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Adsorption and desorption of phosphate by lepidocrocite and hematite, the latter obtained by thermal decomposition of the former, were studied and related to changes in mesoporosity of the solid surface. Differences between the oxides in the accessibility of their surfaces to phosphate seems to control the fractional amounts of 'rapidly' and 'slowly' adsorbed phosphate. Results of isotopic exchange of the phosphate adsorbed at high surface coverage can also be explained in terms of the mesoporosity of the surfaces.

The thermal decomposition of synthetic a-FeOOH in air has becn studied by X~ray powder diffraction (XRD), IR spectroscopy. transmission electron microscopy, and nitrogen adsorption. Three stages may be differentiated during the course ofthe reaction: (i) 200-250°C; slit-shaped micropores of nearly equal width, about 1.4 nm, are formed along the elongated direction of the lath-shaped acicular mícrocrystals of the hematite producÍS, the highest surface area is obtained on a a-FeOOH decomposed at 250°C; (ii) 300-600°C; an internal sintering occurs within the microcrystals transforming the micropores into dosed spherical mesopores with the consequent significant decrease in surface area; and (iii) above 600°C; interparticle sintering takes place and no valuable porosity is detected. Particle shape effects are shown to be the reason for the differences in the IR spectra and for the nonuniform broadening of the XRD peaks observed in the hematite products.

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