MODELING THE IMPACT OF FERRIHYDRITE ON ADSORPTION-DESORPTION OF SOIL PHOSPHORUS

Abstract

Ferrihydrite is an Fe-oxide mineral with a high phosphorus (P) sorption capacity. Modeling the P adsorption and desorption mechanisms of soil amended with ferrihydrite is necessary to predict the movement of dissolved and sediment-bound P. The objective of this study was to model the multi-reaction P sorption properties of soil amended with ferrihydrite. Soil samples were treated with 0, 6.72, and 11.20 Mg ha⁻¹ of ferrihydrite. The <2-mm-size fraction of each treatment was tested for P sorption properties by the batch isotherm method. The Langmuir and Freundlich equations were applied as a single-site, instantaneous equilibrium approach for describing adsorption, and the multi-reaction (MRTM) model was applied with various combinations of equilibrium, reversible-kinetic, and irreversible sorption sites to describe the adsorption-desorption. Application of ferrihydrite increased the rapid P adsorption, but equilibrium was not reached after 1440 h because of the highly kinetic nature of P sorption. The one-site, instantaneous equilibrium approach was deemed inappropriate because of strong time-dependence in the Langmuir and Freundlich parameters. The reversible-kinetic sorption approach was superior to the instantaneous-equilibrium sorption approach for describing the rapid reactivity. The two-site model involving a reversible-kinetic site with either a concurrent irreversible or independently irreversible site was superior to the one-site and multi-site approaches. Application of ferrihydrite increased the rapid adsorption rate coefficients and the irreversible rate coefficients. These results suggest that ferrihydrite can be an effective soil amendment for enhancing P sorption and reducing P release.