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Identification of Noncrystalline (Fe,Cr)(Oh)3 by Infrared Spectroscopy
- James E. Amonette, Dhanpat Rai
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- Journal:
- Clays and Clay Minerals / Volume 38 / Issue 2 / April 1990
- Published online by Cambridge University Press:
- 02 April 2024, pp. 129-136
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Iron-chromium hydroxides are important solid phases governing the aqueous concentrations of Cr(III) in soils and fly ashes. Although direct identification of noncrystalline (Fe,Cr)(OH)3 is difficult, the infrared spectra of noncrystalline Fe(OH)3 and Cr(OH)3, coprecipitated (Fe,Cr)(OH)3, and physical mixtures of Fe(OH)3 and Cr(OH)3 can be distinguished on the basis of the asymmetric stretching doublet (v3) of structural carbonate anions. As the Cr mole fraction of the coprecipitated (Fe,Cr)(OH)3 increases, the position of the low-frequency v3 peak (v3″) changes progressively to higher frequencies, and the carbonate v3 splitting decreases. No change in carbonate v3 splitting or v3″ location was observed for physical mixtures of Fe(OH)3 and Cr(OH)3. The changes in v3 splitting are believed to be caused by different degrees of polarization of the carbonate ligand by the Fe and Cr cations.
Pure Cr(OH)3 exhibits a strong affinity for carbonate and H2O and tends to remain noncrystalline even at very high pHs. In contrast, pure Fe(OH)3 gradually converts to crystalline goethite at high pH, to the exclusion of much of the H2O and carbonate. The presence of Cr in (Fe,Cr)(OH)3 solid solutions seems to inhibit the transformation to crystalline goethite. The strong association of carbonate with Cr and the kinetic inertness of Cr(III) inner-sphere complexes in general may account for the maintenance of noncrystalline solid-solution materials in lieu of transformation to a crystalline end product.
Improvements to the Quantitative Assay of Nonrefractory Minerals for Fe(II) and Total Fe Using 1,10-Phenanthroline
- James E. Amonette, J. Charles Templeton
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- Journal:
- Clays and Clay Minerals / Volume 46 / Issue 1 / February 1998
- Published online by Cambridge University Press:
- 28 February 2024, pp. 51-62
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A method using 1,10-phenanthroline (phen) to quantify Fe(II) and total Fe in nonrefractory minerals was modified to improve the accuracy and precision and to eliminate the inconvenience of performing much of the analysis under darkroom conditions. Reagents were combined to minimize solution-handling errors, volumes of the reagent additions were determined gravimetrically and the acid-matrix solution was preheated to near-boiling before sample contact. The darkness requirement, which stems from the photoreduction of Fe(III) to Fe(II) in the presence of phen, was eliminated by the use of opaque amber-colored high-density-polyethylene bottles during the digestion step and for storage of the digestate and subsequent dilutions before Fe(II) analysis. Reduction of Fe(III) for total-Fe analysis was accomplished either by exposure to light from a Hg-vapor lamp or by reaction with hydroxylamine, NH2OH. Although the minimum periods required for adequate reduction ranged from 1.5 to 4 h, the optimum reduction periods were between 6 and 10 h. When standard samples containing Fe(II) and MnCl2 were digested and analyzed for total-Fe using the light treatment (with incidental heating to 35–45 °C), significant decreases and in some instances, oscillations, in absorptivity were obtained. Similar experiments with NH2OH, or with CrCl3 showed no effect. The absorptivity of most digestates stored in opaque bottles was stable for at least 2 weeks, although digestates with Mn concentrations above 3 µg mL−1 showed proportional decreases in absorptivity. Analysis of 8 geochemical reference materials by the modified method (using NH2OH) yielded excellent agreement with published values and a mean relative standard deviation of 0.6%. Total-Fe results obtained using the light treatment, however, were generally lower (∼2% relative) than the NH2OH values, although this difference decreased with longer irradiation periods. Use of NH2OH was deemed preferable because it was simpler, faster, minimized interferences from Mn and eliminated the need for specialized apparatus. Lastly, MICA Fe was shown to be unreliable as a primary reference material for Fe(II) determinations.
