Transcript Suminda Hapuarachchi and TG Chasteen

Bioremediation of selenium-contaminated
environmental samples
S. Hapuarachchi and T. G. Chasteen
Department of Chemistry
Sam Houston State University
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Abstract
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A great deal of attention is now being focused towards the chemistry of toxic
selenium in water and detoxification of selenium compounds. Selenium is an
animal nutrient and has been used as anti-oxidant process but it can be fatal to
living beings if a high amount of selenium exposure occurs. For an example,
the accumulation of Se in the Kesterson Reservoir of California has been a
serious threat to the animals around that area.
It is important to address environmental problems like this. Therefore,
scientists have been trying to reduce the toxic nature of these selenium
contaminated environmental sites by introducing detoxification methods. One
of the detoxification methods currently being practiced is bioremediation. The
reducing power of bacteria such as Pseudomonas fluorescens has been used to
reduce the toxicity of soluble forms of selenium.
Understanding of the effectiveness of the bioremediation process is key to the
improvement of this process. In this studies the distribution of selenium among
three different physical states generated by a living bacteria culture and new
ways to improve this bioremediation method will be discussed.
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Introduction
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What is Selenium?
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Different forms of Selenium
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An element found in 1817
Name after Greek word, Selene, meaning “the moon”
Metallic form (Se0 like carbon or solid iron or aluminum)
Water soluble forms such as selenate and selenite
Gaseous forms that will bubble out of solution, (CH3)2Se
Uses of Selenium
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Glass manufacturing industry
Electronic applications such as rectifiers, solar batteries
Use in plastics, paints, enamels, ink and rubber
Semi-conductor materials
 Environmental
problems
associated with selenium
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Water contamination
 Power
River Basin, Wyoming
 Kesterson Reservoir of California
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Soil contamination
 Se
contamination affecting plants and animals
Environmental Cleanup Methods
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Biological Treatments
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Filtration after pH adjustment
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Evaporation and soil removal
Bioremediation
Different microbial pathways for the
metabolism of toxic compounds facilitate
the removal of these compounds from the
environment.
 Bacterium like Pseudomonas fluorescens
can detoxify soluble selenium ions by
reducing them to insoluble and other less
toxic forms.
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Aim of this study
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Calculate the distribution of selenium
among three different physical states
generated by a living bacteria culture
exposed to toxic forms of Se.
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Modify this process to improve
effectiveness of bioremediation process.
Experimentation
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Bioreactor experiments
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Anaerobic culture growth
(without O2 present)
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Sequential anaerobic/aerobic growth
(without O2) followed by aerobic (with O2)
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Sample analysis
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Inductively coupled plasma spectrometry
Culture, before starting growth
Figure 1. Bioreactor
Culture after 72 hr growth
Results
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Mass balance with anaerobic culture growth
Se distribution (solid, liquid, gas) after 72 hrs of
growth
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Mass balance with mixed anaerobic
alternating with aerobic growth
12 hrs anaerobic growth
6 hrs aerobic growth
Total of 4 cycles (72 hrs total)
Results
Mass balance with anaerobic culture growth.
Table 1. Ten mM of selenite (n=3)
Phase
% Recovery (SD)
Liquid
92.167(±8.31)
Solid
6.900(±1.32)
Gas
0.004(±0.002)
Total Recovery
99.071(±8.07)
Results
Mass balance with anaerobic culture growth.
Table 2. One mM of selenite (n=6)
Phase
% Recovery (SD)
Liquid
66.680(±18.29)
Solid
32.440(±19.81)
Gas
0.041(±0.07)
Total Recovery
96.161(±0.62)
Results
Mass balance with anaerobic culture growth.
Table 3. Ten mM selenate (n=3)
Phase
Liquid
% Recovery (SD)
95.067(±6.98)
Solid
0.733(±0.06)
Gas
0.001(±0.001)
Total Recovery
95.801(±6.93)
Results
Mass balance w/ sequential anaerobic/aerobic
culture growth.
Table 4. Ten mM selenite (n=1)
Phase
% Recovery
Liquid
80.934
Solid
6.337
Gas
0.001
Total Recovery
87.272
Results
Mass balance w/ sequential anaerobic/aerobic
culture growth.
Table 5. One mM selenite (n=1)
Phase
% Recovery
Liquid
58.472
Solid
33.606
Gas
0.005
Total Recovery
92.083
Conclusions
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Selenite was more effectively reduced by Pseudomonas
fluorescens than selenate. (This may be because selenite is
more toxic and getting rid of it as a solid is more useful.)
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When low amounts of selenite are present in the solution,
reducing efficiency is higher. (Because of toxicity, less
selenite present may allow more detoxification to occur.)
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Sequential anaerobic/aerobic culture growth does not have
a big effect on this detoxification process as carried out.
We saw no real difference in elemental Se product between
cultures grown completely anaerobically as compared to
mixed anaerobic and aerobic periods.
Acknowledgement
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I would like to thank fellow research group
members for their continuous contribution.
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Thanks for Dr. T.G. Chasteen for his
valuable guidance and advice given to me.
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Thanks to the Robert A. Welch Foundation
funding of this work.