A calcite (CaCO₃) permeable reactive barrier (PRB) is proposed for removing fluoride from contaminated groundwater. As the pH increases during the removal reaction, pH control is also necessary. Here the pH is regulated by bubbling air through the barrier thereby providing a CO₂ source that contributes to the carbonate equilibrium. The efficiency of a calcite permeable barrier for removal of fluoride from groundwater is demonstrated in column experiments. Experiments were conducted at room temperature (~25±2 °C) in a polycarbonate column 41.0cm in length and 4.75cm in diameter (id) using groundwater samples containing fluoride concentrations of up to 2,300 mg/L. Such high concentrations are found at some industrial sites. Approximately 1.2 kg of pre-washed (with 18 MΩcm⁻¹ deionised water) and dried 1.18 mm diameter calcite was packed into the column. Approximately four pore volumes of 18 MΩcm⁻¹ deionised water was pumped through the column to ensure any loose calcite powder was washed out. Various fluoride solutions were then introduced into the column via a peristaltic pump at a constant flow rate. The flow rate was such that a residence time of between two and six hours was achieved. The effluent pH and fluoride concentration were logged via multi-channel datalogger connected to Orion ion selective electrodes (ISEs) located in a flow-through cell. Our observations show that fluoride removal is pH dependant with removal increasing with decreasing pH. Barrier performance can be increased via CO₂ addition with the point of injection critical for optimising column performance. Experimental and model results show that approximately 99% of 2,300 mg/L fluoride can be removed when CO₂ is injected directly into the column. This can be compared to approximately 30-50% removal when the influent solution is equilibrated with atmospheric CO₂ before contact with calcite. Results also show that influent water composition affects barrier performance.
Geochimica et Cosmochimica Acta Vol. 68, Issue 11S