Kinetic modeling of fluoride adsorption from aqueous solution onto bone char

The rate of fluoride adsorption from water solution on bone char was interpreted by using a diffusional model as well as kinetic models. The experimental data for the fluoride concentration decay were obtained in a rotating basket adsorber. The diffusional model considered that the overall rate of adsorption was clue to the following steps: external mass transfer, intraparticle diffusion and adsorption on an active site. It was assumed that the rate of adsorption on the active sites was instantaneous. Furthermore, the overall rate of adsorption of fluoride was controlled by the pore volume diffusion. The diffusional model fitted the experimental concentration decay curves satisfactorily and the effective pore volume diffusivity of the fluoride in the bone char varied from 2.73 x 10(-6) to 3.71 x 10(-6) cm(2)/s. The tortuosity factor of the bone char was estimated from the effective diffusivity of the fluoride and varied between 1.7 and 2.3. It was recommended to use an average tortuosity factor of tau(p) = 2.1 to estimate the effective diffusion coefficient of fluoride in bone char. The effective diffusivity of fluoride and the tortuosity factor were not dependent upon the operating conditions. The first-, second- and nth-order kinetic models were fitted to the experimental concentration decay data. The results revealed that the second- and the nth-order kinetic models adjusted the experimental data satisfactorily; nevertheless, the rate constants varied with the operating conditions without a reasonable trend. It was concluded that the diffusional model interpreted the experimental data better than the kinetic models. (C) 2010 Elsevier B.V. All rights reserved.