Field-Relevant Thermal Remediation of Copper-Contaminated Clay Soils: Mechanistic Pathways Revealed by a Custom Heating System

Thermal stabilization is an effective approach for immobilizing heavy metals in contaminated soils, yet its performance depends on coupled thermal–hydraulic–chemical processes under field-relevant conditions. In this study, a custom-designed thermal–hydraulic device was developed to investigate Cu 2+ stabilization in compacted clay. Soils contaminated at 25 and 100 cmol/kg-soil were treated over a temperature range of 25–700 °C. Cu retention increased systematically with temperature, exceeding 99% at 500 °C. This trend is primarily attributed to thermal decomposition of copper nitrate and the formation of stable CuO phases, accompanied by microstructural densification and pore-scale sealing. Hydraulic conductivity decreased significantly under coupled thermal–hydraulic conditions, in contrast to the conventional furnace method. Variations in pH and electrical conductivity (EC) reflected changes in the chemical environment during treatment, while temperature acted as the primary controlling factor. XRD and SEM analyses confirmed CuO formation and indicated possible interactions between Cu species and the silicate matrix, without definitive identification of specific copper silicate phases. A logistic model (R 2 > 0.99) was developed to predict Cu retention as a function of temperature. These results demonstrate that controlled thermal treatment can effectively immobilize Cu while reducing permeability and enhancing soil stability, providing a reliable basis for field-scale remediation of contaminated soils.