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Tc(IV) [6]. Hydrogen is an excellent electron donor
for the microbial reduction of high oxidation state
redox active actinides and fission products.
Aims
1) To quantify hydrogen production from Fe(0) and
hydrogen utilization by microorganisms;
2) To quantify the effects of hydrogen utilization on
microbially-mediated Terminal Electron Accepting
Processes (TEAPs);
3) To assess the effect of hydrogen-driven microbial
processes on the speciation and solubility of priority
radionuclides, and hence to what extent the mobility
of these radionuclides will be retarded within a GDF
and the geosphere.
Methods
Microcosm experiments contained alluvial flood plain
deposits from a site ~2km north north east of the
Sellafield reprocessing facility, England.
Microcosms also contained a synthetic groundwater
analogue and nanoparticulate zero valent Fe (Fe(0)).
No Fe(0) and sterilised (autoclaved) treatments were
included to assess the effects of Fe(0) addition and
the indigenous microbial consortia respectively. Two
sets of microcosms were set up under anaerobic
conditions to replicate GDF conditions post-closure;
geochemical microcosms in triplicate (Figure 3A) and
pressure microcosms, one for each treatment (Figure
3B). An aliquot of the sediment/groundwater slurry
from geochemical microcosms was analysed for
Eh/pH and a separate aliquot was analysed for Fe, S,
Mn and Ca via ICP-AES and nitrate (NO3
-
), sulfate
(SO4
2-
) and sulfide (S2-
) via ion chromatography.
Figure 3 – Microcosm experiment set up
showing geochemical microcosms (top) and
pressure monitoring microcosms using pressure
transducers (bottom).
Results
A reduction in pressure in the added Fe(0) live
microcosm relative to the added Fe(0) sterile
microcosm suggests that hydrogen was consumed by
microorganisms (Figure 4). The lack of hydrogen
production between 0 and 7 days in the red
treatment is likely due to a large volume of hydrogen
de-gassing immediately after this microcosm was
autoclaved. The maximum pressure of the transducer
fitted to the red microcosm was exceeded on Day 20
and recording stopped at this point. Pressure declined
slightly in the no Fe(0) live microcosm (green) from 0
to -10hPa as oxygen was consumed by
microorganisms.
The indigenous microorganisms within the Sellafield
sediment are capable of reducing a number of
Terminal Electron Acceptors (TEAs) including
manganese(IV) (Mn(IV)) (Figure 5). Mn(II) is more
soluble in solution than Mn(IV), hence the total
amount of Mn in solution increases as Mn(IV) is
reduced. The electron donor in these systems is
organic carbon in the sediment (~0.43% by mass),
hydrogen evolved from Fe(0) oxidation or the Fe(0)
itself. Mn(IV) reduction is one of the more
energetically favourable anaerobic reduction reactions
(∆G0
= -349kj mol-1
of sucrose where sucrose is the
electron donor).
Introduction
Over >70 years the UK has built up a substantial
(~370x103
m3
) and complex legacy of Higher Activity
Wastes (HAW) [1]. The UK Government proposes to
dispose of HAW in a Geological Disposal Facility (GDF,
Figure 1) [1].
Figure 1 – A proposed Geological Disposal Facility
(GDF). Source: Modified from [2].
A large volume of cement will be used in a GDF, e.g.
as vault backfill [3] and grout encapsulants for
Intermediate Level Waste (ILW) (Figure 2) [4]. Iron
(Fe) will also be present in the wasteform as metals
resulting from decommissioned reactor components, as
concrete cladding rods and rock bolts used to construct
the GDF, as well as in the form of steel ILW containers
themselves [4]. After closure of a GDF; the presence of
this cement and the exclusion of oxygen will create an
extreme environment in terms of pH and redox
potential, an ecological niche favourable towards
alkaliphilic and anaerobic microorganisms.
Fe present in a GDF will corrode in the following half
reactions:
Reaction 1 (aerobic)
Fe(s) + H2O(l) + ½O2(g) Fe→ 2+
(s) + 2OH-
(aq)
Reaction 2 (anaerobic)
Fe(s) + 2H2O(l) Fe→ 2+
(s) + 2OH-
+ H2(g)
The presence of hydrogen within a GDF will further
promote an ecological niche in which hydrogen-utilising
microorganisms are likely to thrive.
Hydrogen can be generated through the anaerobic Fe
corrosion reaction (Reaction 2), or through the
radiolysis of water [2]. The generation of hydrogen
within a GDF after closure could result in over-
pressurisation of a GDF. A bulk gas phase could move
from a GDF, potentially carrying radiotoxic gases (e.g.
14
C-bearing gases) as it migrates [2].
The host geology within which a GDF will be sited will
play a major part in dictating rates of hydrogen
generation and the effects of gas generation. Lower
permeability sedimentary rocks such as clays will limit
water ingress, thus retarding Fe oxidation (Reactions
1&2) [2]. Evaporites likewise could play host to an
extremely dry, corrosion-inhibiting environment.
