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Concentration of Dissolved Hydrogen Sulfide Species: Dissolved Hydrogen Sulfide in Marsh Water

(a) Anaerobic bacteria living at the bottom of a shallow inlet to a salt marsh are generating hydrogen sulfide as a by product of their metabolism. Although this compound is ordinarily found as a gas, it is reasonably soluble in water. What is the maximum concentration of dissolved hydrogen sulfide species (H2S, HS-, and S2-)that could theoretically accumulate in the waters of the inlet if the pH is 6.0?

(b) A small amount of dibromomethane has been spilled into the above inlet. Assume the partial pressure of the H2S is 0.1 atm. If the principal process that will affect the fate of this species is its reaction with HS-, for which the relevant rate constant is 5.25E-5 /M/sec, how many days will be required for the dibromomethane concentration to decrease to 10% of its initial value?

Solution Preview

a. Remember Henry's constant H = concentration in gas/ conc. in water
Thus the maximum concentration for each of the species would be given by the K value and Henry's constant assuming pH2S= 1 atm. For example, KH = pH2S/ [H2S] aq
which we rearrange and solve:
[H2S]aq = pH2S/ KH = 1 atm x mol/L / 10^ ...

Solution Summary

Concentration of dissolved hydrogen sulfide species resulting from the presence of anaerobic bacteria is calculated.