Stoichiometry of sulfite oxidation by oxygen during the determination of the volumetric mass transfer coefficient

Abstract

The aim of this work was to study the effect of the initial sulfite concentration on the stoichiometric coefficient of sulfite oxidation. A combined method based on the oxidation of sulfite by oxygen in the presence of cobalt(II) (0.5 mM) as catalyst was used to determine the volumetric mass transfer coefficient (kLa) and the oxygen consumption (OC) due to sulfite oxidation. Results demonstrated that the dissolved oxygen (DO) probe response constant (ksensor) was not dependent on the DO concentration or the agitation speed; obtained ksensor values were high enough to not interfere with DO or OC measurements. Using the reaction time to sulfite depletion method, volumetric mass transfer coefficient (kLa) yielded a linear increase as a function of the initial sulfite concentration. On the contrary, from the saturation phase of DO curves, an almost constant k La. value was obtained in accordance to constant air flow rate and agitation conditions that were used in the experiments. The observed stoichiometric coefficient (OC/S0) at high initial sulfite concentrations or low kLa values was lower than the theoretical one, suggesting that the formation of elemental sulfur becomes significant at these conditions.