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The phosphorus fractions and adsorption characteristics of seven floodplain sediment samples collected in the lower reaches of China's Hanjiang River were studied. Most phosphorus fractions showed a marked downstream increase in response to point-source inputs from urban areas. Total phosphorus (TP) contents in the sediments ranged from 603.68 to 945.25 mg x kg(-1). Inorganic phosphorus (IP) was the major component of TP, and calcium-bound phosphorus (Ca-P) was the major fraction of IP. The distribution characteristics of the phosphorus contents were affected by sediment grain size and hydrodynamic conditions. The maximum phosphorus adsorption capacities (Q (max)) and the half-saturation concentration (k) were obtained using an improved Langmuir model. Native adsorbed exchangeable phosphorus content (w (NAP)) and the zero-equilibrium phosphorus concentration value (c (EPC0)) were subsequently calculated. The effects of sediment grain size, temperature, and disturbance on the phosphorus adsorption isotherms were also studied. The results showed that phosphorus adsorption on floodplain sediments was primarily chemisorption; the particle concentration effect played a more important role at a disturbance intensity of 150 r x min(-1) (on a shaker table) than at 100 r x min(-1).

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