Preparation, characterization and analyses of carbons with natural and induced calcium compounds for the adsorption of fluoride

A comparative study of fluoride removal has been made using samples of carbons obtained from single carbonization from pecan nut shells and plum kernels and carbons obtained by impregnation of these wastes, prior to their carbonization, with a calcium acetate solution. The impregnation step was determinant for increasing the ability of carbons to remove fluoride anions from water of carbons prepared from both precursors. The intrinsic calcium content of the precursors left a footprint in the process, with carbons prepared from the higher calcium content precursor (i.e., pecan nut shells) always performing better than the plum kernel carbon counterpart. Outstanding fluoride retention abilities (from 0.5 to 2.5 mg of fluoride adsorbed per gram of carbon) were obtained if pecan nut shells are impregnated with a calcium solution of 3151 mg/L and carbonized at 800 °C (sample denominated IC-PN-25). These results can be associated with the calcium compounds founded in IC-PN-25 which are CaO and CaCO3, according with the X-ray results. Furthermore, dynamic adsorption studies were carried out using beds of carbon denominated IC-PN-25. Reasonably high (0.4) filter usage ratios were obtained in such on-column adsorption studies. Finally, different models were used to fit batch and dynamic adsorption data, with Sips and Yan models fitting better the equilibrium isotherm and breakthrough curve data, respectively, than others.