Ageing and structural effects on the sorption characteristics of Cd2+ by clinoptilolite and Y-type zeolite studied using isotope exchange technique

Abstract

This research investigates the long-term kinetics of Cd2+ sorption and desorption by calcium-exchanged clinoptilolite (CaCpt) and Y-type (CaY) zeolite using isotopic exchange with 109Cd while maintaining pH at circumneutral values. The effects of Si/Al ratio and crystal structure of these zeolitic materials on intracrystalline transport of Cd are discussed. A first-order kinetic model was developed to describe the progressive transfer of Cd2+ to a less reactive form within the zeolite structure, following initial sorption and subsequent desorption of Cd subject to different initial contact times. The kinetic model differentiates between two forms of sorbed Cd2+ designated ‘labile’ and ‘non-labile’ in which the labile form is in immediate equilibrium with the free Cd2+ ion activity in solution. A model combining diffusion and first-order kinetics for cation exchange was also employed to determine Cd2+ diffusivity and intracrystalline exchange rates in CaY and CaCpt. The efficiency of Permeable Reactive Barriers (PRBs) containing zeolitic materials in protecting water systems against lateral flow of metal-contaminated leachate was simulated for three contrasting zeolites. The slow transfer of Cd between labile and non-labile forms was particularly important in moderating high concentration pulses of Cd traversing the PRB. In addition, the reversibility of Cd fixation effectively restored the sorption capability of the zeolite through slow leakage to drainage water.