“Man-made chemical” sounds dangerous if we imagined it in relation to what we eat, put on as clothes, or use with our hands. We should not be obsessed with it, but it is important to be aware that there are a lot of things around us which might be contaminated with what is called “forever chemicals.”

Perfluoroalkyl substances (PFAS) are highly hazardous man-made chemicals that have been used since the 1940s. Some well-known examples of PFAS are perfluorooctanoic acid (PFOA, C8HF15O2), heptadecafluorooctanesulfonic acid potassium salt (PFOSK, CF3(CF2)7SO3K), and GenX (HFPO-DA, C6H4F11NO3). Although these substances are very useful in the chemical industry, the dispersion, low biodegradability, and high stability of the molecules have led to many environmental and health issues. PFAS molecules are very difficult to remove, let alone break down, and these molecules can accumulate over time. This is the reason why PFAS molecules are referred to as “forever chemicals.” They are extremely hard to break down due to the high strength of the carbon-fluorine (C-F) bond found in these compounds. This bond can have a bond dissociation energy of up to 546 kilojoules per mole (kJ/mol). Bond dissociation energy is the energy required to break a bond, and a high bond dissociation energy, as exhibited in a C-F bond, corresponds with a bond that has low energy and is very stable.

Applications of PFAS in industry

Why are these chemicals so extensively used in the chemical industry? PFAS molecules have been found to be very stable, and they exhibit properties that allow them to repel both oil and water. As a result of these properties, these chemicals have been successfully employed to act as a repellent for almost anything. Some examples of the various applicants of PFAS are surface coatings, surfactants, and flame retardants. With so many uses, these harmful toxins can be found in everyday items such as clothing, furniture, food packaging, adhesives, and much more.

PFAS contamination

How exactly are we exposed to PFAS chemicals? Besides the exposure from PFAS containing materials, the answer lies primarily in our water systems and environment. The majority of our drinking water contains these “forever chemicals.” As a result of their widespread manufacturing, usage, and improper disposal, these chemicals can enter our water systems and air through different sites. Another way PFAS has become so widely dispersed is through groundwater and soil receiving rainwater runoff. Since these chemicals are not easily broken down, they can remain in our waters for a great number of years. It is reported that about 110 million people could have PFAS contaminated water at levels of 2.5 parts per trillion (ppt) or even higher. This value is way above the safe level of exposure to PFAS, and this is not where it even ends. Approximately 60 million people have water utilities that are contaminated with PFAS exceeding   5 ppt, and 16 million people with water utilities exceeding 10-90 ppt of the chemical. It is estimated that 242 water utilities throughout the United States are contaminated with PFAS chemicals. These statistics show just how prevalent these harmful chemicals are in the United States. There is currently no federal limitation as to the concentration of these chemicals in our waters which is a huge problem. Most people have shown signs of these chemicals in their bloodstream, which truly indicates the widespread exposure to these harmful chemicals.

Health effects from PFAS exposure

This increased exposure to PFAS can cause and lead to many adverse health concerns. If the chemicals are ingested, they can actually accumulate in the body and remain there for long periods of time. Scientists are still conducting studies about the effects of PFAS, but the existing research has shown that these chemicals may cause developmental effects in infants, lead to decreased fertility in women, interfere with the body’s natural hormones and enzymes, increase cholesterol levels, affect the immune system, and increase the risk of particular cancers. The extensiveness of the health effects in a person depends on how long, how often, and how much PFAS they were exposed to. Due to all of these negative effects, PFAS are environmental pollutants that are included in the United States Environmental Protection Agency (EPA) Contaminant Candidate List.

Removal and breakdown of PFAS

The health concerns surrounding PFAS exposure make finding a solution to the problem a very urgent matter. There have been many past attempts to remove PFAS from the environment. However, not all of these attempts have resulted in the development of environmentally friendly methods. It is extremely important to find a way to remove PFAS in a manner that does not cause further harm to the environment. The current goal for PFAS removal is to formulate and implement an efficient, green, and cost-effective process – a goal that is especially challenging when considering the high strength of the carbon-fluorine bond in PFAS molecules.

