Effects of pH and Cation Composition on Sorption of Per- and Polyfluoroalkyl Substances (PFASs) to Soil Particles
Per- and polyfluoroalkyl substances (PFASs) have drawn great attention recently, due to their environmental persistence, potential toxicity and global distribution. PFAS is a large family of substances, characterized by a perflourinated carbon chain and a functional group. All PFASs are synthetic and have been widely used since the 1950s due to their unique properties of being both hydrophobic and oleophobic, making them useful for many industries.
To be able to predict the fate of PFASs in the environment and to obtain detailed understanding of the transport processes, their partitioning behavior between soil particles and water depending on a range of parameters must be investigated. The aims of this study was to investigate the effects of pH, cation composition, functional group and perfluorocarbon chain length on sorption of PFASs to soil particles, by batch sorption experiment in laboratory scale. The laboratory-scale experiments were combined with modelling of the net charge to evaluate if net charge is a good predictor for sorption of PFASs to soil particles.
14 PFASs of varying length and functional groups were studied (PFBA, PFPeA, PFHxA, PFHpA, PFOA, PFNA, PFDA, PFUnDA, PFDoDA, PFTeDA, PFBS, PFHxS, PFOS and FOSA). The effect on sorption of Na+, Ca2+ (two different concentrations) and Al3+ were investigated at pH-range 3-6. Modelling of net charge was carried out in the geochemical model Visual MINTEQ. The soil had 45% organic carbon content.
The adsorption of PFASs was strongly correlated with perfluorocarbon chain length, showing a stronger adsorption to particles with increasing perfluorocarbon chain length (i.e. more hydrophobic). The relation between sorption (represented by the distribution coefficient log Kd) and perfluorocarbon chain length was linear for all PFSAs and C3 to C10 PFCAs. The PFSAs (sulfonate functional group) sorbed stronger to soil particles than the PFCAs (carboxylic functional group), and FOSA (sulfonamide functional group) sorbed the strongest. For most PFCAs, (C5-C13) there was a trend of decreasing log Kd (i.e. decreased sorption) with increasing pH, due to pH-dependent changes of the soil particle surfaces.
For short and intermediate perfluorocarbon chain length PFCAs (C5-C8) and for PFHxS among the PFSAs, cations had a clear effect on sorption. Aluminium ions (trivalent, Al(NO3)3) had the largest effect, followed by calcium (divalent, Ca(NO3)2) where higher concentration resulted in stronger sorption. Sodium (univalent, NaNO3) had the least influence on sorption.
The net charge modelled with Visual MINTEQ takes into account many parameters (including pH) that affect the surface charge and sorption of PFASs to soil particles. When comparing log Kd for the different PFASs with pH and net negative charge, net charge was a better predictor of sorption of PFASs to soil particles than solution pH alone.
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