Assistant Project Scientist
Dr. Shan Yi is currently an Assistant Project Scientist in Professor Lisa Alvarez-Cohen's group in the Department of Civil and Environmental Engineering, at the University of California, Berkeley. Her expertise is in the area of environmental microbiology and biotechnology involved in the bioremediation of emerging contaminants.
Biotransformation of Fluorotelomer Thioether amido sulfonates in Aqueous Film-Forming Foams
Aqueous film-forming foams (AFFFs) are water-based chemical mixtures applied to liquid-fuel fires, such as petroleum hydrocarbons or chlorinated solvents, to extinguish flames and prevent fuel re-ignition. Poly- and perfluorinated alkyl substances (PFASs) are important AFFF constituents that facilitate rapid fire suppression. Repeated AFFF use at military, industrial, and municipal sites has led to widespread groundwater contamination of PFASs across the United States. Although different redox conditions can be encountered in contaminated groundwater, there is limited information on how biological activities that occur within these various redox zones affect the environmental fate of AFFF PFASs.
The objective of this study was to characterize the biotransformation pathways of a specific group of AFFF PFASs, fluorotelomer thioether amido sulfonates (FtTAoSs), under three different redox conditions: aerobic, nitrate-reducing and sulfate-reducing conditions. FtTAoSs were the principle PFASs in several widely-used AFFF formulations and have a history of application for firefighting over thirty years.
Soil microcosm studies indicated that the aerobic biotransformation of 4:2, 6:2, and 8:2 FtTAoS occurred and produced 4:2, 6:2, and 8:2 fluorotelomer sulfonate (FtS), 6:2 fluorotelomer unsaturated carboxylic acid (FtUCA), 5:3 fluorotelomer carboxylic acid (FtCA), and C4 to C8 perfluorinated carboxylic acids (PFCAs). Two biotransformation products corresponding to singly and doubly oxygenated forms of 6:2 FtTAoS were also identified through high-resolution mass spectrometry (MS) analysis and liquid chromatography tandem-MS. Mass balance analysis produced a near-complete mass recovery of FtTAoS after biotransformation, with 10% (mol/mol) of the amended FtTAoS accounted for in FtS, FtCA, and PFCA products. The transformation rates of the identified products appear to be slow relative to the transformation rate of FtTAoS, indicating that intermediates would persist in the environment.
The anaerobic biotransformation rate of 6:2 FtTAoS was significantly slower than the aerobic transformation rate. The anaerobic biotransformation pathways of 6:2 FtTAoS were also different from the aerobic biotransformation pathway. In sulfate-reducing and nitrate-reducing microcosms, 6:2 FtTAoS was transformed primarily to 6:2 fluorotelomer thioether propionate (6:2 FtTPA), rather than to the perfluoroalkyl carboxylates and fluorotelomer sulfonates that are formed during aerobic transformation. 6:2 FtTPA was transformed under nitrate reducing conditions, but was found to be persistent under sulfate-reducing conditions.
These biotransformation studies confirm some of the sources of FtS and PFCAs in groundwater and soil at AFFF-impacted sites and suggest that fluorinated intermediates that are not routinely measured during the biotransformation of PFASs may accumulate in the environment.