Bioremediation of selenium-contaminated
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Bioremediation of selenium-contaminated environmental samples S. Hapuarachchi and T. G. Chasteen Department of Chemistry Sam Houston State University. Abstract

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Bioremediation of selenium-contaminated environmental samplesS. Hapuarachchi and T. G. Chasteen Department of ChemistrySam Houston State University


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  • Abstract

  • 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

  • What is Selenium?

    • An element found in 1817

    • Name after Greek word, Selene, meaning “the moon”

  • Different forms of Selenium

    • 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

    • Glass manufacturing industry

    • Electronic applications such as rectifiers, solar batteries

    • Use in plastics, paints, enamels, ink and rubber

    • Semi-conductor materials


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  • Soil contamination

    • Se contamination affecting plants and animals


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    Environmental Cleanup Methods

    • Biological Treatments

    • Filtration after pH adjustment

    • Evaporation and soil removal


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    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

    • Calculate the distribution of selenium among three different physical states generated by a living bacteria culture exposed to toxic forms of Se.

    • Modify this process to improve effectiveness of bioremediation process.


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    Experimentation

    • Bioreactor experiments

      • Anaerobic culture growth

        (without O2 present)

      • Sequential anaerobic/aerobic growth

        (without O2) followed by aerobic (with O2)

  • Sample analysis

    • Inductively coupled plasma spectrometry


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    Culture, before starting growth

    Culture after 72 hr growth

    Figure 1. Bioreactor


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    Results

    • Mass balance with anaerobic culture growth

      Se distribution (solid, liquid, gas) after 72 hrs of growth

    • Mass balance with mixed anaerobic alternating with aerobic growth

      12 hrs anaerobic growth

      6 hrs aerobic growth

      Total of 4 cycles (72 hrs total)


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    Results

    Mass balance with anaerobic culture growth.

    Table 1. Ten mM of selenite (n=3)


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    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 2. One mM of selenite (n=6)


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    Table 3. Ten mM selenate (n=3)

    Results

    Mass balance with anaerobic culture growth.


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    Table 4. Ten mM selenite (n=1)

    Results

    Mass balance w/ sequential anaerobic/aerobic culture growth.


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    Phase

    % Recovery

    Liquid

    58.472

    Solid

    33.606

    Gas

    0.005

    Total Recovery

    92.083

    Results

    Mass balance w/ sequential anaerobic/aerobic culture growth.

    Table 5. One mM selenite (n=1)


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    Conclusions

    • 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.)

    • 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.)

    • 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.


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    Acknowledgement

    • I would like to thank fellow research group members for their continuous contribution.

    • Thanks for Dr. T.G. Chasteen for his valuable guidance and advice given to me.

    • Thanks to the Robert A. Welch Foundation funding of this work.


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