Sulfate reduction is a process that some anaerobic microorganisms in soils and i

Question

Sulfate reduction is a process that some anaerobic microorganisms in soils and in wastewater treatment plants use to make energy. In this process the microorganisms use carbon (e.g. ethanol) as a source of electrons for energy and the sulfate as an electron acceptor. Develop a balanced oxidation-reduction reaction for the oxidation of ethanol to CO2 and water, coupled with the reduction of sulfate to form H2S. Note that production of H2S leads to odor problems and we try to avoid allowing the system to go septic like this. (a) Write the oxidation half reaction. Identify the oxidation state of the redox active element on each side of the reaction. (b) Write the reduction half reaction. Identify the oxidation state of the redox active element on each side of the reaction. (c) Combine the oxidation half reaction and reduction half reaction appropriately to obtain a balanced net oxidation-reduction reaction. (d) Identify the oxidizing agent and the reducing agent in the net reaction.

Explanation / Answer

Since sulfate accepts electrons, it undergoes reduction.& Carbon from etahnol donates electrons & undergoes oxidation

a) oxidation haft reaction

C2H5OH ---> CO2

Upon balancing we get (add water & H+ to balance O & H)

C2H5OH + 3H2O ---> 2CO2 + 12 H+

Oxidation state of C changes from -2 to +4. It loses 6 electrons to become +4

C2H5OH + 3 H2O ---> 2CO2 + 12 H+ + 6e-

b) reduction half reaction

SO4^2- + 6H+ ------> H2S + 2H2O

oxidation state of S changes from +6 to -2. It accepts 8 electrons to go from +6 to -2

SO4^2- + 10H+ + 8e-    ------> H2S + 4H2O

equating number of electrons &

adding these reactions we get

4C2H5OH + 3SO4^2- ---> 8CO2 + 18 H+ + 3H2S

d) In the net reaction, sulfate is undergoing reduction, so it acts as an oxidizing agent & C2H5OH is undergoing oxidation, so acts as a reducing agent

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