About the author:
Thomas Smith is laboratory manager for ResinTech Inc. Peter Meyers is technical director for ResinTech Inc. Smith can be reached at [email protected]. Meyers can be reached at [email protected].
Thomas Smith & Peter Meyers
undefinedIt was 1928 when Frigidaire patented a miracle compound called Freon, a new class of chloro-fluoro-carbons (CFCs) that were non-flammable, non-corrosive gasses of extraordinary usefulness as refrigerants. This invention brought on the advent of CFCs and ushered in a new era of organic chemistry exploration. In 1938, the semi-accidental discovery of PTFE (polytetrafluoroethylene) revolutionized the plastics industry and, in turn, gave birth to limitless applications of benefit to mankind. As it turns out, the discovery also gave birth to what are perhaps some of the most ubiquitous and difficult to remove toxic trace contaminants of all time.
Fast forward to 1949. DuPont introduced Teflon, made at their Washington Works Plant near Parkersburg, West Virginia. The precursor chemical was made by 3M. In 1967 the U.S. Food and Drug Administration (FDA) approved Zonyl, a class of PTFE powders, for use in food packaging. Zonyl was remarkable because it repelled both grease and water. It was cheap and effective and became the go-to chemistry for the packaging of many products, such as microwave popcorn, fast-food wrappers and pizza boxes.
Perfluorinated compounds (PFCs) are generally nonflammable and non-corrosive; as close to inert as any chemicals ever synthesized. As a class, PFCs fulfill chemists dreams for materials that are relatively easy to make, and which do not have undesirable chemical properties. PFCs do not burn, degrade or react with other chemicals. Their chemical properties make PFC’s particularly useful as fire retardants.
PFCs repel both water and grease. This property makes PFCs useful for water repellent clothing, waxes and protective coatings of many kinds. PFCs are used in hundreds of consumer and industrial products because of their water and grease repellant properties.
The older PFAS compounds (the C8 compounds such as PFOA and PFOS) have been studied long enough that we know they present very real health risks. These compounds also remain in our bodies for very long periods of time. GenX, DuPont’s replacement for PFOS, is not much if any better, based on current research. As a general statement, the further away from what nature a compound is, the more likely it is to be bad for us, is certainly true. There is still much to be learned about how PFAS compounds affect our health. The longer chain compounds, notably PFOA, PFOS, and especially PFHxS are not excreted rapidly and stay in our bodies for years, eventually reaching concentrations hundreds or even thousands of times higher than the concentrations we were initially exposed to.
Studies of the workers and people who lived near the DuPont site in Parkersburg have higher than normal incidences of cancer, liver enlargement, hormonal changes, thyroid function and elevated cholesterol. Pregnant women and their fetuses subjected to PFAS showed reduced birth weight, structural defects and increased neonatal deformities.
Techniques for Measuring PFAS Levels
The public wants “zero” for all PFAS compounds, regardless of scientific evidence, half-life or demonstrated health risk. There have been many instances where substances that were once thought to be safe, were later found to have health risks. The average citizen is skeptical of science-based information about chemical safety.
Ah, but what is “zero”? As our skill at measuring trace contaminants, such as PFAS, improves, the concentrations we can regulate for marches downward. In the 1980s, the first proposed limit for PFOA was 600 ppt, based solely on the limit of detection at that time. Current technology, at least for potable waters, is a detection limit for 18 PFAS compounds in the low single digit to sub-ppt ppt levels. Analysis of leachates and wastewaters is much more difficult; therefore, there is no approved U.S. EPA methods for wastewaters at this time.
The instrumentation of choice is liquid chromatography and tandem mass spectrometry (LC-MS/MS) sometimes referred to as “triple quad.” Long before the LC-MS/MS method was developed, analytical chemists looked to gas chromatography (GC) as a way to measure PFAS. GC had limitations due to the structure of PFCs, the hydrophilic side of the PFC made for poor chromatography. These limitations increased detection limits to undesirable levels.
In the 1970s, ion chromatography (IC), a type of liquid chromatography, was added to the analytical chemist’s toolbox. An IC first concentrates compounds on an adsorbent then elutes them sequentially, using a conductivity monitor as a detector. This process separates PFAS and other compounds by their retention time. IC worked well for PFAS compounds, but detection limits were in the low parts per million, too high to be useful for the PFAS concentrations typically found in drinking water.
The next advance was solid phase extraction (SPE). SPE concentrates contaminants on to a bed of ion exchange resin. The contaminants are eluted by a solvent which is then evaporated. This technique reduced detection limits for PFAS by ion and gas chromatography from parts per million to parts per billion.
There is a downside to SPE. PFAS are not the only molecules that accumulate on the resin and are eluted off. The concentrated eluate often contains other contaminants along with PFAS. This raises baseline noise, adds matrix interferences and makes low part per trillion analysis difficult, at best. LC-MS/MS allows us to fingerprint and quantify compounds simultaneously and reduce matrix interferences. This brings us to where we are today. The current method EPA 537.1 uses SPE, followed by liquid chromatography and mass spectrometry to identify individual PFAS species and determine their concentration.
As often happens with trace contaminants, our ability to measure contaminants to ever lower levels drives regulations (and vice versa). Our ability to measure more PFAS compounds at lower concentrations is driving states to set limits for an increasing number of PFAS compounds at lower concentrations.
In general, PFAS chemicals have a head and tail. The tail is hydrophobic (water hating). The longer the tail, the more hydrophobic the compound. The head is hydrophilic (water loving).
The head and tail give PFAS compounds their unique and valuable chemical properties and provide clues as to how best to approach their removal. Medias that capture both the head and tail tend to have higher selectivity and longer throughputs to breakthrough than medias that only attract the head or the tail.
Methods to Remove PFAS Chemicals From Drinking Water
Two processes are currently proven successful to remove PFAS chemicals: carbon and anion exchange resins. Both medias are more effective for longer chain PFAS (eight carbons) than shorter chain (less than six carbons). They are also more effective for the sulfonated PFAS compounds than the ones with carboxylic acid groups. For today, carbon or resin (or carbon and resin) remain the best available treatment methods to remove PFAS from water.
Although membrane processes, such as reverse osmosis (RO) and nanofiltration (NF) are also effective, the PFAS compounds remain in a significant fraction of wastewater, which, if returned to the environment, can result in the unfortunate requirement to remove the same molecules of PFAS over and over again.
Advanced oxidation holds what may be the best hope for a solution that permanently removes PFAS from the environment. However, the energy level needed to break the fluorine carbon bonds is very high, and to date no commercially viable advanced oxidation process has been demonstrated. There is much current interest in developing new sorbents that are even more selective for PFAS than the carbon and resin products available today. No doubt there will be improvements. However, the newer medias will still require significant capital expense for tanks, pumps, pipeworks, buildings, operators, maintenance and more.
There is much more to be learned about the effects of PFAS compounds on public health and on the environment. The public is razor focused on PFAS, and this interest will drive regulators, analysts and equipment suppliers to continue exploring new technologies and techniques and of course, ever lower limits.
