A thermodynamic model of chabazite selectivity for Pb2+

Literature data relative to the cation exchange behavior of a Na-exchanged sedimentary chabazite for Pb 2+ were analyzed by means of the recently proposed Double Selectivity Model (DSM). The DSM allowed to interpret the exchange isotherm by assuming that the exchangeable sites present in the zeolite could be divided in two groups: group "I", responsible for 15% of the overall cation exchange capacity, characterized by a very poor selectivity towards Pb 2+ (K 1 = 8.5·10 -3), and group "II", responsible for 85% of the cation exchange capacity, much more selective towards Pb 2+ (K 2 = 16.2). The comparison between the results of the DSM and the experimental results provided very satisfactory and appeared to confirm the hypothesis that site "I" should be identified with the C1 site in the small cage of the chabazitic structure (6-6 pseudohexagonal prism), whereas site "II" should be identified with the C2-C4 cation sites in the large ellipsoidal cage (20-hedron chabazite cage).