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 surface, which are important for sorption processes of cations and anions. Wedge-shaped pores >100 nm long at the surface occurred at the domain boundaries. They were 20-30 nm wide, becoming narrower towards the interior of the crystals to 2 nm and less. The AFM showed these pores to be at least 4-6 nm deep. The shape of the pores suggests that smaller pores continue much further into the interior of the crystals. Besides the irregularities between domains, randomly distributed steps of <1 nm occurred. Hydrothermal treatment did not change crystal size, but made the surfaces smoother and reduced both the size and number of the pores 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 goethite crystals explains the slow diffusion of heavy metals, phosphate and other ions into goethite particles. Therefore, the external and internal surfaces of goethite crystals and their spatial heterogeneity have to be taken into account in models of ion adsorption by goethite.