Per- and polyfluorinated compounds are a broad and heterogeneous group of compounds, that can be divided into many subgroups based on functional groups. It is the different
physicochemical properties of the individual compounds, and the effects of their chemical interaction that determine their fate when released in the environment. Leaching of PFCs from polluted soil is dependent on the soil characteristics, the sorption behavior of the individual compounds, and the compounds interaction, as well as hydrological conditions. The soil characteristics are important since the hydrophobic alkyl tail of these compounds can sorb both to organic carbon in the soil and mineral surfaces. Being anionic compounds, with a strongly hydrophilic organic functional group, sorption of PFC compounds can also be influenced by many other factors; for instance the pH and ion composition of the solution.
It was in this Master thesis, through batch and column experiments, observed that the carbon chain length seemed to be the most important structural feature affecting the leaching of PFCs. At equilibrium partitioning of the PFCs between soil and aqueous phase, compounds with shorter carbon chain length (C4-C7), including 6:2 FTS (C8), had a larger relative contribution to the ΣPFCs in the aqueous phase than in soil. The compounds with longer carbon chain length (C8-C10) had lower relative contribution to the ΣPFCs in water than in soil, and thus seemed to be stronger retained in the soil. Column experiments confirmed the importance of carbon chain length for the leaching behavior of PFCs. There was a general decrease in the relative contribution of shorter carbon chain compounds (< C8), including 6:2 FTS (C8), to the ΣPFCs in leachate from the columns over time, while the relative distribution of the long carbon chain compounds (≥ C8) to the ΣPFCs in leachate from soil increased over time.
The partitioning equilibrium coefficients (Kd) of the acidic PFCs between porewater and the various types of soil showed a strong relationship with carbon chain length of the compound.
The log Koc value increased with 0.1-0.7 log units for each added CF2 in the carbon chain of the acidic compounds, except for PFBA, which had a higher sorption than compounds of equal carbon chain length. The total amount of individual PFCs leached out of the initial amount in soil at equilibrium was decreasing with increasing carbon chain length of the
112 compounds. However, more of the longer carbon chain compounds had leached out from the soil in the columns. This could have been a result of the decreasing calcium concentrations in soil over time, as the long carbon chain compounds seemed to be strongly affected by the calcium concentration in soil and aqueous phase. Water concentration of the long carbon chain compounds (≥ C8) PFOS, 8:2 FTS, PFNA and PFDA decreased with increasing calcium concentration in soil of batch experiments, and increased with decreasing calcium
concentration in soil of column experiments.
There were also found differences in the leaching and sorption behavior between compounds of similar carbon chain length, with different organic functional group. The sulfonic
compounds PFOS and PFHxS were higher retained in soil than the acidic compounds of similar carbon chain length.
The acidic short carbon chain compounds (< C8) showed dependence on the total PFC
concentration in soil for dissolving into the water phase, except PFBA. The compound PFPeA (C5) had a slightly lower retention in soil than PFBA (C4). This was likely because PFPeA depended on the total concentration in soil, and had a higher concentration in all soil samples than PFBA.
TOC level of the soil samples were also found to be important for leaching of PFCs. The compounds were least retained in the soil with lowest TOC level, and leached high levels out of the soil independent of the carbon chain length. The soil types with higher TOC levels in general had much higher retention of PFCs, and more of the short carbon chain compounds present. The long carbon chain compounds PFOS, PFNA and PFDA were the individual compounds most dependent on the TOC level in the soil for sorption. From the soil with very low TOC level, the leaching of PFCs depended on the water flow-velocity. Therefore, in areas where the groundwater flow-velocity is high and the soil has low TOC content, PFCs can spread far from the source independent of carbon chain length. In soil with higher TOC levels, the groundwater flow-velocity would be most important for the leaching velocity of
compounds with low carbon chain length (< C8), and low retention in the soil. Compounds of longer carbon chain length have slower leaching out of soils with a high TOC content.
113 Considering the potential for long term leaching of PFCs, the compound structure, soil
characteristics and the hydrogeological conditions are important. The most important soil characteristics seem to be the TOC level, the concentration level of calcium, as well as the concentration of the PFCs in the soil. The potential for long term leaching from soil is very low for the short carbon chain compounds (< C8) as the compounds were very mobile in soil independent of the TOC level, and they leached out of the soil fast. The compound PFHxS (C6), however, has a higher potential for long term leaching as it was stronger retained in soil than compounds of equal carbon chain length. In general, longer carbon chain compounds ( ≥ C8) have a higher potential for long term leaching. PFOS, 8:2 FTS and PFDA were the compounds that leached out slowest from soil with high TOC level. It was, however, PFOS that had the highest concentrations in soil, and were thus the compound expected to leach out of soil the longest.
The findings of this Master thesis can be used to evaluate the potential for spreading of PFCs with groundwater from soil. It was, however, observed that many factors controlled the leaching of PFCs, and such evaluation is not straight forward. As mentioned, the potential for spreading needs to be based on factors like the compound structure, soil characterstics, soil concentrations and composition of PFCs, hydrological regime, etc. It was clear that the results from the performed batch and laboratory experiments supplemented each other, and more research on the leaching of PFCs would increase the total understanding. More understanding is needed as a basis for evaluating the environmental risk of PFC polluted soil.
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