Elsa H. Rueda

The effect of pH in the adsorption of Alizarin and Eriochrome Blue Black R onto iron oxides.

The adsorption process of two dyes as a function of pH on three different adsorbents (goethite, Co-goethite, and magnetite) has been analyzed. Typical anionic adsorption behavior was observed for both dyes onto goethite and Co-goethite. The adsorption level was practically constant in the range of pH studied when the adsorbent was magnetite. The constant capacitance model (CCM) was employed to fit the experimental results. The surface complexes proposed from the adsorption data were in agreement with the patterns obtained from FTIR spectroscopy and a molecular mechanics calculation. Goethite has very good performance as adsorbent of Alizarin and Eriochrome Blue Black R. The presence of a foreign cation in Co-goethite does not improve the adsorption abilities of goethite. At low pH, the amounts of Alizarin and Eriochrome Blue Black R adsorbed on goethite and Co-goethite are similar. However, a higher dependence with the increase of pH is observed by Eriochrome Blue Black R. On magnetite, the dye adsorption shows less affinity for both dyes. Electronic and steric considerations can explain the trends found in the adsorption of the two dyes on the three iron oxides studied in this work.

Dithionite as a dissolving reagent for goethite in the presence of EDTA and citrate; application to soil analysis

AbstractA synergistic effect of reductant and complexant is observed in the dissolution of goethite by dithionite and citrate or EDTA. The rate data are interpreted using the surface complexation approach to describe the interface of the reacting oxide. Adsorption of both S2O42− (D) and complexant (L) generates three surface complexes that define the dissolution behavior: ≡ Fe-D, ≡ Fe-L, and dimeric surface complexes. The initial rate increases at lower pH values because of increased surface complexation conditional formation constants. At pH values below 4, however, the fast decomposition of S2O42− gives rise to a rapid depletion of reductant, and total dissolution is not observed. It is shown that for best analytical results in soil analysis, EDTA is a better complexant than citrate; the iron extracted in one dithionite-EDTA treatment at pH 5–6, under N2 at 315 K is not increased by increasing the number of extractions, and is equivalent to the total extractable iron found by previous procedures.

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.

Elimination of dyes from aqueous solutions using iron oxides and chitosan as adsorbents: a comparative study

This work investigates the adsorption of Alizarin, Eriochrome Blue Black R and Fluorescein using chitosan, goethite and magnetite as adsorbents. For Alizarin, the best adsorbent is chitosan with a Langmuir parameter of 15.8 mmol dye/g adsorbent. For Eriochrome Blue Black R only 1.94 mmol dye/g chitosan is adsorbed. Langmuir parameters for the Alizarin adsorption on both iron oxides display one or two orders of magnitude lower than for chitosan and two orders of magnitude lower in the case of Eriochrome Blue Black R. Fluorescein does not adsorb in appreciable amounts on chitosan and it presents the lower affinity on the iron oxides.

A discussion of a mechanism for isomerization of n-butane on sulfated zirconia

Although a huge amount of experimental work has been done, there is no clear picture about the superacidity in sulfated zirconia (SZ). Some authors think that sulfated zirconia is not superacid at all. Moreover, the structure of active site in n-butane isomerization is still now under controversy. This work tries to give some light about the mechanism of n-butane isomerization, taking into account experimental and theoretical works from others and from our own. MM2 and DFT simple calculations were performed in order to clarify the nature of the supported active species in sulfated zirconia. It is clear that a penta-coordinated species, monomeric or even dimeric, would be responsible for forming the most active sites at reaction conditions of n-butane isomerization. This idea is the key of this article. Different studies demonstrated that no superacidity is present before contact with n-butane. The sites formed at reaction conditions allow the n-butane dehydrogenation and its isomerization. A bimolecular mechanism of n-butane isomerization on penta-coordinated sites is proposed to occur. Several steps are discussed.

