The iron catalyzed oxidation of sulfur(IV) in aqueous solution: Differing effects of organics at high and low pH

Abstract

We have studied the oxidation of sulfur dioxide by dissolved oxygen in highly dilute solutions with a new differential optical absorption technique. We measured the rate of oxidation catalyzed by iron(III) over a wide range of pH, ionic strength, and in the presence of various organic materials. The studies indicate that noncomplexing organic molecules are highly inhibiting at “high” pH values of 5 and above and are not inhibiting at “low” pH values of 3 and below. Furthermore, the order of the reaction with respect to iron is different in the two pH regimes. This suggests that the mechanism of this reaction differs in the two pH regimes and is probably a free radical chain at high pH and a nonradical mechanism at low pH. Some of the mechanisms proposed in the literature are discussed in the light of these new data. None of the proposed mechanisms give completely satisfactory agreement with the data. We propose a modified free radical chain mechanism for the high pH regime, which correctly predicts the organic inhibitions. For the low pH regime, mechanisms proposed by Conklin and Hoffmann (1988) and by Hoffmann and Jacob (1984) give fair agreement with the pH data and correctly predict the self‐inhibition, the sulfate inhibition, and the ionic strength inhibition. In view of the new data we believe that the iron(III) catalyzed reaction in tropospheric clouds can be a major contributor to the rate of sulfate formation, but there will be significant inhibition of this process by formic acid in some situations.

The role of transition metal ions on HO<sub>x</sub> radicals in clouds: a numerical evaluation of its impact on multiphase chemistry

“…Catalysis of S(IV) autoxidation by iron and manganese has been investigated in many laboratory studies (Conklin and Hoffmann (1988), Kraft and van Eldik (1989), Martin et al (1991), Grgić et al (1991), Berglund et al (1993), Kotronarou and Sigg (1993), Brandt et al (1994), Berglund and Elding (1995), Warneck and Ziajka (1995)). In Grgić et al (1998;, laboratory investigations showed that oxalate has a strong inhibiting effect on the conversion of S(IV) to S(VI) in the presence of Fe(II) or Fe(III) due to the formation of Fe(III)-oxalato-complexes.…”

Química atmosférica do enxofre (IV): emissões, reações em fase aquosa e impacto ambiental

“…A oxidação do S(IV) em fase aquosa, por O 2 , tem sido estudada por vários pesquisadores [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] , sendo que os resultados têm sido objeto de considerável controvérsia, em diversos aspectos. Para reações não catalisadas, a dependência com a concentração do oxigênio é discutida na literatura (Tabela 11).…”

“…Alguns autores estudaram a oxidação de S(IV) catalisada por metais em uma faixa de pH ampla e concluíram que o mecanismo da reação pode inclusive, ser alterado, a depender do pH. Assim, Martin e col. 13 sugeriram um mecanismo onde não há formação de radicais, para a oxidação de S(IV) catalisada por Fe(III), numa faixa de pH de 0 a 3, e um mecanismo por radical, para o mesmo processo, numa faixa de pH de 4 a 7. Segundo os autores, é impossível formular uma lei de velocidade geral, para um larga faixa de pH, sendo as seguintes equações propostas: , T=25 °C.…”

“…As várias reações que podem contribuir para a oxidação do SO 2 a sulfato em fase aquosa têm sido estudadas por muitos investigadores [9][10][11][12][13][14][15] . A oxidação por H 2 O 2 parece ser a mais favorável devido à alta velocidade de reação, porém na ausência de concentrações significativas de H 2 O 2 na atmosfera, a oxidação por ozônio ou por oxigênio catalisada por metais de transição pode se tornar significante 1,14 .…”

Speciation and preconcentration of iron by cloud point extraction combined with fibre optic linear array detection spectrophotometry

“…Iron also plays a central role in the biosphere, as an active centre of proteins responsible for O 2 and electron transfer by oxidase, reductase and dehydrase enzymes (Cotton and Wilkinson, 1988). Iron(II) is probably the preferred nutrient for phytoplankton (Martin et al, 1989) and iron(III) is considered to be an efficient catalyst for the autooxidation of SO 2 to SO 4 2 ; in clouds (Conklin and Haffmann, 1988;Martin et al, 1991). On the other hand, Fe(II) is important in the transport and storage of oxygen in higher animals by means of haemoglobin and myoglobin, while Fe(III) will not bind to oxygen (Nicholls, 1973).…”