Bioremediation of selenium-contaminated environmental samples

S. Hapuarachchi and T. G. Chasteen Department of ChemistrySam Houston State University

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.

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

Environmental problems associated with selenium

Water contamination Power River Basin, Wyoming Kesterson Reservoir of California

Soil contamination Se contamination affecting plants and animals

Environmental Cleanup Methods

Biological Treatments

Filtration after pH adjustment

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.

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.

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

Culture, before starting growth

Culture after 72 hr growth Figure 1. Bioreactor

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 growth6 hrs aerobic growthTotal of 4 cycles (72 hrs total)

Phase % Recovery (SD)

Liquid 92.167(±8.31)

Solid 6.900(±1.32)

Gas 0.004(±0.002)

Total Recovery 99.071(±8.07)

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

Mass balance with anaerobic culture growth.

Results

% Recovery (SD)Phase

96.161(±0.62)Total Recovery

0.041(±0.07)Gas

32.440(±19.81)Solid

66.680(±18.29)Liquid

Table 2. One mM of selenite (n=6)

Mass balance with anaerobic culture growth.

Results

Table 3. Ten mM selenate (n=3)

Phase % Recovery (SD)

Liquid 95.067(±6.98)

Solid 0.733(±0.06)

Gas 0.001(±0.001)

Total Recovery 95.801(±6.93)

Mass balance with anaerobic culture growth.

Results

Table 4. Ten mM selenite (n=1)

Phase % Recovery

Liquid 80.934

Solid 6.337

Gas 0.001

Total Recovery 87.272

Mass balance w/ sequential anaerobic/aerobic culture growth.

Results

58.472Liquid

92.083Total Recovery

0.005Gas

33.606Solid

% RecoveryPhase

Table 5. One mM selenite (n=1)

Mass balance w/ sequential anaerobic/aerobic culture growth.

Results

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.

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.