Adsorption mechanisms of nitrogen and phosphorus by river sediments: patterns, differences, and mechanisms of ammonium (NH₄⁺) and phosphate (PO₄3⁻) adsorption under the influence of sediment concentration

Abstract Sediment adsorption in rivers is a crucial pathway influencing the migration and transformation of biogenic substances at the water–sediment interface, thereby playing a significant role in the eutrophication process of aquatic systems. This study quantitatively investigates the individual contributions of initial nutrient concentration, sediment concentration, and sediment particle size on nitrogen and phosphorus adsorption through generalized laboratory experiments. Comparative analyses were conducted on the adsorption characteristics and mechanisms of phosphate (PO₄ 3 ⁻) and ammonium (NH₄⁺), and the adsorption response to varying sediment concentrations was further examined. Results indicate that the contribution rates of initial concentration, sediment concentration, and particle size to PO₄ 3 ⁻ adsorption were approximately 0.71, 0.23, and 0.06, respectively; while for NH₄⁺ adsorption, these were 0.67, 0.27, and 0.06, respectively. The Sediment Concentration Adsorption Model (SCAM) was subsequently developed to elucidate the adsorption mechanism modulated by sediment concentration. The concept of The Adsorption Critical Sediment Concentration (ACSC) was introduced, identifying it as the threshold at which the adsorption mechanism transitions, along with a description of its calculation method. This study provides quantitative guidance and practical reference for optimizing adsorption systems, contributing to pollutant regulation and removal engineering in riverine and lacustrine environments.