Synthesis of Smectite Clay Minerals: A Critical Review
- J. Theo Kloprogge, Sridhar Komarneni, James E. Amonette
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- Journal:
- Clays and Clay Minerals / Volume 47 / Issue 5 / October 1999
- Published online by Cambridge University Press:
- 28 February 2024, pp. 529-554
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Smectites are one of the most important groups of phyllosilicates found in soils and sediments, and certainly one of the most difficult to study. New information about the formation mechanisms, impact of structural features on surface properties, and long-term stability of smectites can best be gained from the systematic study of single-phase specimens. In most instances, these specimens can only be obtained through synthesis under controlled conditions. Syntheses of smectites have been attempted (1) at ambient pressure and low-temperature (<100°C), (2) under moderate hydrothermal conditions (100–1000°C, pressures to several kbars), (3) under extreme hydrothermal conditions (>1000°C or pressures >10 kbars), and (4) in the presence of fluoride. Of these approaches, syntheses performed under moderate hydrothermal conditions are the most numerous and the most successful in terms of smectite yield and phase-purity. Using hydrothermal techniques, high phase-purity can be obtained for beidellites and several transition-metal smectites. However, synthesis of montmorillonite in high purity remains difficult. Starting materials for hydrothermal syntheses include gels, glasses, and other aluminosilicate minerals. The presence of Mg2+ seems to be essential for the formation of smectites, even for phases such as montmorillonite which contain low amounts of Mg. Highly crystalline smectites can be obtained when extreme temperatures or pressures are used, but other crystalline impurities are always present. Although the correlation between synthesis stability fields and thermodynamic stability fields is good in many instances, metastable phases are often formed. Few studies, however, include the additional experiments (approach from under-and over-saturation, reversal experiments) needed to ascertain the conditions for formation of thermody-namically stable phases. Thorough characterization of synthetic products by modern instrumental and molecular-scale techniques is also needed to better understand the processes leading to smectite formation.
Copper Sorption Mechanisms on Smectites
- Daniel G. Strawn, Noel E. Palmer, Luca J. Furnare, Carmen Goodell, James E. Amonette, Ravi K. Kukkadapu
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- Journal:
- Clays and Clay Minerals / Volume 52 / Issue 3 / June 2004
- Published online by Cambridge University Press:
- 01 January 2024, pp. 321-333
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Due to the importance of clay minerals in metal sorption, many studies have attempted to derive mechanistic models that describe adsorption processes. These models often include several different types of adsorption sites, including permanent charge sites and silanol and aluminol functional groups on the edges of clay minerals. To provide a basis for development of adsorption models it is critical that molecular-level studies be done to characterize sorption processes. In this study we conducted X-ray absorption fine structure (XAFS) and electron paramagnetic resonance (EPR) spectroscopic experiments on copper (II) sorbed on smectite clays using suspension pH and ionic strength as variables. At low ionic strength, results suggest that Cu is sorbing in the interlayers and maintains its hydration sphere. At high ionic strength, Cu atoms are excluded from the interlayer and sorb primarily on the silanol and aluminol functional groups of the montmorillonite or beidellite structures. Interpretation of the XAFS and EPR spectroscopy results provides evidence that multinuclear complexes are forming. Fitting of extended X-ray absorption fine structure spectra revealed that the Cu-Cu atoms in the multinuclear complexes are 2.65 Å apart, and have coordination numbers near one. This structural information suggests that small Cu dimers are sorbing on the surface. These complexes are consistent with observed sorption on mica and amorphous silicon dioxide, yet are inconsistent with previous spectroscopic results for Cu sorption on montmorillonite. The results reported in this paper provide mechanistic data that will be valuable for modeling surface interactions of Cu with clay minerals, and predicting the geochemical cycling of Cu in the environment.