However; both clays and evaporites could retard gas
egress, thus leading to a risk of over-pressurisation
within a GDF sited in such geologies [2]. Conversely; a
fractured higher strength rock, such as granite, would
permit more rapid groundwater ingress, potentially
promoting Reactions 1&2, and would also allow a bulk
gas phase to migrate [2].
Long-lived 'Priority radionuclides' in the radwaste
inventory are often more soluble in high oxidation
states e.g. U(VI), Np(V) and Tc(VII) [5, 6]. However, a
number of anaerobic microorganisms are capable of
reducing and precipitating radionuclides either via
direct enzymatic transformations or indirectly via
biogenic Fe(II) (in the case of Fe(III)-reducing
bacteria). These processes ‘Reductively immobilise’
radionuclides e.g. as poorly soluble U(IV), Np(IV) or
Figure 4 – Over 28 days hydrogen was
consumed by microorganisms in the Fe(0) live
microcosm (blue) relative to the sterile
microcosm (red). At 20 days the added Fe(0)
live microcosm had ~46% less H2(g) evolved in
the headspace than the sterile (autoclaved)
microcosm.
Figure 5 – Mn(IV) reduction occurs in all
treatments except the no Fe(0) sterile
treatment (purple). Mn(IV) reduction is
biotically enhanced as demonstrated by the
excess of Mn(II) in solution in the live
treatments (blue and green) relative to the
sterile treatments (red and purple). Data-points
are a mean of triplicate microcosms and error
bars are 1σ.
Conclusions
Future Work
Future work will involve method development for the
quantitative analysis of hydrogen gas in microcosm
head spaces. Further analogue materials relevant to
geodisposal will be used in a broad range of
microcosm experiments representative of conditions
within a GDF at various stages of its life-cycle.
Finally; work with priority radionuclides [1, 3] in
these complex systems will be examined.
Acknowledgements
We acknowledge funding from NERC and the
Radioactive Waste Management Directorate (RWMD) of
the NDA. The help and support of Dr Steve Boult, Stuart
Rae, Paul Lythgoe, Alastair Bewsher and the
Geomicrobiology Group, particularly Chris Boothman
and Adam Williamson, is also gratefully acknowledged.
References
[1] DEFRA. (2008). Managing Radioactive Waste Safely:
A Framework for Implementing Geological Disposal.
Norwich: TSO.
[2] NDA. (2010). Geological Disposal Gas Status Report.
Nuclear Decommissioning Authority.
[3] Morris, K., Law, G. T. W., & Bryan, N. D. (Eds.).
(2011). Geodisposal of Higher Activity Wastes (Vol. 32).
Cambridge: Royal Society of Chemistry.
[4] Crossland, I. G., & Vines, S. P. (2001). Why a
cementitious repository? United Kingdom Nirex Ltd.
[5] Renshaw, J. C., Handley-Sidhu, S., & Brookshaw, D.
R. (Eds.). (2011). Pathways of Radioactive Substances
in the Environment (Vol. 32). Cambridge: Royal Society
of Chemistry.
[6] Lloyd, J. R., Chesnes, J., Glasauer, S., Bunker, D. J.,
Livens, F. R., & Lovley, D. R. (2002). Reduction of
actinides and fission products by Fe(III)-reducing
bacteria. Geomicrobiology Journal, 19(1), 103-120.
The Hydrogen Driven Geomicrobiology of Cementitious Nuclear Waste
Michael J.C. Crouch1
, Katherine Morris1
, Dirk Engelberg2
, Joe Small3
, Jonathan R. Lloyd1
.
1
Williamson Centre for Molecular Environmental Science and Research Centre for Radwaste & Decommissioning, School of Earth, Atmospheric and
Environmental Sciences. The University of Manchester.
2
Materials Science Centre and Research Centre for Radwaste & Decommissioning, School of Materials. The University of Manchester.
3
UK National Nuclear Laboratory, Risley, Warrington.
• Pressure was 46% less in the live microcosm than
the sterile control.
• Overall this suggests that indigenous microbes in
the Sellafield sediment may be utilizing the hydrogen
generated from Fe(0) corrosion as an electron donor.
• If this is the case; native microorganisms in a GDF
are likely to mitigate against the effects of hydrogen
over-pressurisation.
• Biotic and abiotic processes reduce a number of
TEAs. including Mn(IV), Fe(III) and SO4
2-
. In these
systems reduction is most unequivocal for Mn.
Bioreduction of high valence radionuclides is also
expected in analogous systems.
michael.crouch@postgrad.manchester.ac.uk
Figure 2 – Steel
Intermediate Level
Waste (ILW) drum
containing waste
and cement
grout encapsulant
(Source [4]).