Document Type : Research Article
Authors
Department of Soil Science, Faculty of Agriculture, Tarbiat Modares University (Tehran)
Abstract
Abstract
Phosphate is an essential element for plants, animals, and human. Mobility and availability of phosphate in the natural systems is controlled by the adsorption-desorption and dissolution-precipitation reactions. Iron (hydr)oxides including goethite play an important role in phosphate adsorption reactions in soil. Surface area, crystallinity, and morphology of iron (hydr)oxides are the most important characteristics influencing phosphorus adsorption capacity. With respect to the importance of adsorption process in controlling equilibrium concentration of phosphate in soil and other natural systems, in this research phosphate adsorption reactions on goethite was investigated on three samples of goethite differed in surface area over a wide range of pH and initial phosphate concentration. The goethite charging behavior was calculated from acid-base titration in different levels of ionic strength. The experimental data were analyzed with the CD-MUSIC surface complexation model. Titration data revealed pHpzc= 9.1. Experimental data revealed that phosphate adsorption is strongly pH-dependent. Maximum phosphate adsorption was measured at low pH. It was gradually decreased with increasing pH. Adsorption data were successfully described using two inner-sphere surface complexes, i.e . and . The calculated CD values indicated that 0.28 and 0.46 vu charge is transferred to the goethite surface due to the adsorption of and , respectively. Mole fraction calculation showed that and are the dominant surface species at low and high pH, respectively. Furthermore, experimental data and model calculations revealed that surface area influence phosphate adsorption considerably; however the change in the surface area has no measurable effect on the goethite site density. Therefore, the entire experimental data were successfully predicted by using one set of adsorption parameters.
Keywords: Adsorption, Phosphate, Goethite, CD-MUSIC model, Charging behavior
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