Mn(II)-enhanced oxidation of benzoic acid by Fe(III)/H2O2 system

• Mn(II) enhances the oxidation of BA by Fe(III)/H2O2 system.
• Mn(II) does not change the HO mechanism for the oxidation of BA.
• The variations of initial kobs and t1/2 of BA are both nonlinear with the concentration of Mn(II).
• Mn(II) in Fe(III)/H2O2 system promotes the production of HO2/O2-.

This study investigated the enhanced oxidation of benzoic acid (BA) by Fenton-like system (Fe(III)/H2O2) with manganese ion (Mn(II)). Experiments were conducted at pH 3.0, with Fe in the low concentration, H2O2 in excess and Mn(II) in a wide range. In all cases, Mn(II), which is very inert to H2O2, significantly enhanced the oxidation of BA, accelerated the formation of Fe(II) and the decomposition of H2O2. The pseudo-first-order rate constant for the initial lag phase and the half-life of BA were calculated to evaluate the degree of enhancement in the presence of Mn(II). As a primary product, the concentration of p-hydroxybenzoic acid (pHBA) was monitored during the oxidation. The ratio between formed pHBA and oxidized BA indicated that HO dominated the oxidizing process, and this was further confirmed by the inhibition of tert-butanol on the oxidation of BA. Finally, the role of Mn(II) in Fe(III)/H2O2 system was analyzed, and a series of reactions including manganese species, HO, HO2/O2- and H2O2 were proposed to be the possible reason for the enhancement.