Rapid oxidation of paracetamol by Cobalt(II) catalyzed sulfite at alkaline pH

Abstract

In this study we have investigated the efficiency Cobalt (II) (Co(II)) for the activation of sulfite ions following the oxidation of paracetamol used as model contaminants. Physico-chemical parameters that can impact the paracetamol degradation (pH, initial paracetamol concentration, Co(II)/S(IV) molar ratio, oxygen concentration) and contribution of various radicals were investigated in order to elucidate the chemical mechanism. Main results show that the pH is a key factor controlling the efficiency in the system Co(II)/Sulfite. Higher efficiency is observed for pH between 9.0 and 10.0. Increasing S(IV) concentrations, until 1 mM, slightly promoted the degradation of paracetamol. In fact, an excess of sulfite ions inhibits the reaction through the scavenging of SO4− and SO5−. Moreover, degradation efficiency drastically decreases from ∼ 85% to less than 5% in absence of oxygen. SO4− was confirmed to be the main oxidant responsible for the paracetamol degradation. For the first time we determined the second order rate constant between SO4− and paracetamol (1.33 ± 0.79 × 109 M−1 s−1 (at pH 5) and 6.14 ± 0.99 × 108 M−1 s−1 (at pH 11.0)). Moreover, radical-scavenging experiments also suggest the possible implication of SO5−. Hence, this work provides a precise understanding of the overall mechanism and a new promising strategy by using sulfite and transition metal such as Co(II) to promote organic compounds degradation in water under neutral and alkaline pH conditions.