MICROPORE FORMATION IN HEATED SYNTHET IC AL-GOETHITES FROM A FERROUS SYSTEM

ABSTRACT: Nitrogen adsorption and desorption isotherms were obtained for microcrystalline, platy crystals of synthetic Al-substituted goethites synthesized from the ferrous system and heated to 260 ~ Specific surface area, cumulative pore area and frequency of mesopore wall-separations of 2-15 nm in size increased as AI substitution increased. Nanometre-size micropores developed at heating temperatures between 200 and 240 ~ and decreased uring the growth of hematite crystals between 240 and 260 ~ Aluminium substitution had little effect on micropore development and a moderate ffect on mesopore development. The increases in micropore volume and specific surface area due to heating are much smaller than is reported in the literature for large, lath-like crystals of goethite that develop abundant slit-shaped micropores. Slit-shaped micropores develop during dehydrox-ylation of goethite and crystallization of hematite resulting in a large increase in the specific surface area (e.g. increase of 35-139 m 2 g- l) (Naono & Fujiwara, 1980; Watari et al., 1983). Many naturally occurring goethite samples contain abun