Quantification of Allophane from Ecuador

Abstract

Allophane is a very fine-grained clay mineral which is especially common in Andosols. Its importance in soils derives from its large reactive surface area. Owing to its short-range order, allophane cannot be quantified by powder X-ray diffraction (XRD) directly. It is commonly dissolved from the soil by applying extraction methods. In the present study the standard extraction method (oxalate) was judged to be unsuitable for the quantification of allophane in a soil/clay deposit from Ecuador, probably because of the large allophane content (>60 wt.%). This standard extraction method systematically underestimated the allophane content but the weakness was less pronounced in samples with small allophane contents. In the case of allophane-rich materials, the Rietveld XRD technique, using an internal standard to determine the sum of X-ray amorphous phases, is recommended if appropriate structural models are available for the other phases present in the sample. The allophane (+imogolite) content is measured by subtracting the amount of oxalate-soluble phases (e.g. ferrihydrite). No correction would be required if oxalate-soluble Fe were incorporated in the allophane structure. The present study, however, provides no evidence for this hypothesis. Mössbauer and scanning electron microscopy investigations indicate that goethite and poorly ordered hematite are the dominant Fe minerals and occur as very fine grains (or coatings) being dispersed in the cloud-like allophane aggregates.Allophane is known to adsorb appreciable amounts of water, depending on ambient conditions. The mass fraction of the sample attributed to this mineral thus changes accordingly; the choice of a reference hydration state is, therefore, a fundamental factor in the quantification of allophane in a sample. Results from the present study revealed that (1) drying at 105ºC produced a suitable reference state, and (2) water adsorption has no effect on quantification by XRD analysis.

Removal of barium, cobalt, strontium and zinc from solution by natural and synthetic allophane adsorbents

“…These values are in the range of N2-BET values previously reported for natural allophanes (~200-400 m²/g) and synthetic allophanes (~250-900 m²/g) [10,29,33]. Noteworthy, the BET values obtained before and after the adsorption experiments were identical to within the analytical precision of the BET analyses, which indicates that changes in the surface area and surface properties of the adsorbents are negligible throughout the experiments.…”

“…XRD analysis confirmed the dominance of allophane in NatAllo (~78 wt.%; [24,29]) which also contains minor quantities (each < 5 wt.%) of quartz, cristobalite, feldspar (albite and orthoclase), hornblende, halloysite, gibbsite and goethite (Fig. 1).…”

Removal of Barium, Cobalt, Strontium, and Zinc from Solution by Natural and Synthetic Allophane Adsorbents

“…The specific surface areas are 294 m 2 /g for NatAllo, 358 m 2 /g for SynAllo-1, and 370 m 2 /g for SynAllo-2, respectively. These values are in the range of N 2 -BET values previously reported for natural allophanes (~200-400 m 2 /g) and synthetic allophanes (~250-900 m 2 /g) [10,29,33]. It is noteworthy that the BET values obtained before and after the adsorption experiments were identical to within the analytical precision of the BET analyses, which indicates that changes in the surface area and surface properties of the adsorbents are negligible throughout the experiments.…”

“…XRD analysis confirmed the dominance of allophane in NatAllo (~78 wt.%; [24,29]), which also contains minor quantities (each <5 wt.%) of quartz, cristobalite, feldspar (albite and orthoclase), hornblende, halloysite, gibbsite, and goethite ( Figure 1). The XRD patterns of SynAllo-1 and SynAllo-2 exhibited two broad peaks centered at ~3.4 Å and ~2.3 Å , which is typical for short-range ordered aluminosilicates such as allophane.…”

XAFS Study of Fe-Substituted Allophane and Imogolite

“…Iron-substituted natural and synthetic allophane, imogolite, and halloysite samples have been described (Kitagawa, 1973). Natural allophane from Ecuador contains oxalate-soluble Fe that may be present in isomorphic substitution in the allophane, as well as dithionite-extractable Fe present in a finely dispersed goethite phase (Kaufhold et al, 2009(Kaufhold et al, , 2010. The X-ray absorption spectrum (XAS) of Fe-substituted imogolite is distinct from that of Fe-adsorbed imogolite (Ookawa et al, 2006), but the structures have not been fully modeled.…”