Effects of a variable air–water interface on injection of PFAS—Lessons learned from column tests with porous media

Abstract

Five column experiments were conducted using a mixture of 10 per- and polyfluo- roalkyl substances (PFASs) to explore sampling techniques for the PFAS injection solution concentration in order to accurately determine the tracer mass within the experimental system and close the mass balance. Samples of the injection solution were first taken by the method of pouring from the injection container into sampling tubes and analyzed for PFAS concentration. The results showed a drastic increase in the concentration of certain PFASs over the course of injection (up to 763%), leading to incomplete mass balances and low recovery rates. By contrast, when the injection solution was sampled by pipetting away from the air–water interface, the mass bal- ance greatly improved (PFAS recovery percentages increased from an average of 62%–102%). The observed concentration increase by the method of pouring can be attributed to the attraction of certain PFASs to the air–water interface. Factors such as carbon chain length and solubility were found to impact this attraction most, while surprisingly, the air–water interfacial sorption coefficients did not appear to be signif- icantly influential. This phenomenon is important to column experiments, as elevated local concentrations may promote micelle or hemi-micelle formation, consequently affecting the transport and sorption characteristics of the compounds under study. These aspects should be thoroughly considered in future studies to ensure a correct calculation of mass balance and therefore to correctly characterize the behavior of PFASs in the environment, and may also be relevant in remediation investigations, such as column tests with granular activated carbon.