Transport of degradable contaminants in a composite vertical cut-off wall under nonlinear adsorption conditions

Barrier materials frequently exhibit nonlinear adsorption characteristics for contaminants in leachate, with this nonlinearity becoming increasingly pronounced at higher concentrations. A composite vertical cut-off wall, constructed from an adsorptive soil-bentonite mixture and a geosynthetic clay liner provides an effective solution for barrier systems and emergency responses to organic contaminants. Understanding the transport mechanisms of degradable contaminants (e.g., organic pollutants) within the composite cut-off wall is essential for evaluating barrier effectiveness against contamination. This study established a general theoretical model for the one-dimensional transport of degradable contaminants in the composite vertical cut-off wall, considering nonlinear adsorption, advection, diffusion, mechanical dispersion, and degradation processes during the transport period simultaneously. Parametric studies show that the relative concentrations of degradable contaminants in the composite cut-off wall, based on the Freundlich adsorption assumption, significantly differ from those predicted by the linear adsorption assumption. The influence of the Freundlich adsorption model parameters on contaminant transport and boundary conditions was thoroughly analyzed. The degradation process can effectively reduce the relative concentration of contaminants in the composite cut-off wall. Notably, the time required for the contaminant concentration to traverse the composite cut-off wall and reach the limit value (breakthrough time), when considering degradation (t_1/2 = 10 a), is 1.7 times that calculated without accounting for degradation. Furthermore, increased permeability of the aquifer under the boundary conditions results in lower contaminant concentrations within the outlet range of the transport model, leading to a corresponding increase in breakthrough time.