Synthesis and Properties of Poorly Crystalline Hydrated Aluminous Goethites

Abstract

Abstraet--AI-substituted goethites were prepared by rapid oxidation of mixed FeCI2-AICI 3 solutions at pH 6.8 in the presence of CO2 at 25~ A combination of AI substitution and adsorption of CO2 reduced crystal size (except for an increase at small additions of AI) and produced unusual thin, porous particles. Product goethites had surface areas up to 283 mZ/g and unit-cell expansions induced by hydration. Substitution of AI for Fe reduced the 111 spacing and increased infrared OH-bending vibrational frequencies. A1 substitution split the goethite dehydroxylation endotherm during differential thermal analysis into a doublet and increased the temperature of all reactions. Both cold and hot alkali solutions dissolved AI from the goethite structure.After drying the product in vacuo at 110~ X-ray powder diffraction data indicated minimal deviation from Vegard's law for the goethite-diaspore solid solution up to about 30 mole % AI substitution. Goethite prepared in the presence of 40 mole % AI had a 111 spacing of 2.403/~ corresponding to 36 mole % structural AI if Vegard's law was obeyed. Rapid oxidation of mixed FeCI2-AICI 3 solutions appears to be conducive to a higher degree of AI substitution in goethite than alkaline aging of hydroxy-Fe(III)-Al coprecipitates.

Distinguishing Between Surface and Bulk Dehydration-Dehydroxylation Reactions in Synthetic Goethites by High-Resolution Thermogravimetric Analysis

“…Peak maxima coincide with a maximum rate of weight-loss, whereas the minima between peaks occur during the transition between weight-loss events. Multiple weight-loss events immediately preceding bulk dehydroxylation are attributed to dehydration or dehydroxylation of surface Fe (oxo)hydroxo functional groups and similar events were observed in several investigations with goethites of varying crystallinity (Paterson and Swaffield, 1980;Fey and Dixon, 1981;Mackenzie et al, 1981;Schulze and Schwertmann, 1984;Schwertmann, 1984b;Schulze and Schwertmann, 1987;Gasser et al, 1996). In this study, all goethites showed this complex weight-loss pattern to varying degrees as illustrated by thermograms for samples prepared in the absence of Mn or Pb and aged at different pH and temperature ( Figure 2).…”

The Effect of Al on Fe Oxides. XIX. Formation of Al-Substituted Hematite from Ferrihydrite at 25°C and pH 4 to 7

“…Note, however, that the strong goethite-excluding effect of A1 does not occur in soils, probably because goethite is formed by oxidation of Fe(II). For example, in synthesis experiments using Fe(II) as the Fe source, goethite was the only phase at room temperature (RT) and pH 7, even at high R~ (-0.4 mol mo1-1) (Taylor and Schwertmann, 1978;Goodman and Lewis, 1981;Fey and Dixon, 1981).…”

Fourier Transform Infrared Studies of Aluminous Goethites and Hematites

“…Rendon and Serna (1981) and Serna et al (1982) showed that morphological variations may also be a major cause of spectral differences in the case of microcrystalline materials. Fey and Dixon (1981), in fact, noted that differing degrees of structural hydration may lead to shifts in the 900-cm -~ band of goethite. J6nfis and Solymfir (1970) used a third preparative method so that it seems likely that the three different sets of results shown in Figure 3 may be due to variations in the degree of structural hydration and/ or morphology resulting from the mode of preparation.…”

“…This conclusion is supported by the present results--a band at 460 cm was noted only for the three highest A1 concentrations (see Figure 6), and can in fact be attributed to a-Al203 (Farmer, 1974). De Grave et al (1982) recently detected corundum in the X-ray powder diffraction pattern of 15 mole % Al-substituted hematite after the sample was heated to 900~ Thus, FTIR analysis may indicate the extent of solid solubility in some mineral systems, despite previous assertions to the contrary (Tarte, 1968;Fey and Dixon, 1981).…”

“…The ionic replacement of Fe by AI in each has been widely reported (e.g., Norrish and Taylor, 1961;Janot and Gibert, 1970;Davey et al, 1975;Schwertmann et al, 1977;Bigham et al, 1978;Mendelovici et al, 1979), and a variety of techniques have been employed to characterize such substituted minerals Clark, 1982a, 1982b, and references therein). Dispersive infrared (IR) spectroscopy has been used by several workers for the characterization ofaluminous goethites (e.g., J6n~is and Solymfir, 1970;Fey and Dixon, 1981), but we know of no published infrared data relevant to aluminous hematites. Infrared analysis of specimens containing these minerals is an attractive alternative to other means of characterization, as it offers the possibility of quantitatively examining with a single measurement all of the species present.…”