Effects of thermal desorption on the complex contaminated soils of polycyclic aromatic hydrocarbons and heavy metals

Thermal desorption technology is widely used for the remediation of contaminated sites. However, the combined effects of thermal desorption on the complex contaminated soils of polycyclic aromatic hydrocarbons (PAHs) and heavy metals were still unclear. The complex contaminated simulated soils of PAHs and heavy metals were selected to investigate the effects of thermal desorption temperature (220-400 ℃) and residence time (5-60 min) on PAHs in the soil and to analyze the effects of thermal desorption temperature (310, 340, and 370 ℃) on the morphological distribution of heavy metals (Cu, Pb, As, and Cd) in soil under air and nitrogen atmosphere. The results showed that the removal of PAHs from contaminated soil increased significantly with the increase in thermal desorption temperature and residence time. The proportion of low-ring PAHs gradually decreased while that of high-ring PAHs gradually increased. After thermal desorption treatment in both atmospheres, the proportion of Cu, Pb, and As weakly acid-extracted states increased slightly, while the proportion of Cd weakly acid-extracted states decreased significantly. The conversion trends of the reducible and oxidizable states were different. The proportion of residue states of four heavy metals, Cu, Pb, As, and Cd, increased gradually with the increase of thermal desorption temperature, which indicated that thermal desorption was beneficial to the immobilization of the four heavy metals. Compared with air, the proportions of oxidizable and residual states of four heavy metals increased under nitrogen conditions. The proportions of Cu and Pb reducible states decreased significantly, while the proportions of As reducible states decreased and the proportions of Cd reducible states did not change significantly. These results showed that nitrogen was more favorable for the stabilization of Cu, Pb, and Cd. On the contrary, the air was more beneficial to the stabilization of As.