Role of oxalate in permanganate oxidation of 4-chlorophenol

The role of oxalate in 4-chlorophenol (4-CP) oxidation by permanganate (Mn(VII)) was explored in this study. The performance of oxalate was heavily depended on pH and oxalate concentration. 4-CP degradation by Mn(VII) was significantly enhanced at pH 4.0-6.0 in the presence of oxalate, while negligible influence was observed at pH 7.0-9.0. The oxalate plays a dual role in Mn(VII) oxidation over the pH range of 4.0-6.0: one is the chelate, which coordinates with Mn(III) to form Mn(III)-oxalate complexes, and the other is the reductant, which reacts with Mn(III) to form Mn(II). The stable Mn(III)-oxalate complexes can work as an efficient oxidant for 4-CP. While their unstable counterparts, due to the lower concentration of oxalate or the higher pH, would auto-decomposed to MnO₂ and Mn(II), and then the MnO₂ works as both a catalyst and an oxidant for the decomposition of 4-CP.