Transcript Chabalala_S - Energy Postgraduate Conference 2013

Reduction of Uranium(VI) under Micro-aerobic
Conditions using an Indigenous Mine
Consortium
University of Pretoria
Energy Postgraduate Conference 2013
Aim & Objectives
To utilise indigenous cultures of bacteria from the local
environment to biologically reduce U(VI) to U(IV)
– Isolation and purification of microorganisms for
use in further experiments
– Characterization of microorganisms in order to
identify and classify the microorganisms involved
in the reduction of uranium (VI)
– Investigation of the reduction potential of
microorganisms that reduce uranium (VI) to
uranium (IV) using the consortium in a batch
system to establish kinetic parameters for use in
reactor scale-up
Introduction
– Among all elements currently in use in the
energy industry worldwide, uranium is the most
abundant
– Uranium-containing wastes are produced at
various steps of the nuclear fuel cycle, and vary
considerably from low level radioactive effluents
produced during uranium mining to intensely
radioactive levels in nuclear power plant, spent
fuel, and liquid wastes
– Discharge of radio-nuclides such as uranium
from contaminated sites and their subsequent
mobility in the environment is a subject of
paramount concern
– The primary radiation health effect of concern is
an increased probability of the exposed
individual developing cancer during their lifetime
Introduction
Physical/chemical processes
Treatment Options
Pump and treat processes of U(VI)-contaminated water
Involves extraction of contaminated water, followed by a
separation process on the surface
Separation processes include:
• Ion exchange
• Chemical precipitation
• Reverse osmosis
Limitation:
• Expensive to apply
Introduction
Biological processes
Offer the potential for removing metal/radionuclide
pollutants from dilute solutions, where physical
chemical methods may not be feasible.
4 mechanisms by which bacteria immobilize metals or
Radio-nuclides namely;
•
•
•
•
Bio-sorption,
Bioaccumulation,
Precipitation by reaction with inorganic ligands
Microbial reduction
Introduction
Bioreduction
• Involves the reduction of an element from a higher
to a lower oxidation state or to an elemental form
affects its solubility, resulting in its precipitation
using bacteria
Advantages:
•
•
It is not limited by saturation
Many radio-nuclides are less soluble when
reduced
Materials & Methods
Elemental Analysis of Soil
• Uranium contaminated soil was collected from a
closed uranium mine was analyzed by ICP-OES
•
Isolation of Indigenous Bacteria
• Mixed culture was obtained by inoculating basal
mineral medium (BMM) amended with glucose with
mine soil
• Bacterial cultures were purified and then incubated
Batch studies
• Pure culture batch studies were conducted in basal
mineral medium (BMM) supplemented with glucose
as a carbon source with different concentrations of
U(VI) 75, 100, 200, 400, 600 and 800 mg/L
Materials & Methods
Sampling
• A 0.5 mL sample of the homogenous solution was
collected using a syringe and then centrifuged
• The sample was then diluted with 4.5 mL of BMM
(1:10 dilution), mixed with 2 mL of complexing
reagent and analyzed in duplicate for U6+
immediately on the UV spec
• Total uranium level in each sample (U(IV) and
U(VI)) was determined by oxidizing an unfiltered
sample
with
nitric
acid
prior
to
uranium
measurement
Results
• Elemental analysis of soil
• Soil was found to contain 168 mg/kg Uranium
• Preliminary culture characterization
• Gram staining – Gram- & Gram+
• Culture characterization
• Purified and sequenced rRNA genes from
bacteria 4 species were found:
• Pantoea agglomerans
• Enterobacter cloacae
• Pseudomonas stutzeri
Removal of Uranium at Low Concentrations
100
Pseudomonas stutzeri
Pantoea agglomerans
Concentration in mg/L
80
Enterobacter cloacae
60
40
20
0
0
5
10
15
20
25
30
Time in hours
Figure 2. Uranium(VI) reduction for the three pure cultures of
bacteria Pseudomonas stutzeri, Pantoea agglomerans and
Enterobacter cloacae under an initial concentration of 75 mg/L
Removal of Uranium at High Concentrations
120
250
(A)
(B)
Pantoea agglomerans
Enterobacter cloacae
80
Pantoea agglomerans
60
40
Enterobacter cloacae
150
100
50
20
0
0
5
10
15
20
25
0
30
0
10
Time in hours
20
30
40
50
60
Time in hours
1000
500
(C)
(D)
800 mg/L
600 mg/L
Pseudomonas stutzeri
Pantoea agglomerans
400
800
Enterobacter cloacae
Concentration in mg/L
Concentration in mg/L
Pseudomonas stutzeri
200
Concentration in mg/L
Concentration in mg/L
100
Pseudomonas stutzeri
300
200
600
400
200
100
0
0
0
10
20
30
Time in hours
40
50
60
0
5
10
15
20
Time in hours
Figure 3. Uranium(VI) reduction for the three pure cultures of bacteria Pseudomonas
stutzeri, Pantoea agglomerans and Enterobacter cloacae under varying
concentrations; A: 100 mg/L B: 200 mg/L C: 400 mg/L and D: 600 and 800 mg/L
Conclusion
• Microorganisms play important roles in the
environmental fate of toxic - array of physicalchemical and biological mechanisms effecting
transformations between soluble and insoluble
phases
• Further work - Identify and characterize of
cytosolic and outer membrane proteins involved
in U(VI) reduction at a cellular level
• Kinetic modelling of uranium reduction and
cumulative removal studies should help us to
better predict and model how uranium will
behave in situ
Contribution to Scientific Community
• Achieving U(VI) reduction reaction under near zero
oxidation reduction potential (ORP) under facultative
conditions
• Developing a simplified and more reliable method for
measuring U(VI) using the Arsenazo III method
Acknowledgements
oProf EMN Chirwa (University of Pretoria)
oNational Research Foundation (NRF)
oSouth African Nuclear Human Asset & Research
Programme (SANHARP)