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

Cláudia R Martins,Cristiane S.C Cardoso,Janete J.F Alves,Jailson B De Andrade,Wanessa B.D Carvalho

doi:10.1590/s0001-37652008000200006

Cláudia R Martins, Cristiane S.C Cardoso + Show 3 more

Open Access

https://doi.org/10.1590/s0001-37652008000200006

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Abstract

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.

Highlights

The process of sulfur dioxide oxidation plays an important role in atmospheric chemistry (Eathoug et al 1994, Cox et al 1986)
The S(IV) oxidation reaction was monitored under different conditions, which consisted of passing a purified air or NO2 flow (15 or 130 ppbv) through the S(IV) solution (10−4 or 10−5 mol/L) in the absence and presence of transition metal ions
The results indicated that the addition of NO2 to the system hindered the oxidation of S(IV) at both pH 3.7 and pH 5.2, and that this effect was more marked at the higher NO2 concentration

Summary

Introduction

The process of sulfur dioxide oxidation plays an important role in atmospheric chemistry (e.g., formation of acid rain and degradation of visibility) (Eathoug et al 1994, Cox et al 1986). SO2(g) oxidation in the atmosphere, either homogeneously in the gas phase or heterogeneously in an aqueous-phase, must be associated with other reaction processes such as photochemical oxidation or catalytic oxidation. These processes depend on a variety of factors, such as the intensity of incident solar radiation, relative air humidity, and the presence of various oxidants (e.g., O3, H2O2, NOx , OH radicals) and transition metal ions that are known to act as catalysts (Martin 1984, Zuo and Zhan 2005). Special care was taken to ensure the purity of the water, because traces of transition metals remaining in the water as a result of deficiencies in the purification process can accelerate the reaction

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