Public awareness and our assembled knowledge of the negative health effects of poly- and perfluoroalkyl substances (PFAS) have increased rapidly in recent years. The unique properties of PFAS compounds that make them useful in fire-fighting foam, food packaging, clothing and much more are also the properties that make them extremely persistent in nature and allow them to accumulate in humans and all other living beings.
A large source of PFAS contamination in nature comes from fire-fighting foam from airport practice sites. While undesirable as contaminants in the soil at the practice sites, even more problematic are PFAS that are mobilized into the surrounding ground water.
PFAS stands for poly- and perfluoroalkyl substances and loosely refers to organic compounds containing a smaller or larger chain with some or all of C-H replaced with C-F. PFAS can be sorted into severeal subclasses. Some examples of common subclasses are perfluoro- sulfonic acids, carboxylic acids, phosphonic/phosphinic acids, sulfonamides, and alcohols.
Figure 1. Generic structures for polyfluorinated compounds. The n = 8 linear carbon structures are shown for many of these examples, but n = 4 to 14 linear and/or branched carbon units are generally possible. Image reproduced from Lindstrom et al. (Environ. Sci. Technol., 2011.)
Contrary to what one may be excused for believing, PFAS compounds were not originally intended to pollute people and the planet. They share some unique features that give them their useful properties and also causes problems when they are not contained and become pollutants. First, the C-F bond is unusally strong, making (the highly fluorinated moiety of) PFAS stable to heat, many reducing and oxidizing conditions, processing by microorganisms, etc. Further, PFAS compounds have both hydrophobic and lipophobic properties. An unusal property that can be visualized as a three-phase system with water and organic solvent and likely stems from the extremely low polarizability of the fluorine atom and corresponding per/polyfluorinated compounds. The stability, water and fat repelling properties are why PFAS compounds are used in food packaging, frying pans, DWR-coated clothing, and fire-fighting foam among many other applications.
The problems for humans and nature stems from the same properties: they accumulate in (human) tissue due to hydrophobicity and enzymatic stability that make the clearance from the body extremely slow. Once thought inert and safe to health, PFAS compounds have now been shown to cause various diseases, e.g., cancer and thyroid disease, when they accumulate in the body.
Analogously in nature, these so-called "forever chemicals" accumulate in organs and tissues in organisms at sea and land. Often taken up by microorganism and then moving up the food chain, creating a biomagnification on top of the bioaccumulation.
Of interest in our current PFAS project are PFAS in fire-fighting foam that contaminate soil and ground water. The contaminated water in this study is leachate from soil from fire fighting practice areas at airports.
Strategies for PFAS remediation of contaminated soil can broadly be classified into i) immobilization ii) mobilization iii) removal/degradation. Point ii) and iii) are commonly combined, i.e., liquid that is used to wash soil from PFAS is further treated to remove and degrade PFAS. This is the direct procedure for contaminated lakes, sea, and ground water.
Point i) involves restricting the hydrodynamic movement in the soil by in situ barriers or mixing with Portland cement or transport to crates. It can also involve adding additives to the soil, e.g., activated carbon, that bind stronger than the soil to PFAS and prevents high levels of PFAS in soil leachate. However, it can be argued that additives do not contain any PFAS but simply extend the leakage.
Point ii) consists of washing/removing the PFAS from the soil. Water is the most obvious and arguable most reasonable liquid for washing. Water with a range of additives has been tried, such as salts, acids, bases, and detergents. Organic solvents and supercritical carbon dioxide has also been attempted. Desorption by heating is another strategy for removing PFAS from soil and other solids. A complication is that the there are 4000+ known PFAS compounds which have different physicochemical properties, notably solubility and vapor pressure in this context. Thus, some are easier than others to remove. Generally, the smaller PFAS compounds are more soluble and have higher vapor pressure.
Point iii) finally commonly involves capturing and concentrating PFAS from water and storing or destructing the concentrate. Optionally, PFAS could be destructed without prior concentration in contaminated water. Also, attempts has been made to destruct PFAS still in the soil.
