Functionalizing naturally occurring kaolinite by anatase nanoparticles and its effects on the adsorption of fluoride

The development of useful materials for removal of pollutants from water is of great importance for the world's health. In this study, naturally occurring coal-bearing kaolinite (CBK) without thermal pre-activation was successfully modified with TiO2 by a simple one-step hydrothermal method to improve its ability to remove fluoride. The synthesized samples were characterized by X-ray diffraction analysis (XRD), N-2 adsorption-desorption analysis and transmission electron microscopy (TEM), confirming the formation of anatase nanoparticles (< 10 nm) on the surface of the CBK bulk. The obtained CBK/TiO2 nanocomposites preserved the crystalline structure of CBK and exhibited a Brunauer-Emmett-Teller (BET) surface area as high as 118.0 m(2) g(-1). The removal efficiency of fluoride increased from 15 to 87%, when the TiO2 content in the nanocomposite was increased from 0 to 50 mass%, indicating the fluoride-removal ability of CBK improved significantly after modification with TiO2. The fluoride-adsorption kinetics fitted well with pseudo-second-order kinetic model, while the adsorption isotherm data could be described by either the Langmuir adsorption model or the Freundlich adsorption model. Moreover, the synthesized CBK/TiO2 nanocomposites exhibited excellent adsorption capacities for fluoride in groundwater and it could decrease fluoride to a lesser extent (e.g., from 5.88 to 0.98 mgL(-1)).