Laboratory experiment on phosphorus adsorption characteristics of different-sized sediment particles in the Middle Yangtze River

In alluvial rivers, sediment particles exhibit a strong affinity to phosphorus (P), with the adsorption capacity varying across different particle sizes, and the experimentally derived adsorption parameters can be used in water quality models that consider the effect of sediment adsorption processes. However, there exist limited systematic experimental studies on how particle size influences P adsorption by sediment. To investigate the P adsorption characteristics of different-sized sediments, surface bed sediment samples were collected from two sections in the Middle Yangtze River (MYR), with particle size ( D ) ranging between 2 and 500 μm. These sediment mixtures were then separated into 7 size fractions (down to D < 8 μm), with the corresponding mean particle size ( ) ranging from 5.9 to 424.2 μm. The native adsorbed P (NAP) amounts for different-sized sediment fractions were measured, and P adsorption experiments were conducted for the 7 sediment fractions under 3 different sediment concentrations ( S ) with a similar environment of pH and temperature when sampled from the MYR. Experimental results show that: (i) NAP values for sediment particles with D < 500 μm ranged from 0.15 to 0.47 mg·g −1 , exhibiting a negative exponential relation with ; (ii) P adsorption by sediment primarily occurred within the first 4 h, with the highest adsorption rate, and both the amount and rate of P adsorption were negatively correlated with S and ; (iii) key adsorption parameters in the Langmuir isotherm and kinetic equations were calibrated using the experimental data, achieving high fitting accuracy. The maximum P adsorption amount ( Q max ) obtained in isothermal experiments was well described by the equation considering the combined effects of S and : Q max = 1.6189 S -0.228 -0.283 , with particle size having a greater influence than sediment concentration; and (iv) in addition, the kinetic adsorption data for most sediment fractions ( D < 250 μm) were closely distributed and displayed a consistent linear trend, leading to the determination of unified adsorption kinetic parameters for nonuniform sediment under different S values: k 1 = 0.257 L·mg −1 ·h −1 and k 2 = 0.057 h −1 .