Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/22/2003
Publication Date: 1/30/2004
Citation: Wilson, G.V., Rhoton, F.E., Selim, H.M. 2004. Modeling the impact of ferrihydrite on adsorption-desorption of soil phosphorus. Soil Science. 169: 271-282.
Interpretive Summary: Phosphorus (P) can be adsorbed by soil particles and then later be released back into solution. The rates at which P is adsorbed and released are needed in order to predict the movement of P from agircultural lands. Ferrihydrite is a form of iron that is so common in groundwater in the southeastern United States that it is available in large quantitites from water treatments plants. This form of iron has a high capacity to sorb P. The objectives of this study were to determine the impact on P sorption and release of using ferrihydrite as a soil amendment. The Multi Reaction Transport Model (MRTM) was applied with various combinations of equilibrium, time-dependent, and irreversible sorption sites to describe the sorption and release processes. Application of ferrihydrite increased the rapid P adsorption but equilibrium between the amount in solution and the amount sorbed was not reached after 1440 h due to the highly time-dependent nature of P sorption. An attempt was made to describe the P sorption as occurring with one-type of sorption site that was immediately in equilibrium but this was found to be inappropriate. The time-dependent sorption approach was better than the immediate-equilibrium sorption approach for describing the rapid reactivity. Describing P sorption and release with two types of sorption sites was superior to the one-site approach. The two sites included a time-dependent site in which P could adsorbed then released, along with a site in which the sorption of P was not reversible. Ferrihydrite increased the rapid adsorption rates and the irreversible rates. These results suggest that ferrihydrite can be an effective soil amendment for increasing P removal from runoff waters and for reducing P release from sediment back into the water.
Technical 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. Two aggregate size fractions (< 2 mm and > 2 mm) of each treatment were 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 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 due to the highly kinetic nature of P sorption. The 1-site, instantaneous equilibrium approach was deemed inappropriate due to strong time dependence in the Langmuir and Freundlich parameters. The reversible-kinetic sorption approach was superior to the instantaneous sorption approach for describing the rapid reactivity. The 2-site approach with a reversible-kinetic site and either a concurrent irreversible or independently irreversible site were superior to the single-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 reducing P release.