Modeling adsorption rate of tetracyclines on activated carbons from aqueous phase

The adsorption rate of three tetracyclines (TCs) (tetracycline, oxytetracycline, chlortetracycline) on two activated carbons (ACs) were investigated in this work. The experimental adsorption equilibrium data of the TCs on both carbons was obtained in a batch adsorber at T = 298 K and pH range of 4-5, and the Langmuir isotherm better interpreted the experimental data than the Freundlich isotherm. The adsorption of TCs on the ACs was mainly due to π-π interactions. The rate of adsorption of TCs was interpreted using kinetic models along with diffusional models. The pseudo-first-order and pseudo-second-order were fitted to the experimental concentration decay curves of the TCs for the adsorption of TCs on ACs. The first-order kinetic model matched reasonably well the experimental concentration decay data, but the rate constant, k1, considerably decreased with time. Thus, the rate of adsorption of TCs on ACs cannot be interpreted by the first-order kinetic model. The pore volume diffusion model (PVDM) and the pore volume and surface diffusion model (PVSDM) were also applied to interpret the rate of adsorption of TCs. The PVDM overpredicted the experimental concentration decay data indicating that intraparticle diffusion was due to both pore volume and surface diffusion mechanisms. The PVSDM fitted quite well the experimental concentration decay data of the TCs on both ACs, showing that the intraparticle diffusion of TCs is due to the pore volume diffusion, as well as the surface diffusion. Furthermore, the contribution of surface diffusion is directly dependent on the adsorption capacity of the carbons because the concentration of TCs adsorbed on the surface is the driving force of surface diffusion. Additionally, the contribution of surface diffusion is affected by the time and radial position.