A Study of the Adsorption of Ni(II) onto an Amorphous Silica Surface by Chemical and NMR Methods

Citation

Young, James Robert (1982) A Study of the Adsorption of Ni(II) onto an Amorphous Silica Surface by Chemical and NMR Methods. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Q574-CG34. https://resolver.caltech.edu/CaltechETD:etd-09062006-143724

Abstract

The physical structure of the electrical double layer at a metal-oxide surface is not well-known. Adsorption models, which require a knowledge of the location of the adsorbed species, have integrated different descriptions of the electrical double layer into a common coordination chemistry framework.

The goal of this work was to determine the hydration number of Ni(II) species adsorbed unto a silica surface by extending a nuclear magnetic resonance (NMR) technique used to determine the hydration number of paramagnetic metal ions in solution. Information about the environment of the adsorbed Ni(II) species at the molecular level could then be incorporated into state-of-the-art adsorption models.

In order to conduct the NMR experiment, it was necessary to determine the speciation of a Ni(II)-silica system at a high metal-ion loading. Titrations, in 0.1 N NaClO₄, were conducted on silica suspensions, nickel solutions and a system where the total amount of Ni(II) was twice the total number of available surface binding sites. The pH dependence of the adsorption process and the maximum adsorption density achievable were investigated.

The amount of charge on a silica surface was found to increase logarithmically with pH. Formation of surface complexes with Na⁺ provided the necessary mechanism to account for this behavior. All of the Ni(II) present in the Ni(II)-silica system was removed by adsorption processes at a pH of approximately 8. In the same system, removal of Ni(II) by a precipitation process does not occur until Ni(OH)₂(s) forms at a pH of approximately 8.5. The adsorption process was not limited by the number of surface hydroxyl groups present. It was shown that the NiOH moiety of a hydrolyzed nickel surface complex could provide the new adsorption centers necessary for a multilayer adsorption process, if the NiOH group functions chemically like an SiOH group. The observation of adsorption of Ni(II) onto a silica surface at coverages greater than one monolayer indicates siliceous soils and sediments may have a greater capacity to bind Ni(II) than previously thought.

The capability of the NMR spectroscopic technique to determine hydration numbers of adsorbed metal ions was demonstrated. Usefulness of the technique was limited by the slowness of water exchange between the first coordination sphere of the adsorbed Ni(II) species and bulk solution. A lower limit for the hydration number of the adsorbed Ni(II) species was calculated to be 4.4. This is at the lower end of the predicted range for this value (4.5 to 6.0). Suggestions were made to improve upon the design of the NMR experiment so as to overcome the limitations encountered.

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