ATOMIC-FORCE MICROSCOPY (AFM) INVESTIGATIONS OF THE SURFACE-TOPOGRAPHY OF A MULTIDOMAIN POROUS GOETHITE

A multidomain goethite was analysed with atomic force microscopy (AFM) to obtain information about the size and shape of pores on the surfac e, which are important for sorption processes of cations and anions. W edge-shaped pores >100 nm long at the surface occurred at the domain b oundaries. They were 20-30 nm wide, becoming narrower towards the inte rior of the crystals to 2 nm and less. The AFM showed these pores to b e at least 4-6 nm deep. The shape of the pores suggests that smaller p ores continue much further into the interior of the crystals. Besides the irregularities between domains, randomly distributed steps of <1 n m occurred. Hydrothermal treatment did not change crystal size, but ma de the surfaces smoother and reduced both the size and number of the p ores and also the BET surface area (from 75.3 to 35.1 m(2) g(-1)). The existence of meso- (2-30 nm width) and micropores (<2 nm width) in go ethite crystals explains the slow diffusion of heavy metals, phosphate and other ions into goethite particles. Therefore, the external and i nternal surfaces of goethite crystals and their spatial heterogeneity have to be taken into account in models of ion adsorption by goethite.