Defluoridation performance of nano-hydroxyapatite/stilbite composite compared with bone char

Fluoride contamination of drinking water is a serious problem in the Rift Valley region of Ethiopia. In this work, a nano-hydroxyapatite/stilbite (nHAST) composite was prepared based on locally available stilbite zeolites and its fluoride removal performance was compared with that of Bone Char (BC). The reaction mechanism involving the fluoride adsorption is analyzed based on kinetics and isotherm studies. In both nHAST and BC the kinetic data fitted well to a pseudo-second order kinetic model of similar characteristics. In contrast, the adsorption isotherm on the nHAST composite fitted best with the Freundlich model, whereas on BC, it correlated well with the Langmuir model, suggesting a different mechanism: adsorption of fluoride on BC was homogeneous, whereas on the composite was heterogeneous, possibly related to the higher load of fluoride on the hydroxyapatite component in the composite. At low concentrations, both adsorbents behave similarly; however, the maximum adsorption capacity, measured at high concentrations, is higher in the nHAST composite than in BC. The intrinsic HAp capacity of nHAST, normalized to the amount of HAp on the adsorbent, is significantly higher (9.15 mg F-/gHAp) than that of BC (1.08 mg F-/gHAp) (measured at low F- initial concentrations where the OH/F-exchange mechanism predominates), showing a much higher F- removal efficiency of HAp on the composite. These results foresee a high potential of nHAST composite toward F(-)removal implementation, specially taking into account the low-cost, ease of production, local availability and social acceptance. (C) 2015 Elsevier B.V. All rights reserved.