| The sulfite oxidation reaction was studied by measuring the rate of oxygen absorption across an unbroken interface into 1 liter of unsparged, agitated sulfite (10 mM) and catalyst (0.01 to 100 mM) solution at pH 4-6 and 50(DEGREES)C. At catalyst concentrations enabling enhanced absorption all the incoming oxygen was consumed in a thin interfacial reaction zone and the bulk solution oxygen concentration was zero. Multivalent Fe, Mn, Co, Cu and Cr ions were potent catalysts under these conditions, univalent Ni was inactive. At 10 mM these catalysts gave pseudo-first order (in oxygen) rate constants of 8.6, 43, 4.7, 95 and 11 s('-1), respectively. However, values for Cu and Cr are uncertain because of long transient times. Dry catalyst added in its upper valence state (ferric, cupric, chromic) produced high initial rates that fell in one to six hours to steady state while catalyst added in its lower state (ferrous, manganous, cobaltous) showed no high rates and reached steady state in less than one minute. Ferric and ferrous eventually resulted in the same rate so the catalytically active agent was probably some ferric species--possibly FeOOH--but was solubility-limited at a total Fe concentration of about 0.01 mM at pH 5 resulting in an enhancement factor of 2.4 at all higher concentrations. The iron ions probably cycle, being reduced during initiation and re-oxidized in the reaction zone. Fe was a much stronger catalyst than Mn or Co but its rate was limited by ferric solubility. Thiosulfate (0.05-1 mM) had a stronger inhibiting effect and efficiency on Mn than on Fe. EDTA was an effective inhibitor for Fe at equal or greater concentrations. A generalized free radical mechanism is proposed to model the initiation, termination and inhibition of radical chain reactions. Rates for Fe and Co increase with pH from 4 to 5, perhaps due to an effect upon the catalytic regeneration reaction, while Mn was unchanged. Strong positive Mn-Fe synergisms were found to cause absorption rates of up to five times those expected. |
