The behaviour of PFOS (synonym for PFOS, PFOA and other perfluorinated compounds in soil and water is partly determined by their solubility, degradation, volatilization and sorption.A complicating factor in this is that the so-called derivatives may have significantly different properties
|solubility in surface water (mg/l)||370||9500|
Compared with the solubility of other organic contaminants, and in relation to the concentrations of these substances measured at localised pollutions, the solubility of both compounds is relatively good. The solubility is affected by the salt content, pH, redox conditions, and salt formation / precipitation
| ||PFOS (K salt)||PFOA (NH4 salt)|
|vapour pressure (Pa, at 20 °C)||3.31 * 10-4||13 * 10-3|
Literature shows that PFOS (potassium salt) is substantially non-volatile (see table). And the ammonium salt of PFOA has a very low volatility, too. But PFOA is relatively volatile with a vapour pressure of 70 Pa. We cannot help noticing that large differences between the different forms are being reported. Literature sometimes gives different values for the same substance. In line with the low volatility are reports that the chemicals are detectable worldwide and even in the most remote areas such as the Arctic and Antarctic. However, there is still much uncertainty about the way in which the distribution and long distance transport takes place. One of the questions is to what extent differences in the volatility of the various forms play a role here
Sorption of perfluorinated compounds in soil and sediments determines their behaviour and distribution in the environment, but there is little consensus on the distribution coefficients to be used in the assessment of the behaviour in soil and water.
From literature (lit. 13) it appears that log K oc values determined in the lab, are systematically lower than log K oc values derived from field data.Based on laboratory experiments, the adsorption can be reasonably well described by an average log K oc of 3.0 for PFOS and a log K oc 2.8 for PFOA.However, for a field situation average log K oc values are derived from 4.2 for PFOS and 3.7 for PFOA.These differences show the importance of field research alongside laboratory studies.
Literature also suggests that these substances are not so much 'lipophilic' but rather 'proteinphilic’ (protein-loving). A possible connection with the protein content of organic substance is reported to be probably accompanied by a correlation between the amount of protein and the amount of soil organic matter.
On the basis of the bandwidth in laboratory values and field values, and the lack of consensus on these, there is a wide bandwidth in the predictability of dispersing behaviour and the concentrations in the aqueous phase. Actually, we do not have sufficient knowledge of the behaviour in the soil.
In view of the strong covalent bond between the fluorine and the carbon atoms, biological degradation is very inconvenient. Literature has no known cases of (biological) degradation in the environment. In the 2008 EFSA publication (lit. 28) PFOS is designated as extremely resistant to thermal, chemical and biological degradation processes. In order for PFOS to combust completely a temperature of 1,100 ˚C is required. Chemical oxidation occurs only under strongly oxidizing conditions, for example in the presence of activated persulfate or Fenton's reagent.
SAMPLING AND ANALYSIS
Proper characterization of the behaviour of PFOS in soil and water starts with thorough sampling and analysis. This demands attention and a good preparation. In practice often low concentrations are involved and contamination is an important issue when sampling, for example by trace contaminants from sample material (Teflon) or the environment. Also, during the analysis contamination of the sample is possible and therefore a critical inspection of the analysis results is important. In the Netherlands we use the following reporting limits for PFOS: in soil 0.1 µg / kg and in groundwater 5 ηg / L