Abstract
Effective removal of hexavalent chromium, Cr(VI) from aqueous solution by adsorption on the used black tea leaves (UBTLs) as a low cost adsorbent has been reported. In the present study, kinetic investigations were carried out to study the transport mechanism of Cr(VI) ion to the UBTLs during the removal process. Adsorption of Cr(VI) on the UBTLs would include diffusion of Cr(VI) from the bulk solution to the surface of the UBTL, adsorption, intraparticle diffusion and chemical reaction. Different transport models were applied to find out the controlling transfer process of Cr(VI) uptake by the UBTLs. The experimental data obtained from the batch treatments show that the Cr(VI) uptake occurs rapidly in the first day, followed by a slow uptake process for about 15 days. A significant effect of particle size of the UBTLs on the adsorption rate indicates the following: the first rapid step and followed slow step of adsorption in this system are controlled by chemical reaction and the intraparticle diffusion, respectively. A parabolic diffusion model suggested that this adsorption mechanism shifts from the reaction controlled to the diffusion controlled with an increase in initial Cr(VI) concentration in the solution. Non-linearity of the ln(Ct/Co) vs. t plot indicates that the film resistance is insignificant to the transfer of Cr(VI). The intraparticle diffusion is the rate-determining step in the uptake of Cr(VI) by the UBTLs.
