Kinetics and Mechanism of the Oxidation of Aqueous Hydrogen Sulfide by Peroxymonosulfate

The stoichiometry and mechanism of the oxidation of aqueous S(−II) by HSO₅⁻ is similar to the oxidation of S(−II) by H₂o₂, but the rate of oxidation by HSO₅⁻ is 3-4 orders of magnitude faster than the corresponding reaction with H₂O₂. A two-term rate law of the following form is found to be valid for the pH range of 2.0-6.3: −d[S(−II)]/dt = k₁[H₂S][HSO₅⁻] + k₂K_a1 [H₂S] [HSO₅⁻]/ [H⁺], where k₁ = 1.98 × 10¹ M⁻¹ s⁻¹, k₂ = 1.22 × 10⁴ M⁻¹ s⁻¹, and K_a1 = [H⁺][HS⁻]/[H₂S] = 2.84 × 10⁻⁸ M at 4.9 °C, μ = 0.2 M, and [S(−II)] = [H₂S] + [HS⁻] + [S²⁻]. At high pH and high [HSO₅⁻]/[S(−II)] ratios SO₄²⁻ and H⁺ formation are favored, whereas at low pH and low [HSO₅⁻]/[S(−II)] ratios elemental sulfur (S₈) is favored as the principal reaction product. Peroxymonosulfate is a monosubstituted derivative of hydrogen peroxide that is thermodynamically more powerful as an oxidant than H₂O₂ and kinetically more reactive. These properties make HSO₅⁻ a potentially important oxidant in natural systems such as remote tropospheric clouds and also a viable alternative to H₂O₂ for the control of malodorous sulfur compounds and for the control of sulfide-induced corrosion in concrete sewers.