Phosphorus adsorption in Fe-loaded activated carbon: Two-site monolayer equilibrium model and phenomenological kinetic description

This study investigates the impregnation of iron oxide in a granular activated carbon (GAC-Fe), to employ for the adsorption of phosphorous from aqueous solution. Physical and chemical characterization of GAC-Fe was performed including Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy (SEM-EDS), Thermogravimetric analysis (TGA), N-2 physisorption, and zero point charge (pH(ZPC)) analyses. Adsorption isotherms were developed considering the presence of two active sites on the adsorbent surface. The two-site monolayer model was shown as the best isotherm model to describe the system equilibrium. The results indicated the high relation of the adsorption with the pH solution, showing the best adsorption capacity in pH 3.0 (q(m) = 2.874 mg g(-1)). An analysis of the surface charge of the adsorbent indicated a positive value at pH lower than 3.5, explaining the higher affinity to phosphate. The process was spontaneous and favorable at the temperatures studied (25, 35 and 45 degrees C). The thermodynamic results suggest two types of adsorption mechanisms on the surface, related to chemisorption processes. The best kinetic model of adsorption considers the internal mass transfer as the rate-limiting step. The temperature and initial concentration do not show any influence on the effective diffusion coefficient. Hence, the study brings an alternative material for phosphorus adsorption and contributes for a better knowledge of the mechanism involved in the process and equilibrium of the system.