Incineration

One technique that is being explored is incineration or destruction of PFAS through the use of heat. This is done by directing heat at objects contaminated by PFAS, such as soil, waste, and water. Incineration is a method that has been used for the destruction of many other chemicals. Even though incineration can destroy PFAS, there are many other risk factors that need to be taken into consideration. For example, incomplete combustion of PFAS could lead to problems with the resulting products of the reaction. The process of incineration leaves behind waste residues that need to be properly disposed of, which needs a solution of its own. Incineration also requires an extensive amount of energy and fuel. In addition, incineration can result in PFAS emissions entering the air, the effects of which have not been researched in depth.

Reverse osmosis and nanofiltration

Reverse osmosis and nanofiltration are other methods that are being investigated for the removal of PFAS. They have been shown to be effective in removing PFAS. In simple terms, this is a pressure-driven process in which water is filtered by pushing the water through a semipermeable membrane. This process is useful for removing PFAS from water sources, and recent advances in manufacturing have made it possible to use this process more efficiently. The methods involved with reverse osmosis and nanofiltration have previously been used with removing many other chemicals from water. The process is commonly used in water purification systems and industrial applications, and it is most often employed for desalination purposes. Despite the wide usage of the process, there are some operating considerations that must be looked at for reverse osmosis and nanofiltration systems. Membrane technology comes with the challenge of fouling, where the accumulation of matter from the filtration process can result in a reduction in the flux performance of the membranes. To avoid fouling, the membranes must be constantly modified, and the operating conditions need to be continuously changed. Additionally, replacing the membranes results in waste products that contain PFAS residue, and cleaning the membranes requires there to be a way to dispose of the used cleaning solution. The “reject stream” or stream of water post-filtration will contain concentrated amounts of PFAS and will need to be disposed of or treated. Thus, waste management is a big issue that needs to be addressed if the processes of reverse osmosis and nanofiltration are to be used for PFAS removal.

What’s next?

It is clear that a method for removing and destroying perfluoroalkyl substances urgently needs to be found. These molecules have harmed the environment and our lives in countless ways. Many past attempts have come with drawbacks and failed to be cost-effective and environmentally friendly. Until we have found a way to successfully eliminate PFAS from the environment without causing further damage, these “forever chemicals” will continue to affect us detrimentally. So, what can you do? Fortunately, there are ways to be aware of these harmful chemicals in our water. If you are living in the United States, you can have your water checked for PFAS by contacting your state for a list of certified laboratories that are using the EPA Method 537. You can visit EPA’s website to search online for EPA certified labs for drinking water testing (https://www.epa.gov/dwlabcert/contact-information-certification-programs-and-certified-laboratories-drinking-water).

References

• “Basic ” EPA, Environmental Protection Agency, https://www.epa.gov/pfas/basic-information-pfas.
• “Drinking Water Health Advisories for PFOA and PFOS.” EPA, Environmental Protection Agency, 18 Feb. 2021,
• epa.gov/ground-water-and-drinking-water/drinking-water-health-advisories-pfoa-an d-pfos
• John Hahladakis, Costas A. Velis, Roland Weber, Eleni Iacovidou, Phil Purnell, An   overview of chemical additives present in plastics: Migration, release, fate and environmental impact during their use, disposal and recycling, Journal of Hazardous Materials, Volume 344, 2018, Pages 179-199, ISSN 0304-3894, https://doi.org/10.1016/j.jhazmat.2017.10.014.
• “Pfas – per- and Polyfluoroalkyl ” PFAS Per and Polyfluoroalkyl Substances, https://pfas-1.itrcweb.org/12-treatment-technologies/#12_2_2.
• “Report: Up to 110 Million Americans Could Have Pfas-Contaminated Drinking ”
• Environmental Working Group,
• https://www.ewg.org/research/report-110-million-americans-could-have-pfas-contaminat ed-drinking-water.
• Yao Y, Volchek K, Brown CE, Robinson A, Obal T. Comparative study on adsorption of perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA) by different adsorbents in water. Water Sci Technol. 2014;70(12):1983-91. doi: 10.2166/wst.2014.445. PMID:
• “Potential Health Effects of Pfas ” Centers for Disease Control and Prevention, Centers for Disease Control and Prevention, 24 June 2020, https://www.atsdr.cdc.gov/pfas/health-effects/index.html.

