Biological sulfide production

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Hydrogen sulfide by biological sulfate reduction

It is generally known that sulfate reducing bacteria (SRB) can convert dissolved sulfate into sulfide in a process called biological sulfate reduction. These rapid sulfide-generating bacteria are able to conserve energy by the reduction of sulfur oxyanions like sulfate, sulfite and thiosulfate. The energy required for the reduction of sulfate (SO42-) is supplied by feeding an energy source (‘electron donor’) like ethanol or acetic acid (CH3COOH). A typical overall conversion equation is (neglecting the small amount of organic material required to produce biomass):

SO42- + CH3COOH + 2 H+ → HS- + 2 HCO3- + 3 H+

The sulfide that is formed can react with dissolved metals (Me2+) like copper to form metal sulfide:

HS- + Me2+ → MeS + H+

This shows that biological sulfate reduction and metal sulfide precipitation can be combined.

There are several applications where metal sulfide precipitation is the best solution but where the effluent or process water characteristics are not compatible with a biological process. An example is when water properties like pH, temperature, salinity and overall composition, are outside the working range of the bacteria. An example is electrolyte bleed in copper refining. Sulfide precipitation using biologically produced sulfide is still possible but the process stream and the bioreactor must be separated.

Example: Thioteq technology - off line biogenic sulfide production

The Thioteq process is based on a different concept than straight biological sulfate reduction. The biological process and the metal precipitation are split over two compartments that are connected with a gas recycle. The water to be treated passes only through the chemical stage (contactor). Sulfide is produced in the biological stage (bioreactor) and transported to the chemical (precipitation) stage with a carrier gas. The sulfide is then used to recover metal as its metal sulfide.

Schematic flowsheet

THIOTEQ water treatment and recovery of one metal product.

  • Click the details on the schematic flowsheet to learn more.
Alkali sourceEffluentMetal sulfide productTHIOTEQ clarifierTHIOTEQ contactorTHIOTEQ contactorTHIOTEQ bioreactorTHIOTEQ bioreactorH2SSulfurNutrientsContaminated water
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Animation of the process regulation

Example of biological sulfide production: Flash animation showing the regulation of the Thioteq process.

  • Start animation by pressing any of the radio buttons in the top left corner.
  • Stop animation by reloading the page.

The normal case is Design. The compartment on the lower side is the bioreactor. Bacteria convert elemental sulfur (S0) and Electron donor (i.e. acetic acid or ethanol) into hydrogen sulfide. The hydrogen sulfide (H2S) is stripped from the bioreactor with a gas. This H2S-rich gas is transported to the contactor (orange rounds). A metal-rich stream is pumped through the contactor (indicated by the yellow rounds). The H2S from the rich gas reacts in the contactor with dissolved metal to form a metal sulfide. The suspended metal sulfide flows out of the contactor to the ‘TPS’ section. Here, the metal sulfide is allowed to settle and is then withdrawn as a slurry for further processing. The ‘rich gas’ that has supplied it H2S to the contactor becomes a ‘lean gas’. The lean gas is recycled back to the bioreactor (indicated by the green rounds). This is the process under design conditions.

But how can it be avoided that the contactor is oversupplied with H2S where there is less than the design quantity of the metal in the feed to the contactor? How this works can be seen when the button 50% is clicked. Part of the rich gas is now recycled past the contactor and flows directly back to the bioreactor. Through proper controls, the right amount of H2S without overdosing is still supplied to the contactor. The gas that is recycled back to the bioreactor now has a higher H2S concentration. This results in a higher dissolved H2S concentration in the bioreactor. This doesn’t harm the bacteria. But it does slow them down as they cannot produce H2S so easily anymore due to the higher H2S concentration. The feeding of the electron donor should be adjusted to the lower request for H2S in order not to waste this.

The situation when no H2S is required can be simulated when the button 0% is clicked. Now, all H2S is recycled directly back to the bioreactor. The H2S concentration goes to high and the bacteria stop producing H2S. The electron donor should be completely stopped now. When the H2S requirement increases again (press for example 50%), the H2S goes down and the bacteria start working again.

Read more

  • Peters, R. W., and Ku, Y., AIChE Symposium series separation of heavy metals 81 (1985), 9-27.
  • Bhattacharyya, D., Jumawan, A.B., Sun, G., Sund-Hagelberg, C., Schwitzgebel, K., in: Bennett, G. F. (ed.) Water-1980, AIChE Symposium Series, 77, American Institute of Chemical Engineering, New York (1981), 31-38.
  • Widdel, F., and Hansen, T. A.. , in: A. Balows, H. G. Trüper, M. Dworkin, W. Harder and K.-H. Schleifer, (eds.), The prokaryotes, 2nd. Edn., Springer-Verlag, New York (1992), 583-624.

See also

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