A critical analysis of cation adsorption from single and binary solutions on low surface area β-MnO2

Generally it is advocated that high surface area materials are better adsorbents due to availability of more surface active sites. In the present study, potential of a low surface area (0.81 m2/g) β-MnO2 has been examined for removal of Pb(II), Cd(II), Cu(II) and Zn(II) ions from aqueous solutions. To understand the adsorption mechanism of cations the various techniques used include SEM, FTIR, XRD and Raman spectroscopy. Inner or outer sphere complex formation was envisaged due to +ve/−ve shifts in FTIR bands. Raman spectra showed shifting of band (red/blue shift) at 636 cm−1 (A1g symmetric mode) for all the cations except Cd(II). Another important observation made was the decrease in scattering intensity of A1g symmetric mode after cation adsorption except for Pb(II) loaded β-MnO2. This was attributed to the breakdown of long-range translational crystal symmetry caused by the incorporated defects. Results for adsorption on β-MnO2 from binary solutions namely Pb(II)–Cd(II), Pb(II)–Cu(II), Pb(II)–Zn(II), Cd(II)–Cu(II), Cd(II)–Zn(II) and Cu(II)–Zn(II) are discussed. Pb(II) having a low adsorption capacity of only 26 mg/g showed tenfold increase in presence of Cd(II). The concentration dependent selectivity of certain cations in binary system has been explained on the basis of observed changes in the Raman spectra of loaded β-MnO2.