Applied Geochemistry 51 (2014) 55–64
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Applied Geochemistry
journal homepage: www.elsevier.com/locate/apgeochem
Microbial reduction of uranium(VI) in sediments of different lithologies
collected from Sellafield
Laura Newsome a,⇑, Katherine Morris a, Divyesh Trivedi b, Nick Atherton c, Jonathan R. Lloyd a,b
a
Research Centre for Radwaste and Decommissioning and Williamson Research Centre for Molecular Environmental Science, School of Earth,
Atmospheric and Environmental Sciences, University of Manchester, Manchester M13 9PL, UK
b
National Nuclear Laboratory, Chadwick House, Birchwood Park, Warrington WA3 6AE, UK
c
Sellafield Ltd., Land Quality, Sellafield, Seascale, Cumbria CA20 1PG, UK
a r t i c l e
i n f o
Article history:
Available online 28 September 2014
Editorial handling by M. Kersten
a b s t r a c t
The presence of uranium in groundwater at nuclear sites can be controlled by microbial processes. Here
we describe the results from stimulating microbial reduction of U(VI) in sediment samples obtained from
a nuclear-licensed site in the UK. A variety of different lithology sediments were selected to represent the
heterogeneity of the subsurface at a site underlain by glacial outwash deposits and sandstone. The natural sediment microbial communities were stimulated via the addition of an acetate/lactate electron
donor mix and were monitored for changes in geochemistry and molecular ecology. Most sediments
facilitated the removal of 12 ppm U(VI) during the onset of Fe(III)-reducing conditions; this was reflected
by an increase in the proportion of known Fe(III)- and U(VI)-reducing species. However U(VI) remained in
solution in two sediments and Fe(III)-reducing conditions did not develop. Sequential extractions, addition of an Fe(III)-enrichment culture and most probable number enumerations revealed that a lack of bioavailable iron or low cell numbers of Fe(III)-reducing bacteria may be responsible. These results highlight
the potential for stimulation of microbial U(VI)-reduction to be used as a bioremediation strategy at UK
nuclear sites, and they emphasise the importance of both site-specific and borehole-specific investigations to be completed prior to implementation.
Ó 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://
creativecommons.org/licenses/by/3.0/).
1. Introduction
Contamination of groundwater by aqueous uranium is a common problem at sites where mining, milling and reprocessing of
uranium for nuclear fuel has taken place. In the UK, Sellafield is a
nuclear reprocessing site in the north west of England (Fig. S1),
with the largest contaminated land liability in the UK Nuclear
Decommissioning Authority’s estate (Cruickshank, 2012). Uranium
is known to be present as a contaminant in groundwater underlying the Sellafield site (McKenzie et al., 2011). In oxidising groundwaters uranium is generally present as the mobile uranyl ion
(UO2+
2 ) or uranyl hydroxide complexes below pH 6.5 or as uranyl
carbonate complexes at higher pH, whereas under reducing conditions, insoluble U(IV) predominates (Choppin et al., 2002;
Newsome et al., 2014). Bioreduction, whereby indigenous soil bacteria are supplied with an electron donor which they oxidise coupled to reduction of aqueous U(VI) to insoluble U(IV), could offer a
promising in situ remediation strategy to prevent further migration
and dispersal of uranium and other radionuclides in groundwater.
⇑ Corresponding author.
E-mail address: laura.newsome@postgrad.manchester.ac.uk (L. Newsome).
Over twenty years of research has identified a diverse range of
relatively common soil bacteria that are able to facilitate the enzymatic reduction of U(VI) to U(IV), mainly, but not limited to Fe(III)and sulphate-reducing bacteria (Lovley and Phillips, 1992; Lovley
et al., 1991; Newsome et al., 2014; Williams et al., 2013). Although
abiotic reduction of U(VI) by Fe(II) minerals is possible e.g. Latta
et al. (2012), Veeramani et al. (2011), most studies show that under
environmental conditions the dominant mechanism is direct enzymatic reduction (Bargar et al., 2013; Law et al., 2011; Singer et al.,
2012; Williams et al., 2013, 2011). The form of microbially reduced
U(IV) is often stated to be uraninite e.g. Suzuki et al. (2002), however, more recently another form, commonly termed monomeric
U(IV), has been identified e.g. Bernier-Latmani et al. (2010), Kelly
et al. (2008). Field trials stimulating microbial reduction of U(VI)
in the subsurface have successfully demonstrated uranium immobilisation over periods of up to a year, although the long-term stability of biogenic U(IV) in situ warrants further investigation
(Anderson et al., 2003; Bargar et al., 2013; Tang et al., 2013a,b;
Williams et al., 2011).
Laboratory microcosm experiments using sediments representative of the Sellafield and Dounraey nuclear facilities, and a sediment sample from the Low Level Waste Repository have
http://dx.doi.org/10.1016/j.apgeochem.2014.09.008
0883-2927/Ó 2014 The Authors. Published by Elsevier Ltd.
This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/3.0/).
56
L. Newsome et al. / Applied Geochemistry 51 (2014) 55–64
demonstrated the potential for bioreduction of U(VI) as a remediation technique (Begg et al., 2011; Law et al., 2011; Wilkins
et al., 2007). To date, there are no data on U(VI) behaviour in sediments from the Sellafield site. The geology underlying Sellafield is
complex, with a varying thickness of made ground underlain by a
mixture of Quaternary glacial outwash deposits and till, channel
sands and gravels (drift deposits), and Sherwood Sandstone
(Smith and Cooper, 2004). The Sherwood Sandstone also outcrops
at certain locations on site. It is a major aquifer, with the regional
groundwater flow southwest towards the Irish Sea. Groundwater
in the Sherwood Sandstone is probably in hydraulic continuity
with the overlying drift deposits, which have more variable
groundwater flow directions. In certain areas of the site there are
localised bodies of perched groundwater (Cruickshank, 2012;
McKenzie and Armstrong-Pope, 2010). Contaminant flow pathways vary across the site due to the complex heterogeneity of
the subsurface (Hunter, 2004).
