One-dimensional non-isothermal diffusion model for organic pollutant in an unsaturated composite liner considering the degradation effect

In this article, a one-dimensional non-isothermal diffusion model for organic pollutant in an unsaturated composite liner (comprising a geomembrane and an unsaturated compacted clay liner (CCL)) considering the degradation effect is established, which also includes the impacts of temperature on diffusion-related parameters, and employs a water content and pore-water pressure head relationship equation that better matches the experimental results. Subsequently, this model is addressed through a finite-difference technique, and its reasonableness is proved by comparing with the experiment measurements and two other calculation approaches. Following this, the analyses suggest that the diffusion coefficients' change induced by a rising temperature accelerates the diffusion rate, whereas such an alteration on partitioning coefficients has an opposite effect. Furthermore, the evaluation reveals that the non-isothermal state caused by an increasing upper temperature overall lowers the anti-fouling performance. The unsaturated composite liner's barrier function is weakened by an increment in residual water content of CCL, but enhanced by unsaturated layer thickness. It is also detected that the degradation effect should be considered if the degradation half-life <= 100 years. Lastly, a simplified approach for assessing the unsaturated composite liner's barrier performance is presented, which can provide guidance for its engineering design in a non-isothermal scenario.