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  1. School of Earth and Environment FACULTY OF ENVIRONMENT Redox cycling of technetium in estuarine sediments. Ian Burke, Chris Boothman, Jon Lloyd, Francis Livens, John Charnock, Joyce McBeth, Rob Mortimer and Katherine Morris

  2. School of Earth and Environment FACULTY OF ENVIRONMENT Contents Why look at technetium behaviour in Humber Estuary Sediments Reduction microcosm experiments Fate of technetium in reduced sediments Reoxidation microcosm experiments

  3. School of Earth and Environment FACULTY OF ENVIRONMENT Technetium-99 Fission product produced in high yield in nuclear reactors Historically Tc(VII)O4–discharged to sea TcO4– highly mobile, but Tc found in sediments Switch in redox Tc(VII) oxic / Tc(IV) anoxic – mechanism ?

  4. School of Earth and Environment FACULTY OF ENVIRONMENT Technetium-99 – how does it stick? • Our approach sediment microcosms – redox cycling • Wanted fresh, 99Tc uncontaminated sediments • Delights of Humber Estuary…

  5. School of Earth and Environment FACULTY OF ENVIRONMENT Sediment microcosms – model approach Get oxic, surface mud (!) Into bottles + 99Tc Cap Let bugs do work…

  6. School of Earth and Environment FACULTY OF ENVIRONMENT Tc(VII) removal • + sterile; • live • Tc(VII) removal occurred; • After nitrate-reduction • After Mn(II)(aq) - ingrowth • Before Fe(II)(aq) - ingrowth • During Fe(II) (s) – ingrowth • Before SO42- - removal • Tc(VII) removal very rapidly in Fe(III)- and sulfate- reducing sediments Burke et al. (2005), Burke et al. (in prep)

  7. School of Earth and Environment FACULTY OF ENVIRONMENT Microbial community Humber Estuary sediments very diverse – bug heaven close matches to known nitrate-, metal- and sulfate-reducing bacteria no change during incubation – geochem brown  black; bugs same Burke et al. (2005)

  8. School of Earth and Environment FACULTY OF ENVIRONMENT Pure cultures + sediment • Where in cascade? • NO3--reducer NO3- reduction; no Tc removal • Fe(III)-reducer and sulfate-reducer were able to mediate Fe(III)-reduction • Fe(II) ingrows into solids • Technetium removed from solution Burke et al. (2005)

  9. School of Earth and Environment FACULTY OF ENVIRONMENT Fate and Mechanism? • 99Tc removal from solution during Fe(III)-reduction • Hypothesis – reductive precipitation on Fe(II) solids • 4H+ + 3Fe2+(s)+ TcO4- 2Fe3+ + 2H2O + TcO2 • 1 – 5 μM Tc(VII); 20 – 30 mM Fe(II) in microcosms • X-ray absorption Spectroscopy - mechanism • Fe-R sediment + 1000 μM Tc(VII) - ca 250 ppm Tc on sed (0.03 wt %) • Sulfate-reducing sediment + 1000 μM Tc(VII) – ca 250 ppm on sed • Oxic sediment + 1000 μM Tc(VII) – ca 250 ppm on sed • TEM (0.5 wt %) •99mTc gamma camera (10-12 M)

  10. School of Earth and Environment FACULTY OF ENVIRONMENT 99Tc in anoxic mud (0.03 wt %) EXAFS FT Biotic-R SO42--R Fe(III)-R TcO4- Burke et al. (2005), Morris et al. (in review)

  11. School of Earth and Environment FACULTY OF ENVIRONMENT Imaging TEM – EELS; Mapping (ca 0.5 wt % Tc) Tc Fe(II) Si Tc associated with Fe(II) in sediment 4H+ + 3Fe2+(s)+ TcO4- 2Fe3+ + 2H2O + TcO2 Burke et al. (in prep)

  12. School of Earth and Environment FACULTY OF ENVIRONMENT Imaging 99mTc (10-12 M) Fast reactions occurring in realsediments at < ppt Concentrations lower than predicted solubility of Tc(IV) Sorption important process Lear et al. (in prep), Burke et al. (in prep)

  13. School of Earth and Environment FACULTY OF ENVIRONMENT Redox Cycling – Reoxidation – Air and Nitrate Humber sediment microcosms + TcO4- XAS  99% removal as Tc(IV)O2 Fe-R and S-R sediments labelled with 99Tc Added air or nitrate

  14. School of Earth and Environment FACULTY OF ENVIRONMENT Air reoxidation results • Fe(III)-R and S-R reducing sediments: • Remobilisation of ~ 60 % Tc over 2 months • Remobilised as TcO4- (aq) (XAS) • Fe(II) reoxidised  Fe(III) • % Fe(II)  less than 10% in 60 days • S-R sediment – ingrowth of sulfate Burke et al., 2006

  15. School of Earth and Environment FACULTY OF ENVIRONMENT Nitrate reoxidation results • Fe(III)-R and S-R reducing sediments: • NO3- reoxidation of sediment biotic • Significant reoxidation of Fe(II) (40% or 100%) • Tiny reoxidation of Tc(IV) (< 10%) • Suggests Tc is recalcitrant to remobilisation with NO3- reoxidation even though xs Fe(II) (and in S-R sediment, sulfide) reoxidise Burke et al., 2006

  16. School of Earth and Environment FACULTY OF ENVIRONMENT Summary TcO4– removed from solution in; sediment experiments during microbial Fe(III)-reduction sediment experiments where Fe(II) is present Reduced Tc is found;as Hydrous Tc(IV)O2 associated with Fe(II) in sediments Upon Reoxidation; sediments are reoxidised by air and nitratesome Tc(IV) is reoxidised and remobilised as TcO4– air has much more effect on Tc solubility than nitrate

  17. School of Earth and Environment FACULTY OF ENVIRONMENT Publications Burke, I. T., Boothman, C., Lloyd, J. R., Mortimer, R. J. M., Livens, F. R. and Morris, K. (2005). Effects of progressive anoxia on the solubility of technetium in sediments. Environmental Science and Technology. 39, 4109-4116. Burke I. T., Boothman C., Lloyd J. R., Livens F. R., Charnock J. M., McBeth J. M., Mortimer R. J. M. and Morris K. (2006) Reoxidation behavior of technetium, iron and sulfur in estuarine sediments. Environmental Science and Technology, 40 3529-3535. Begg J. D. C., Burke I. T. and Morris K. (2007) The behaviour of technetium during microbial bioreduction in amended soils from Dounreay, UK. Science of the Total Environment. 373 pp 297-304. McBeth, J.M., Lear, G., Morris, K., Burke, I.T., Livens, F.R., Lloyd, J.R. (2007) Technetium reduction and reoxidation in aquifer sediments. (In press, Geomicrobiology Journal) Morris K., Livens F.R., Charnock J. M., Burke I. T., McBeth J. M., Begg J. D. C., Boothman C. and Lloyd J. R. The use of X-ray absorption spectroscopy (XAS) to investigate the fate of technetium in environmental media. (in review, Applied Geochemistry). Lear G., McBeth, J. M., Morris, K., Burke, I. T., Gunning, DJ., Ellis, B., Lawson, R., Livens F.R. and Lloyd J.R. Novel imaging techniques for technetium contaminated sediments. In preparation. Burke I. T., Livens F. R., Lloyd J. R., Brown, A., McBeth J. M., Ellis, B., Lawson R.S. and Morris K.Fate of technetium in reduced sediments: comparison of direct and indirect analysis. In preparation.