Modelling The Adsorption Of Fluoride Onto Activated Alumina In The Presence Of Other Ions

Removal of fluoride (F-) contamination in drinking water by adsorption has been studied experimentally, but often with synthetic solutions in the absence of other ions commonly present in a natural source. Selection of an adsorbent and design of adsorption columns for removal of F- from variety of feeds requires data on adsorption in the presence of other ionic species as well as models to predict the adsorption behaviour. A model is developed in this work which takes into account the speciation reactions that occur during adsorption onto activated alumina, and the presence of the ions H+, OH-, Na+, Cl-, and NO3- along with F-. The model accounts for the diffusional resistances in the multicomponent ionic system and the rates of adsorption reactions. Nernst-Planck equation is used to obtain the fluxes of the species. Data on adsorption are obtained using a differential bed adsorber. The data are used to determine the equilibrium constants and rate constants for the speciation reactions that appear in the model. The model was tested against independently determined batch adsorption data. A reasonable fit was obtained at low F- concentrations, except at short times. A significant discrepancy between predictions and data was observed at higher initial concentrations of F-, which is caused by an uncertainty in the amount of impurity present in the commercial adsorbent used. The model is general in nature and can be applied to any adsorbent with a charged interface.