Macroscopic and molecular scale assessment of thermal effects on soil-water interactions of unsaturated diesel-contaminated soils

The soil-water interactions of unsaturated diesel-contaminated soil are crucial for assessing pollution transport during thermal remediation. This paper aims to improve our understanding of this issue by measuring the matric suction of unsaturated contaminated kaolin and carrying out molecular dynamics simulations under thermal conditions. Results show that the increase in pollutant concentration could reduce the water retention capacity of diesel-contaminated kaolin due to changes in electrochemical properties and pore characteristics of samples, as well as a decrease in interfacial tension. On the other hand, pollutants formed a protective film on the kaolinite surface to act as a liquid bridge and prevent water loss at higher temperatures, as confirmed by Fourier transform infrared spectroscopy. With rising temperatures (50-60 °C), kaolin matric suction generally decreased with higher pollutant concentrations, but this trend was not very evident at lower pollution concentrations (0-10,000 mg/kg). In addition, molecular dynamics simulations were used to demonstrate the validity of these findings. The presence of pollutants might strengthen the interaction energy between kaolinite and water (for example, increasing from 276.52 kcal/mol (25 °C) and 267.95 kcal/mol (40 °C) at 8000 mg/kg to 296.54 kcal/mol (25 °C) and 292.46 kcal/mol (40 °C) at 10,000 mg/kg), thereby enhancing the water retention capacity of kaolin. In short, the study revealed that the coating of pollutants on kaolinite could act as a protective film, which binds water molecules through van der Waals and electric field forces and thereby reduces the sensitivity of water retention capacity to temperature.