Urban runoff is increasingly becoming an indispensible water source in spite of its degraded quality. Of potential concern are emerging contaminants such as perfluorinated chemicals, which are non-biodegradable, bioaccummulative and potentially toxic. This study investigated sorption of perfluorinated compounds to sediments of a surface water body located in an urban section of Singapore. We studied the spatial distribution in the water column and sorption and desorption of PFCs in laboratory batch experiments. Total PFC concentrations in the water column ranged from 183 to 253 ng/L. Perfluorooctanoate (PFOA) was most abundant followed by PFOS, PFNA (C=9), perfluorohexane sulfonate (PFHxS) and PFDA (C=10). Transport of the strongly and irreversibly sorbing PFDA and PFUnA (C11) perfluoroalkanoic acids (PFAAs) and the perfluorooctane sulfonamids was governed by suspended solids behavior. Strongly sorbing PFCs were removed from the water column by settling particles. Sorption isotherms were nearly linear in all cases. However, desorption of PFOS and the C9, C10 and C11 PFAAs was hysteretic with hysteresis increasing with chain-length. For example, the thermodynamic irreversibility index (TII) of PFDA approached 1.0 after desorption while the KD increased approximately 25 fold from approximately 40 to 1000. Desorption removed 100 and 52% of PFHxS and PFOS and 100, 82, and 27% of PFOA, PFNA, and PFDA, respectively. Sorption of PFOA and PFHxS was completely reversible. Data indicate that the frequently used KD parameter determined from batch sorption experiments is of limited value for modeling PFCs behavior in surface waters. Similarly, the carbon-nornalized KOC value, which is assessed assuming contaminants partition into a homogenous organic fraction, are inadequate to predict the sorption capacity of hysteretically sorbing PFC compounds. KOC varied by more than an order of magnitude depending on whether KOC values are derived from sorption or desorption isotherms.

Sorption and desorption of perfluorinated compounds to sediments of an urban water body

251st National Meeting (2016)

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