Phosphorus fractions and phosphate sorption-release characteristics of the sediment in the Yangtze River estuary reservoir

The relationship between turbidity and phosphorus (P) removal in the water of the Qingcaosha reservoir was studied in the field. The reservoir is located on the Yangtze River estuary in China. The characteristics of P fractions and P sorption-release in Qingcaosha reservoir sediment were investigated in the laboratory. The field results showed that the settlement of suspended matter due to turbidity could lead to the deposition of the P in water onto the sediment surface. The laboratory results indicated that the total phosphorus (TP) content in the sediment varied from 550.33 to 844.48 mg/kg. In addition, the calcium (Ca) bound P (HCl-P) fraction had the highest proportion of TP, followed by organic phosphorus (OP) and phosphorus bound to aluminum (Al), iron (Fe), and manganese (Mn) oxides and oxyhydroxides (NaOH-P) in reservoir sediment. The sorption capacity of sediment ranged from 9.78 to 39.84 mg/kg. The concentration of soluble reactive phosphorus (SRP) in the overlying water was below the zero equilibrium phosphorus concentration (EPC0). This showed that the original adsorbed exchangeable P on the sediment was released back to the water. The amount of P released from the sediment was strongly related to the content of NaOH-P. The fact that NaOH-P could easily release P in the Qingcaosha reservoir was noted. Thus, NaOH-P could give some useful information regarding the potential release of P from sediment.