Hydrogen sulfide (H2S) is an acid that can corrode the steel in expensive equipment installed in wastewater treatment plants. The Henrys law constant for H2S at 25C is 3.49 mg/L-atm. H2S gas is also toxic and the lowest-observed-adverse-effect-level (LOAEL) for asthmatic individuals is 2.8 mg/m3 . It dissociates in water with an acid dissociation constant Ka1 = 0.86 x 10-7 to yield a proton and a bisulfide anion: H2S (aq) H+ + HSIf the total sulfide concentration (H2S + HS-) in the raw wastewater in summer (WW temperature = 25C) is 3.0 mg/L,

(a) Find the fraction of H2S in the H2S (aq) form (this is the strippable fraction) as a function of pH, and draw a graph of the H2S fraction versus pH (pH 2 to pH 10).

(b) If the pH of raw wastewater is 7.5, determine whether H2S will be smelled in the air around the preliminary treatment units. Would the concentration of H2S in the air be a cause of concern?

1) determine the fraction of ammonia as a function of pH. You'll have two equations, one for the equilibrium constant that relates products and reactants and one for the fraction of H2S divided by the total sulfide species (i.e., %H2S = [H2S]/([H2S]+[HS-]) which is equal to 1/(1+[HS-]/[H2S]) when you divide numerator and denominator by [H2S] ). You can substitute one equation into the other and then solve for the %H2S at different pH values.

2) Then for part (b), if the total sulfide concentration is 3.0 mg/L, then multiply the fraction that is H2S(aq) at pH 7.5 (from part a) times the total sulfide concentration.

3) but this only gives you the H2S dissolved in water, and you'll need Henry's law to solve for the partial pressure in air (which is the concentration of H2S in the air).

4) When you do step #3 above, you'll end up with H2S in units of atm. This value needs to be converted to ppmv (note: 1 ppmv = 10^-6 atm)

5) then the H2S concentration in ppmv needs to be converted to mg/m3 so that you can compare the concentration in air with the LOAEL concentration (which is the concentration of concern).