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研究报告

摘要：通过室内混合置换实验对Cu、Cd、Zn、Pb在石英砂中的运移特性进行了研究,并运用非平衡、非线性吸附模型对重金属运移的动态过程进行了数值模拟.结果表明,耦合Freundlich等温吸附的两点非平衡模型(Two Site Nonequilibrium Model,TSN)对重金属运移穿透曲线(BTCs)的拟合效果较好(R²>0.9,均方根误差RMSE<0.0008),石英砂对重金属的吸附作用以动力学吸附为主(平衡吸附点位分数f较小).通过模拟计算得到的阻滞因子R_d值明显小于用静态吸附实验得到的R_d值,说明采用静态吸附实验得到的吸附特征参数不能贸然地用于重金属运移的模拟,尤其是当动态吸附过程存在时.参数的敏感度分析结果表明,影响峰值大小的最敏感参数为吸附特征参数β,其次为饱和水力传导率K_s;影响浓度峰值时间最敏感的参数为K_s,其次为β;K_s和β对模拟结果的均方根误差(RMSE)影响较大,控制着模拟结果的浓度分布,其他参数则对模拟结果影响较小.

Abstract：Displacement studies on the transport of bromide and heavy metals (Cu,Cd,Zn,Pb) were conducted under saturated steady-state flow conditions in quartz sand columns. The bromide breakthrough curves (BTCs) for three velocities were analyzed with the equilibrium convection-dispersion equation (CDE) to obtain the longitudinal dispersivity λ and saturated water content θ_s. The two-site nonequilibrium model (TSN) coupled with the Freundlich adsorption equation was used to simulate heavy metal movement. The results indicated that the TSNmodel gave good fits to the observed BTCs (R²>0.9,RMSE<0.0008),but failed to capture the peak concentrations. It was inferred from the estimated parameters that heavy metal adsorption was mainly controlled by time ( the fraction of exchange sites assumed to be in equilibrium with the solution phase fless than 10% for Cu,Cd,Pb,and 13.2% for Zn) and the retardation factor R_d calculated from the transport simulation parameters was significantly less than that obtained from batch experiments,indicating that heavy metal sorption did not reach equilibrium in the columns. Sensitivity analysis results showed that the most sensitive parameter for the peak value of BTCs was the sorption parameter β and that for the peak time was the saturated hydraulic conductivity K_s. Both K_s and β controlled the fit between simulated and observed concentrations,as indicated by the RMSE.