Tailored MnO2 Nanorods as Highly Efficient Materials for Methyl Orange Adsorption/Degradation

Nowadays, organic dyes are used in various industries and so they are one of the main water pollutants with severe environmental impact1. Particularly, Methyl Orange (MO) is an azo-dye widely recognized as potential carcinogen1. Therefore, the treatment of wastewater, prior to disposal, is becoming more and more urgent2. Many technologies have been exploited to remove/degrade aqueous MO and among them, adsorption is one of the most effective methods2. In this context, we investigated the adsorption capacities of four different MnO2 nanopowders (prepared by tailoring the synthetic route), which showed very promising and novel results in terms of MO adsorption/degradation. EXPERIMENTAL Synthesis of MnO2 nanoparticles We adopted a one-pot hydrothermal procedure already optimized in our laboratory3, by using stoichiometric MnSO4 ́H2O or MnCl2 ́4H2O as the salt precursors, and (NH4)2S2O8, KMnO4 or KBrO3 as the oxidizing agents. Samples were labeled as MX_Y according to either the salt precursor (X = S for MnSO4 ́H2O or Cl for MnCl2 ́4H2O) or the oxidant (Y = N for (NH4)2S2O8, K for KMnO4 or Br for KBrO3) used. Sample characterizations X-Ray Powder Diffraction (XRPD) analyses were performed on a Philips PW 3710 Bragg-Brentano goniometer, with graphite-monochromated Cu Kα radiation, between 20° and 90° (0.1° step). Transmission Electron Microscope (TEM) analyses were performed on LIBRA 200 EFTEM (Zeiss) instrument operated at 200 kV accelerating voltage. The BET surface area and total pore volume was determined by a multipoint BET-BJH method (Coulter SA3100 apparatus). Methyl Orange removal tests The MO adsorption/removal capability was achieved by mixing 150 mL of MO aqueous solution (100 mg L-1) and 75 mg of MnO2 nanopowders. All the experiments were carried out at spontaneous pH (~3), under vigorous stirring. The kinetics were monitored for 2 h by UV/Vis spectroscopy (MO peak at 465 nm, Shimadzu UV/Vis spectrophotometer UV-2600). The amount of MO adsorbed during time (qt, mg g-1) was calculated using the following equation: qt = V(c0 – ct) , where c0 and ct (in mg L-1) at the initial and certain time t, respectively; V (in L) is the volume of the solution and W (g) is the mass of the adsorbent used. Furthermore, to investigate the MO degradation mechanism, aliquots of MO eluate (after 2 h) were examined by using HPLC-MS technique (HPLC Agilent Technologies 1200 series instrument coupled with a Thermo Scientific LTQ Orbitrap XL analyzer), and the used powders were investigated by Fourier Transformed Infrared (FTIR) spectroscopy. RESULTS AND DISCUSSION By varying the starting salt precursors/oxidizing agents, we succeeded in tailoring MnO2 structural, morphological and surface features. Particularly, novel MnO nanorods (from manganese sulphate and potassium bromate, namely MS_Br, inset of Fig. 1) showed the highest MO adsorption efficiency, probably due to both i) its polymorphic composition, i.e. b-ramsdellite enhances the adsorption capability and ii) its highest percentage of pores with diameter under 20 nm, i.e. smaller pores promote the entrapment of the small MO molecules, avoiding any adsorption/desorption equilibria. Moreover, by means of HPLC-MS and infrared spectroscopy analyses, we also observed the MO degradation by MS_Br. Thus, we hypothesized a novel pathway, based on: i) the loss of one/two methyl groups from the nitrogen atom; ii) the addition of –OH groups to the phenyl ring; iii) the cleavage of the azo-bond; and iv) the removal of the amine group from the p-sulfanilic acid (Fig. 1). No other species characterized by lower m/z values have been detected. CONCLUSION Herein, we successfully prepared tailor-made MnO2 nanorods with ad hoc physico-chemical features, to be applied as MO adsorbents. Due to its peculiar properties, novel MS_Br sample showed the most performing MO adsorption/degradation behaviour. REFERENCES 1. T. Dang et. al, J. Phys. Chem. Solids 98, 50 (2016) 2. D. Zhao et. al, Procedia Environ. Sci. 18, 890 (2013) 3. E. Pargoletti et. al, J. Power Sources 325, 116 (2016)