Kinetic Modeling of Phosphate Adsorption by Preformed and In situ formed Hydrous Ferric Oxides at Circumneutral pH

Kinetics of phosphate removal by Fe(III) was investigated by both preformed and in situ formed hydrous ferric oxides (HFO) at pH 6.0-8.0. A pseudo-second-order empirical model was found to adequately describe phosphate removal in the two cases. The Elovich and intra-particle diffusion models, however, were only capable of describing phosphate adsorption to preformed HFO (PF-HFO). By using surface complexation kinetic models (SCKMs) to describe phosphate adsorption to PF-HFO, the adsorption rate constant (0.0386-0.205 mM-1 min-1 for SCKM-1 and 0.0680-0.274 mM-1 min-1 for SCKM-2) decreased with increasing pH while the protonation reaction rate constant in SCKM-2 (0.0776-0.0947 mM-1 min-1) increased over the pH range 6.0-8.0. Using the rate constants obtained from the process of phosphate adsorption to PF-HFO, the amount of active surface sites on the in situ formed HFO were calculated as 0.955 ± 0.170, 1.46 ± 0.39 and 2.98 ± 0.78 mM for pH = 6.0, 7.0 and 8, respectively. Generally, as the SCKMs incorporate phosphate complexation on HFO surface sites and protons competiting for the surface sites, they could provide a good description of the rate and extent of phosphate removal by both preformed and in-situ formed HFO over a wide range of conditions.