Catalytic Mechanism of the “Noncatalytic” Autooxidation of Sulfite

Iron ions are shown to play a special role among transition metal ions in the oxidation of sulfite by oxygen. The thermodynamically favorable formation of chain carriers S : FeOH²⁺+ HSO₃ ^– Fe²⁺+ H₂O + , H _{r 298} ⁰ –250 kJ/mol accompanied by the regeneration of the active Fe(III) form in the reactions of Fe(II) with and HSO₅ ^–provides the efficient catalytic mechanism for sulfite consumption even at [Fe]₀ 10^–8mol/l. Any aqueous solution contains iron ions in this amounts. Thus, the noncatalytic oxidation of sulfite is in fact the catalytic reaction involving unavoidable microadmixtures of iron ions. Other transition metal ions (Mn²⁺, Co²⁺, etc.) can only enhance the catalytic effect of iron admixture.     

Iron Catalysis of SO2 Oxidation in the Atmosphere

The mechanisms of SO₂ oxidation catalyzed by iron ions in the droplet phase of the convective cloud in the lower atmosphere were examined. The relations of the catalytic SO₂ decrease to the concentration of the iron ions and to the intensity of fluxes to the droplet of the OH _(g) and HO _2(g) radicals were characterized. The determining role of the replacement of the low-reactive HO _2(g)(O^– _2(aq)) radical by the reactive SO^– _5(aq) radical in the sulfite medium during daytime was revealed. This process occurred due to the coupling of the decay of the radicals and their regeneration in the liquid-phase reactions O^– _2(aq) + FeOH²⁺ _(aq) Fe²⁺ _(aq) + OH^– _(aq) + O_2(aq), HSO^– _5(aq) + Fe²⁺ _(aq) FeOH²⁺ _(aq) + SO^– _4(aq)HSO ₃ ^- _(aq),O_{2 (aq)} SO^– _5(aq).