Enhanced removal of organics by permanganate preoxidation using tannic acid as a model compound - Role of in situ formed manganese dioxide

The effect of permanganate preoxidation on organic matter removal during the coagulation with aluminum chloride was investigated using tannic acid as a model compound. Results showed that a small amount of KMnO₄ (0.75 mg/L) increased the removal eficiency of tannic acid up to 20%, as compared to the process of coagulation by aluminum chloride alone. The key factor enhancing the removal eficiency of tannic acid in preoxidation process was the in situ formation of a reductant manganese dioxide. The complexation model was used to describe the reaction between MnO₂ and tannic acid. Under weak pH condition, tannic acid was dificult to be adsorbed by MnO₂ due to the static electrical repulsive forces. The presence of Ca²⁺ served as a bridge to hold the negative charged MnO₂ and tannic acid together, which could be a crucial factor influencing tannic acid adsorption by in-situ manganese dioxide.