Mechanisms and extended kinetic model of thermal desorption in organic-contaminated soil

Thermal desorption is a preferred technology for site remediation due to its various advantages. To ensure the effective removal of different pollutants in practical applications, it is necessary to understand the kinetic behaviors and removal mechanisms of pollutants in thermal desorption process. This paper explored the thermal desorption processes of five organic pollutants (nitrobenzene, naphthalene, n-dodecane, 1-nitronaphthalene, and phenanthrene) at 50-350 °C in two different subsoils with 6-18% moisture content. The results suggested that the thermal desorption process was well-fitted by the exponential decay model (R² = 0.972-0.999) and could be divided into two distinct stages. The first stage was relatively fast and highly influenced by soil moisture, while the second stage showed a slower desorption rate due to the constraints imposed by the soil texture and structure. The influence of soil moisture on thermal desorption depended on the octanol/water partition coefficient (K_OW) of pollutants. Pollutants with log K_OW values lower than the critical value exhibited enhanced thermal desorption, while those with log K_OW values higher than the critical value were inhibited. The critical value of log K_OW might be between 3.33 and 4.46. Changes in soil texture and structure caused by heating promoted thermal desorption, especially for naphthalene, 1-nitronaphthalene and phenanthrene. The differences in texture and structure between the two soils diminished as the temperature increased. Finally, an extended kinetic model under changing temperature conditions was derived, and the simulation results for the two subsoils were very close to the actual thermogravimetric results, with the differences ranging from -1.28% to 0.94% and from -0.67% to 1.35%, respectively. These findings propose new insights into the influencing mechanisms of soil moisture and structure on the thermal desorption of organic pollutants. The extended kinetic model can provide reference for future kinetic research and guide practical site remediation.