Using dark-field microscopic scattering imaging to study the interaction between TNT and soil minerals and its impact on TNT bioavailability

After dispersal into soil, explosive compounds such as TNT undergo dissolution and rapid aging through interactions with soil components. In arid, nutrient-poor soils, the adsorption and transformation of TNT by soil minerals critically influence its environmental behavior. Adsorption onto minerals and metals triggers redox and hydrolysis reactions, facilitating its degradation. A dark-field imaging methodology was established to visualize particle-organic pollutant interactions by leveraging light-scattering properties and chromogenic reactions. Studies revealed that TNT interactions with minerals and metals primarily involve three mechanisms: Physical adsorption (e.g., Fe₂O₃, Al₂O₃, Fe₃O₄, SiO₂, CuO, Pb), adsorption-reduction (e.g., Cu, Al), adsorption-hydrolysis (e.g., MgO). Dark-field microscopy enables semi-quantitative assessment of TNT contamination in soils but highlights uneven adsorption across particle surfaces and crystal facets. Mineral adsorption alters TNT pyrolysis behavior, with chemically adsorbed TNT exhibiting higher thermal decomposition temperatures than physically adsorbed forms. Soil magnesium content is a key factor influencing TNT bioavailability. TNT adsorbed onto MgO resists extraction by hydroxypropyl-β-cyclodextrin (HPCD), significantly reducing bioavailability of TNT. Magnesium-rich minerals (e.g., montmorillonite) further suppress TNT bioavailability, positioning montmorillonite as a potential remediation agent. Combined with XRD for mineral identification and pollutant fractionation, dark-field microscopy and scattering spectroscopy provide high spatial resolution and reveal adsorption heterogeneity among soil particles. • Visualization of the adsorption reaction between TNT and mineral/metal particles. • TNT reduced to ADNT on the surfaces of Cu and Al. • The higher the content of Mg-bearing montmorillonite, the lower the bioavailability of TNT.