NO₂-Catalyzed Sulfite Oxidation

Rates of sulfite (SO 3 2– ) oxidation during nitrogen dioxide (NO 2 ) absorption into sulfite solutions were measured using a high-gas-flow sparging apparatus. SO 3 2– absorbs NO 2 via a free-radical mechanism that produces a sulfite radical (SO 3 •– ) and nitrite. The radical then catalyzes SO 3 2– oxidation to form sulfate. Free-radical scavengers such as thiosulfate can be added to suppress radical concentrations and inhibit sulfite oxidation. The effects of thiosulfate, sulfite, NO 2 absorption, temperature, and dioxygen (O 2 ) partial pressure on sulfite oxidation were investigated under sodium hydroxide (NaOH) scrubbing conditions. Oxidation is inverse half-order in thiosulfate, first-order in sulfite, and half-order in absorbed NO 2 . Sulfite oxidation shows little dependence on O 2 partial pressures above 5 kPa but has a strong dependence on O 2 at lower partial pressures. Oxidation from 20 to 65 °C was fit using the Arrhenius equation with an activation energy of 24.1 kJ/mol. The addition of 0.01 mM iron increased oxidation rates by a factor of 3 compared to solutions with 0.01 mM ethylenediaminetetraacetic acid added to chelate trace metals. A standard NaOH scrubber would need approximately 50 mM thiosulfate in the circulating solvent to maintain 10 mM SO 3 2– while removing 100 ppm of SO 2 and 2.5 ppm of NO 2 .