Kinetics of the Oxidation of Hydrogen Sulfide by Atmospheric Oxygen in an Aqueous Medium

Hydrogen sulfide is an important acid rain precursor and this led us to investigate the kinetics of its oxidation in aqueous phase by atmospheric oxygen. The kinetics was followed by measuring the depletion of oxygen in a reactor. The reaction was studied under pseudo order conditions with [H2S] in excess. The kinetics followed the rate law: -d[O2]/dt = k[S][O2]t (A) Where [S] represents the total concentration of hydrogen sulfide, [O2]t is the concentration of oxygen at time t and k is the second order rate constant. The equilibria (B - C) govern the dissolution of H2S; the sulfide ion in water forms different species: H2S K1 HS- + H+ (B) HS- K2 S2- + H+ (C) Where K1 and K2 are first and second dissociation constants of H2S. Although, H2S is present as undissociated H2S, HS- and S2- ions, nature of [H+ ] dependence of reaction rate required only HS- to be reactive and dominant. The rate law (A) on including [H+ ] dependence became Equation (D). -d[O2]/dt = k1K1[H+ ][S][O2]t / ([H+ ] 2 + K1[H+ ] + K1K2) (D) Our results indicate anthropogenic VOCs such as acetanilide, benzene, ethanol, aniline, toluene, benzamide, o-xylene, m-xylene, p-xylene and anisole to have no significant effect on the reaction rate and any observed small effect is within the uncertainty of the rate measurements.