The aim of this work was to assess the potential for microbial
U(VI) reduction within these different lithological units. Here we
present the results from biostimulated U(VI)-reduction experiments using unique on-site sediments obtained from a Sellafield
site drilling programme and covering a range of different lithologies underlying the site. In brief, sediments were incubated with
an artificial groundwater representative of the site, and containing
acetate and lactate as electron donors to stimulate microbial U(VI)
reduction. Changes in geochemistry were monitored. Generally,
the indigenous microbial community could facilitate the removal
of 12 ppm (50 lM) U(VI) from solution through reduction to
U(IV). Some variations in the rate and extent of U(VI) reductive
precipitation were observed, and the reasons for this are explored
further.
2. Materials and methods
2.1. Sediments
A range of sediments from the 2009 to 2010 Sellafield site drilling programme which were either non-active or had very low levels of radionuclides were shipped to The University of Manchester
in 2011 and stored in the dark in a cool store room. A fresh sediment sample was obtained from a 2012 site excavation and stored
at 4 °C in the dark. Seven sediment samples (Table 1) were selected
as representative of key lithologies on sites and were further
characterised using X-ray fluorescence (PANalytical Axios),
X-ray diffraction (Bruker D8 Advance), for surface area using BET
(Gemini 2360 Surface Area Analyser), pH (measured after equilibrating 1 g sediment in 1 ml deionised water for an hour, after
ASTM (2006)), total organic carbon (Leco TruSpec) and total bioavailable iron using the ferrozine assay (Lovley and Phillips, 1987).
2.2. Stimulation of microbial U(VI) reduction
Sediments were incubated in triplicate in sterile glass serum
bottles with a 1: 10 ratio with sterile anaerobic artificial groundwater representative of the Sellafield area and composed of (g/l)
KCl, 0.0066; MgSO47H2O, 0.0976; MgCl26H2O, 0.081; CaCO3,
0.1672; NaNO3, 0.0275; NaCl, 0.0094; NaHCO3, 0.2424 (Wilkins
et al., 2007). Electron donors were also supplied as 5 mM acetate
and 5 mM lactate, while control bottles contained no added electron donor. Uranium as U(VI) in 0.001 M HCl was added to the
microcosms to give a final concentration of 12 ppm, representative
of elevated concentrations of uranium reported in Sellafield site
groundwaters (McKenzie et al., 2011). The bottles were crimp
sealed with butyl rubber caps, the headspace purged with N2,
and the experiments incubated in the dark at room temperature
over three months. In an additional experiment, one replicate set
of sediment was stored at 10 °C to represent the average temperature of UK groundwater, in order to investigate the impact of
reduced temperature on the rate of microbial U(VI) reduction.
Sediment slurry was extracted at set time points using N2
flushed syringes and aseptic technique. An aliquot was immediately added to 0.5 N HCl for analysis of Fe(II) as a fraction of total
bioavailable iron (Lovley and Phillips, 1987). Porewaters were then
separated via centrifugation (14,800g) and monitored for U(VI) by
the bromo-PADAP assay (Johnson and Florence, 1971), nitrite
(Harris and Mortimer, 2002), pH and Eh. Surplus porewaters and
sediment pellets were frozen at 80 °C for later analysis of nitrate
and sulphate via ion chromatography (Dionex) or microbial community composition via pyrosequencing (see below).
2.3. X-ray absorption spectroscopy (XANES)
Uranium speciation in samples of microbially-reduced clay
(RB23) and gravelly sand (RB27) frozen back at day 90 was analysed at the DIAMOND Lightsource, Harwell, UK on Beamline
B18. U LIII-edge spectra were collected in fluorescence mode using
a 9 element Ge detector (Dent et al., 2009) with samples loaded in
the cryostat. Data were calibrated, background subtracted and nor-
Table 1
Characteristics of Sellafield sediment samples.
8.0–8.5 m
2008
Brown GRAVELLY SAND Red brown
(RB27)
SANDSTONE
(IS16)
Unknown
9.5–10 m
2012
2009
Red brown
SAND and
GRAVEL (IS18)
9.5–10 m
2009
5.94 ± 0.03
2.1 ± 0.045
5.43
0.57
0.119
3.42
0.244
3.4
0.230 ± 0.010
8.55 ± 0.07
0.46
2.22
0.38
0.075
2.32
0.065
1.2
0.0384 ± 0.009
8.31 ± 0.03
0.75 ± 0.11
5.66
9.72
0.079
2.52
0.080
3.9
0.158 ± 0.010
5.05 ± 0.01
0.27
3.51
0.066
0.009
0.666
0.044
1.9
0.0074 ± 0.001
8.56 ± 0.09
0.29
5.00
0.878
0.121
3.72
0.153
3.9
0.138 ± 0.022
Quartz Clinochlore
Muscovite Albite
Orthoclase Pyrite
Quartz Clinochlore
Muscovite Albite
Orthoclase Pyrite
Quartz Clinochlore
Muscovite Albite
Orthoclase Calcite
Quartz Calcite Quartz
Clinochlore
Muscovite
Albite
Brown fine
SAND (RB24)
8.0 m
2008
Friable brown
gravelly CLAY
(RB23)
2.0–2.5 m
2008
8.38 ± 0.03
0.35
3.57
1.23
0.117
4.46
0.073
3.5
0.0392 ± 0.003
8.36 ± 0.03
0.43
5.91
0.377
0.08
2.99
0.102
3.6
0.0307 ± 0.003
Quartz Clinochlore
Muscovite Albite
Orthoclase
Quartz Clinochlore
Muscovite Albite
Orthoclase Pyrite
Sediment
description,
depth and
year of
excavation
Dark brown SAND
Red brown clayey
and GRAVEL (RB10) SANDY SILT (RB18)
1.5–2.5 m
2009
pH
TOC (%)
BET (m2/g)
Ca (%)
Mn (%)
Fe (%)
P (%)
U (ppm)
Bioavailable
Fe (%)
Mineralogy
(XRD)
L. Newsome et al. / Applied Geochemistry 51 (2014) 55–64
malised for drift to a standardised E0 position using ATHENA (Ravel
and Newville, 2005). Linear combination fitting was performed on
XANES spectra compared to data from the Actinide Reference Database for Spectroscopy (Scheinost et al., 2013) for U(IV) as uraninite
(Opel et al., 2007) and U(VI) as a uranyl carbonate complex
(Rossberg et al., 2009). EXAFS was not performed due to the low,
but environmentally-relevant, concentrations of uranium included
in the experiment.
