Role of cuprous oxide in the autoxidation of aqueous sulphur dioxide and its atmospheric implications

Copper smelters emit sulphur dioxide and cuprous oxide concurrently. In humid atmosphere, aqueous cuprous oxide is likely to catalyse the autoxidation of sulphur dioxide. This led us to undertake the study of the kinetics of the autoxidation of aqueous SO2 in aqueous Cu2O suspensions. In unbuffered suspensions, the reaction is characterised by an induction period and the rate law observed is: Robs=k2[Cu2O]0.5[S(IV)][O2]0.5. The value of k2 was determined to be 1.26×10-2 ℓ mol-0.5 g-0.5 s-1 at 30°C and pH 5.5. Addition of 0.18 mol ℓ-1 ethanol did not affect the reaction. A free radical mechanism has been proposed to explain the results. In acetate-buffered suspensions, in addition to the oxidation of sulphur(IV) into sulphate, copper(I) is also autoxidised into copper(II). The atmospheric implications of this study are discussed.