Study On Kinetic Curve Fluctuation Of Phosphorus Adsorption In Resuspension Sediment

Phosphorus is a limiting factor of eutrophication in most freshwater environments and some coastal waters(such as Bohai Sea),and its concentration in water is regulated by adsorption and desorption of resuspension sediment.In some natural or engineering waters with bigger waves and shallower water depth,sediment re-suspension occurs frequently.The physicochemical properties of resuspension sediment are more active than sediment,and the adsorption and desorption of phosphorus in water is more prominent.Adsorption kinetics can provide important information such as adsorption reaction mechanism,adsorption rate,adsorption equilibrium time and so on.Therefore,it is of great significance to study the kinetics of resuspension sediment-phosphorus interactions for water eutrophication prevention and control.In previous study,it was found that the kinetic curve of phosphorus adsorption by resuspension sediment fluctuated under low phosphorus concentration,which was significantly different from the conventional kinetic curve.The fluctuation phenomenon has important effects on the adsorption rate,equilibrium time and adsorption kinetic parameters.From the perspective of mass rebalancing,this paper takes the resuspension sediment in Chengdao oilfield as an example,and discusses the causes of fluctuation through batch experiments and numerical simulations.Based on the fact that resuspension sediment is composed of complex minerals,a multi-component kinetic model is established.The numerical simulations of two-component clay minerals show that the redistribution of phosphorus among different adsorbent components is the main cause of the fluctuation phenomenon.The main insights gained are as follows:(1)The exact determination of all the original exchangeable phosphorus(Qi)as the initial condition of the kinetic model is particularly important.Affected by solid concentration,the amount of exchangeable phosphorus obtained by conventional intercept method is not Qi,but part of the original exchangeable phosphorus under experimental solid concentration(Q0),and the obtained Q0-CS often show a positive correlation(should be negative correlation).In order to describe the influence of solid concentration on Q0,the intercept method is improved:On the basis of MEA model,Cs-F formula is proposed,which can not only correctly express the negative correlation of Q0-Cs,but also express the other two types of solid concentration effect:EPC0-CS correlation(positive correlation,negative correlation or no correlation)and the influence of solid concentration on the position of adsorption isotherm.The results show that the formula can describe the adsorption isotherms under different solid concentrations well,and can be used to determine the Qi accurately with infinite dilution extrapolation(IDE)method.(2)Based on Langmuir adsorption theory,One component dynamics model(differential equations)was established.There are three unmeasurable parameters to be determined in the equation:Qe,Ka,Kd.Collinearity analysis of model parameters shows that there will be"over-parameterization problem" if we invert Ka,Kd and Qe at the same time,and the most suitable inversion double parameters are Ka and Kd,so Qe needs to be determined by other methods.Experiments and simulations show that the pseudo-second-order kinetic model is suitable for determining the Qe of clay minerals.The single-component model can describe the adsorption kinetics of the clay minerals(montmorillonite,illite,chlorite and kaolinite),and there is no monochromatic mathematical relationship between CS and Ka(Kd).(3)Based on the one component kinetics model,a multi-component kinetics model was established.The kinetic parameters of each mineral in the multi-component model can be obtained from the one-component model.However,due to the influence of additional phosphorus(Q0),the Qe of each component in the mixed components is not equal to the Qe of the corresponding one-component model,which needs to be determined through the inversion of the multi-component kinetics model.(4)In the proper range of initial phosphorus concentration,the kinetic experiment curves of phosphorus adsorption by dual mineral adsorption system(illite,montmorillonite)showed obvious fluctuation phenomenon.The Qi of minerals has an important effect on the fluctuation of phosphorus concentration:larger Qi,higher fluctuation of phosphorus concentration;Qi is small,and the concentration of fluctuation phenomenon is low.The sensitivity analysis results show that Qi has high sensitivity to the state variables of the model.(5)Two-components kinetics model can describe the fluctuation phenomenon well.By comparing the Qi of minerals with Qe determined by inversion,it can be seen that illite desorbed phosphorus and montmorillonite absorbed phosphorus during the adsorption process of mixed minerals,and the rate of illite desorption was higher than that of montmorillonite in the initial stage,which caused the temporary accumulation of dissolved phosphorus in the kinetic process,thus forming the fluctuation phenomenon.This confirms that the redistribution of phosphorus among different components is an important cause of the fluctuation phenomenon.