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Permeable reactive barriers (PRBs) are a technology for remediating groundwater contaminated with hazardous substances. Fluoride compounds are specified as hazardous substances by Japanese environmental standards. Currently there are no suitable materials applicable to PRB for removing fluoride; hence, we present an adsorbent prepared inexpensively from volcanic ash soil (VAS) as a PRB material. The purpose of this work was to clarify whether VAS is an applicable PRB material to remove fluoride from groundwater. VAS consists of aluminum oxide, silicon oxide, and iron oxide. Previous reports have suggested that the mechanism of fluoride adsorption onto VAS involves an ion-exchange reaction with the active hydroxyl groups of the metal oxides. The present study successfully confirmed the ion-exchange mechanism through batch-wise adsorption experiments. We also confirmed that VAS has a higher adsorption selectivity for fluoride than for the common components in groundwater. Because of the ion-exchange mechanism, the ordinary Langmuir equation cannot be applied to describe the adsorption behavior when the pH differs between batch-wise and column experiments. Therefore, we introduced the modified Langmuir adsorption isotherm, which considers the pH of the aqueous solution at equilibrium to predict the amount of fluoride adsorbed in the column experiment. Column experiments were also conducted to obtain the rate of fluoride adsorption onto VAS. The overall mass transfer coefficient was estimated from experimental breakthrough data by numerical analysis. We obtained both the equilibrium and the kinetic parameters of fluoride adsorption onto VAS for the design of a PRB for removing fluoride from groundwater.

Investigation of Permeable Reactive Barrier Design Parameters for Remediating Fluoride-Contaminated Groundwater using a Volcanic Ash Soil Adsorbent