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Surface Tension, pH, and Specific Conductance of Select PFAS and their Mixtures
Citation:
Chen, J., A. Adegbule, J. Huang, AND M. Brooks. Surface Tension, pH, and Specific Conductance of Select PFAS and their Mixtures. American Geophysical Union Fall Meeting, New Orleans, LA, December 13 - 17, 2021.
Impact/Purpose:
The intent of this presentation is to communicate research that is being conducted to broadly investigate the source zone characteristics of per- and polyfluoroalkyl substances (PFAS) in the subsurface. Specifically, this research focuses on interfacial partitioning of select PFAS in source zones, as characterized through surface tension measurements. Measurements of pH and specific conductance were also collected to assist in the evaluation of screening-level characteristics. Information on these topic is very important to the management of PFAS contaminated sites.
Description:
Widespread application of per- and polyfluoroalkyl substances (PFASs) in a variety of industrial and commercial fields has resulted in environmentally persistent contamination with potential health risks on a national and global scale because of their unique chemical properties. Understanding the unique chemical properties (i.e., surface-active behavior) is needed for further evaluation of the risk posed by PFASs. In particular, studies of shorter-chain alternatives are less frequent, but their application potential is increasing as possible replacements for longer chain compounds. In this study, we selected 7 PFASs depending on different carbon chain lengths (n = 4, 6, and 8) and functional groups (COO- and SO3-). They are perfluorobutanoic acid (PFBA), perfluorobutane sulfonic acid (PFBS), perfluorohexanoic acid (PFHxA), potassium perfluorohexane sulfonate (PFHxS-K), perfluorooctanoate (PFOA), potassium perfluorooctane sulfonate (PFOS-K), and hexafluoropropylene oxide dimer acid (GenX). We tested characteristics of surface tension, pH, and specific conductance in reverse osmosis water as a function of concentration for individual compounds and their mixtures. For the range of concentrations investigated, only one compound (PFHxA) suggested the possible formation of micelles based on a stable, low surface tension at high concentrations. All others indicated a sharp decline in surface tension with increasing concentration without evidence of approaching a critical micelle concentration. The PFASs with the lowest and highest surface tension curves were PFBA (n = 4, COO- functional group) and PFOS-K (n = 8, SO3- functional group), respectively. This indicates that surface-activity behaviors may relate to PFAS carbon chain length and functional groups. All mixtures showed surface tension measurements intermediate to individual compounds. Solutions of PFHxS-K and PFOS-K had the highest pH and lowest specific conductance, possibly because the anions originated from potassium salts, while PFHxA and GenX solutions showed similar and lowest pH. Specific conductance characteristics of all PFASs and their mixtures showed similar behavior: exponentially increasing specific conductance as solution concentration increased.