Mariana Alvarez

Structural studies on the manganese for iron substitution in the synthetic goethite-jacobsite system

Abstract A set of synthetic oxides were prepared from Fe(III) and Mn(II) nitrate solutions in alkaline media with a Mn/(Mn+Fe) ratio up to 0.5. The structural characterization of the solid phases obtained was carried out by X-ray diffraction (XRD). XRD analyses showed that in preparations with a Mn mol fraction (xMn) lower than 0.14 (samples G1–G4) a Mn-substituted goethite was the only crystalline phase present. A second phase, jacobsite appeared together with goethite in the range 0.45≥xMn≥0.14 (samples G5–G8). For xMn=0.55 the majority and minority phases detected were jacobsite and hausmannite. Atomic and cell parameters for the nine samples synthesized were obtained by the Rietveld refinement of the XRD data. The relative proportion of the different phases in the system was also estimated by the refinement program and indicates that at xMn=0.28, jacobsite and goethite-like phases are in the same proportion. Refined cell parameters show that the unit cell in the goethite-like phase is distorted as a function of xMn. Deviations from the Vegard are observed and suggest a non-homogeneous substitution of Mn in the goethite phase.

Structure and reactivity of synthetic Co-substituted goethites

Abstract A set of synthetic goethites were prepared from Fe3+- and Co2+-nitrate solutions in alkaline media with a Co/(Co + Fe) ratio (xCo) up to 10 mol%. The structural characterization of the resultant solid phases was carried out by X-ray diffraction (XRD). XRD analyses showed that in preparations with xCo <10 mol%, Co-substituted goethite was the only crystalline phase present. Atomic and cell parameters for the samples synthesized were obtained by the Rietveld refinement of the XRD data, and showed that the unit cell in the goethite-like phase is contracted as a function of xCo. Little deviation from the Vegard rule was observed for all unit-cell parameters. Cobalt substitution produces an increase in the surface area of the goethite, as well as an increment in the dehydroxylation temperature. The acid dissolution of all Co-goethites showed an increase in dissolution rate with the Co content, and a congruent behavior was observed. The activation energy for dissolution was obtained two samples. A modified first-order Kabai equation best describes the dissolution data.

Hybrid nanomaterials based on gum Arabic and magnetite for hyperthermia treatments

In this study, one-step co-precipitation method was conveniently adapted to obtain novel nanomaterials based on Gum Arabic and magnetite. Two synthesis procedures were evaluated: one employing the solid biopolymer in the co-precipitation media; a second using an aqueous solution of the polysaccharide. An exhaustive characterization of both formulations was performed using several specific techniques. The obtained data confirmed the successful incorporation of the gum Arabic on the magnetic core. Values of hydrodynamic diameters, measured by dynamic light scattering, in aqueous dispersions were about 70-80nm, while sizes lower than 20nm were registered by TEM microscopy. Surface charge of gum Arabic coated magnetic nanoparticles was significantly different from the corresponding to raw materials (magnetite and GA). This fact confirmed the formation of hybrid nanosystems with novel and specific properties. The potential utility of these materials was tested regarding to magnetic hyperthermia therapy under radiofrequency fields. Magnetocalorimetric measurements were performed in a wide range of field amplitude and frequency. Specific absorption rate of 218W/gFe was determined at field frequency of 260kHz and amplitude of 52kA/m. These results demonstrate their viability to be applied in tumor ablation treatments. Using the linear response theory and restricting field parameters to the accepted biomedical window, maximum useful value of 74w/gFe is predicted at 417kHz and 12kA/m.

The Influence of Mn(II) and Aging Time in the Ferrihydrite to Goethite Transformation

The ferrihydrite-goethite transformation in samples of pure- and Mn-ferrihydrite was investigated in order to elucidate the effect of Mn(II) in the aging process. Samples of pure- and Mn-ferrihydrites were aged for different times, and their structural changes were described on the basis of Rietveld structure refinements. A comparison of two series of samples with (A-series) and without Mn (F-series) shows b- enlargement and a- and c- shortening due to the presence of Mn during the complete aging process. However, with aging different trends are observed in both the series. In the sam- ples synthesized without Mn(II) the variations in all the unit-cell constants are virtually negligible, whereas in the series with Mn(II) more significant changes are reflected by the enlargement of a- and c- and the shortening of the b. These re- sults confirm that the dimensions of the lattice parameters depend not only on the substitution ratio but also on the aging history of the sample.

Metal-Modified Mesoporous Silicate (MCM-41) Material: Preparation, Characterization and Applications as an Adsorbent

Mesoporous silicate (MCM-41) materials, modified with CuII, FeIII or AlIII were prepared by a hydrothermal method and were characterized by nitrogen adsorption at 77 K, X-ray powder diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), temperature programmed reduction (TPR), adsorption of pyridine as monitored by Fourier transform infrared spectroscopy (FTIR) and n-butylamine titration. The degree of ordering of the mesoporous material was decreased by the presence of Cu, Fe, or Al. The added Fe or Cu gave rise to new acidic and oxidation-reduction sites, while Al resulted in new acidic sites. The hydrothermal stabilities of selected samples were studied as a function of temperature and pH. All the samples were stable in acidic and neutral media at temperatures from 303 to 363 K. Adsorption of phenol on MCM-41 at 303 K was strongly influenced by the pH and by the modification with Cu, Fe, or Al. Phenol adsorption was particularly strong at the Al-modified material at pH 3, apparently due to the acidic character of phenol.

EFFECT OF Cd(II) ON THE RIPENING OF FERRIHYDRITE IN ALKALINE MEDIA

To acquire a better understanding of the influence exerted by the presence of Cd2+ during the process of transforming ferrihydrite to goethite, the morphological and structural changes of several samples obtained by the addition of Cd2+ to a suspension of nascent goethite were explored, and their chemical reactivity in acid media assessed. The samples (series Gi) were obtained by adding, at different times during the synthesis process, Cd2+ ions to ferrihydrite (Fe5HO8.4H2O) formed in alkaline media. The suspensions were aged for 5 days at 70°C, and the amorphous materials were extracted using a HCl solution (series GHCl-i). The X-ray diffraction (XRD) patterns showed that a goethite-like phase was formed, and chemical analyses indicated that the Cd content, xCd, increased with the earlier addition of the Cd2+ ions to the Fe oxyhydroxide suspension. Lattice parameters and cell volume, obtained by the Rietveld simulation of XRD data, indicated an enlargement of the cell parameters of goethite in line with the Cd-for-Fe substitution. In order to determine the influence of oxalate ions on the non-extracted solids, a second set of samples was also prepared that was kept in contact with an ammonium oxalate solution for 4 h (series Gox-i). The dissolution behavior of two series of Cd goethites and of a third series, obtained from coprecipitation of Fe3+ and Cd2+ ions in alkaline media, was observed. Kinetics measurements in 4 M HCl showed that the initial dissolution rate of samples Gox-i decreased with aging time, while the opposite effect was observed for series GHCl-i. Dissolution–time curves were well described by the Kabai equation, and activation energies were calculated using the Arrhenius equation. The results indicate that the presence of Cd during the crystallization process of goethite leads to the formation of a Cd goethite with modified morphology, structural parameters, and chemical reactivity.