction to hydrous Tc(IV)O2 in Fe(III)- and sulfate-reducing estuarine sediments.

Laboratory study of spectral induced polarization responses of magnetite — Fe2+ redox reactions in porous media

“…This is particularly relevant to some nuclear legacy sites such as areas of Hanford (Washington), Oak Ridge (Tennessee), and Rifle (Colorado) in the USA and Sellafield in the UK (Hunter, 2004;Catalano et al, 2006;Kelly et al, 2008;Williams et al, 2009), where invasive surveys may be very difficult due to radiotoxicity of contaminant plumes and the possibility of compromising the hydraulic integrity of the subsurface. The need for monitoring subsurface geochemistry such as redox conditions at contaminated sites has been recognized, because they play a vital role in controlling the mobility of key redox active contaminants such as Cr, U, Tc and As (Finneran et al, 2002;Islam et al, 2004;Burke et al, 2005;. Thus, remote sensing of redox indicators, such as the presence of Fe (II) in groundwater or the formation of sulfide minerals (Ntarlagiannis et al, , 2010aPersonna et al, 2008), would be a highly valuable complementary approach to conventional sampling and geochemical analysis of groundwater.…”

Developments in Bioremediation of Soils and Sediments Polluted with Metals and Radionuclides. 3. Influence of Chemical Speciation and Bioavailability on Contaminants Immobilization/Mobilization Bio-processes

“…Whether metal oxides retard, promote, or have no impact on the reduction of contaminants largely depends on their mineralogy and free energy, surface area and surface properties, and physical distribution within sediments and soils (Burke et al 2005).…”

Biogeochemical Reduction Processes in a Hyper-Alkaline Leachate Affected Soil Profile

“…Where sufficient organic matter is available for oxidation, progressively more anoxic conditions develop and a cascade of terminal-electron-accepting processes occur in sequence (but dependent on the availability of electron acceptors), with either increasing incubation time (Burke et al 2005), or depth within sediments (Froelich et al 1979). Microbial processes releasing most energy are favoured, so the sequence in which electron acceptors are used typically follows the decreasing order of redox potentials shown in Table 1 (calculated from standard thermodynamic data using the Nernst equation).…”