The Effect Of The Physical Properties Of The Sediments On Contaminant Adsorption/desorption And Interfacial Diffusion

With the rapid development of chinese industry and agriculture and accelerationof urbanization process, the pollution of the water environment is serious, and thedisasters of the water environment, such as eutrophication and heavy metalspollution, occur frequently, especially in the developed coastal zones. In recent years,after the external pollution sources have been controled due to the effort of the waterenvironment regulation, and more attention is paid to the effect of the internalsources from the contaminated sediments on the water environment. By means of theexperimental measurement and numerical simulation, the effect of the phusicalproperties of the suspended and porous deposited sediments on the contaminant(phosphorus) adsorption/desorption and interfacial diffusion have been studied inthis thesis, which can provide a scientific reference to the prediction, control andregulation for the internal sources from the contaminated sediments in rivers andlakes.The experimental study in this thesis is divided into three main parts:(1) Thephosphorus (P) adsorption isotherms and kinetics and influencing factors of thedifferent grain-sized sediment suspensions. It can be ovserved that the equilibriumadsorption and kinetics of the fine-grained sediments are considerably higher inmagnitude than those of the coarse-grained sediments, which is predominantlyattributed to the finer grain size and thus larger specific surface area for the former.The P adsorption isotherms and kinetics for different grain-sized sediments insuspension exhibit a clear size-dependent feature, namely the decreasing trend withthe increasing grain size, and the effect of the initial aqueous concentration andsediment content is also significant. The obtained data in terms of the adsorptionquantity and the aqueous equilibrium concentration for all cases fit well with theLangmuir equation.(2) The P desorption and release features from the poroussediment layers under quiescent condition. It can be seen that the static contaminantrelease from the porous sediment layers in different period of time is dependent onthe relative importance of the direct desorption from the uppermost sediments andthe release from the pore-water within the underlying sediment layer on theinterfacial diffusion process. In an immediate release stage, the direct desorption from the uppermost sediments is the dominant contribution to the higherconcentration of the overlying water column, due to the larger desorption from thecoarser sediment layer; In a posterior release stage, the release from the pore-waterwithin the porous sediment layer may exert more pronounced effect on theconcentration variation of the overlying water column, due to the larger porosity inthe finer sediment layer.(3) The temporal release from the permeable sediment layerunder different flow velocity conditions. It can be observed that under thehydrodynamic condition of the overlying water column, the interfacial diffusion fluxtends to increase with the increasing flow velocity. This enhancement drived by thehydrodynamic disturbance in mediating the interfacial diffusion flux is somewhatinsignificant in an immediate release stage, and the diffusive boundary layer, withinwhich there exists large concentration gradient in its initial formation stage, results inthe resistance to the intensified interfacial diffusion. While in a posterior releasestage, the concentration profile within the thicker diffusive boundary layer becomesmore and more uniform, and accordingly, the transport process across thesediment-water interface is gradually dominated by the hydrodynamic disturbance,instead of the molecular diffusion.The numerical simulation in this thesis is carried out to study the flow andconcentration fields formed by the interaction of the overlying water column and theporous sediment seepage. Based on the coupling computation of the overlying watercolumn and porous sediment seepage in the case of the uneven bedform, the flowpattern and interfacial pressure distribution have been analyzed. It can be seen thatthe flow and concentration fields within the porous sediment layer are affected bynot only the interfacial pressure distribution induced by the overlying water flowvelocity, but also the porosity and permeability of the porous sediment layer.