Evaluating the biotransformation potential of commercial model fluorotelomer monomers in soils

Kavitha Dasu, Purdue University

Abstract

Fluorotelomer (FT) compounds are commonly used to produce commercial polymers for making surfaces stain-resistant in textile fabrics, carpets, furniture and paper as well as use in pesticide carriers, paints, heat-stable lubricants and fire-fighting foams and floor polishes. The FT polymers consist of a non-fluorinated backbone linked to FT compounds through ester, ether or urethane bonds. These linkages are subject to microbially mediated cleavage releasing volatile fluorotelomer alcohols (FTOHs), which subsequently degrade to suite of acid metabolites of concern including the globally detected perfluorooctanoic acid (PFOA). In my research, rates of aerobic biotransformation of 8:2 fluorotelomer stearate (FTS), 8:2 fluorotelomer citrate (TBC), toluene-2,4-di(8:2 fluorotelomer urethane) (FTU) and hexamethylene-1,6-di(8:2 fluorotelomer urethane) (HMU) were quantified in different soils, and analog-induced biotransformation was probed for FTU using a non-fluorinated urethane with the same backbone. At each sampling time, triplicate soil microcosms were extracted by sequential extraction and analyzed with LC/MS/MS or GC-MS/EI for FT monomers, FTOHs and acid metabolites. Relative rates of biotransformation followed: FTS >>> TBC > FTU and HMU. FTS half-lives ranged from 5 d to 28 d. Slower degradation of TBC, FTU, and HMU is likely due to enzyme access hindered by the additional FT chains. Biodegradation of urethane linkages also appeared slower compared to the ester linkages and a half-life was not reached within the 6-month studies. PFOA was the dominant persistent acid metabolite produced in all studies and overall metabolite production was higher in the forest soil. Biotransformation of FTU in forest soil, where a non-fluorinated toluene-2,4-dicarbamic acid diethyl ester, TDAEE was added, enhanced degradation of FTU was not observed; however, PFOA production was greatly enhanced with each addition. Results of this research clearly show that FT monomers used as surfactants and in producing polymers can serve as an indirect source of persistent perfluorinated acids in the environment. The rates of cleavage were shown to be affected by the type of hydrocarbon backbone and the number of FT chains present. In addition, although these studies were not conducted long enough to see PFOA concentrations plateau, PFOA production was much less than observed in FTOH biotransformation studies. This mitigation is directly associated to the rate of cleavage, thus FTOH release rates, multiple competing metabolite pathways, increases in irreversible binding with increasing residence time, and microbial community diversity. Hence, overall PFOA production is likely to be much less from monomers, and therefore, even less from polymers. The data for these FT monomers will aid in estimating expected PFOA loads from such monomer residuals that may be present in FT polymers or formed by polymer breakdown as well as aiding in the prediction of FT polymer stability in soils.

Degree

Ph.D.

Advisors

Lee, Purdue University.

Subject Area

Soil sciences|Environmental science

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