Insight into slag-enhanced thermal desorption at the organo-mineral interface of clay soils

Multiple active components in steel slags can optimize soil texture and catalyze the thermal desorption of refractory organic pollutants, especially polycyclic aromatic hydrocarbons (PAHs). In this study, a safety and risk assessment was conducted on toxic metals present in collected steel slags, and the refining slag with a high content of free-CaO from the XG iron and steel plant (XGRS) was selected and used to regulate the plasticity index (PI) of clay soils. For the clay soil with high liquid limit and high plasticity (CH), the PI reduced from 34.3% to 24.8% with steel slags at an addition ratio of 10% (mass percent), and for the clay soil with low liquid limit (CL), the PI decreased from 22.8% to 16.5%, resulting in silt soil with optimal thermal properties. The thermal removal efficiency of PAHs in CH decreased from 81.9% to 74.0% at 350°C as the soil water content increased from 0 to 150 g kg⁻¹, due to the hydration and swelling of clay minerals and the heat loss associated with dehydroxylation and dehydration. The efficiency further decreased to 68.7% using 2% (mass percent) CaO as an additive due to the delayed dehydr(oxyl)ation of clay minerals and enhanced aromatization of soil organic matter. In contrast, addition of 2% high-content titania-bearing slag (HTBS) enhanced the removal efficiency of high-ring PAHs, with the efficiency increasing for 5-ring from 52.6% to 59.7% and for 6-ring from 76.6% to 81.8%. This is due to the presence of amorphous TiO₂ and crystalline CaTiO₃ in HTBS, which can degrade water to produce reactive oxygen species. The 2% XGRS addition facilitated the complete removal of PAHs by air-supplemented lattice oxygen, leading to the oxidation of C–C bonds in CH to C–O, C=O, and O–C=O functional groups. This study provides valuable insights into the use of additives to enhance the thermal remediation of PAH-contaminated clay soils via activating oxygen species and oxidizing C–C bonds in organo-mineral complexes within clay soils.