Model Study of Water Vapor and VOCs Adsorption on Bulk Jet Engine Soot Particles: Thermodynamic and Kinetic Aspects

Understanding the interactions between jet engine soot and exhaust plume vapor components, such as water vapour and benzene, is crucial for assessing post-combustion soot modification and the resulting impact on the environment and human health. This study focuses on the adsorption and desorption of single component exhaust plume species, namely, water vapor and volatile organic compounds (VOCs) with varying physicochemical properties, on soot particles derived from different combustion conditions, thus providing fundamental insights into the vapor-solid partitioning process and, therefore, the thermodynamic and kinetic mechanisms governing soot aging in jet exhaust plumes.

Model jet soot particles were synthesized by two methods: (1) enclosed spray combustion using Jet-Al fuel, which contains surface-adsorbed organic compounds, and (2) controlled oxidation of carbon black to mimic the physicochemical properties of jet soot without any adsorbed organic compounds to separate the role of soot core-shell structure, porosity, and surface chemistry on the solid-vapour partitioning process. Water vapor and VOCs adsorption isotherms, including components relevant to jet exhaust (e.g., toluene and benzene), were measured on soot powders using gravimetric dynamic vapor sorption under precisely controlled temperature and partial pressure conditions. Preliminary results indicated Type II isotherms for VOCs, driven by soot’s functional groups and particle surface area. Thermodynamic analysis of adsorption isotherms showed a moderate enthalpy of adsorption (31.8–45.4 kJ/mol) at low surface coverage and ambient temperature, consistent with a physisorption mechanism. Kinetic modeling using the linear driving force (LDF) and stretched exponential (SE) diffusion models showed that single aromatic species followed the LDF mechanism, displaying rapid adsorption kinetics (average k=0.016 s^-1), indicative of interparticle void filling. In contrast, water vapor adsorption mainly followed the Fickian diffusion mechanism and was much slower (average k=0.001 s^-1), possibly due to intraparticle diffusion of water vapor to oxygen functional groups on the edges of graphitic planes.

This study highlights how soot's physicochemical properties, such as pore size distribution, surface area, and surface chemistry, govern adsorption characteristics that control solid-vapor partitioning. By investigating the fundamental sorption mechanisms, these findings could advance our understanding of atmospheric jet soot aging and provide a foundation for modeling multicomponent vapor interactions in complex, real-world environments. The results could also inform strategies for mitigating the environmental and health impacts of aviation emissions through modifications to the combustion process.