Physicochemical Vapor Sorption Properties of Geologic Materials Quantified by Inverse Gas Chromatography

Abstract:

Sorption of chemical compounds in the environment is an important factor in assessing how volatile compounds are transported through geological materials. We used inverse gas chromatography (IGC) to quantity gas-solid sorption properties of particles of various sizes and mineralogical makeup (e.g., soil, clay, tuff, and sand). The level of interaction between the uncharacterized material and known probe gases was measured by varying temperature, humidity, and carrier gas flow. The probe gases can be surrogates for additional compounds of interest (e.g., volatile radionuclides and organic contaminants). The results (e.g., diffusion and partition coefficients and heat of sorption) help to decipher sorption effects for geologic materials under different temperature and humidity conditions. Further, these results can help define boundary conditions for inputs in computational models that simulate subsurface gas transport.

The inherent physical and chemical heterogeneity of soil and many geologic materials can make prediction of sorption properties difficult. Characterizing the properties of individual organic and inorganic components can elucidate the primary factors influencing sorption interactions in more complex mixtures. IGC is effective in quantifying key sorption parameters used for modeling of subsurface gas transport. Based on heat of sorption, non-polar alkanes had significantly greater affinity for soil and clay than simpler mineral media (e.g., salt and calcium carbonate), while tuff and quartz sand fell between these two endmembers. Diffusion coefficient positively correlated with temperature, but differences between materials were less pronounced and particle size may be more impactful. We discuss the capabilities and challenges of using IGC to quantify vapor sorption of geologic materials and to inform gas transport.