REDUCTIVE DISSOLUTION KINETICS OF Al-SUBSTITUTED GOETHITES

Several Al-substituted goethites were synthesized by hydrolysis of Fe3+ salt solutions. The kinetics of the reductive dissolution of these goethites in dithionite-ethylenediaminetetra acetic acid (D-EDTA) was studied at pH 5.5, at 303, 323 and 333 K. The initial dissolution rate (R) per unit of surface area decreases with Al substitution. In the sample with greater Al content (G″7), the kinetic profiles of the dissolved Fe fraction vs. time gave a small positive intercept. The kinetic profile of R as a function of EDTA initial concentration shows a significant weakening in the presence of Al. The maximum is flatter and wider in Al-substituted goethite than that of pure goethite. In sample G″7, where the Al content is 11.3 mol.% the maximum is obtained when the [D]:[EDTA] initial ratio is ∼4.5 vs. 2 in un-substituted goethite. These results can be attributed to the lesser density of the more active dimeric sites, the presence of more strongly bonded Al-O-Fe with regard to Fe-O and the small value for the ≡ Al-EDTA surface species constant. Activation energy (Ea) increases with Al substitution. Its value is doubled from GO (pure goethite) to G″7 (11.3 mol.% of Al). The frequency factor (A) acts in the opposite sense to Ea, but it is not sufficient to outweigh the effect of Ea.

Theoretical characterization of alumina and sulfated-alumina catalysts for n-butene isomerization

A theoretical study was carried out in order to characterize the changes produced by the presence of sulfate on the alumina surface. A molecular orbital approach of the extended Huckel (EH) type together with dynamic molecular calculation were performed to evaluate adsorption and desorption energies of the intermediate species for skeletal and double-bond isomerization reactions of n-butene The bimolecular and monomolecular mechanisms for skeletal isomerization were evaluated.

Iron-oxide mineralogy of a Mollisol from Argentina: a study by selective-dissolution techniques, X-ray diffraction, and Mössbauer spectroscopy.

Selective-dissolution techniques by ammonium oxalate (OX), dithionite-citrate-bicarbonate (DCB), and dithionite-ethylenediaminetetraacetic acid (D-EDTA), and X-ray diffraction and Mössbauer spectroscopy were used to identify and characterize iron oxides and oxyhydroxides in the <2-mm, <50-μm, and <2-μm size fractions of a Mollisol from Bahia Bianca, Argentina. Iron compounds are present at low concentrations in mixtures with quartz, Na-rich feldspar, illite, interstratified illite-montmorillonite, and traces of kaolinite. Total Fe and Al content increases as soil particle size decreases, from 4.3 and 13.3 wt. % in the <2-mm size fraction to 8.5 and 22.8 wt. % in the clay fraction (<2 μm), respectively. No more than 25–30% of the total Fe is associated with the crystalline and the amorphous Fe oxides. Weakly ferromagnetic hematite and goethite were identified in the different fractions. These phases have small particle sizes and/or low crystallinity. They may also have Al for Fe substitutions. Crystalline magnetite or maghemite is rare. These Fe-rich phases are probably coating coarser particles.The efficiency of Fe removal is highest for the D-EDTA treatment and least efficient for the OX method, for all fractions. The opposite is true for Al removal. Poorly crystalline hematite and goethite, which are soluble in oxalate, are only present in the coarser fractions. Poorly crystalline and crystalline hematite and goethite, which are soluble in DCB and EDTA, are present in coarser fractions, but do not occur in the clay fraction. DCB treatment probably dissolves Al in the 2:1 type phyllosilicates occurring in this soil, whereas D-EDTA dissolves Fe in the hydroxy interlayers of the smectite minerals or in the silicate phases.

Removal of Fluorescein using different iron oxides as adsorbents: effect of pH.

In this work the adsorption process of Fluorescein (dye with aril-methane group) as a function of pH on three different adsorbents: goethite, Co-goethite, and magnetite has been studied experimentally and theoretically. FTIR and Raman spectroscopy have been performed in an attempt to confirm the structure of surface complexes formed by sorption of the Fluorescein to different iron oxides. Typical anionic adsorption behaviour was observed for this dye onto goethite and Co-goethite whereas the adsorption level was practically constant in the range of pH studied when the adsorbent was magnetite. The diffuse layer model was employed to fit the experimental results. The surface complexes proposed from the adsorption data were in agreement with the patterns obtained from FTIR and Raman spectroscopy. The surface structure of the oxides affects the adsorption process and the final adsorbed amount at the equilibrium. Our model of diffuse double layer with the addendum of the effect of hydrophobic forces fits well the adsorption data of Fluorescein on iron oxides at different pH in the studied range. At lower pH electrostatic forces by ligand-exchange are predominant. In the range of pH 9-11 hydrophobic forces are managing the Fluorescein adsorption on the iron oxides, with the formation of outer-sphere complexes through van der Waals/hydrophobic forces. It is interesting that in the three iron oxides studied, the adsorbed amount in this range is similar.