Modeling the Sorption of Fluoride onto Alumina

Fluoride is a potentially toxic ion that occurs in aquifers both naturally and as a result of anthropogenic activity. Sometimes remediation of the aquifer is required. One potential aquifer decontamination strategy is an "interceptionsorption trench"—one of a number of "reactive wall" technologies. This remediation strategy relies upon natural hydraulic gradients to transport the fluoride through the aquifer to the interception-sorption trench where it partitions onto a strong sorbent—alumina. In this paper, the focus is on the development and calibration of an equilibrium-based geochemical model that will be employed in the development of a quantitative reactive transport model, which in turn will be used for the design of an interception-sorption trench. The geochemical model described here takes into account a variety of ions likely to be present in a sandy aquifer, chemical activities, and the surface charge on the alumina. The model is calibrated over a wide pH range and for high initial fluoride concentrations using experimental results obtained from batch tests. It is found that pH dependent equilibrium constants are needed to capture the behavior of the experimentally observed fluoride sorption. The presence of sodium sulfate in solution is investigated, and it is found that sodium significantly interferes with the sorption of fluoride onto alumina under alkaline conditions. The geochemical model indicates that under acidic conditions, the alumina may release potentially large and unacceptable concentrations of aluminum into the aquifer. As a way of managing this potential problem, it is proposed that aluminum concentrations in the pore fluid may be mitigated by the inclusion of tree bark within the interception-sorption trench.