Subsurface fate and transport of poly- and perfluoroalkyl substances

Abstract

Poly and perfluoroalkyl substances (PFASs) are fluorinated chemicals that have been the focus of many recent studies because of their widespread use, persistence, bioaccumulative potential, toxicity, and distribution in the environment. Because of their unique properties, PFASs are used in a wide variety of products including food paper packaging products, stain repellants, nonstick coatings, and fire fighting foams. Releases of PFASs to the environment and impact to groundwater has occurred through land application of biosolids as well as through use of fire fighting foams known as aqueous film-forming foam (AFFF). Because of the potential risk associated with exposure to these compounds, it is important to understand their subsurface fate and transport. The present study investigated the occurrence and fate of PFASs from land-applied municipal biosolids by evaluating the levels, mass balance, desorption, and transport of perfluoroalkyl acids (PFAAs) and PFAA precursors in soils receiving application of municipal biosolids at various loading rates. PFOS was the dominant PFAS in both biosolids and biosolids-amended soil. Concentrations of PFASs in soil increased linearly with increasing biosolids loading rate, enabling development of a model for predicting PFAS soil levels based on cumulative biosolids loading rates. Mass balance calculations showed a loss of PFAA precursors in soil relative to the mass applied in biosolids, suggesting precursor transformation. Laboratory desorption experiments indicated that the leaching potential of PFASs decreases with increasing chain length and that previously derived organic-carbon normalized partition coefficients may not be accurate predictors of the desorption of long-chain PFAAs from biosolids-amended soils. Trace levels of PFAAs were also detected in soil cores from biosolids-amended soils to depths of 120 cm, suggesting potential movement of these compounds within the soil profile over time and confirming the higher transport potential for short-chain PFAAs in soils amended with municipal biosolids. This study also investigated PFAA sorption to multiple soils in the presence of nonaqueous phase liquid (NAPL) and nonfluorinated AFFF surfactants. Sorption of small-chain PFAAs did not follow the chain-length dependent trend observed for longer chain-length PFAAs. NAPL and nonfluorinated AFFF surfactants all had varying impacts on sorption on longer chain (>6 CF2 groups) PFAAs. The primary impact of NAPL was observed in low foc soil where Freundlich n-values increased when NAPL was present. Impacts of nonfluorinated AFFF surfactants varied with surfactant and soil. In general, the anionic surfactant sodium decyl sulfate (SDS) had chain-length dependent impacts on sorption. Increases in sorption were noted for the smallest compounds and these increases diminished in magnitude with increasing chain length. An amphoteric surfactant, n,n-dimethyldodecylamine n-oxide(AO), significantly increased sorption for the longer chain PFAAs in a positively charged soil. Changes in sorption caused by SDS and AO may be due to mixed hemimicelle formation, competitive sorption, or changes to PFAA solubility. Short-chain PFAA sorption generally increased in the presence of NAPL, SDS, and AO. These results demonstrate detailed site-specific information will likely be needed to model PFAA transport at AFFF-impacted sites. Finally, column studies in multiple solid phases were used to understand 1-D advective transport of PFAAs with respect to the equilibrium batch sorption data. Overall, behavior was chain-length dependent, though short chain PFAA behavior was again notable, confirming equilibrium studies. Comparison of equilibrium and column sorption results showed the potential for nonequilibrium behavior, particularly in soils with appreciable organic carbon content and for longer chain PFAAs. Nonequilibrium was confirmed to be the result of rate-limited sorption. Mass transfer coefficients were fitted from the data and found to vary with organic carbon content. This may be due to intraparticle diffusion into the organic matter matrix . This study initiates an understanding of the subsurface fate and transport of PFASs at the equilibrium and 1-dimensional scales. Additional research is needed to understand how these results translate to larger scales towards the end goal of reliable site characterization and remediation of PFASs.