Ferrihydrite (Fh) is a naturally occurring mineral with high specific surface area and consequently a high surface reactivity. Fh plays an important role in controlling the fate and mobility of contaminants in the environment. Owing to its abundance and similarity in size to the Iron (Fe) atom, Aluminum (Al) has been believed to substitute into the Fh structure by up to 20-30 mol% Al. Less certain is the extent to which Al is dispersed or clustered within the Fh bulk or surface structure.  Also, there has been no attempt to capture this substitution in surface complexation models (SCM) aimed at setting up a predictive framework. SCMs combine chemical descriptions of surface ion/electrolyte and protonation reactions with an electrostatic double-layer model to link the microscopic realm of adsorption to the macroscopic level of solution chemistry. This research aims to develop such an SCM to model and describe surface reactions on Al substituted Fh (Al-Fh).<br/>Our generic approach was to modify an existing SCM model, based on the Hiemstra 11-site Fh model.  The new model focuses on determining the most likely Fe surface sites to be replaced with Al, based on theoretical and experimental evidence. We hypothesize that the singly coordinated FeOH sites which belong to octahedrally coordinated Fe of distorted structure, and less stable, are the ones being substituted by Al. Our research group has zeta potential data for Fh with no Al, 12% Al and 24% mol % Al by molar content obtained from batch experiments in a 200mM NaNO<sub>3 </sub>background electrolyte.  The zeta potential data was used as a benchmark to which the model was fit.  Chemical equilibrium constants (log K) from the existing Fh model were used as a starting point. As “Al properties” are added to the model to simulate the Al-Fh surface, the log K values are expected to increase. The Basic Stern Layer model was utilized to account for electrostatic interactions. Sulfate batch adsorption data will be modelled to determine the accuracy of the model in describing observed macroscopic properties.  Preliminary iterations of the model show a divergence in the zeta potential values measured, and those calculated, at higher pH for 12% Al-Fh (pH&gt;8.2) and at lower pH for 24% Al-Fh (pH&lt;9.7). These deviations may be due to the capacitance value, Al protonation constants and the slipping plane distance which have not been optimized for the model.<br/>

Redefined surface complexation model describing the surface structure of aluminum substituted ferrihydrite

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.