hydrometallurgy conference

Hydrometallurgy Conference - 2009 1

Indigenous microorganism strains as bio-extractants of Ca, Fe and Mg from metallurgical and mine drainages

By

E. Fosso KankeuA.F. Mulaba-Bafubiandi

B.B. MambaT.G. Barnard


Introduction

Metal enriched soils brought to the surface by mining activities could release metals into the environment following natural oxidation by water or air (Wang and Chen, 2006).

An excess of calcium and iron in water could cause aesthetic or operational problems while excess of magnesium in water could cause aesthetic and health problems (SABS, 2005).


Introduction continues

Remediation of higher level of metals in surface water by physico-chemical techniques has been reported to be ineffective and costly, hence the need to shift to biological techniques which are cheaper and ecofriendly (Alluri et al., 2007; Cohen, 2006).

Microorganisms possess inherent abilities suitable for the removal of metals from solutions (Nies, 1999)


Introduction continues

Indigenous microorganisms adapted to conditions in-situ are suitable for use in the removal of metals from water.


Principles of microbial activities

Metal removalIt consists to take the metal out of the

solution. Microorganisms perform this task through absorption (by physico-chemical interaction between cell membrane and metal) or accumulation (active transport of metal into the cytoplasm).


Figure 1 Absorption of copper on the cell surface of bacteria

Principles of microbial

activities


Figure 2 Active transport of Arsenate into bacterial cytoplasm (accumulation)

Principles of microbial

activities


Methodology

Water and soil samples collected around mine areas were analysed for their metal content using Inductively Coupled Plasma Spectrometer (ICP)

Microorganisms were isolated from soil and water samples using selective nutrient agar and strains were identified by gene sequencing at Inqaba Biotechnical Industries (Pty) Ltd-South Africa

Synthetic solutions of metals were prepared by diluting crystal or powder form into sterile distilled water to make a stock solution of 1000ppm.


Methodology continues

To prepare biomass of microorganisms, strains initially isolated on site were inoculated in nutrient broth (Merck SA) and incubated with shaking (150 rpm) at 37oC overnight. Cells were concentrated by centrifugation for 15 minutes at a speed of 8000 rpm. Non-living biomass was prepared by autoclaving lyophilized cells.


Methodology continues

Metal removal experiment was done by mixing microorganisms (100 mg wet cell) with 30ppm and 50ppm synthetic metal solution in 250 ml Erlenmeyer flask, the mixture was then incubated under the above conditions and 5 ml of solution was taken after one, two and twenty-four hours.

The amount of metal removed was determined by measuring the concentration of metal after exposure using ICP


Results and discussions

Gene sequencing allows to identify isolated microorganisms as Bacillus subtilis, Shewanella sp and Brevundimonas sp.

Using these microorganisms to remove metal from synthetic solution, we found that Bacillus subtilis and Shewanella sp absorbed the higher amount of each of the three metal from solution (Figs 3,4 & 5)

Calcium was the metal easily removed



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At higher concentration (50ppm) of metals in solution there was a decrease in the removal of metal by microorganisms.

Combining the metals in solution contributed to affect microorganisms affinity for the metals therefore reducing the removal efficiency.



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In many circumstances microorganisms have the tendency to release the metals in solution after the second hour of incubation.



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Exposure time (hour)

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Time (hour) vs Fe 30 ppm


Time (hour) vs Ca 30 ppm


Time (hour) vs Mg 30 ppm


Figure 3 Separate removal of Fe, Ca and Mg by Bacillus subtilis



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Figure 4 Separate removal of Fe, Ca and Mg by Shewanella sp



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0 5 10 15 20 25 30

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Figure 5 Separate removal of Fe, Ca and Mg by Brevundimonas sp



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Microbial affinity seemed to influence the removal efficiency of metals as cell wall composition (active group) could determine the binding of metal.

The decrease of percentage removal of metal after the second hour was certainly due to an efflux transport system developed by microorganism to reject the excess metal from their body or breakage of bonds between cell wall and metals.



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The inhibition of microorganisms could have affected the removal of metals as we recorded higher removal in solution containing 30ppm of metal than in solution containing 50ppm of metal.

There was a decrease in the performance of microorganisms when removing simultaneously the three metals from solution.



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Experiment on the metal tolerance by microorganisms in solution of mixed metal showed that biomass of Shewanella sp and Brevundimonas sp was adversely affected after one hour, while Bacillus subtilis resisted the presence of metal.

Relatively low metals’ removal was recorded in general, certainly due to a combination of factors such as: inhibition, efflux transport and use of small biomass (metal uptake is directly proportional to microbial biomass). Use of non-living biomass did not improve the results.


Conclusion

Metal removal in this study is largely dependent on the affinity between microbial cell wall and the metal.

Regarding the removal ability, the resistance to metal and the retention (efflux mechanism minimised) of metal, Bacillus subtilis appeared to be the microorganism that performed the best.

Possible recovery of metals from microorganisms could be done by ion exchange mechanism after concentration of microorganisms through filtration.

Hydrometallurgy Conference - 2009

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Documents

icp slide

barnard slide

surface water

methodology water

metal content

concentration of metal

water samples