Theoretical model for one-dimensional transport of heavy metal contaminants in a triple-layer composite liner considering temperature change

Aiming at the problem of one-dimensional transport of heavy metal contaminants (HMCs) in a triple-layer composite liner composed of geomembrane (GM), geosynthetic clay liner ( GCL) and compacted clay liner (CCL) under the consideration of temperature change, the governing equations for thermal conduction and HMCs transport are developed based on the relevant assumptions, and a corresponding theoretical model is established. The theoretical model constructed in this study is solved numerically by the finite difference. Subsequently, a comparative analysis with the experimental measurements, the analytical solution calculations and other numerical solution calculations is performed to validly verify the reasonableness of this model. Finally, the effects of different factors on the transport laws are analyzed and discussed using Cd2+ as an example. The results show that an increase in the upper temperature increases the transport flux and bottom concentration of Cd2+ in the composite liner, but decreases its upper concentration. The thermal diffusion effect on the transport behaviors becomes significant with an increasing in Soret coefficient S-T, and the defined breakthrough times t(b) are 37.9, 37.2, 36.4, 31.2 and 26.5 a for ST of 0.001, 0.005, 0.01, 0.05 and 0.1 K-1 in this order. The increase in the maximum adsorption capacities of GCL and CCL leads to the approximately linear growth in tb, and under a certain leachate head h(b), the tb almost increases linearly with the increase of GCL thickness and CCL thickness. Furthermore, as h(b) increases from 0.5 m to 1.0 m, the tb reduces by an average of 8.33 a. In engineering practice, the adsorption performance and thickness of both GCL and CCL can be combined to design a composite liner, where the influence of temperature variation on barrier effectiveness should be incorporated.