Continuous Column Studies for Water Defluoridation Using Synthesized Magnesium-incorporated Hydroxyapatite Pellets: Experimental and Modeling Studies

Abstract

This study presents the synthesis of magnesium-incorporated hydroxyapatite (M-i-HAPa) pellets and evaluation of its defluoridation potential via column studies. Characterization of M-i-HAPa was conducted to analyze the phase composition, bonding patterns, elemental details and microstructural characteristics. An adsorption-regeneration experimental setup was used to study the breakthrough performances and the effect of varying process parameters such as flow rate, bed height, feed fluoride concentration, particle size and particle shape. The experimental studies of varying flow rate and size of particles were evaluated with simulated runs from an adsorption simulator “ADSORB” developed in MATLAB software. The column adsorption capacity was observed to be 1.46 mg/g at feed fluoride concentration of 10 mg/L, flow rate of 1 L/h and bed height of 30 cm. Studies on the column breakthrough performances under different process conditions indicated that bed height of 30 cm with 1–1.5 mm pellets of spherical particle sizes and flow rate of 1 L/h for an initial feed fluoride concentration of 10 mg/L was optimum for efficient column performance. Regeneration of M-i-HAPa pellets was achieved using 0.1 M NaOH solution; however, its capacity was reduced to 40% of its initial capacity after the third cycle. The treated water quality parameters tested after the defluoridation process were found to be under permissible limits as per WHO and BIS guidelines, ensuring the promising nature of M-i-HAPa pellets as a viable adsorbent for defluoridation. A domestic defluoridation unit was also designed which can treat approximately 348 L of fluoride water at a concentration of 10 mg/L using 3 kg of M-i-HAPa pellets with a fluoride uptake capacity of 1.16 mg/g.