NO2-Catalyzed Sulfite Oxidation

Rates of sulfite (SO32-) oxidation during nitrogen dioxide (NO2) absorption into sulfite solutions were measured using a high-gas-flow sparging apparatus. SO32- absorbs NO2 via a free-radical mechanism that produces a sulfite radical (SO3 center dot-) and nitrite. The radical then catalyzes SO32- 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, NO2 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 NO2. 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 degrees 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 ethylenediarninetetraacetic acid added to chelate trace metals. A standard NaOH scrubber would need approximately 50 mM thiosulfate in the circulating solvent to maintain 10 mM SO32- while removing 100 ppm of SO2 and 2.5 ppm of NO2.