NO2‑Catalyzed Sulfite Oxidation

Rates of sulfite (SO3 2–) oxidation during nitrogen dioxide (NO2) absorption into sulfite solutions were measured using a high-gas-flow sparging apparatus. SO3 2– absorbs NO2 via a free-radical mechanism that produces a sulfite radical (SO3 •–) and nitrite. The radical then catalyzes SO3 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, NO2 absorption, temperature, and dioxygen (O2) 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 O2 partial pressures above 5 kPa but has a strong dependence on O2 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 SO3 2– while removing 100 ppm of SO2 and 2.5 ppm of NO2.