Coupled Thermal Desorption–Thermal Plasma Methods for Diesel-Contaminated Soil Remediation and Syngas Production

Diesel is a major soil contaminant that poses significant environmental risks, making its removal essential. This study investigates the synergistic application of thermal desorption (TD) and thermal plasma for the remediation of diesel-contaminated soil, while simultaneously converting desorbed contaminants into valuable gaseous products. Artificially contaminated soil (25 g/kg) was treated by TD at 250–300 °C and the resulting off-gas and volatilized diesel were subsequently processed in a thermal plasma system. Soil samples were characterized using CHNS, EDX, FTIR, and TGA/DTG analyses, while gas composition was determined using a gas analyzer. The results demonstrate that TD achieved diesel removal efficiencies of up to 86% at 300 °C and 65% at 250 °C. TD off-gas and volatilized diesel were predominantly converted into synthesis gas (H2 + CO) in a thermal plasma environment, with H2 and CO concentrations reaching up to 15.49 vol% and 7.61 vol%, respectively, depending on the plasma-forming gas, carrier gas flow rate, and remediation temperature. Thermal treatment of diesel-contaminated soil significantly altered key physicochemical properties, including reduced organic matter content, increased soil compaction, and temperature-dependent shifts in pH and nitrogen speciation (decreased NO3−-N and increased NH4+-N). These changes were accompanied by enhanced phosphorus availability, indicating substantial thermally induced transformation of soil nutrients. Phytotoxicity assessment using Lepidium sativum in a soil leachate-based bioassay indicated that higher treatment temperature (300 °C) increased toxicity and inhibited plant growth, whereas treatment at 250 °C resulted in lower phytotoxicity. These findings highlight the adaptability of the proposed combination of methods enabling effective soil remediation while supporting energy recovery.