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Preparation and Properties of Hematite with Structural Phosphorus

Published online by Cambridge University Press:  28 February 2024

Natividad Gálvez ,
Vidal Barrón  and
José Torrent
Natividad Gálvez
Affiliation:
Departamento de Ciencias y Recursos Agrícolas y Forestales, Universidad de Córdoba, Apdo. 3048, 14080 Córdoba, Spain
Vidal Barrón
Affiliation:
Departamento de Ciencias y Recursos Agrícolas y Forestales, Universidad de Córdoba, Apdo. 3048, 14080 Córdoba, Spain
José Torrent
Affiliation:
Departamento de Ciencias y Recursos Agrícolas y Forestales, Universidad de Córdoba, Apdo. 3048, 14080 Córdoba, Spain
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Abstract

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Synthetic hematites prepared in the presence of phosphate can incorporate phosphorus (P) in forms other than phosphate adsorbed by ligand-exchange on the crystal surface. To investigate the nature of such occluded P, which is also found in some natural specimens, we prepared 13 hematites by aging ferrihydrite precipitated from Fe(NO3)3-KH2PO4 solutions. The P/Fe atomic ratio of the resulting hematites ranged from 0 to 3% and all incorporated significant amounts of OH. As P content is raised, particle morphology changes from rhombohedral to spindle or ellipsoid-shaped. Despite the grainy appearance in transmission electron microscope images, X-ray diffraction data indicate that the particles are single crystals. Specific surface area ranged from 66 to 91 m2g−1, partly in micropores. The intensity of the absorption bands due to Fe3+ ligand field transition in the visible region, as measured by the second derivative of the Kubelka-Munk function, suggests that both OH and P contribute to an Fe deficiency in the structure. Such a deficiency is also apparent from the 104/113 peak intensity ratio in the X-ray diffraction patterns. The c unit-cell length increases with increasing P content. The infrared spectra exhibit four bands in the P-OH stretching region (viz., at 936, 971, 1005, and 1037 cm−1) which suggest that occluded PO4 possesses a low symmetry. Congruent dissolution of P and Fe was observed on acid treatment of the hematites, the dissolution rate being negatively correlated with the P content. All observations are consistent with the occluded P in the hematites being structural. A model is proposed where P occupies tetrahedral sites in the hematite structure, thus resulting in an Fe deficiency and facilitating proton incorporation.

Keywords

α-Fe2O3HematiteColorCrystal ChemistryDissolutionIR SpectraStructural PStructure
Type
Research Article
Information
Clays and Clay Minerals , Volume 47 , Issue 3 , June 1999 , pp. 375 - 385
Copyright
Copyright © 1999, The Clay Minerals Society

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