57
removed from further analysis. Denoising and chimera removal
was performed during OTU picking (at 97% sequence similarity)
with USEARCH (Edgar, 2010) in Qiime, and a representative
sequence for each OTU was identified. Taxonomic classification
of all reads was performed in Qiime using the Ribosomal Database
Project (RDP) at 80% confidence threshold (Cole et al., 2009), while
the closest GenBank match for the OTUs that contained the highest
number of reads (the representative sequence for each OTU was
used) was identified by Blastn nucleotide search.
2.4. Molecular ecology
2.5. Sequential extractions
To compare changes in the microbial community during development of U(VI) reducing conditions, DNA was extracted from two
sediments (clay RB23 and gravelly sand RB27) at the start of the
experiment and after approximately 90 days incubation. DNA
was extracted from soil/slurry samples (200 ll) using a PowerSoil
DNA Isolation Kit (MO BIO Laboratories INC, Carlsbad, CA, USA).
The 16S-23S rRNA intergenic spacer region from the bacterial
RNA operon was amplified using primers ITSF and ITSReub as
described previously (Cardinale et al., 2004). The amplified PCR
(polymerase chain reaction) products were separated by electrophoresis in Tris–acetate–EDTA gel. DNA was stained with ethidium
bromide and viewed under short-wave UV light. Positive microbial
community changes identified by RISA (ribosomal intergenic
spacer analysis) justified further investigation by 16S rRNA gene
sequencing.
A pyrosequencing methodology was then applied to investigate
the microbial diversity within the samples. Isolated DNA from each
sample was subjected to PCR amplification of the V1–V2 hypervariable region of the bacterial 16S rRNA gene, using universal bacterial primers 27F (Lane, 1991) and 338R (Hamady et al., 2008).
The primers were synthesised by IDTdna (Integrated DNA Technologies, BVBA, Leuven, Belgium) and their design was based on
Roche’s guidelines for one way amplicon sequencing with the
454 Life Sciences GS Junior system. The fusion forward primer
(50 -CCATCTCATCCCTGCGTGTCTCCGACTCAGNNNNNNNNNNAGAGTTTGATGMTGGCTCAG-30 ) contained the 454 Life Sciences
‘‘Lib-L Primer A’’, a 4 base ‘‘key’’ sequence (TCAG), a unique tenbase multiplex identifier ‘‘MID’’ sequence for each sample
(NNNNNNNNNN), and bacterial primer 27F. The reverse fusion primer (50 -CCTATCCCCTGTGTGCCTTGGCAGTCTCAGTGCTGCCTCCCGTAGGAGT-30 ) contained the 454 Life Sciences ‘‘Lib-L Primer B’’, a
4 base ‘‘key’’ sequence (TCAG), and bacterial primer 338R. The
PCR amplification was performed in 50 ll volume reactions using
0.4 ll (2.0 units) FastStart High Fidelity DNA polymerase (Roche
Diagnostics GmbH, Mannheim, Germany), 1.8 mM MgCl2, 200 lM
of each dNTP, 0.8 lM of each forward and reverse fusion primers,
and 2 ll of DNA template (8.6–11.0 ng DNA per reaction). The
PCR conditions included an initial denaturing step at 95 °C for
2 min, followed by 35 cycles of 95 °C for 30 s, 55 °C for 30 s,
72 °C for 45 s, and a final elongation step at 72 °C for 5 min. PCR
products were then loaded in an agarose gel, and following gel
electrophoresis, bands of the correct fragment size (approximately
410 base pairs) were excised, cleaned up using a QIAquick gel
extraction kit (QIAGEN, GmBH, Hilden, Germany), and eluted in
30 ll of DNAse free H2O. The cleaned up PCR products were quantified using an Agilent 2100 Bioanalyzer (Agilent Technologies, Inc.,
Santa Clara, CA, USA), and pooled so that the mixture contained
equal amounts of DNA from each sample. The emulsion emPCR
and the pyrosequencing run were performed at the University of
Manchester sequencing facility, using a 454 Life Sciences GS Junior
system (Roche).
The 454 pyrosequencing reads were analysed using Qiime 1.6.0
release (Caporaso et al., 2010). Low quality reads (mean quality
score less than 25) and short sequences (less than 300 base pairs)
were discarded, and both forward and reverse primers were
Sequential extractions were performed on sediment samples to
provide insight into the mineral phases that Fe and microbiallyreduced U(IV) were present in. The modified BCR sequential
extraction procedure was used (Rauret et al., 1999). Briefly the
extraction steps included 0.11 M acetic acid ‘‘exchangeable’’,
0.5 M hydroxylamine hydrochloride ‘‘reducible’’, 8.8 M hydrogen
peroxide and 1.0 M ammonium acetate ‘‘oxidisable’’, and aqua
regia ‘‘residual’’ fractions. The Fe-extractions were performed on
0.5 g of sediment. For the U-extractions, 0.5 g of microbiallyreduced sediment slurry was used, and the first two extractions
were performed under anaerobic conditions (Keith-Roach et al.,
2003). Extracts were diluted in 2% nitric acid and analysed for uranium via ICP-MS (Agilent 7500CX).
