PHOSPHATE ADSORPTION ON GOETHITE (?-FEOOH)

Abstract

Goethite, the most frequent and abundant form of iron oxide in soils and sediments, is an important component influencing phosphate adsorption in natural soil or aquatic environment (Schwertmann and Taylor, 1989). The mechanism of phosphate adsorption by goethite has been discussed for at least 25 years. Some researcher have found abnormalities which can not be fitted into adsorption models. Surface precipitation and coagulation may affect the adsorptive interaction between phosphate and the goethite surface during adsorption experiments. The effects of surface precipitation and coagulation on adsorption reactions are not included in the current thermodynamic models, which may account for some of the deviation of experimental data from the model calculated data. In this work, the interaction between phosphate and goethite was investigated to evaluate phosphate adsorption behavior and to evaluate changes in zeta potential and particle size during adsorption. Scanning electron microscopy was used to observe the goethite particle micro-structure. The experimental adsorption data were fitted to Langmuir, Freundlich, and surface complexation models at different surface coverage ranges. Phosphate adsorption on goethite was investigated at varying pH and phosphate concentrations. The adsorption isotherms approached linearity at high surface coverage and deviated from Langmuir or surface complexation adsorption isotherms. A plot of the change in zeta potential with surface coverage showed a break in the slope, which may represent a transition from adsorption (monolayer coverage) to precipitation (multilayer coverage). The transition from adsorption to precipitation is a smooth and continuous process, so it is difficult to determine when plotting the adsorption isotherms. The measurement of zeta potential in adsorption experiments may provide a better method to distinguish adsorption reaction and surface precipitation. Particle size measurement of goethite showed that diameter of goethite varied from about 400 nm for the highly dispersed individual particle to about 20,000 nm as pH and phosphate concentration changed, which suggests that the particles are coagulating during adsorption. The Langmuir and Freundlich models provide good descriptions of the adsorption experimental data over limited concentration ranges. The surface complexation model using constants for P adsorption on goethite determined by Sigg et al. (1981) and Goldberg et al. (1984a) did not describe the experimental data very well. The surface complexation model provides a quantitative description of surface potential change with pH and phosphate concentration. It provides the theoretical basis for surface complexation reactions, surface charge, and surface potential changes in adsorption reactions. However, it fails to account for the physical changes occurring to the goethite particles, such as precipitation and coagulation.