Congener-specific organic carbon-normalized soil and sediment-water partitioning coefficients for the C₁ through C₈ perfluoroalkyl carboxylic and sulfonic acids

Abstract

Organic carbon-normalized soil and sediment-water partitioning coefficients (K_oc) were estimated for all C₁ through C₈ perfluoroalkyl carboxylic (PFCA) and sulfonic (PFSA) acid congeners. The limited experimental K_oc data set for the straight chain C₇ through C₁₀ PFCAs and C₈ and C₁₀ PFSAs was correlated to SPARC and ALOGPS computationally estimated octanol-water partitioning/distribution constants and used to predict K_oc values for both branched and linear C₁ through C₈ isomers. Branched and linear congeners in this homologue range are generally expected to have K_oc values > 1, leading to their accumulation in organic matter on sediments and soils, retardation during ground and pore water flow, and the preferential association with dissolved organic matter in aquatic systems. Both increasing perfluoroalkyl chain length and linearity increase K_oc values with substantial intra- and inter-homologue variation and interhomologue mixing. Variability in K_oc values among the PFCA and PFSA congeners will likely lead to an enrichment of more linear and longer-chain isomers in organic matter fractions, resulting in aqueous phases fractionated towards shorter-chain branched congeners. The expected magnitude of fractionation will require inclusion in source apportionment models and risk assessments. A comparison of representative established quantitative structure property relationships for estimating K_oc values from octanol-water partitioning constants suggests that these equilibrium partitioning frameworks may be applicable towards modeling PFCA and PFSA environmental fate processes.

Keywords:

• Perfluorinated acids
• perfluoroalkyl carboxylic acids
• perfluoroalkyl sulfonic acids
• soil and sediment partitioning
• organic carbon

Acknowledgments

S.R. thanks the Natural Sciences and Engineering Research Council (NSERC) of Canada for financial support.

Notes

^a Range given spans the upper and lower limits of the regression and log normal average log K_oc values as defined by the respective standard errors in ref. ^{[ 27 ]}

^b Range given spans the upper and lower limits as defined by the respective standard errors in ref. ^{[ 29 ]}

^c Calculated at a soil pH of 7 and pK _a values of 0.5 for TFA,^{[ 40 ]} 3.8 for n-PFOA,^{[ 88 ]} and −5 for n-PFOS.^{[ 43 ]}