A rapid method for the removal of fluoride in contaminated groundwater using natural crystalline apatite: a laboratory and field study

Natural apatite from Eppawala located in the north central province of Sri Lanka was investigated for its ability to adsorb fluoride from aqueous solutions under laboratory conditions to develop an inexpensive and environmentally friendly adsorbent material. The adsorption of fluoride was evaluated as a function of pH, adsorbent dosage, contact time and initial fluoride concentrations. The solid materials were characterized using FTIR and XRD methods and their geochemical composition was obtained from XRF analyses. The adsorption studies were carried out under controlled conditions in which the fluoride activities of the solution were measured using an ion-selective electrode. The fluoride adsorption onto apatite is strongly pH dependent and the highest fluoride removal was observed at pH 6.0, while adsorption decreases both at low and high pH levels. The percentage fluoride removal increases with the absorbent dosage mainly due to increasing the number of adsorption sites, but the optimum adsorption can be obtained using small initial amounts of apatite in the suspension. The sorption process was very rapid and the maximum adsorption was achieved within the first 10 min in a solution containing 15.0 mg/L fluoride at pH 6.0. The kinetic studies showed that the fluoride removal process follows pseudo-second-order kinetics. The Langmuir isotherm best fitted (R ² = 0.9537) with the adsorption over a range of fluoride concentrations from 5.0 to 25.0 mg/L. The monolayer coverage of fluoride on apatite was estimated as 0.212 mg/g. The simulated experimental conditions were tested for natural water with high fluoride levels with promising results and hence the results obtained from this study can be used to design an inexpensive, easy to use and environmentally friendly method for removal of fluoride in water supplies at village level.