Oxidation of sulfur (IV) by oxygen in aqueous solution: role of some metal ions

Abstract

No presente trabalho foi avaliado o papel de íons metálicos tais como: Cr(VI), Cr(III), Cd(II), V(V) e do íon cloreto, no processo de oxidação do S(IV) em solução aquosa. As concentrações dos íons metálicos, íon cloreto e do S(IV) foram escolhidas de modo a representarem as frações solúveis destes em aerossóis urbanos. Os estudos relativos ao processo de oxidação do S(IV) foram conduzidos em condições experimentais controladas tais como: (pH (2,1-4,5), T (11,0-35,0 °C), vazão de O2, concentração de reagentes, etc...). A constante cinética, K, determinada a 25,0 °C e na faixa de pH estudada (2,1-4,5), utilizando-se água pura, apresentou um valor médio de 8,0 ± 0,5 x 10-4 s-1 , e este valor foi tomado como referência para reação de oxidação não catalisada. Os valores das constantes cinéticas determinadas, na presença de Cr(VI), revelaram que a reação de oxidação do S(IV) é bastante influenciada pela acidez do meio. Em pH 2,1 (K = 2,3 x 10-2 mg-1 L s-1) a reação se processa com velocidade específica cinco vezes superior quando comparado a pH 2,6 (K = 4,3 x 10-3 mg-1 L s-1) e trinta vezes superior quando comparada a pH 3,4 (K = 8,0 x 10-4 mg-1 L s-1). A seguinte expressão de velocidade foi obtida para o sistema envolvendo Cr(VI) em pH 2,6:-r S(IV) = K [Cr(VI)] [S(IV)] e a energia de ativação encontrada foi: Ea = 70,3 kJ/ mol. Não foi observado efeito catalítico para o íon metálico Cd(II) e para cloreto, porém efeitos catalíticos inibitórios foram observados para os íons Cr(III) e V(V). Catalytic effect of metal ions: Cr(VI), Cr(III), Cd(II), V(V) and chloride anion, on the oxidation of S(IV) in aqueous solution, at concentrations of metal ions and S(IV) usually found in urban atmospheres, were studied under controlled experimental conditions (pH (2.1-4,5), T (25.0-35.0°C), air flow rate, concentration of reactants, etc...). The kinetic constant determined at 25.0 °C and pH range (2.1-4.5), using ultra pure water was 8.0 ± 0.5 x 10-4 s-1. This value was considered as a reference for the oxidation reaction rate. The kinetic constants determined in the presence of Cr(VI) revealed that the oxidation reaction of S(IV) is quite influenced by the acidity. At pH = 2.1 (K = 2.3 x 10-2 mg-1 L s-1) the reaction is carried out with a rate five times greater when compared to pH = 2.6 (K = 4.3 x 10-3 mg-1 L s-1) and thirty times greater when compared to pH = 3.4 (K= 8.0 x 10-4 mg-1 L s-1). The following rate expression was obtained at pH = 2.6:-r(S(IV) =K [Cr(VI)] [S(IV)] and the activation energy found was: Ea =70.3KJ/mol. No catalytic effects were observed for Cd(II) or chloride ion, while inhibitory effects were observed for Cr(III) and V(V) ions.

Use of Cu2+ as a metal ion probe for the EPR study of metal complexation sites in the double sulfite CuI 2SO3.CdII SO3 .2H2O

“…The results of the elemental analysis were in good agreement with the Cu 2 The X-ray diffraction data (Table 1) provided evidence for Cu II replacement by Cd II in Chevreul's salt. As can be seen, the patterns of the substituted sulfite resemble those of Chevreul's salt with minor changes in "d" spacings.…”

“…cloud droplets and haze aerosols), where trace metal catalysis of the autoxidation of S IV has been postulated as an important pathway for SO 2 conversion. 2,3 An X-ray study revealed the presence of two types of copper atoms, one with a distorted octahedral coordination, CuO 4 (H 2 O) 2 , and the other with a distorted tetrahedral coordination, CuO 3 S (Figure 1). 4 The former is in In such cases, the X-ray diffraction data were similar to those of Chevreul's salt (Figure 1), with minor changes in "d" spacings, forming an isomorphic series.…”

Isomorphic series of double sulfites of the Cu2SO3.MSO3.2H2O (M = Cu, Fe, Mn, and Cd) Type: a review

“…[28][29][30][31][32][33][34][35][36][37][38][39][40][41] The Cu(II)-S(IV) system was proposed by Conklin …”

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

“…Despite the lack of a consensus about which metals are more active catalyzers in this type of reaction, Fe(III) and Mn(II) ions appear to be the strongest candidates, followed by others such as Cu(II), Co(II) and Ni(II) (Martin 1984, Brandt andvan Eldik 1995). Studies conducted by Brandt and Elding (1998) and Martins et al (1999) also revealed the catalytic activity of Cr(VI) ions in this type of reaction. The mechanism whereby homogeneous catalysis by transition metals occurs has been widely discussed in the literature, with the majority of reports explaining the process based on the free radical mechanism, a necessary condition being that the ion display multiple oxidation states.…”