For capturing and concentrating PFAS from water, there are some more or less established methods in use. Recently in the news, soil remediation by plant uptake has been discussed. This involves planting in contaminated soil and harvesting vegetable material that has taken up some of the PFAS that is mobilized in the water in the soil. The smaller PFAS are more mobile in the soil and are preferentially taken up, which is useful since the methods discussed below are more effective against the larger PFAS compounds. The vegetable material is subsequently burned in a facility suited for PFAS contaminated material. For removing PFAS from leachate and other contaminated waters, two major concepts are the SAF method and capture on adsorbents. The SAF method firstly involves airating the water to make the PFAS float to the surface as a foam, which can be skimmed off. As mentioned, the method is most effective for the larger PFAS compounds. Finally, adsorbents come in many variants and applications. The major classes are granulated activated carbon and ion exchange materials. Activated carbon is general-purpose for organic compounds, which is both its strength and weakness. Spent activated carbon can be sent back to the manufacturer to be re-activated but is commonly single-use (by the customer/user). Synthetic ion exchange (IEX) beads have the benefit that they can be designed to be somewhat selective and that they are less friable than granualted activated carbon. IEX beads are naturally only effective against ionic PFAS compounds such as perfluoroalkyl sulfonic acids, carboxylic acids, phosphonic acids and more. Both single-use and regenerable (by the customer/user) IEX beads are available. IEX beads have traditionally been used in column systems. A long established and working technology, which however requires a large hardware installment and CAPEX. SpinChem is offering an alternative for applying adsorbents to liquids with a different working principle to column technology and a few unique advantages. More on this below.
As for the destruction of PFAS compounds, the mentioned stability of the compounds makes this a difficult tasks. High temperature incineration at suitable facilities can be used for vegetable material and PFAS concentrate. Alternatively, for PFAS in water, strongly oxidizing conditions may be used. Oxidants such as persulfate is an option for concentrate, for dilute water solutions ozone and Fenton conditions may be more suitable.
While IEX resin in column systems are well-established, rotating bed reactors have their own set of advantages that make them a strong option in many cases:
Additionally, the RBR operation can be automated which further improves utility and efficiency.
The graphs below show experimental data for perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) contaminants in 680 L of contaminated water in an IBC-tank during treatment with PFAS-tailored Sorbex regenerable resin in an RBR S5. Fitting theoretical decay curves to the concentration data indicate remediation half-lives around 15 min for PFOA and 6 min for PFOS, respectively.
Figure 2. Remediation of PFOS and PFOA contaminated water stored in an IBC-tank (680 L water) with PFAS-tailored IEX resin (1.6 kg) in RBR operating at 250 rpm.
Kinetics experimentSorbex regenerable resin (1.6 kg) was added to a SpinChem S5 RBR, which was spun at 250 rpm in PFOS and PFOA-containing leachate water from contaminted soil. Samples were taken after 15 and 30 minutes as well as 1, 2, 4, and 24 hours. The sample concentration was done according to the sample preparation instructions.
Sample preparation and samplingSamples were taken in a 1 L high density polyethylene bottle. 5 g of Trizma® Base Primary standard and Buffer ≥ 99.9 % was added to ensure 5 g/L and pH was adjusted to 7 ± 0.5 using hydrochloric acid. These samples were concentrated for analysis with LC/MS by following the EPA METHOD 537.1 (Shoemaker, 2018). The EPA method explains the following on how to elute PFAS properly through a column. The Supelco Supelclean ENVI-Chrom P 6 ml column was conditioned by allowing 15 mL of methanol to run through and afterwards 18 mL of distilled water. The column was not allowed to dry at these steps. A few millilitres of distilled water was added to the columns. The water from the bottles were moved to the column by vacuum suctioning. A septum with a tube through it was placed in the column and the other end of the tube was placed in the liquid for aspiration. The vacuum was turned on and the flow through of the column was adjusted to approximately 15 ml/h. The column was not allowed to dry at this point either. After all sample was pulled through the column the bottle and column was rinsed twice with 7.5 ml distilled water. Air was run through the column at the same vacuum capacity for 5 minutes. A 15 ml falcon tube was inserted on a rack to collect the elute. The bottle was rinsed twice with 4-5 ml of methanol and was then run through the column to elute the PFAS collected in the column. The eluted methanol was evaporated by placing the falcon tube in a water bath heated to 60 C by a circulator. A gentle nitrogen gas stream was run into the falcon tube to assist the evaporation. The dried sample was redissolved in 1 mL methanol or the appropriate volume to ensure a concentration between 100 - 1000 ng/ml PFAS. To ensure complete dissolution, the samples were places in a ultrasound water bath for five minutes or until the visible dried sample was dissolved. 250 µl of the samples were transferred to a 300 µl polypropylene vial which was analyzed at an external lab.
Analysis of PFAS by LC/MSThe samples were analysed by liquid chromatography tandem mass spectrometry LC/MS by the Chemistry department at Umeå University. The instruments used were Thermo Scientific™ TSQ Quantiva Triple Quadrupole Mass Spectrometer with a Thermo Scientific™ Hypersil GOLD™ C18 50x2.1, 3 µm column and a precolumn of similar quality 10x2.1, 3 µm. 2 µl of the sample was injected. The LC was run with a gradient consisting of water with 0.1% formic acid and acetonitrile with 0.1% formic acid. The collision gas used was argon and the ionization for the MS was done by a heated electrospray at 2.5 kV under negative conditions. Both PFOA and PFOS were calibrated from PFOA standards.
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