Influence of NO2 and metal ions on oxidation of aqueous-phase S(IV) in atmospheric concentrations

An investigation was made of the influence of atmospheric concentrations (15 or 130 ppbv) of NO2 on the aqueous-phase oxidation rate of S(IV) in the presence and absence of Fe(III), Mn(II) and Cr(VI) metal ions under controlled experimental conditions (pH, T, concentration of reactants, etc.). The reaction rate in the presence of the NO2 flow was slower than the reaction rate using only clean air with an initial S(IV) concentration of 10-4 mol/L. NO2 appears to react with S(IV), producing a kind of inhibitor that slows down the reaction. Conversely, tenfold lower concentrations of S(IV) ([S(IV)]º = 10-5 mol/L) caused a faster reaction in the presence of NO2 than the reaction using purified air. Under these conditions, therefore, the equilibrium shifts to sulfate formation. With the addition of Fe(III), Mn(II) or Cr(VI) in the presence of a NO2 flow, the reaction occurred faster under all the conditions in which S(IV) oxidation was investigated.

A reação de oxidação de S(IV) em fase aquosa foi estudada em laboratório em presença de NO2 dos íons metálicos Fe(III), Mn(II), e Cr(VI) sob condições experimentais controladas (pH, T, concentração dos reagentes, etc.). Na presença de corrente de ar com NO2 (15 ou 130 ppbv) a reação de oxidação de S(IV) ocorreu mais lentamente do que na presença de ar purificado, para uma concentração inicial de S(IV) de 10-4 mol/L. Ao contrário, para concentração inicial de S(IV) dez vezes menor ([S(IV)]° = 10-5 mol/L) a reação ocorreu mais rapidamente na presença de NO2. A explicação está relacionada com o equilíbrio envolvendo a formação de espécies intermediárias de longa vida, que impedem o prosseguimento da reação, porém a depender das concentrações relativas de S(IV) e NO2, essas espécies se decompõem deslocando o equilíbrio no sentido de formação de sulfato. A adição dos íons Fe(III), Mn(II) ou Cr(VI) em presença de corrente de ar com NO2 indicou atividade catalítica para esses íons, em todas as condições nas quais a oxidação de S(IV) foi investigada.

Influence of NO2 and metal ions on oxidation of aqueous-phase S(IV) in atmospheric concentrations.

An investigation was made of the influence of atmospheric concentrations (15 or 130 ppbv) of NO2 on the aqueous-phase oxidation rate of S(IV) in the presence and absence of Fe(III), Mn(II) and Cr(VI) metal ions under controlled experimental conditions (pH, T, concentration of reactants, etc.). The reaction rate in the presence of the NO2 flow was slower than the reaction rate using only clean air with an initial S(IV) concentration of 10(-4) mol/L. NO2 appears to react with S(IV), producing a kind of inhibitor that slows down the reaction. Conversely, tenfold lower concentrations of S(IV) ([S(IV)]o masculine=10(-5) mol/L) caused a faster reaction in the presence of NO2 than the reaction using purified air. Under these conditions, therefore, the equilibrium shifts to sulfate formation. With the addition of Fe(III), Mn(II) or Cr(VI) in the presence of a NO2 flow, the reaction occurred faster under all the conditions in which S(IV) oxidation was investigated.