NH4+, Cu2+, Zn2+, Cd2+ and Pb2+ exchange for Na+ in a sedimentary clinoptilolite, North Sardinia, Italy

Recent findings of clinoptilolite-bearing volcanoclastic deposits in northern Sardinia promoted a study aimed at evaluating the cation exchange properties of this zeolite in view of a possible future utilization of the parent rock. Isotherms of NH4+, Cu2+, Zn2+, Cd2+ and Pb2+ exchange for Na+ at 25 degrees C and 0.1 total normality were obtained and the related thermodynamic quantities K-a and Delta G(0) computed. The maximal exchange level was in any case less than 100% for each cation investigated. Good selectivities were recorded for NH4+ and Pb2+, whereas clinoptilolite proved to be unselective for Cu2+, Zn2+ and Cd2+ The cation selectivity sequence NH4+ > Pb2+> Na+> Cd2+ >Cu2+ congruent to Zn2+ was obtained. The results are discussed in the light of literature data, and contrasting performances of various clinoptilolites were ascribed to different cationic compositions of the minerals, due to different minerogenetic conditions, which were proved to affect both cation exchange capacity and selectivity. This led to the conclusion that any practical utilization of a clinoptilolite-rich rock as cation exchanger should be preceded by basic studies on representative samples from the deposit under examination. (C) 2000 Elsevier Science B.V. All rights reserved.