Retardation mechanism and migration of Cr(VI) by the active porous material in modified soil-bentonite barriers

Vertical cut-off walls made of soil-bentonite materials are often used as engineered barriers to prevent the spread of heavy metal pollution. In this study, the potential use of two common active porous materials (APM), active carbon and zeolite, as modifiers for the soil-bentonite barrier to retard hexavalent chromium Cr(VI) in contaminated sites, and the microscopic mechanism was investigated by consolidation permeability tests, isothermal adsorption tests, soil column tests, and scanning electron microscope tests. Additionally, to evaluate the effectiveness of the modified materials in practical engineering, the migration process of heavy metal pollutants in engineered barriers and contaminated sites was simulated using finite element numerical software. The results show that the active porous material can only improve the impermeability of the soil-bentonite barrier to a limited extent. However, the barrier material modified by the APMs showed a greatly enhanced adsorption capacity for Cr(VI), and the Langmuir and the Freundlich models described the nonlinear adsorption process well. The active porous material improved the retardation of the engineering barrier to Cr(VI), and the effect of adding active carbon was better than that of zeolite. The microscopic mechanism lies in the incorporation of active porous materials can impede pore development and combine with soil particles to form agglomerates possessing a large specific surface area, thereby enhancing the heavy metal ion adsorption. The results of the simulation show that increasing the thickness of the engineering barrier can effectively prolong the breakthrough time of Cr(VI), and the breakthrough time of the APMs-modified barriers was more sensitive to the change in thickness. Based on the migration simulation of the actual site, the 10% active carbon-modified soil-bentonite barrier can protect the surrounding densely populated areas from pollution for 37.14 years and improve the service life of the engineered barrier by at least 51.1%.