Mechanism of thermal activation of sulfite and its application in the heat–electro-S(IV) system for As(III) oxidation in water

Abstract

In recent years, a new strategy of using sulfite (S(IV)) to replace persulfates (peroxydisulfate (PS) and peroxybisulfate (PMS)) for sulfate radical based advanced oxidation process (SR-AOP) has emerged as one of the potential alternatives in the field of AOPs. Although the thermal activation of PS and PMS has been investigated extensively, that of S(IV) has not yet been explored. In this work, As(III) oxidation in a heat–S(IV) system was investigated first to demonstrate the feasibility of thermal activation of S(IV). Results showed that S(IV) can be activated to oxidize As(III) by heating the reactant solution from room temperature to above 50℃. Almost complete oxidation of 5 μM As(III) in water at pH 7 with addition of 1 mM S(IV) was achieved at 70 °C within 30 min. As(III) oxidation in the heat–S(IV) system was strongly dependent on the dissolved oxygen in water. Radical-scavenging experiments and electron spin resonance analysis demonstrated that the main active species responsible for As(III) oxidation was SO4•−. On the basis of thermal activation, a new system of SR-AOPs, namely, heat–electro-S(IV) system, the coupling thermal and electrochemical activations was established with ruthenium-plated iridium electrodes at a voltage of 3 V. It achieved complete oxidation of 50 μM As(III) in water at pH 7 at 50 °C with the addition of 2 mM S(IV) after reaction for 30 min. This work provides a new activation method for SR-AOPs with S(IV) as the precursor of reactive species and helps in understanding the reaction of S(IV) autooxidation.