Investigation of phosphorus fractions and isotherm equation on the lake sediments in Ekbatan Dam (Iran)

Phosphorus (P) is one of the main elements involved in the eutrophication of surface water bodies, which negatively affect the water quality. Reservoir dams are local for the accumulation of sediments transported from upstream. The amount of P release from sediments to water is potentially controlled by the retention characteristics of sediments. In this regard, 100 sediment samples were taken from surface sediments to determine the P retention capacity of sediments in Ekbatan Dam, Hamadan, Iran, and 15 water samples were collected from the interface of bottom sediments with lake water to determine different types of water P, including water-soluble P (WSP), dissolved reactive P (DRP), suspended solids P (SSP), and total P (TP). Firstly, some physical and chemical properties of sediments such as pH, cation exchangeable capacity (CEC), organic carbon (OC) content, equivalent CaCO3 (TNV), aluminum (Al) and iron (Fe) content were measured, and then WSP, TP, SSP, and DRP as well as P isotherms were determined. Moreover, three isotherm equations of Freundlich, Langmuir, and Temkin were fitted to the experimental data. Finally, the map of spatial variation of degree of P saturation (DPS) was plotted. The results revealed that the amount of DRP in water was at high level (0.02 mg L−1). The maximum adsorption in the sediments ranged from 83.93 to 634.13 mg kg−1 that was significantly correlated with the physical and chemical properties of sediments, such as clay percentage, OC, and TNV. The equilibrium concentration of P at zero point was significantly higher than DRP. This demonstrated the release of P from sediments into water. P binding energy in the sediments ranged from 0.03 to 5.48 L mg−1. There was a significant correlation between the binding energy and oxalate-extractable Fe, representing that the poorly crystalline or amorphous Fe would play a dominate role in the P adsorption under the slightly alkaline condition. The results suggested that the sediments not only had high P adsorption capacity and binding energy but released loosely bound WSP to water as well, which can cause eutrophication in the long term.