Defluoridation of Groundwater Using Termite Mound

Abstract

High concentration of fluoride in groundwater is a longstanding health problem. Fluoride adsorption capacity of termite mound (TM), containing mainly silicon, aluminum, iron, and titanium oxides, was investigated under a batch adsorption system. The influence of parameters such as contact time, solution pH, adsorbent dose, initial fluoride concentration, and the presence of competing anions was investigated. Equilibrium was achieved within 10 min of agitation time. A high percentage (~90 %) of fluoride removal was obtained in a wide pH range 3–8, which is important in the practical application. Kinetics data followed the pseudo-second-order model (R2 > 0.99). The Dubinin–Radushkevich isotherm described most satisfactorily (R2 = 0.968, χ2 = 0.09) the equilibrium adsorption, giving a sorption capacity of 2.70 mg/g. The obtained mean free energy (EDR = 11.62 kJ/mol) suggested that chemisorption should be mainly responsible for fluoride adsorption. Fluoride removal was significantly decreased in the presence of carbonate and phosphate ions, whereas slightly increased in the presence of chloride, nitrate, and sulfate. The adsorbent reduced 7.56 mg/L fluoride content of groundwater to below 1.5 mg/L. The fluoride-loaded TM was successfully regenerated using calcined eggshell or NaOH solution with insignificant loss of metals. The adsorption efficiency of the regenerated TM was comparable to the fresh TM. The results obtained from this study could provide important information for evaluating the application of TM for defluoridation.