2.6. Enrichment culture
To enrich for Fe(III)-reducing bacteria present in the clay sediment, 1 ml of microbially reduced sediment slurry was inoculated
into 100 ml of anaerobic sterile freshwater minimal medium
(Lovley et al., 1991; Thorpe et al., 2012) at pH 7, with 5 mM acetate
and 5 mM lactate as an electron donor and approximately
15 mmoles per litre ferrihydrite as the electron acceptor (Cornell
and Schwertmann, 2006; Schwertmann and Cornell, 2000). The
proportion of Fe(II) in the experiment was monitored using the ferrozine assay (Lovley and Phillips, 1987). When the Fe(II) had
reached 30% or greater (usually occurring between two and four
weeks of incubation), 1% of the enrichment culture was inoculated
into the next batch of freshwater minimal medium. This was
repeated until the thirteenth enrichment. From each enrichment
culture, the DNA was extracted and the microbial diversity
assessed using RISA (as above). The microbial diversity of the first,
eighth and thirteenth enrichments was investigated by pyrosequencing (as above). The identity of the reduced Fe(II) mineral
was characterised using XRD and ESEM.
To investigate whether sediments IS16 (sandstone) and IS18
(sand and gravel) contained sufficient bioavailable Fe to support
a robust population of Fe(III)-reducing bacteria, an aliquot of the
Fe(III)-reducing enrichment culture (1%) was added to sediment
microcosms with no added U(VI). These were then monitored for
changes in Fe(II)/Fe(III) over 100 days. Due to the low concentrations of bioavailable Fe(III) present in the sandstone, it was added
at a 2:5 sediment to artificial groundwater ratio instead of 1:10.
This ensured that measurements were well above the limit of
detection and so could be analysed with greater confidence. The
acid-digested sediment slurry (Lovley and Phillips, 1987) was filtered before adding to ferrozine solution in order to avoid turbidity
affecting colorimetric measurements.
2.7. Most probable number (MPN) enumerations
Most probable number (MPN) enumerations were used to
assess the abundance of Fe(III)-reducing bacteria in sediments
RB23 (clay), RB27 (gravelly sand), IS16 (sandstone) and IS18 (sand
and gravel). Here, 1 g of sediment was added to 10 ml anaerobic
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L. Newsome et al. / Applied Geochemistry 51 (2014) 55–64
sterile freshwater minimal medium at pH 7 in triplicate, with
3.9 mM nitrilotriacetic acid (as the trisodium salt), 2 mM acetate
as the electron donor and approximately 4 mmoles per litre ferrihydrite as the electron acceptor. Serial dilutions were performed
by adding 1 ml of the slurry to 9 ml of the medium, until a dilution
factor of 10 7 was reached. Fe(III) reduction was monitored using
the ferrozine assay (Lovley and Phillips, 1987) after 7 and 11 weeks
incubation, parallel to reduction times seen in sediment incubations. MPN estimates were then made using published MPN tables
(Man, 1983).
3. Results
3.1. Characteristics of Sellafield sediment samples
Prior to assessing the potential for microbial U(VI) reduction,
the sediment samples were characterised using a range of techniques. The seven sediment samples comprised mainly silicate
minerals, namely quartz, mica, chlorite and feldspar (Table 1).
Two sediments contained calcite, and three contained pyrite.
Lithology ranged from clay and silt, to sand and gravels, broadly
representative of glacial till and the buried channel and fluvioglacial units. One sample (IS16) was of the upper sandstone bedrock.
The mean (±1 SD) composition of the seven soils determined by
XRF was: Ca 1.89% (3.5), Mn 0.086% (0.04), Fe 2.87% (1.2), P
0.11% (0.07), U 3.06 ppm (1.1), by total organic carbon analysis
was 0.84% (0.74), total bioavailable Fe by ferrozine assay was
0.092% (0.08), with a BET derived surface area of 4.47 m2/g (1.4)
and with sediment pH ranging from 5.1 to 8.6 (Table 1). In contrast
to the other sediments, after 1 h equilibration with artificial
groundwater the clay (RB23) released approximately 1.0 mM
nitrate and 0.4 mM sulphate to solution, indicating it contained
significant quantities of labile electron acceptors. The clay also contained the highest concentration of organic carbon and bioavailable iron.
3.2. Microbial U(VI) reduction
To determine whether the range of sediments and their extant
microbial communities underlying Sellafield were able to support
microbial reduction of U(VI), samples of different lithology were
incubated with added electron donors for approximately 90 days.
In five of the seven incubations, U(VI) was removed from solution,
and Fe(II) produced (Fig. 1). Sediments were observed to darken in
colour and geochemical indicators demonstrated the development
of progressively reducing conditions. An initial peak of nitrite was
detected up to around 300 lM, representing reduction of the
nitrate present in the artificial groundwater. Ion chromatographic
analyses confirmed nitrate removal, and subsequent sulphate
removal (Fig. S2). All microcosms remained around circumneutral
pH throughout the incubation period. Final Eh measurements ranged from 79 to 153 mV. For the additional experiment incubated at 10 °C (RB27), a slight time lag before U(VI) reduction
occurred compared to the 21 °C incubation (Fig. 1), but the overall
extent of U(VI) and Fe(III) reduction was the same after 90 days.
Control samples with no added electron donor generally did not
exhibit U(VI) removal or Fe(III) reduction; nitrate and sulphate
remained in solution and final Eh values were between +102 and
+188 mV. The exception was the clay (RB23) control, in which after
90 days U(VI), nitrate and sulphate were completely removed from
solution, Fe(II) was generated, and the Eh was 116 mV.
