Investigation of chemical processes at mineral surfaces using accelerator-based and surface analytical techniques: Heavy metal sorption on zeolite crystals
Natural HEU-type zeolite crystals were exposed to copper, mercury, lead and uranium aqueous solutions. The reacted zeolite samples obtained were examined using SEM-EDS and further investigated by means of -RBS12C-RBS and XPS. The -RBS12C-RBS study proved an inhomogeneous depth distribution of sorbed metals on the mineral surface and particularly an intense metal accumulation in the near-surface layers of the material, which can be attributed to an anomalous diffusion of metal cations into the micropores due to ion-exchange reactions. This phenomenon strongly depends on the size of the zeolite crystals, the treatment time and the type of the exchangeable cations. In addition in the case of mercury and uranium, metal-rich "layers" supported on the surface could also be identified. The XPS results showed that these "layers" consist of metal oxides and hydroxides adsorbed and precipitated during the interaction of the zeolite with the heavy metal-containing aqueous solutions. The surface morphology and microtopography seem to play an important role to the chemical processes taking place at the zeolite surface.