Phosphorus fractionation and adsorption characteristics in drinking water reservoir inlet river sediments under human disturbance

Purpose: Sediment phosphorus fractionation, adsorption kinetics, and adsorption isotherm characteristics were studied in two rivers, Muyang River and Lengshui River in China, emptying into a drinking water source reservoir, Songhua Reservoir. The relationship between phosphorus fractions and adsorption characteristics was further investigated to explore possible management options for maintaining river water quality. Methods: Sediment samples were collected from six and five sampling sites located along the Muyang and Lengshui Rivers, respectively. Sediment physical–chemical properties and phosphorus fractions were quantified. Adsorption characteristics were modeled with adsorption kinetics and adsorption isotherms. Results: The results from sedimentary sequential chemical extraction (SEDEX) method showed that organic phosphorus (Org-P) had the highest portion of total phosphorus in sediments, followed by detrital phosphorus and authigenic phosphorus (Auth-P), residual phosphorus, iron-bound phosphorus (Fe–P), aluminum-bound phosphorus (Al-P), and exchangeable phosphorus. Notably, the Lengshui River had a higher portion of Fe–P. The kinetics of phosphorus adsorption in sediments was simulated using a pseudo-second-order kinetic model. The results showed that sediment phosphorus adsorption in the upper reaches of the rivers was primarily dominated by single-layer adsorption, whereas that in downstream reaches gradually became irregular and complex deep adsorption. Adsorption isotherms could be well fitted using the modified Langmuir model and the Freundlich model, which indicated that the Lengshui River sediment had higher maximum phosphorus adsorption capacity and strength than Muyang River sediments. The EPC₀ values of sediments in both rivers indicated that sediments acted as the source of phosphorus in surface water. Principal component analysis showed that (i) the portion of Fe/Al-P in sediments positively correlated with equilibrium adsorption capacity, rate, and strength; (ii) Auth-P in sediments could increase the speed and amount of physically adsorbed phosphorus in sediments; and (iii) Org-P positively impacted the maximum adsorption capacity of phosphorus in sediments. Conclusion: Sediment phosphorus fractions and sorption could be affected by human disturbance. At the same time, sediment phosphorus fraction features correlated with several P adsorption characteristics. The findings from this study provide useful guidance for water quality protection of the drinking water source reservoir.