Today, more than 1 billion people are suffering from the lack of potable water. About 2.3 billion people (41 percent of the earth’s population) live in regions with water scarcity; this number is estimated to be 3.5 billion by 2025.1

96.5 percent of the world’s water is found in seas and oceans, and the remainder is found as ice caps, brackish water, and fresh water sources (e.g. lakes, rivers, and ground waters). To overcome water shortage problems, methods such as water conservation and dam construction have been applied for several years, but they are not enough against increasing water demand and decreasing fresh water sources.2

Water is also very important for generating energy, and vice versa. The largest portion of U.S. electric production is provided by thermoelectric power generation, where steam-driven turbine generators are used to generate electricity. In 2000, thermoelectric power plants used 39 percent of all fresh water sources in the United States.3 All these reasons make the production of drinking water a worldwide issue.

Desalination

Since most of world’s water supply is found in oceans and seas, desalination is the process of removing salts and minerals from either ocean or brackish water to make it safe for human consumption and use. The most widely applied desalination processes are divided into two main categories, thermal distillation processes and membrane processes.

Desalination via thermal distillation methods, which separate liquid mixtures based on their boiling points, mainly fall into three categories: multi-stage flash (MSF), multi-effect distillation (MED), and mechanical vapor compression (MVC). Thermal distillation processes require the evaporation of water while leaving the salt in a concentrated brine. Middle Eastern countries mainly use thermal-based desalination plants to produce fresh water because of their easily accessible fossil fuel sources.2, 4

Membrane-based separations are the main choice of producing potable water in countries outside the Middle East. More than 50 percent of the newly installed desalination plants have been using reverse osmosis (RO) membrane technology (since 2001).2

Membrane separations

A membrane is an interphase between two adjacent phases acting as a selective barrier, regulating the transport of substances between the two compartments. It is a very thin film that allows passage of some types of substances while preventing the passage of other substances, depending on their sizes. Membranes used for separation technology gave rise to an interdisciplinary area including many fields of science and engineering such as chemistry, chemical engineering, material science, process engineering, environmental science, ecology, and economics.5, 6 Today, the membrane industry is impressively large. The membrane separation technology market is quite diverse and ranges from medicine to the chemical industry, and the most important markets are medical devices and water treatment. There was a $2 billion sale of synthetic membranes worldwide in 2003.6

Water purification membranes

Water treatment processes employ several types of membranes. They include microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) membranes. They are designed to remove materials of increasing sizes. MF membranes have the largest pore size and typically reject large particles and various microorganisms. UF membranes have smaller pores than MF membranes and, therefore, in addition to large particles and microorganisms, they can reject bacteria and soluble macromolecules such as proteins. RO membranes are effectively nonporous and therefore exclude particles and even many low molar mass species such as salt ions, organic substances, etc.7 NF membranes are relatively new and are sometimes called “loose” RO membranes. They are porous membranes, but since the pores are ten of angstroms or less, they exhibit performance between that of RO and UF membranes.8 Of these membranes, NF and RO membranes constitute the dominant technology for desalination of water.9

2.1 Nanofiltration Membranes

Membranes for nanofiltration (NF) are usually comprised of cellulose acetate or aromatic polyamides. NF allows diffusion of organic compounds, and rejects some salts with low pressures being applied. NF itself cannot purify seawater to drinking water standards, but it is a process that can be used to produce mildly salty water, or as a water-softening technique.2, 4 When NF is coupled with RO, then it can be used to turn seawater into drinking water.10