In contrast U(VI) remained in solution in two of the electron
donor stimulated sediment incubations over the 90 day incubation
period; these were the sandstone (IS16) and the sand and gravel
(IS18) (Fig. S3). The colour remained orange–brown and the final
Fig. 1. Results from microbial reduction experiments demonstrating removal of
U(VI) and reduction of Fe(III) in sediments of different lithology. Triplicates with
electron donor (h), controls without added electron donor (s), error bars represent
+/-1 standard deviation. Dashed lines for gravelly sand RB27 represent samples
incubated at 10 °C. Results for two sediments which failed to remove U(VI) are
presented in Fig. S3.
Eh measurements of +89 and +143 mV indicated that bioreducing
conditions did not develop. Possible reasons for this were investigated further using sequential extractions and adding an inoculum
of the Fe(III)-reducing Sellafield enrichment culture, to determine
if Fe(III) reduction was feasible in these sediments (see below).
XANES spectra were collected for two sediments to identify uranium speciation after 90 days incubation. Both the clay (RB23) and
gravelly sand (RB27) samples had a very similar shape to the U(IV)
standard, with the same edge position and clearly lacking the postedge ‘‘shoulder’’ of the U(VI) standard (Fig. 2). Linear combination
fitting of these spectra indicated that 100% of the uranium in each
sample was present as U(IV).
3.3. Molecular ecology
A rapid molecular profiling approach (gel electrophoresis of the
PCR amplified products of the bacterial 16S-23S rRNA intergenic
L. Newsome et al. / Applied Geochemistry 51 (2014) 55–64
59
than doubled, as did sequences of genera belonging to the Bacteroidales, Sphingobacteriales (Flavisolibacter), Clostridiales, Rhodospirillales
(Azospirillum),
Burkholderiales
(Limnohabitans,
Polaromonas, Rhodoferax), Hydrogenophilales, Desulfuromonadales
(Geobacter) and Spirochaetales (Spirochaeta) bacterial orders. Of
these, Clostridiales, Burkholderiales and Desulfuromonadales contain known U(VI)- or Fe(III)-reducers (Finneran et al., 2003; Gao
and Francis, 2008; Lovley et al., 1991).
The gravelly sand sediment (RB27) was characterised by a large
increase in sequences belonging to the Pseudomonadales order
(from 11.6% to 65.3%), with most of these being closely affiliated
to Pseudomonas peli strain R-20805 isolated from a nitrifying inoculum (Table 2, Fig. S5). Other bacterial orders containing genera
which increased by a factor of two or more were Bacteroidales,
Neisseriales (Vogesella), Desulfuromonadales (Geobacter) and Alteromonadales (Shewanella), and within these were sequences affiliated to known U(VI)- and Fe(III)-reducing Geobacter (1.0%) and
Shewanella (1.5%).
Fig. 2. Uranium LIII-edge XANES spectra for uranium amended sediments and U(IV)
uraninite and U(VI) uranyl carbonate reference standards. For both sediments the
edge position and shape of the spectra closely resemble the uraninite standard
indicating that U is present as U(IV).
spacer region) indicated that there were clear shifts in the structure of bacterial communities in the two sediment samples after
90 days of incubation (Fig. S4). The phylogenetic diversity of the
bacterial communities within these samples was investigated further by 16S rRNA gene sequencing, using pyrosequencing (Fig. 3
and Table 2). The results indicated that both sediments were characterised by a diverse range of bacterial phyla prior to incubation
(Fig. 3), with most of the sequences affiliated to terrestrial or soil
environments (Table 2) and belonging to ubiquitous bacterial
orders such as Rhodospirillales, Acidobacteriales, Solibacterales,
Xanthomonadales, Rhizobiales, Sphingobacteriales, Hydrogenophilales, Pseudomonadales and Burkholderiales.
In the clay sediment (RB23), after 90 days incubation with acetate and lactate there was a clear shift in the structure of the bacterial community, as sequences affiliated with nitrogen-fixing
Azospirillum sp., iron-reducing Rhodoferax ferrireducens, organisms
from agricultural soils or environments associated with U(VI)
reduction and organic degradation became enriched (Table 2,
Fig. S5). Furthermore, the number of sequences affiliated with
the known Fe(III)- and U(VI)-reducing Geobacter (2.9%) genus more
3.4. Sequential extractions
As the sandstone (IS16) and sand and gravel (IS18) were unable
to generate microbial U(VI) reduction, one possible explanation is
that they contained insufficient bioavailable Fe(III) to maintain
an active Fe(III)- and consequently U(VI)-reducing microbial community. BCR sequential extractions were performed alongside XRF
and 0.5 N hydroxylamine hydrochloride extractions to gain insight
into the operationally defined sediment associations of Fe in IS16
and IS18 compared to two sediments which successfully removed
U(VI) from solution; the clay (RB23) and gravelly sand (RB27). The
proportion of Fe in the four sediments, measured via aqua regia
digestion, ranged from 0.1% to 1.8%; similar to but slightly lower
than the results obtained by XRF, probably due to refractory Fe
present in the mineral lattices of clays not dissolving with aqua
regia. The results for total bioavailable iron measured using the ferrozine assay were broadly comparable to the sequential extraction
reducible fraction, ranging from 0.07 to 2.3 milligrams per gram of
sediment. While the sand and gravel (IS18) contained similar concentrations of total Fe and ‘‘bioavailable’’ Fe to the clay (RB23) and
gravelly sand (RB27), the concentration in the sandstone (IS16)
were much lower (Fig. 4).
An additional set of sequential extractions were performed on
the clay (RB23) and gravelly sand (RB27) to investigate which sediment fraction microbially reduced U(IV) was associated with. In
the clay, most uranium was found in the reducible (38% ± 2.0%)
and oxidisable (50% ± 7.6%) fractions, while in the gravelly sand,
most uranium was in the exchangeable (41% ± 3.7%) and reducible
(44% ± 19%) fractions (Fig. 5). This suggests that microbially
reduced U(IV) may partition to a more recalcitrant fraction in the
clay compared to the gravelly sand.
