Study On The Effect Of Flow Turbulence On The Adsorption-desorption Of Phosphorus Onto Sediment Particles

The sediment suspension, settling and transport have important influence onpollutants movement. The influence mainly manifests as the adsorption anddesorption behaviors of pollutants onto sediment particles. The spatial and temporalvariation process of pollutants movement can be correctly simulated only if theadsorption and desorption effects has been reasonably considered. Therefore, thephosphorus adsorption and desorption behaviors onto sediment particles, especiallythe effects of flow turbulence were investigated through experiments and numericalsimulations in the present dissertation.In order to quantify the influence of flow turbulence and sediment movement on thesediment adsorption and desorption of phosphorus, an oscillating griddevice wasapplied to simulate the vertical uniform turbulence approximately. The experiment isdeveloped in view of different actions of water flow turbulence properties based onvaried oscillating parameters, different suspended sediment concentration anddifferent sediment particle size. The experimental result shows that the suspendedsediment concentration and oscillating grid parameters have a good correlation. Thewater turbulent structure and the vertical sediment concentration distribution can bewell quantified by the numerical model. It is concluded that the device can be furtherused to investigate thesediment adsorption and desorption of pollutants in turbulentwater.The equilibrium experiment and dynamic experiment of sediment adsorption/desorption of phosphorus is carried in a vibrator with constant temperature. Theexperimental data was fitted by the Langmuir adsorption model, which shows that theLangmuir model can reasonably describe the dynamic process and equilibrium stateof sediment adsorption/desorption of phosphorus. The sediment concentration andsediment particle size have different influence on the phosphorus adsorption anddesorption behaviors. When the sediment concentration is low and the particle size issmall, the sediment adsorption ability per unit mass is relatively stronger.The dynamic experiment of phosphorus adsorption and desorption onto sedimentparticles was carried out by adjusting the oscillatory parameters of the oscillating gridto simulate different strength of turbulence. The adsorption experiments show that thedissolved phosphorus concentration in turbulent water is influenced by both turbulent diffusion and sediment adsorption/desorption effects. If there is stronger turbulence,the sediment content in water and the total quantity of phosphorus adsorption on tosediment particles tends to be higher, and it needs less time to get to the equilibriumstate. The similar results was also obtained from the desorption experiments.Based on the joint hydrodynamic model, sediment movement model and thepollutants transport/diffusion model,1D and3D simulation model which consider thesediment adsorption and desorption effects were developed. The3D model is amutli-fraction sediment model which takes consideration of the different movementbehaviors and adsorption/desorption characteristics of different sediment particle size.The dynamic process and the effects of flow turbulence on the adsorption anddesorption of phosphorus onto sediment particles due to combined waves and currentis simulated in the numerical model. The numerical results and analyses reveal themechanism of the desorption behaviors of the high concentration layer of cohesivesediment under the action of waves and currents.Because of different diameter grains and settling velocities, the polluted sedimenttransport into lake by river flow will have different settling distance. The finer parts ofsediment absorbed more pollutants will settle down later than the coase parts. Whensimulate and estimate the effect of adsorption and desorption behavior of non-uniformsediments, a mutli-fraction sediment-pollutants numerical model is necessary.