The potential of Zeolite Clinoptilolite for Arsenic immobilisation: a computational study
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2015
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Abstract
Clinoptilolite has been identified as a candidate for arsenic removal from water, but the exact adsorption sites, their relative stabilities and the characteristics of the environment where the arsenic is located within the zeolite framework still remains elusive. We investigated the arsenic acid AsO(OH)3 and arsenous acid As(OH)3 adsorption by Al(III)-modified natural zeolite clinoptilolite (CL) under vacuum and hydrated conditions using first-principles density functional theory calculations. Density functional theory (DFT) calculations within the generalized gradient approximation (GGA) were carried out using the plane-wave pseudopotential PWscf code in the Quantum-ESPRESSO package using ultrasoft pseudopotentials with the PBE functional. We also considered the effect of Si/Al ratio on the adsorption potentials of Al(III)-modified natural zeolite clinoptilolite. Essentially the adsorption energies for both arsenic and arsenous acids on dehydrated CL zeolite (Si/Al = 1.7 - 8) at all the adsorption sites are favorable (7 – 252 kJ/mol, exothermic) except at the 8 MR in the (100) plane (site 3) for AsO(OH)3 at Si/Al ratio of 5 and the site locate between the 8MR in the (001) plane and the 8 MR in the (100) plane (site 4) for AsO(OH)3 at Si/Al ratio of 6 and 8. However, in the hydrated CL, the exothermicity is maintained only in the 8 MR in the (001) plane (site 2) and site 4 with increase in the extent of exothermicity (32 - 161 kJ/mol) at these sites with an interatomic distance of the adsorption of single AsO(OH)3 and As(OH)3 molecules being 2.309 Å.
The unfavorableness of the adsorption in the hydrated systems increases with increasing water molecules added. We attribute this to the water molecules filling the pores in the zeolite, therefore leaving no space for the adsorbates to fit in. The strength of binding of the arsenic species is shown to depend sensitively on the Si/Al ratio in the Al(III)-modified CL zeolite, adsorption energies decreases (252 - 7 kJ/mol) as the Si/Al ratio increases (1.7 - 8), this is as a result of increase in the interaction between the framework cations and the arsenic species. The
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calculated high adsorption energies indicated a great potential for Al(III)-modified clinoptilolite for arsenic immobilization. This preliminary work improves our understanding of the role that Al(III)-modified clinoptilolite zeolite may play in the remediation of Arsenic contaminated sites and may help the development of reliable forcefields that can be employed in classical MD simulations to simulate complex systems.
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A thesis presented to the Department of Physics, in partial fulfillment of the requirements for the award of M.Phil. Material Science degree,