Abstract
Classical molecular dynamics simulations have been employed to study the exchange of Na + for Hg 2 + 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 Hg 2 + 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 Hg 2 + ions are consistently adsorbed at the supercage windows in both the LTA and FAU framework types. The reduced pore size of zeolite A leads to an average hydration number per Hg 2 + ion of ¡1.0, whilst the wider pore of zeolite Y exerts less steric hindrance, and thus the Hg 2 + hydration number reaches values between 1.0 and 2.0 in zeolite Y. These observations might indicate that Hg 2 + ions are more strongly immobilized in zeolite A than in zeolite Y. Preliminary measurements of mercury removal using these zeolites, as synthesized from bauxite and kaolin, seem to support these findings. • A new set of interatomic potentials is provided to describe the interaction between Hg 2+ and O. • The hydration of Hg 2+ is affected by the Si/Al ratio and size of the supercage windows. • Smaller supercage windows and lower Si/Al ratio should be more effective exchanging Hg 2+ . • Preliminary experiments show Na-A is more robust than Na-Y for the removal of Hg 2+ .
Highlights
The contamination of rivers by mercury is a serious environmental and health issue, which predominantly affects less developed countries and is derived from the mining of precious metals such as gold [1,2,3]
In the absence of Hg2+, the slowest water diffusion is observed for zeolite Na-A, followed by zeolite Na-Y(2), with the fastest diffusion occurring in zeolite Na-Y(5), as expected since zeolite Na-A has both the highest concentration of Na+ ions and the narrowest windows between larger cages, 8MR (∼ 4 Å) compared to 12MR (∼ 7.5 Å) in framework type FAU
In the present work we have combined classical molecular dynamics simulations and ion-exchange experiments to study the efficiency of zeolites Na-A and Na-Y to remove Hg2+ from water
Summary
The contamination of rivers by mercury is a serious environmental and health issue, which predominantly affects less developed countries and is derived from the mining of precious metals such as gold [1,2,3]. Even if effective actions are adopted to eliminate the industrial and artisanal methods that produce the undesirable waste, the mercury already in the water bodies must be physically extracted. In this regard, ion-exchange matrices could be a viable method for the removal of the poisonous cations [4,5]. Zeolites are microporous alumino-silicate materials widely known for their outstanding ion exchange characteristics and environmental benignity [6] Their efficacies as ion-exchange matrices are derived from the replacement of Si(IV) at tetrahedral positions by Al(III), which leaves a net negative charge that is balanced by extra-framework counter-ions commonly belonging to the alkali and alkaline earth groups [6].
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