3.5. Enrichment culture
Fig. 3. Bacterial phylogenetic diversity within clay (RB23) and gravelly sand (RB27)
sediments before and after development of bioreducing conditions (at the phylum
level/class for the Proteobacteria). Phyla/classes detected at greater than 1% of the
bacterial community are illustrated.
An Fe(III)-reducing enrichment culture was generated from the
clay sediment. This enrichment culture reduced more than 30% of
bioavailable Fe(III) in the medium within 2–4 weeks, forming a
golden coloured platy mineral. This was identified by XRD as a
mixture of the Fe(II) bearing minerals siderite (FeCO3) and vivianite (Fe3(PO4)28H2O) (Fig. 6).
RISA indicated that after eight subcultures, a relatively diverse
microbial community remained present (Fig. S4). This was confirmed with the 16S rRNA gene sequencing results, with 582
sequences present in the eighth subculture (compared to 898 in
the initial sample). A relatively stable microbial community had
developed by the eighth enrichment subculture which remained
broadly similar at the thirteenth subculture (Fig. 7 and Table 3),
60
L. Newsome et al. / Applied Geochemistry 51 (2014) 55–64
Table 2
Closest phylogenetic relatives of the five most abundant OTUs from clay (RB23) and gravelly sand (RB27) sediments before and after the development of bioreducing conditions.
Number
of clones
% of
clones
RB23 day 0
155
10.3
103
76
6.9
5.1
60
4.0
55
3.7
RB23 day 90
349
13.8
181
7.1
180
115
7.1
4.5
63
2.5
RB27 day 0
288
6.3
216
4.7
203
4.4
195
4.3
106
2.3
RB27 day 96
6963
27.9
*
5182
20.8
897
3.6
497
1.9
435
1.7
Closest phylogenetic relative
Accession
number
Uncultured bacterium clone
NC42e10_14934
Bacterium Ellin643
Uncultured Rhodospirillaceae
bacterium
Uncultured bacterium clone
WC1_a1
Uncultured bacterium clone
WC2_31
JQ384246.2
98
DQ075307.1
EF018478.1
100
99
GQ263698.1
98
GQ263951.1
100
Azospirillum sp. 7C
Type strain Pseudorhodoferax
caeni strain SB1
Flavisolibacter ginsengisoli
Uncultured bacterium clone
5MhU1878D11
Type strain Rhodoferax
ferrireducens T118 strain DSM
15236
AF411852.1
NR_042216.1
97
97
NR_041500.1
JF395212.1
97
97
Nitrogen-fixing bacteria from rhizosphere/fuel contaminated Antarctic soils
Activated sludge/anaerobic digester/organics degrader/drinking water
treatment
Ginseng cultivating soil/organics degrader/rhizosphere
Microbial community response to U(VI) bioremediation/organics degrader
NR_074760.1
99
Acetate amended soils/organics degrader
Algoriphagus terrigena strain
DS-44
Thiobacillus denitrificans ATCC
25259
NR_043616.1
99
Korean soil
NR_074417.1
97
Type strain Pseudomonas
mohnii
Type strain Polaromonas sp.
JS666
Thiobacillus thiophilus strain
D24TN
NR_042543.1
100
Chemolithoautotrophic, facultatively anaerobic bacterium. Acetate amended
soils/thiosulphate or sulphur oxidiser/acid mine draining/PCB contaminated
soil
Degrader of chlorosalicylates or isopimaric acid
NR_074725.1
99
NR_044555.1
97
Type strain Pseudomonas peli
strain R-20805*
Type strain Pseudomonas peli
strain R-20805*
Type strain Pseudomonas
mohnii
Type strain Pseudomonas peli
strain R-20805*
Pseudomonas sagittaria sp.
nov.
NR_042451.1
99
Nitrifier/organic degrader/nitrate reducer
NR_042451.1
99
Nitrifier/organic degrader/nitrate reducer
NR_042543.1
100
NR_042451.1
96
JQ277453
ID similarity (%)
100
Environment
Elevated atmospheric CO2/warmed Antarctic soils/methane oxidisers
Elevated atmospheric CO2/anaerobic cellulose degraders/rice paddy soil
Elevated atmospheric CO2/activated sludge/drinking water treatment
Cellulosic waste/elevated CO2 soils/Antarctic soils
Cellulosic waste/elevated CO2 soils
Microbial community response to U(VI) bioremediation/anaerobic digester/
drinking water treatment/snow/glaciers
Chemolithoautotrophic, thiosulphate-oxidising bacterium. Acetate amended
soils/hydrocarbon bioremediation
Degrader of chlorosalicylates or isopimaric acid
Nitrifier/organic degrader/nitrate reducer
Siderophore producer/oil or herbicide contaminated soil/phenol degrader/
magnetite mine drainage
These represent bacteria assigned to different operational taxonomic units, but each are most closely related to Pseudomonas peli strain R-20805.
Fig. 4. Profile of Fe in sediments which were able to facilitate U(VI) bioreduction
(RB23, RB27) and were not able to (IS16, IS18), as determined by sequential
extraction. Bars represent the average of triplicate samples, error bars are +/-1
standard deviation.
Fig. 5. Profile of bioreduced U in clay and gravelly sand sediments, as determined
by sequential extraction. Bars represent the average of triplicate samples, error bars
are +/-1 standard deviation.
L. Newsome et al. / Applied Geochemistry 51 (2014) 55–64
61
reduction from taking place, unless manipulated through the addition of electron acceptor (Fe(III) or sulphate) and/or microbial
inocula.
3.6. Most probable number enumerations
After 11 weeks of incubation, approximately 1 105 – 1 107
cells of Fe(III)-reducing bacteria per gram of sediment were measured in the sediments which had previously been shown to
reduce U(VI) (Table 4). As expected from biogeochemical measurements, this exceeded the numbers present in the sediments which
could not reduce U(VI), with approximately 1 104 cells per gram
of sandstone (IS16) and sand and gravel (IS18).
