The role of sulfate radicals and pH in the decomposition of persulfate in aqueous medium: A step towards prediction

Abstract

The decomposition of persulfate (S2O42−) is a relevant part of free-radical polymerizations in aqueous media, advanced oxidation processes and nanocellulose production. For the first time, dispersed information reported in the literature about the different reactions that occur in the process were consistently integrated into a kinetic model. A sequence of calculations is proposed to evaluate some unknown rate coefficients mainly at 50°C using experimental kinetic data reported in the literature. It is proposed that the decomposition of persulfate occurs by a set of (at least) four parallel reactions one of which is first order (the thermal path) and the others second order. According to the model results, the predominance of each of them depends on the pH except for the thermal path that is present at any pH. Under strong acid and alkaline conditions at constant pH, first-order/pseudo first-order kinetics is obtained (the observable rate coefficient kd0 is constant). At intermediate pHs, where the concentration of sulfate radicals [SO4*−] is maximized, the second order character of the reaction between S2O42− and SO4*− and the autocatalytic-like behavior of the latter become evident (kd0 decrease with time). Notwithstanding the differences between theory and experiment for certain conditions observed in this work, the proposed mathematical model is able to describe the overall kinetic features of the process under study, at least at 50°C. Recommendations are provided so that future experimental work carried out in different laboratories, is comparable.