REDUCING SELENATE TO SELENIUM

[w0124148]

I desperately need help with the following problem:

A number of bacteria, including Bacillus, Acinetobacter, and Pseudomonascan convert selenate (SeO₄ 2-) to nontoxic elemental selenium. The bacteria do this for their own survival - to prevent accumulation of toxic levels in their cells.

What is the chemical reaction that shows how the bacteria make Se0 using hydrogen sulfide (H₂S) and selenate?

I think that it has something to do with ferroselite (FeSe₂) oxidizing the sulfur, but I'm not sure. Thanks in advance for the help!!!

[expert]

I can only propose that it’s not hydrogen sulfide but rather Cysteine as a part of a protein that reduces the selenate:
2 Protein-SH + SeO₄ (2-) + H(+) = Protein-S-S-Protein + Se + H₂O

Two hydrosulfide groups in protein turn into disulfide group resulted in crosslink in the protein (could be an enzyme) and reduction of selenate.
Hydrogen sulfide will react in a similar fashion forming sulfur and selenium:
H₂S + SeO₄ (2-) + H(+) = S₈ + Se + H₂O

[w0124148]

The problem specifically says using hydrogen sulfide and selenate. I've racked my brain and talked to many of my old professors; they were just as perplexed. This is all I can come up with, and I'm not sure if it's even balanced. Thanks for the help!!

SeO₄(2-) + 4H₂S ⎯⎯→ Se0 +4H₂O + 4S0

[expert]

Your equation is impossible since you have double negative charge on the left and no charge on the right. You need protons for this reaction, so go with my equation above. The real formula for free sulfur is S₈

http://en.wikipedia.org/wiki/Sulfur

If you don’t like S₈ then use formula S and go with the following equation:

3 H₂S + H₂SeO₄ = 3 S + Se + 4 H₂O

If you better like an ionic form the use the following:
3 H₂S + SeO₄(2-) + 2 H(+) = 3 S + Se + 4 H₂O