Modelling the adsorption of fluoride onto activated alumina in the presence of other ions

Most of the studies on the adsorption of F$^-$ are experimental, and have been done with synthetic solutions. Such studies rarely mimic the field situation. Therefore, selection of an adsorbent that can remove F$^-$ from any kind of feed requires models that can predict the adsorption behavior for any given set of input conditions. From our observations and as also reported by many authors, the adsorption of F$^-$ is affected by the presence of many ions. When modelling the adsorption of F$^-$, it is usually taken as a single entity getting adsorbed on the adsorbent. As this is not a proper assumption, a model was developed which takes into account all the speciation reactions that take place during adsorption, and all the species like H$^+$, OH$^-$, Na$^+$, Cl$^-$, and NO$_3^-$ present in the solution along with F$^-$. As an electrolyte system is involved, the Nernst-Planck equations were used to obtain the flux of each species. Using the model, the equilibrium constants and rate constants for the reactions were obtained. For one initial concentration of F$^-$, a reasonable fit was obtained to the batch adsorption data, except at short times. Because of an uncertainty in the amount of impurity present in the commercial adsorbent used, there was a significant discrepancy between predictions and data at higher initial concentrations of F$^-$. The present model can be applied to any charged adsorbent.