Enhanced removal of phenolic compounds via irreversible sorption using manganese oxides immobilized on oxidized humin

In this study, to resolve the formation of manganese ions (Mn²⁺) and radical products of the 1-napthol removal reaction via birnessite, humin was oxidized in a potassium permanganate solution to synthesize humin–manganese oxides (Ox–Hu/δ-MnO₂). Experiments were performed to compare the radical products and 1-napthol removal efficiencies of Ox–Hu/δ-MnO₂ with different humin ratios (250–1000 mg), Ox–Hu, and δ-MnO₂. In the 12.5 mg loading/batch experiment, the 1-napthol reaction rate constant (k₁ = 0.12) was over three times higher for Ox–Hu/δ-MnO₂ (250 mg) than for Ox–Hu/δ-MnO₂ (500–1000 mg) and the absorbed Mn²⁺ was effectively eluted in the reacted solution (Mn²⁺ < 0.025 mg/L). High-performance liquid chromatography results of extracted mediator reaction products revealed that most of the 1-napthol and polymer reaction products (e.g, 1,4-napthoquinon) were recovered from Ox–Hu and δ-MnO₂ but not from Ox–Hu/δ-MnO₂, confirming that the residues were strongly bound in Ox–Hu/δ-MnO₂. Therefore, Ox–Hu/δ-MnO₂ enhanced the 1-napthol removal efficiency via the δ-MnO₂ mediated oxidation coupling reaction and by irreversible sorption to humin. In this study, the stability of bound residues contributes significantly to the proposed method by disposing mediators after the treatment.