Adsorption of Fluoride from Water Using Sintered Clay-Hydroxyapatite Composites

This paper proposes a new method of removing fluoride from drinking water. It presents the results of an experimental study on the effects of the temperature and mixture ratio on the adsorption characteristics of fluoride on clay-hydroxyapatite (C-HA) composites. The writers fabricated the C-HA materials from composites of hydroxyapatite (HA), redart clay, and fine wood chips (sawdust) in well-controlled proportions. Five different ratios of clay to HA at five different temperatures (500, 600, 700, 800, and 900 degrees C) were sintered. The results of the adsorption tests demonstrated that the defluoridation capacity of the adsorbent that contains only HA decreases with increasing sintering temperature. The adsorption capacities of the C-HA adsorbents also increase with increasing HA volume fraction. The adsorption capacity of the porous C-HA mixtures and HA are well modeled by the Freundlich equation. The changes in the free energy reveal the spontaneous nature of the adsorption process. A particle and intraparticle (pore) model was also used to characterize the batch kinetics. The potential implications of the results are then discussed for the development of future point-of-use water filtration devices for the removal of fluoride and microbial pathogens from C-HA filters. (C) 2013 American Society of Civil Engineers.