Radionuclide contaminated land scenarios at UK nuclear legacy sites

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Radionuclide contaminatedland scenarios at
UKnuclear legacy sites

• C.L. Thorpe
• G.L. Law, I.T. Burke, J.R. Lloyd and K. Morris

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The Project

• Project summary
• Model site
• Sellafield nuclear facility
• Cumbria, UK.
• Focus
• The treatment of co-contaminants strontium and
  technetium in a variety of conditions including
  high nitrate and low pH.
• Scenarios
• Bio-stimulation by electron donor addition in
  high nitrate/ low pH sediment
• Nano-scale/micro-scale Zero Valent Iron addition

Figure 1 www.visitcumbria.com
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Geochemical Conditions

• Nuclear facility conditions
• pH range (5.5-10)
• High nitrate (0-gt100 mM)
• Localised high bicarbonate concentrations
• Contamination of problematic radionuclides
• Technetium-99
• Strontium-90
• Uranium
• (Rifle, CO, US Sellafield, UK Naturita, CO, US
  ORFRC, US Hanford, US Dounreay, UK Drigg, UK)

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My Project Outline

• Aim to investigate the potential methods for
  remediation of strontium and technetium
  contamination in the far field at a range of pH
  and potentially high nitrate.
• The effect of nitrate as a co-contaminant on
  terminal electron accepting processes (TEAPs)
• The potential co-treatment of strontium and
  technetium with nano-scale zero valent iron
  (nZVI)
• The effect of nZVI on the microbial community and
  progression of TEAPs

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Nitrate Co-contamination

• Question What is the effect of high nitrate (100
  mM) on the progression of terminal electron
  accepting processes (TEAPs) in Sellafield
  sediments?
• Nitrate is present at nuclear facilities due to
  the use of nitric acid in the nuclear fuel cycle
• Nitrate can inhibit TEAPs as it competes with
  metals and radionuclides as an electron acceptor
  in anaerobic respiration.
• Previous studies show that metal/radionuclide
  reduction does not take place until nitrate
  reduction is complete

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Experimental Methods

• Monitoring progression of TEAPs
• Spectrophotometrically
• nitrite,
• Mn
• Fe(II)
• Fe(II)/Fe(III) in sediments (acid extraction)
• Ion Chromatography
• Nitrate
• Sulphate
• Probe
• Eh
• pH

Microcosms
Headspace gas Argon
Sellafield representative groundwater (Wilkins et
al. 2006) 100 ml
Sellafield sediment 10g
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Nitrate Scenarios
Microcosm work the effect of 100 mM nitrate on
terminal electron accepting processes (TEAPs) in
Sellafield Sediment at pH 5.5 and pH 7.
Microcosm conditions Nitrate concentrations Initial pH
1 0.3 mM nitrate 5.5
2 2 mM nitrate 5.5
3 10 mM nitrate 5.5
4 100 mM nitrate 5.5
5 0.3 mM nitrate 7
6 100 mM nitrate 7
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Results
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Results 10 mM nitrate, pH 5.5
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Results 100 mM nitrate, pH 7
Fe (II) sediment
pH
Nitrite (mM)
Eh
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Discussion pH 5.5 (0.3, 2, 10 and 100 mM
nitrate)

• Iron reduction did not commence until nitrate
  reduction had reached completion and all nitrite
  was reduced.
• Results at pH 5.5
• In microcosms containing 0.3, 2 and 10 mM nitrate
  TEAPs progressed to iron reduction within 25
  days. In microcosms containing 100 mM nitrate,
  nitrate reduction is ongoing at 80 days
• A rise in pH was observed in all microcosms, the
  extent of pH rise was dependent on initial
  nitrate concentration but 10 mM was sufficient
  for pH to rise from 5.5 to 7 during production of
  nitrite.

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Discussion pH 7 (0.3 mM and 100 mM nitrate)

• Iron reduction did not commence until nitrate
  reduction had reached completion and all nitrite
  was reduced
• Results at pH 7
• Microcosms containing 0.3 and 100 mM nitrate
  progressed to iron reduction within 15 and 60
  days respectively.
• In sediment containing 100mM nitrate pH rose to
  9.4 during nitrate reduction and a darkening of
  the pore water was observed thought to be
  leaching of organic matter.

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Microbial Analysis
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Future Questions
Further work- scenarios to treat strontium and
technetium co-contaminants Question What is
the optimum pH for Sr sorption to Sellafield
sediment? Question How does nZVI affect the pH
and Eh of a system when it is initially added and
over time? Question What effect does nZVI
addition have on iron mineral phases and on Sr
Sorption and mobility? Question What effect
does nZVI addition have on the microbial
community? Can hydrogen production act as an
electron donor?
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Rationale for nZVI Work

• Fe0 2H2O--- Fe2 H2 2OH-
• nZVI reduces everything indiscriminately so will
  affect nitrate, Mn and Fe species as well as
  redox sensitive radionuclides Tc and U.
• OH? produced during iron corrosion may raise the
  pH and aid bioreduction. Increased pH aids
  strontium sorption and co-precipitation with Ca
  minerals.
• Hydrogen production may stimulate the in situ
  microbial community and help maintain reducing
  conditions.

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Eventual Aim

• 24 month aim to design flow through columns
  suitable for use with Tc and Sr to demonstrate
  bioremediation scenarios
• Electron donor addition
• pH neutralisation
• Zero valent iron addition
• Combinations of the above

Magga et al. 2008- column to measure pesticide
contaminate spread
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