Influence of NO2 and dissolved iron on the S(IV) oxidation in synthetic aqueous solution

The influence of dissolved NO2 and iron on the oxidation rate of S(IV) species in the presence of dissolved oxygen is presented. To match the conditions in the real environment, the concentration of iron in the reaction solution and trace gases in the gas mixture was typical for a polluted atmosphere. The time dependence of HSO3−, SO42−, NO2− and NO3− and the concentration ratio between Fe(II) and total dissolved iron were monitored. Sulphate formation was the most intensive in the presence of an SO2/NO2/air gas mixture and Fe(III) in solution. The highest contribution to the overall oxidation was from Fe-catalysed S(IV) autoxidation. The reaction rate in the presence of both components was equal to the sum of the reaction rates when NO2 and Fe(III) were present separately, indicating that under selected experimental conditions there exist two systems: SO2/NO2/air and SO2/NO2/air/Fe(III), which are unlikely to interact with each other. The radical chain mechanism can be initiated via reactions Fe(III)–HSO3− and NO2–SO32−/HSO3−.