Manganese dioxide γ-MnO 2 was synthesized via the reduction-oxidation reaction between KMnO 4 and C 2 H 5 OH at room temperature and characterized with x-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Brunauer-Emmet-Teller nitrogen adsorption (BET-N 2 adsorption). The results showed that γ-MnO 2 was about 10-18 nm in size and the BET surface area was about 65 m 2 g −1 . The feasibility of γ-MnO 2 used as a low cost adsorbent for the adsorption of Pb(II), Co(II) and Cu(II) from aqueous solutions was explored. During the adsorption process, batch technique was used, and the effects of contact time and pH on adsorption efficiency under room temperature were studied. The adsorption data showed that the Freundlich, Langmuir and Redlich-Peterson isotherms are a good model for the sorption of Co(II) and Cu(II), while the Langmuir and Redlich-Peterson isotherms provide a reasonable fit to the experimental data for Pb(II). By using the Langmuir isotherm, the adsorption capacities for Pb(II), Co(II) and Cu(II) are found to be 200 mg g −1 , 90.91 mg g −1 and 83.33 mg g −1 , respectively. The effectiveness of γ-MnO 2 in the sorption of the three metal ions from aqueous system has the order Pb(II) > Co(II) > Cu(II). Kinetic studies showed that a pseudo-second-order model was more suitable than the pseudo-first-order model. Also, the intra-particle diffusion models were used to ascertain the mechanism of the sorption process. It is concluded that γ-MnO 2 can be used as an effective adsorbent for removing Pb(II), Co(II) and Cu(II) from aqueous solutions.

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Manganese dioxide nanomaterials have wide applications in many areas from catalysis and Li−ion batteries to gas sensing. Understanding the crystallization pathways, morphologies, and formation of defects in their structure is particularly important but still a challenging issue. Herein, we employed an arsenal of X-ray diffraction (XRD), scanning electron microscopy (SEM), neutron diffraction, positron annihilation spectroscopies, and ab initio calculations to investigate the evolution of the morphology and structure of α-MnO 2 nanomaterials prepared via reduction of KMnO 4 solution with C 2 H 5 OH prior to being annealed in air at 200−600 °C. We explored a novel evolution that α-MnO 2 nucleation can be formed even at room temperature and gradually developed to α-MnO 2 nanorods at above 500 °C. We also found the existence of H + or K + ions in the [1 × 1] tunnels of α-MnO 2 and observed the simultaneous presence of Mn and O vacancies in α-MnO 2 crystals at low temperatures. Increasing the temperature removed these O vacancies, leaving only the Mn vacancies in the samples.

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