Migration Behavior of NH4+ and Na+ in a Bentonite-Clay Mixed Soil Column and Numerical Simulation

Abstract

The landfill barriers effectively prevented the migration of high-concentration pollutants, such as NH4+ and Na+, from the landfills to the surrounding environment. However, due to the high hydraulic head inside the landfill compared to the surrounding environment, NH4+ and Na+ can migrate towards the outside of the landfill barrier with the infiltrating solution, potentially causing harm to the surrounding environment. To address this, saturated mixed soil column samples made of bentonite and Shanghai clay, with bentonite contents of 3% and 10%, were used in this study. Permeability coefficients of the column samples in solutions are obtained by using permeation tests, and using NaCl and NH4Cl solutions with concentrations of 37.4 mmol/L and 74.8 mmol/L, respectively. The concentration-depth result of the column samples after permeation tests was determined using Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) and Ion Chromatography (ICS-1100). Numerical simulations are used to investigate the effect of downstream solute concentration of the barriers on upstream solute concentration, dry density, and bentonite content of the barriers. The results indicate that the permeability coefficient of the soil column samples exposed to NH4Cl solution is greater than that of samples exposed to NaCl solution. This can be attributed to the stronger cation exchange of montmorillonite for NH4+, resulting in less swelling of the bentonite and more micro-pores, leading to an increase in the permeability coefficient. The concentration of Na+ is higher than that of NH4+ at the same depth of the column samples, indicating that Na+ has a higher migration rate in the column sample. This could be attributed to the relatively fast diffusion of Na+ on the surface of the bentonite and larger hydration radius of Na+. According to the simulation results, the recommended values for the bentonite clay mixed-soil barrier wall are as follows in this study: a thickness of 43 cm, a dry density of 1.5 g/cm3, and a bentonite content of 5%.