Development of Water Purification Sorbent from Pumice Volcanic Rock for Removal of Fluoride

Abstract

Groundwater is extensively used for household purposes in Kenya’s Rift Valley, particularly in drought-prone regions such as Tiaty, Baringo County. However, groundwater often contains harmful geogenic pollutants such as fluoride. Majority of conventional defluoridation techniques while effective, they are costly and complicated to operate. Hence, there is a need to develop simple and low-cost adsorbents using locally available materials for fluoride remediation. In this study, a silica-based defluoridation sorbent was developed from pumice rock obtained from the Paka volcano in Baringo County, Kenya. Alkaline leaching technique was used to extract silica particles from pumice rock, which was subsequently modified with iron to enhance its affinity for fluoride. To assess its efficacy, randomly sampled borehole water was used. Temperature, electrical conductivity (EC), total dissolved solids (TDS) and pH were determined onsite using a portable multi-parameter meter. Fluoride and sulphate levels were determined using potentiometry and turbidimetry, respectively. Flame photometry was used to determine potassium and sodium. Calcium and magnesium were analyzed using flame atomic absorption spectrophotometry. Bicarbonates, chloride, alkalinity, and hardness levels were determined using titrimetry. The scanning electron microscopy, X-ray diffraction, Fourier transform infrared and X-ray fluorescence spectroscopy were used to characterize the sorbent. Batch experiments were used to determine the optimal pH, sorbent dose, and contact time for fluoride removal. Temperature, pH, EC, and TDS were in the range, 27.4 - 40.7 ºC, 6.8 - 8.7, 402.89 - 4212.99 µS/cm, and 201.24 - 2111.77 mg/L, respectively. The levels of major ions were in the range, HCO3- (127.1 - 1453.2 mg/L), Na+ (46.81 - 856.37 mg/L), Cl- (19.85 - 288.02 mg/L), SO42- (15.62 - 95.31 mg/L) Mg2+ (1.38 - 29.41 mg/L), Ca2+ (1.73 - 30.84 mg/L), K+ (6.08 - 26.49 mg/L) and F- (0.88 - 16.4 mg/L). The extracted silica particles were 96.71 % pure and amorphous, whereas the iron-functionalized silica particles contained 93.67 % SiO2 and 2.93 % Fe2O3. The optimal pH, sorbent dose, and contact time for defluoridation using initial fluoride concentration of 20 mg/L were 6, 1 g, and 45 minutes, respectively. Defluoridation fitted the Freundlich's isotherm model (R2 = 0.989), indicating multilayer adsorption on a heterogeneous surface, and it followed pseudo-second-order kinetics (R2 = 0.992), implying chemisorption. Fluoride levels in borehole water decreased significantly upon deflouridation; Intex 4.57 - 1.1 (75.27 %), Kadokoi 2.46 - 0.54 (78.05 %), and Naudo 5.39 - 1.2 mg/L (77.74 %). The Fe-modified silica sorbent has been demonstrated to significantly remove fluoride from water samples. Hence, it can be used as a mitigation strategy against the adverse health effects associated with elevated fluoride levels in drinking water.