Chemical, structural and hyperfine characterization of goethites with simultaneous incorporation of manganese, cobalt and aluminum ions
MARIANA ALVAREZ; ANA E. TUFO; CRISTINA ZENOBI; CINTHIA P. RAMOS; ELSA E. SILEO
ELSEVIER SCIENCE BV
Lugar: Amsterdam; Año: 2015 vol. 414 p. 16 - 27
To elucidate the influence of bi-substitution on the structural and hyperfine properties of goetites, two series of (Al,Co)- and (Mn,Co)-substituted goethites were synthesized in alkaline media by aging several ferrihydrites with different Al/Co and Mn/Co ratios. The samples were fully characterized by chemical analyses, X-ray diffrac-tion (XRD) and Mössbauer spectroscopy; scanning electron microscopy (SEM), zeta potential and BET surface area measurements were also performed. All the solids presented only anα-FeOOH-like structure, with the ex-ception of twopreparationswithhighCo concentrations that developed twophases, goethite and small amounts of the Co-ferrite (CoFe2O4). The cell parameters in the Co-substituted goethiteswere markedly smaller than that of the pure sample indicating a oxidation of Co(II) to Co(III) before the incorporation step. In the Co +Mn series the metal substitution followed the trend Co ~ Mn, and in the Co + Al series the trend was AlNCo, and in both cases the incorporation of Co decreased the crystallite size of the samples. The metal-for-Fe incorporationchanged the specific surface areas and the morphology of the acicular formed particles. Cobalt containing samples had the highest SSA, while Mn-containing samples had the lowest SSA. The IEP values of the bisubstituted samples were similar to that of pureα-FeOOH, but mono-substitution by Mn and Al diminishedthe isoelectric points. The low IEP values detected in Mn-goethite (5.8) and Al-goethite (5.2) could be respectively ascribed to an inhomogeneous distribution of Mn(III), and to the different basicity properties of the surface Fe?OH and Al?OH groups. The hyperfine magnetic field Bhf, increased quasi linearly with the incorporation of Co in both series. In the Co?Mn series the effect was attributed to variations in particle size distribution, in contrast themarked increase observed in the Co?Al series can be attributed to the decrease in the content of diamagnetic ion Al(III). The results indicate that simultaneous substitutions produce substantial changes in the structural, surface and hyperfine properties of goethites. As the characteristics of the dissolution and adsorption processes of the goethites greatly depend on particle size, BET areas and surface charge of the solids, the reported results willallow us to predict changes in the chemical reactivity and adsorption of the multi-substituted goethites. Also the data on hyperfine properties will help to elucidate the probable substitution in natural samples. The fact that Co-incorporation in bi-substituted samples greatly decreased the particle size increasing the specific surface area is an important parameter for technological applications in adsorption removal processes.