Mercury exchange in zeolites Na-A and Na-Y studied by classical molecular dynamics simulations and ion exchange experiments

Classical molecular dynamics simulations have been employed to study the exchange of Na+ for Hg2+ in zeolite Na-A, with a Si/Al ratio of 1, and zeolite Na-Y, with Si/Al ratios of 2 and 5, in dry and hydrated conditions within the temperature range 330 – 360 K, to understand factors underpinning the performance of zeolites for water decontamination. A classical forcefield based on DFT energies has been developed for the interaction between the Hg2+ ions and the zeolite O atoms. In terms of water diffusion, zeolite Na-A shows the lowest calculated diffusivity, followed by zeolite Na-Y (Si/Al=2) and Na-Y (Si/Al=5), as a consequence of differing pore dimensions and extra-framework ion loadings. In the absence of speciation anions, the Hg2+ ions are consistently adsorbed at the pore windows in both the LTA and FAU framework types. The reduced pore size of zeolite A leads to an average hydration number per Hg2+ ion of <1.0, whilst the wider pore aperture of zeolite Y exerts less steric hindrance, and thus the Hg2+ hydration number reaches values between 1.0 and 2.0 in zeolite Y. These observations might indicate that Hg2+ ions are more strongly immobilized in zeolite A than in zeolite Y. Preliminary measurements of mercury removal using these zeolites, as synthesised from bauxite and kaolin, seem to support these findings.