4. Discussion
Fig. 6. ESEM image showing cubic siderite ‘‘Sid’’ and acicular/platy vivianite ‘‘Viv’’
minerals. Mineral identification was confirmed using XRD.
with 41–46% of sequences closely related to four species of Thaurea, Geobacter, Sporotalea (reassigned as Pelosinus (Moe et al.,
2012)), Acidovorax and Simplicispira; genera found in anaerobic
environments and typically associated with organic degradation
or Fe(III)-reduction (Fig. S6).
An alternative explanation for some sediments not being able to
support stimulated microbial reduction of U(VI) is that the extant
microbial community did not contain viable Fe(III)-reducing bacteria. Results showed that adding an inoculum of enrichment culture
to sediment microcosms of the sandstone (IS16) and sand and
gravel (IS18) increased the extent of Fe(III)-reduction compared
to controls without the added inoculum and Eh data were consistent with this interpretation (Fig. S7). As the sequential extractions
demonstrated that the sand and gravel (IS18) contained comparable amounts of bioavailable iron to the sediments which did stimulate U(VI)-reduction, it is possible that this sediment lacks an
active Fe(III)-reducing microbial community. Given the low concentration of bioavailable iron in the sandstone (IS16), it is not surprising that when the proportion of sediment was increased and an
enrichment culture added, some Fe(III)-reduction could be measured. This indicates that the lack of bioavailable Fe(III) and
Fe(III)-reducing bacteria in this sediment might preclude U(VI)-
Fig. 7. Bacterial phylogenetic diversity within the enrichment culture (at the
phylum level/class for the Proteobacteria). Phyla/classes detected at greater than 1%
of the bacterial community are illustrated.
Sellafield site sediments representing a range of different lithologies underlying the site have been characterised in terms of their
potential to develop reducing conditions and consequently remove
U(VI) from solution. The indigenous microbial community in most
of the sediments was able to reduce U(VI) to U(IV) and remove it
from solution over approximately 90 days after stimulation with
added electron donor, including at 10 °C representative of UK
groundwater temperatures. The clay sediment (RB23) contained
sufficient indigenous and labile organic matter to stimulate microbial U(VI) reduction without addition of an electron donor.
As expected, over the incubation period a cascade of anaerobic
redox processes was observed in most of the sediments, as the available nitrate was depleted quickly (within 1 week) followed by periods of Fe(III)- and sulphate reduction. In addition, within the
identified 16S rRNA sequences, there were sequences closely related
to known nitrogen-fixing (Azospirillum) nitrifying (Pseudomonas),
and Fe(III)-/metal-reducing (Geobacter, Shewanella, Rhodoferax)
genera as well as a number of bacteria closely related to those
known to degrade a range of organic compounds. Thus both geochemical and molecular ecology results indicate clearly that the
indigenous microbial population in the majority of these Sellafield
sediments have the capacity to utilise a wide range of electron
donors and acceptors and carry out diverse biogeochemical
processes.
Regarding U(VI) reduction, it is noteworthy that during the
incubation period the proportion of sequences related to known
U(VI)-reducing bacterial genera more than doubled, including
Geobacter and Shewanella (Lovley et al., 1991), albeit they remained
in low abundances (1–3%). It is not clear whether U(VI) reduction
was carried out by these microorganisms or by more dominant
members of the microbial communities, since many of the
sequences of this study were not closely related to cultured organisms with known physiological properties. Moreover, in the gravelly sand sediment there was a significant enrichment of
sequences closely related to P. peli after incubation (Fig. 3 and
Table 2). Although Pseudomonas is known predominately as a facultative anaerobic denitrifying genus, sequences related to Pseudomonas stutzeri dominated similar U(VI) bioreduction experiments
established with soil from the nearby Low Level Waste Repository
site (Wilkins et al., 2007). Future anaerobic microcosm studies
should explore in more detail the potential of these indigenous
microbial communities for the reduction of U(VI) and other
radionuclides.
Important differences in sediment composition include the clay
content, and the amount of total organic carbon and bioavailable
Fe(III) present. Although calcium may inhibit microbial U(VI)
reduction through the formation of stable Ca-uranyl-carbonate
complexes (Brooks et al., 2003), this effect was not observed in
these sediments. That is, while calcium was present in the artificial
62
L. Newsome et al. / Applied Geochemistry 51 (2014) 55–64
Table 3
Closest phylogenetic relatives of the five most abundant OTUs from the initial sediment enrichment and the eight and thirteenth enrichment subcultures.
Number
of clones
% of
clones
Closest phylogenetic relative
First enrichment culture
349
3.5
Uncultured Acidobacteria
bacterium clone 4OLI_9
281
2.8
Geothrix fermentans
*
281
2.8
229
2.3
189
1.9
Uncultured bacterium clone
B0610D002_F03
Uncultured bacterium clone
ORFRC-FW102–726d-55
Flavisolibacter ginsengisoli
Accession
number
ID similarity (%)
Environment
GQ342374.1
99
NR_036779.1
97
AB660901.1
99
Biostimulated U mining soil/drinking water treatment/hydrocarbon
contaminated aquifer/bioremediation of U(VI) or TCE/rice paddy soil
Fe(III)-reducer/biostimulated U mining soil/rice paddy soil/drinking water
treatment
Rice paddy soils/acetate amended soils/activated sludge/Arctic sediments
FJ451813.1
99
In situ uranium bioremediation
NR_041500.1
97
Ginseng cultivating soil/organics degrader/rhizosphere
Eighth enrichment subculture
1878
17.9
Thauera sp. G3DM-88
EU037291.1
99
1036
9.9
NR_075007.1
99
Landfill sediments contaminated with Cr/anaerobic digester/activated
sludge/iron reducer/organic degrader
Fe(III)-reducer/rice paddy soil/acetate amended soils/wetlands/mining soils
828
7.9
NR_026506.1
100
542
5.2
NR_044941.1
99
300
2.9
NR_042451.1
99
Thirteenth enrichment subculture
2029
15.9
Thauera sp. G3DM-88
EU037291.1
99
1981
15.6
AB660562.1
98
Landfill sediments contaminated with Cr/anaerobic digester/activated
sludge/iron reducer/organic degrader
Rice paddy soil
1137
8.9
NR_042513.1
97
U(VI) removal by anaerobic microbial communities
1116
8.8
NR_026506.1
100
783
6.2
NR_044941.1
99
Geobacter bemidjiensis strain
Bem
Acidovorax defluvii strain
BSB411
Simplicispira metamorpha
strain DSM 1837
Type strain Pseudomonas peli
strain R-20805
Uncultured bacterium clone
B0610D003_J14 (Geobacter)
Type strain Sporotalea
propionica strain TmPN3*
Acidovorax defluvii strain
BSB411
Simplicispira metamorpha
strain DSM 1837
Activated sludge/ammonium rich aquifer/wastewater or drinking water
treatment
Activated sludge/anaerobic digester/water treatment/nitrifying chemostat/
ammanox
Nitrifier/organic degrader/nitrate reducer
Activated sludge/ammonium rich aquifer/wastewater or drinking water
treatment
Activated sludge/anaerobic digester/water treatment/nitrifying chemostat/
ammanox
Sporotalea propionica has since been reassigned as Pelosinus propionicus comb. nov. (Moe et al., 2012).
