Polymerization and Carbonization Behaviors of 2-Methylnaphthalene in Contaminated Soil During Thermal Desorption

Abstract

Despite the important niche and broad applicability of thermal remediation (TD), little work has been done to discuss chemical reactions of methylnaphthalene contaminated soil. The 2-methylnaphthalene desorption amount (MDA) of TD is studied here under different conditions, and the carbonation (chemically polymerized or condensed) behavior of 2-methylnaphthalene is explained by analyzing the changes of soil organic carbons (SOCs), off-gas products, and surface chemical properties. It indicates that the influence sequence of MDA from high to low is heating time, heating temperatures, and flow rates of carrier gas. MDA increases steadily with the increase of temperatures (200–300 °C) but decreases slightly after 300 °C; the reason may be the chemical conversion of 2-methylnaphthalene. GC-MS analysis of off-gas confirms that partial 2-methylnaphthalene is polymerized to form 2-methylbenzo[b]thiophene and 2,4-di-tert-butylpheno at 400 °C, which is the first step of carbonization process. The x-ray photoelectron spectroscopy results of soil indicate that the C content decreases, but C–(C, H) chemical structure increases, indicating that new carbonaceous substances are generated. A layer of “char” is seen by scanning electron microscope to be left on the surface of the soil particles. As the temperature increases (200–400 °C), the SOCs generally decreases from 1.14 to 0.82%, which is the result of the equilibrium between SOCs pyrolysis and 2-methylnaphthalene carbonization. Therefore, partial 2-methylnaphthalene turns into smaller organic molecules in desorption gas of TD, meanwhile is accompanied by its chemical conversion to non-volatile products, which are attached to remediated soils and then improve soil properties and increase their fertility.