Investigation of HEU-type zeolite crystals after interaction with Sr2+ cations in aqueous solution using nuclear and surface analytical techniques

Pure HEU-type zeolite (heulandite) crystals were allowed to interact with S r2+ cations in aqueous solution. The powdered solid experimental products o btained from batch-type sorption experiments, using solutions of Sr2+-conce ntrations between 10 and 1000 mg/l, were investigated using INAA, RI-XRF an d SEM-EDS. The Sr-uptake by the mineral which can adequately be described w ith a Freundlich-type isotherm, varies from 3.14 to 6.22 mg/g. The distribu tion coefficients increase progressively by decreasing the solutions concen tration reaching a value of 1800 ml/g. The investigated zeolite interacts w ith Sr2+ cations through ion exchange reactions and initial exchangeable Ca 2+ cations are replaced into the structural micropores. However, in the bes t case, only ca. 43% of the theoretical CEC can be covered because of the l imited availability of the extra framework Ca2+ cations that can be removed from the lattice under ambient treatment conditions. The XPS investigation of Sr-loaded single crystals indicated that adsorption of Sr2+ cations on the outer surface also occurs while surface precipitation phenomena must be excluded. Similar surface analyses by means of C-12-RBS showed that the Sr depth-distribution at near-surface layers is quite homogenous in contrast to a previous relevant study revealing an intense surface Sr-accumulation o n a natural Ca-zeolite of different structural characteristics (scolecite).