Determining air-water interfacial areas for the retention and transport of PFAS and other interfacially active solutes in unsaturated porous media

The objective of this work was to determine the methods that produce the most representative measurements and estimations of air-water interfacial area specifically for the retention and transport of PFAS and other interfacially active solutes in unsaturated porous media. Published data sets of air-water interfacial areas obtained with multiple measurement and prediction methods were compared for paired sets of porous media comprising similar median grain diameters, but one with solid-surface roughness (sand) and one without roughness (glass beads). All interfacial areas produced with the multiple diverse methods were coincident for the glass beads, providing validation of the aqueous interfacial tracer-test methods. The results of this and other benchmarking analyses demonstrated that the differences in interfacial areas measured for sands and soil by different methods are not due to errors or artifacts in the methods but rather the result of method-dependent differential contributions of solid-surface roughness. The contributions of roughness to interfacial areas measured by interfacial tracer-test methods were quantified and shown to be consistent with prior theoretical and experiment-based investigations of air-water interface configurations on rough solid surfaces. Three new methods for estimating air-water interfacial areas were developed, one based on the scaling of thermodynamic-determined values and the other two comprising empirical correlations incorporating grain diameter or NBET solid surface area. All three were developed based on measured aqueous interfacial tracer-test data. The three new and three existing estimation methods was tested using independent data sets of PFAS retention and transport. The results showed that the method based on treating air-water interfaces as smooth surfaces as well as the standard thermodynamic method produced inaccurate air-water interfacial areas that failed to reproduce the multiple measured PFAS retention and transport data sets. In contrast, the new estimation methods produced interfacial areas that accurately represented air-water interfacial adsorption of PFAS and associated retention and transport. The measurement and estimation of air-water interfacial areas for field-scale applications is discussed in light of these results.