Redox

Question 1

How do you balance redox reactions?

Answer 1

a) Oxidised (e- freed) vs reduced (e- absorb).
b) Balance e-
c) Balance final charge with H+
d) Balance H with H20
e) Balance O with O2

Question 2

(1) Explain how and why dissolved Al and Fe differ in geochemical mobility in natural freshwaters that are a) oxic and b) anoxic.

Answer 2

Anoxic: Fe immobile with sulfides. Fe2+ can be mobile without sulfides.
Oxic: Fe forms hydroxides -> immobile.

Iron mobility dependent on redox.
Al is not a redox element. Al forms oxyhydroxides in oxic and anoxic environments, depending on pH.

Question 3

Does sulphate diffuse up or down in the sediment.

Answer 3

Sulphate diffuses downward. Diffusion happens from high to low concentration, and since sulphate decreases with depth due to sulphate reduction the diffusion is downward. Sulfate works as electron donator for the reactions in the redox ladder.

Question 4

In which order does the redox ladder happen?

Answer 4

02 -> H20
NO3- -> N2
MnO2 -> Mn2+
Fe(OH)3 -> Fe2+
SO42- -> H2S
CH20 -> CH4

Question 5

Explain the concept redox ladder

Answer 5

A redox gradient is a series of reduction-oxidation (redox) reactions sorted according to redox potential. The redox ladder displays the order in which redox reactions occur based on the free energy gained from redox pairs. These redox gradients form both spatially and temporally as a result of differences in microbial processes, chemical composition of the environment, and oxidative potential. Common environments where redox gradients exist are coastal marshes, lakes, contaminant plumes, and soils.

Question 6

How does weathering of pyrite (FeS2) affect pH? Show in a balanced reaction how pyrite is weathered . (B) Why does weathering of ZnS have no effect on pH?

Answer 6

a) Weathering of pyrite causes a big release of H+ -> lowered pH.

4FeS2 + 14H2O + 15O2 -> 4Fe(OH)3 + 8SO42- + 16H+

b) Other sulfides than iron go into solution and does not form hydroxides and therefore have no effect on pH.