Low-Temperature Pyrolysis of PFOS-Contaminated Soil Enhanced by Additives: Thermodynamic Insights, Transformation Products, and Remediation Implications

Perfluorooctanesulfonate (PFOS) is a persistent pollutant in soils due to its exceptional chemical and biological stability. Pyrolysis has been recognized as an effective technology for the remediation of PFOS-contaminated soil. However, its large-scale application faces challenges such as the requirement of high temperatures, long residence time, and corrosive off-gas treatment. The application of additives during pyrolysis is a promising strategy to overcome these challenges. In this study, six additives (Fe2O3, Fe3O4, CaO, Ca(OH)2, kaolinite, and MgO) were employed to improve PFOS removal from soil by pyrolysis. The effects of temperature, residence time, and removal efficiency with additives on the PFOS decomposition mechanism and economic benefits were systematically investigated. The results showed that all additives could allow for effective PFOS removal at a relatively low temperature (350 °C) and with a short residence time (30 min). Fe2O3 and CaO at a 5% dosage exhibited PFOS removal efficiency reaching 95.19% and 95.49%, respectively, which were 21.00% higher than that of the no-additive system. The thermodynamic analysis showed that the additives could reduce the activation energy (Ea) of PFOS pyrolysis, among which Fe2O3 showed the most significant effect (54.24 kJ/mol). Although additives exerted no significant effect on the type of PFOS decomposition products in soil, they effectively reduced the emission of acidic off-gases. Among them, CaO and Ca(OH)2 showed the most significant reduction by forming inorganic fluorides, followed by Fe2O3 and Fe3O4, through providing active sites. Economic analysis indicated that CaO had the lowest cost for PFOS removal (2.86 CNY/mg), followed by Fe2O3 (2.88 CNY/mg). Comprehensively considering PFOS removal efficiency, decomposition mechanism, economic cost, and pH of treated soil, Fe2O3 was identified as the optimal additive. This study provides new insights into the PFOS pyrolysis in soils, and proposes an energy-efficient remediation approach by reducing temperature, residence time, Ea, and off-gas emissions, which offers support for the large-scale application of this technology.