Table 4
MPN enumerations of Fe(III)-reducing bacteria.
Sediment
Sand (RB10)
Clay (RB23)
Gravelly sand (RB27)
Sandstone (IS16)
Sand and gravel (IS18)
7 weeks
MPN (cells g
7.4 103
1.5 105
1.5 104
2.3 103
9.2 102
1
)
11 weeks
MPN (cells g
1
)
2.4 105
7.4 106
9.3 104
9.3 103
2.4 104
groundwater (1.6 mM) and within the sediments (up to 9.7% Ca
measured by XRF (Table 1)), complete U(VI) reduction was
observed in most sediments after 90 days incubation. Indeed,
microbial reduction of U(VI) as Ca-uranyl-carbonate complexes
has also been observed in situ at the US DOE Rifle site (US
Department of Energy, 2011), perhaps suggesting that the influence of calcium on microbial U(VI) reduction is less important in
natural soil systems compared to microbial pure cultures.
Sorption effects were observed in the clay (RB23) sediment
incubations; after one hour just over half the added U(VI) had been
removed from solution. As other geochemical indicators clearly
demonstrated development of reducing conditions (e.g. Fe(II)
ingrowth, and nitrate and sulphate reduction) it is likely that the
sorbed U(VI) was also reduced. Indeed, reduction of U(VI) sorbed
to soils has been observed previously (Begg et al., 2011; Law
et al., 2011) and XANES data confirmed that uranium in the solid
phase was present entirely as U(IV) in the sample with added electron donor (Fig. 2). Furthermore, results from the sequential
extractions indicate that microbially reduced U(IV) partitioned to
more recalcitrant phases of the clay compared to the gravelly sand.
Relatively high concentrations of organic matter in the clay sediment (RB23) allowed for bioreducing conditions to be established
even when no additional electron donor was supplied. Together
this suggests that this clay could act as a natural attenuant of
U(VI), through both sorption and, if the organic fraction is bioavailable, microbial reduction. Although groundwater does not flow
through clay strata, many soils contain a clay mineral component
which may be able to offer some capacity for uranium retention.
In certain sediments, the paucity of bioavailable Fe(III) might
preclude an active Fe(III)-reducing microbial community from
developing and consequently being able to reduce U(VI), such as
in the sandstone IS16. However, the ability to reduce U(VI) is not
restricted to just Fe(III)-reducers, and this does not explain the lack
of U(VI) reduction in the sand and gravel (IS18) as this sediment
contained comparable amounts of bioavailable Fe(III) to the others.
The issue of certain amendments working in some locations but
not others, or in the laboratory but not in the field, is common
and it is often difficult to determine the reasons why, especially
in complex heterogeneous systems (Lovley, 2003). Evidence
obtained by adding an active Fe(III)-reducing enrichment culture
to the sandstone (IS16) and sand and gravel (IS18) sediment incubations, and also from MPN enumerations suggested that these
sediments contain fewer Fe(III)-reducing bacteria than the U(VI)reducing sediments, which may have contributed to their failure
to reduce U(VI) in these experiments.
This work highlights the potential for stimulated microbial
U(VI) reduction to be a suitable technique for treating uranium
contaminated groundwater in situ, both at Sellafield, and given
the range of different lithology sediments tested, at other UK
nuclear sites. Should remediation of uranium in groundwater
L. Newsome et al. / Applied Geochemistry 51 (2014) 55–64
become a priority, further development of this work such as scaleup to columns prior to field deployment would clearly be
warranted.
Acknowledgements
We thank Athanasios Rizoulis and Christopher Boothman (University of Manchester) for assistance with processing and interpretation of pyrosequencing data, Paul Lythgoe, Alastair Bewsher and
John Waters (University of Manchester) and David McKendry
(Manchester Metropolitan University) for analytical support with
XRF, IC, XRD, BET and Leco Truspec, Haydn Haynes (University of
Manchester) for help with monitoring the MPN samples and Nicholas Atherton and Julian Cruickshank (Sellafield Ltd) for providing
soil samples. Beamtime at beamline B18 was funded by Grant
SP8941-2 from the Diamond Light Source. We acknowledge financial support from the Nuclear Decommissioning Authority via a
PhD student bursary, managed by the National Nuclear Laboratory.
JRL acknowledges the support of the Royal Society via an Industrial
Fellowship. We also acknowledge financial support from NERC via
the BIGRAD consortium (NE/H007768/1) and also via a BNFL
Endowment which has supported the development of the Research
Centre for Radwaste and Decommissioning.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.apgeochem.2014.
09.008.
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