Manganese doped goethite: Structural, optical and adsorption properties

Pure and manganese doped (0.22–1.87 wt.%) goethite (α-FeOOH) samples were prepared by co-precipitation method. The structural properties of the samples were studied using XRD, Raman, TG-DTA, UV–vis and TEM techniques. The XRD patterns of all the samples confirmed goethite to be the only crystalline phase. The lattice parameters ‘ a ’ and ‘ c ’ steadily decreased, whereas ‘ b ’ increased with the increase of Mn(II) content due to Jahn Teller effect. Raman spectra showed a strong peak at 399 cm −1 with other prominent peaks at 242, 292, 479, and a very small broad peak 550 cm −1 for the pure goethite sample. The observed blue and red shifts of peak positions after Mn(II) doping in Raman spectra have been attributed to phonon confinement, strain, defects and distortion in crystal structure. The DTA peak corresponding to decomposition of goethite to hematite shifted to higher temperature (320–335 °C) due to Mn(II) doping. Distinct changes in the size and geometrical shape of Mn-substituted α-FeOOH particles were observed in SEM micrographs. The TEM image of pure goethite revealed the acicular particles to be nano range. Depending on Mn(II) concentration in goethite matrix, red shifts were observed in the UV–vis spectra. The indirect bandgaps for all the samples were at relatively lower energy levels than the direct bandgaps. Adsorption capacities of Pb(II), Cd(II), Cu(II) and Zn(II) on Mn doped sample (0.43% Mn) showed two- to three-fold increase when compared to undoped goethite.