Fate of technetium in reduced sediments

Technetium -99 is an important fission product in radioactive wastes. As Tc(VII)O4-, Tc is highly mobile in oxic environments but, under reducing conditions, Tc becomes strongly associated with sediments as hydrous Tc(IV)O2. In order to examine further the behaviour of Tc over a range of concentrations in estuarine sediments, anoxic incubation experiments were combined with a range of both direct (XAS, transmission electron microscopy and gamma camera imaging) and indirect (incubation experiments and chemical extractions) experimental techniques. When TcO4- was incubated in progressively anoxic sediment microcosms at 10-6 M concentrations, typically >99 % TcO4- was removed from solution over the course of 36 days in systems undergoing microbial Fe(III)-reduction. By contrast, when spiked into pre-reduced Fe(III)- and sulfate- reducing estuarine sediments, >99 % TcO4- was removed from solution in under 10 minutes in both microbially active and heat sterilised systems. Sequential extraction techniques showed that 70 ± 3 % of Tc bound to sediments was remobilised when sediments were exposed to the first strong chemical oxidant (H2O2) in the extraction scheme. At higher Tc concentrations (0.05 - 0.5 wt % on solids) X-ray absorption spectroscopy and scanning transmission electron microscopy, combined with energy dispersive X-ray mapping, were used to examine the speciation and associations of Tc in sediments. X-ray absorption spectroscopy showed that Tc was reductively precipitated in all microcosms with modeling of EXAFS spectra suggesting hydrous Tc(IV)O2 phases were formed in all reduction experiments but with subtly different local environments in different systems. Electron microscopy showed that Tc was localised and associated with nano-metre size Fe(II)-rich particles, suggesting that the removal of Tc may be controlled by reduction of Tc(VII) to Tc(IV) by biogenic Fe(II) in sediments. In addition, gamma camera imaging, using the gamma emitting 99mTcO4- (half-life 6.01 hours) and requiring only 10-12 M Tc concentrations, was used to visualise the interaction of Tc in sediments at very low concentrations which are relevant to far field contamination scenarios. Here, over the course of 24 hours the scavenging of Tc to sulfate-reducing sediments was observed. Interestingly, as the Tc concentrations used in the 99mTc experiments were below the solubility limits for hydrous Tc(IV)O2 (ca 10-9 mol l-1 at pH 7-9), sorption of Tc(IV) species is likely to be a significant control on Tc behaviour in these sediments. Overall, the results of this study show that multiple approaches help in understanding Tc speciation in complex heterogeneous sediments over a wide range of concentrations.