The Effect of Smectite on the Corrosion of Iron Metal
- Barbara A. Balko, Stephanie A. Bossé, Anne E. Cade, Elise F. Jones-Landry, James E. Amonette, John L. Daschbach
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- Journal:
- Clays and Clay Minerals / Volume 60 / Issue 2 / April 2012
- Published online by Cambridge University Press:
- 01 January 2024, pp. 136-152
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The combination of zero-valent iron (ZVI) and a clay-type amendment is often observed to have a synergistic effect on the rate of reduction reactions. In the present study, electrochemical techniques were used to determine the mechanism of interaction between the iron (Fe) and smectite clay minerals. Iron electrodes coated with an evaporated smectite suspension (clay-modified iron electrodes, CMIEs) were prepared using five different smectites: SAz-1, SWa-1, STx-1, SWy-1, and SHCa-1. All the smectites were exchanged with Na+ and one sample of SWy-1 was also exchanged with Mg2+. Potentiodynamic polarization scans and cyclic voltammograms were taken using the CMIEs and uncoated but passivated Fe electrodes. These electrochemical experiments, along with measurements of the amount of Fe2+ and Fe3+ sorbed in the smectite coating, suggested that the smectite removed the passive layer of the underlying Fe electrode during the evaporation process. Cyclic voltammograms taken after the CMIEs were biased at the active-passive transition potential for varying amounts of time suggested that the smectite limited growth of a passive layer, preventing passivation. These results are attributed to the Brønsted acidity of the smectite as well as to its ability to sorb Fe cations. Oxides that did form on the surface of the Fe in the presence of the smectite when it was biased anodically were reduced at a different electrochemical potential from those that form on the surface of an uncoated Fe electrode under otherwise similar conditions; this difference suggested that the smectite reacted with the Fe2+ formed from the oxidation of the underlying Fe. No significant correlation could be found between the ability of the smectite to remove the Fe passive film and the smectite type. The results have implications for the mixing of sediments and Fe particles in permeable reactive barriers, underground storage of radioactive waste in steel canisters, and the use of smectite supports in preventing aggregation of nano-sized zero-valent iron.
An Electron Paramagnetic Resonance Spectroscopy Investigation of the Retention Mechanisms of Mn and Cu in the Nanopore Channels of Three Zeolite Minerals
- Daniel R. Ferreira, Cristian P. Schulthess, James E. Amonette, Eric D. Walter
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- Journal:
- Clays and Clay Minerals / Volume 60 / Issue 6 / December 2012
- Published online by Cambridge University Press:
- 01 January 2024, pp. 588-598
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The adsorption mechanisms of divalent cations in zeolite nanopore channels can vary as a function of their pore dimensions. The nanopore inner-sphere enhancement (NISE) theory predicts that ions may dehydrate inside small nanopore channels in order to adsorb more closely to the mineral surface if the nanopore channel is sufficiently small. The results of an electron paramagnetic resonance (EPR) spectroscopy study of Mn and Cu adsorption on the zeolite minerals zeolite Y (large nanopores), ZSM-5 (intermediate nanopores), and mordenite (small nanopores) are presented. The Cu and Mn cations both adsorbed via an outer-sphere mechanism on zeolite Y based on the similarity between the adsorbed spectra and the aqueous spectra. Conversely, Mn and Cu adsorbed via an inner-sphere mechanism on mordenite based on spectrum asymmetry and peak broadening of the adsorbed spectra. However, Mn adsorbed via an outer-sphere mechanism on ZSM-5, whereas Cu adsorbed on ZSM-5 shows a high degree of surface interaction that indicates that it is adsorbed closer to the mineral surface. Evidence of dehydration and immobility was more readily evident in the spectrum of mordenite than in that of ZSM-5, indicating that Cu was not as close to the surface on ZSM-5 as it was when adsorbed on mordenite. Divalent Mn cations are strongly hydrated and are held strongly only in zeolites with small nanopore channels. Divalent Cu cations are also strongly hydrated, but can dehydrate more easily, presumably due to the Jahn-Teller effect, and are held strongly in zeolites with medium-sized nanopore channels or smaller.