A detailed investigation of MnO2 nanorods to be grown onto activated carbon. High efficiency towards aqueous methyl orange adsorption/degradation

Abstract

Herein, we report a one-pot wet chemical method adopted to synthesize ad hoc MnO2 nanoparticles. By varying both the manganese salt precursors (e.g. sulphate or chloride) and the oxidizing agents (e.g. ammonium persulphate, potassium permanganate or potassium bromate), we succeeded in tailoring MnO2 structural, morphological and surface features. Hence, owing to nanopowders peculiar properties, they were exploited as adsorbents for aqueous Methyl Orange (MO) removal. Particularly, novel MnO2 nanorods (from manganese sulphate and potassium bromate, namely MS_Br) showed the highest removal efficiency probably due to both its polymorphic composition and its highest percentage of pores with diameter under 20 nm. Then, this powder was grown on Activated Carbon (AC40, sample MS_Br@AC40) pellets to either enhance its adsorption properties or to facilitate the adsorbent removal at the end of the kinetic test. Novel MS_Br@AC40 shows superior MO removal capabilities, achieving the almost total pollutant disappearance, thanks to the synergistic adsorption/oxidation features between carbon (high surface area, i.e. 1200 m2 g−1) and MnO2. By means of HPLC-MS on eluates, we also managed to investigate MS_Br and MS_Br@AC40 degradative power towards MO molecules, thus leading to a novel degradation pathway. Finally, the adsorbent regeneration capability has been evaluated, showing very promising results.