Mineral surfaces play an important role in the mobility of ions in groundwater as sorption reactions limit ion transport through the phreatic zone. Sorption reactions are often utilized to prevent contaminate transport after an accidental release when removal is prohibitively expensive. However, this process is still not well characterized as surface interfaces are difficult to measure or model. To better understand the thermodynamics of this system, the interface of a mineral surface with an aqueous environment is being investigated. Rutile (TiO<sub>2</sub>) was used as a test mineral to explore how the alkali metal series goes from a hydrated ion in solution to an ion bound to the surface. Isothermic calorimetry experiments measuring heat flow were done with 5g/L rutile in 0.1M of each alkali chloride salt (MCl; M=Li,Na,K,Rb,Cs) and 0.01M HCl. Each solution started at pH 2 with 0.1M NaOH titrated in until the final pH was about 11.5. At a pH well below the pH<sub>znpc</sub> of rutile, the surface sites are taken by protons however the gradual increase in pH causes the surface to slowly deprotonate, opening up surface sites for the alkali cation to bind to. By accounting for the heat of formation of water from the acid-base neutralization reaction, the surface binding energies of the alkali metals to the rutile surface can be calculated, assuming the chlorine does not interact with the rutile surface. These results can then be compared to an electronic double layer (EDL) model of the rutile surface using equilibrium constants for rutile from literature. Molecular dynamic simulations of the hydrated rutile (110) surface were also performed to probe the sorption at the nanometer scale. A 7x4x4 rutile (110) slab was hydrated with each alkali metal cation in solution. The extent of surface hydroxylation for rutile (110) remains in contention. Therefore several surface structures were modeled from a non-hydroxylated surface with water above the terminal 5-coordinated Ti and bridging O up to complete hydroxylation on the surface. Simulations were run for 5 nanoseconds with the alkali metal in bulk water, near the surface, and bound to the surface to measure total energy change as the metal approaches the surface. The difference in the energy of the system as the cation goes from a hydrated ion in bulk water to bound to a surface site can then be compared to the sorption heat measured in earlier calorimetry experiments.

Alkali metal sorption to rutile (110): Experimental and theoretical approach

2019 Fall National Meeting

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