CCL非线性吸附特性及孔隙率变化对污染物运移影响分析

Numerical analysis for contaminant transport in variable porosity of porous medium with nonlinear sorption in CCL

假设压实黏土衬里(compacted clay liner,CCL)中的孔隙在空间均匀分布,土颗粒表面可均匀吸附重金属离子或有机污染物,且可用Langmuir方程表示其非线性吸附特性,依据体积与质量关系,提出了土体因吸附污染物而引起的孔隙率降低的估算公式.在饱和稳态和考虑土体孔隙率变化的条件下,建立了污染物一维运移的控制方程,在控制方程中,分别考虑垃圾降解效应、CCL、下卧有限厚度含水层等情况.应用所建立的控制方程对某一假想填埋场进行了数值计算与分析.变动参数计算与对比分析表明,因土颗粒对污染物的非线性平衡吸附所引起的孔隙率降低对污染物的运移具有显著的影响,污染物穿过CCL的能力显著下降.与不考虑孔隙率变化的情况相比,穿透曲线的峰值浓度降低近10%.考虑吸附引起的孔隙率变化情况下,线性吸附与非线性吸附对污染物运移影响规律相似.吸附强度参数K1和弥散系数对污染物运移影响较为显著.

It is assumed that the voids are uniformly distributed in the substance space occupied by soils and contaminant sorption behaviour onto the soil particle is characterized by equilibrium and nonlinear features can be described by Langmuir＇s sorption isotherm equation. Based on the analysis of basic interrelationship between volume and mass, an approximate formula is presented. Under the condition of variable porosity and steady state seepage, the governing differential equation of one dimensional transport of contaminant along vertical direction is established and the initial condition and boundary conditions corresponding to actual profile of landfill underlain by compacted clay liner （CCL） and aquifer are formulated considering degradation of wastes in landfill and the aquifer of finite thickness. Then the initial boundary-value issue is numerically solved. Through parametric computations and comparative analyses, it is shown that sorption-induced porosity reduction has a considerable effect on the contaminant transport processes and will decrease breakthrough potential of contaminant into CCL. Compared with the case of constant porosity, the peak concentration of contaminants in CCL will reduce 10% when the variable porosity induced by sorption is considered. Both effects of linear and nonlinear sorption on the contaminant transport are almost identical. The influences of the sorption intensity parameter K1 in the Langmuir＇s sorption isotherm equation and hydrodynamic dispersion coefficient on contaminant transport are considerable under the condition of variable porosity.