Using Atomistic Simulation Techniques to Study the Adsorption of Cations and H2O on Zeolite Surfaces

Abstract

Heavy metals and radionuclides are both hazardous cationic species, and their interaction with soils have received increasing attention in recent years. Ion exchange is regarded as one of the key processes that regulate these hazardous species within the environment, but it can also be the process used for the decontamination of polluted soil, (ground)water or effluent. An ion-exchanger, often a zeolite, which is specific to the type of contamination, can be used to selectively remove the pollutant. For example, Clinoptilolite, which is used to remove Cs-137 and Sr-90 from soils and milk, or Phillipsite, which has been used to recover Cd from acid mine drainage. Due to this selective recovery of the cationic contamination, zeolites concentrate and therefore reduce the waste volume. For these reasons we decided to model cationexchange and adsorption behaviour of a range of zeolites with varying structural properties, including ring size and Si/A1 ratio. The zeolites studied are; zeolite A (LTA), zeolite RHO, Heulandite and its isostructure Clinoptilolite. The cationic species used in our calculations were selected for their varying ion size, charge along with their environmental importance (Table 1). Zeolite A was chosen as a model structure to verify our simulation techniques as there is ample experimental crystallo-graphic data available to compare our results with. In addition, atomistic calculations have been used successfully before to study structural and reactivity properties of zeolites, including Na-exchanged zeolite A. Atomistic simulations could aid the understanding of the ion-exchange properties of zeolites by studying the bulk and surface structures of the different zeolites for; (i) cation adsorption sites, (ii) effect of aluminium distribution, (iii) cation substitution, (iv) cation diffusion pathways and activation energies, (v) effect of water at neutral, acid and basic pH.