Phosphorus Fractions and Release Factors in Surface Sediments of a Tailwater River in Xinmi City, a Case Study

The Shuangji River in Xinmi City is a tailwater-type river. Its main water sources are the effluent from the domestic sewage plant, the effluent from the paper industry sewage plant and the coal well. The construction of wastewater treatment facilities in Xinmi city has significantly reduced the amount of total phosphorus (TP) discharged into Shuangji River. However, phosphorus control in rivers where the overlying waters are predominantly tailwaters is still a challenge, especially as the sediment–water interface’s phosphorus exchange mechanism needs to be investigated in detail. In this study, the content and proportion of each phosphorus fraction in the sediment of a tailwater-type river, the Shuangji River, were determined. It was found that the organic phosphorus (OP) and iron-bound phosphorus (Fe-P) content and proportion were high, and the risk of release was relatively high in the section of the river where the overlying water was the tailwater of a sewage plant. Temperature, pH, dissolved oxygen, and hydraulic disturbance were also found to control phosphorus forms’ transformation and release in the sediment. Elevated temperatures mainly stimulated the release of OP and Fe-P from the sediments. The dissolution of calcium-bound phosphorus (Ca-P) is the main pathway for phosphorus release under acidic conditions, whereas, under alkaline conditions, phosphorus release is mainly controlled by ion exchange between OH − and Fe-P and metal oxide-bound phosphorus (Al-P). Aerobic versus anaerobic conditions cause changes in Fe-P content in the sediment mainly by changing Fe ions’ chemical valence. Hydrodynamic disturbance accelerates labile-P release, but once the hydrodynamic disturbance stops, the overlying water dissolved total phosphorus (DTP) concentration rapidly decreases to a similar concentration as before.