Nanofiltration membranes usually have negative charges (e.g., carboxylate groups, sulfonate groups, etc.), and as a result, ion repulsion is a major factor in determining salt rejection. More highly charged ions, such as sulfate, are more highly rejected than monovalent ions, such as chloride, by a negatively charged nanofiltration membrane. In particular, NF membranes are used to remove divalent ions such as calcium and magnesium, which are mainly responsible for water hardness. These membranes also usually display good rejection of organic compounds with molecular weights above 200 to 500 grams.2,11,12

2.2 Reverse osmosis membranes

Osmosis is a natural process in which water molecules move across a semipermeable membrane from a lower solute concentration area to the higher solute concentration area. Water flows until a chemical potential equilibrium of water is established. When equilibrium is reached, the pressure difference between the two sides of the membrane is equal to the osmotic pressure of the solution.12

Reverse osmosis (RO) is the process of forcing water from a region of high solute concentration through a membrane to a region of low solute concentration by applying a pressure that is greater than the osmotic pressure. As a result, separation of water from the solution occurs as pure water from the high concentration side to the low concentration side. The RO process includes a feed water source, feed pre-treatment, a high-pressure pump, RO membrane modules and post-treatment steps.

RO membranes are capable of rejecting monovalent ions such as sodium and chloride, which makes the RO process a valuable method for desalination. Membranes used for RO processes have salt rejections of more than 99 percent. RO membranes do not have distinct pores, but rather rely on free volume within the polymer film.

RO membrane separations depend highly on the properties of the polymer film such as the chemical and physical structure of the membrane material. Desired RO membranes should be resistant to chemical substances and microbial organisms, stable over a long time both mechanically and structurally, and have ideal separation properties such as high water flux, high salt rejection, chlorine, and fouling (clogging of membrane pores) resistance.

Approximately one billion of six billion people in the world live in water-stressed areas, and RO membrane technology is the leading desalination technology to overcome the problem of insufficient clean water and estimated to continue its leadership in the near future.13 Scientists and engineers are extensively investigating the development of the most efficient membrane desalination technology to produce the cheapest potable water.

On the other hand, cells use membranes, though scientists do not try to further develop them, since they were already designed in a perfect manner. Cellular membranes have a phospholipid structure with embedded proteins. They control many different kinds of transportations of substances in and out of cells (e.g. sugar, drugs, ions). They are so well designed that they know which substances are helpful or harmful for the cell, and decide on the passage of substances based on that. Many researchers have tried countless times for many years to produce an equally wonderful membrane technology for making clean water. But cellular membranes, consisting of hundreds of functions in living organisms, do not form spontaneously.

REFERENCES

1) R.F. Service, Freshwater resources, desalination freshens up. Science, (2006). 313, 1088- 1090.

2) L.F. Greenlee, D.F.Lawler, B.D. Freeman, B. Marrot, P. Moulin, Reverse osmosis desalination: Water sources, technology and today’s challenges. Water Research (2009), 43, 2317-2348.

3) T.J. Feeley, T.J. Skone, G.J.Stiegel, A. McNemar, M.Nemeth, B. Schimmoller, J.T. Murphy, L. Manfredo, Water: A critical resource in the thermoelectric power industry.Energy (2008), 33, 1-11.

4) G. A. Tularam, M. Ilahee, Environmental concerns of desalinating seawater using reverse osmosis. J. Environ. Monit.(2007), 9, 805–813.

5) P. Vandezande, L. E. M. Gevers, I. F. J. Vankelecom, Solvent resistant nanofiltration: separating on a molecular level. Chem. Soc. Rev.(2008), 37, 365–405.

6) M. Ulbricht, Advanced functional polymer membranes. Polymer (2006), 47, 2217–2262.

7) R.H. Perry, D.W.Green, Eds., Perry’s Chemical Engineers’ Handbook, 7th ed., McGraw-Hill: New York, 1997.

8) Sagle, A., and B. Freeman, “Fundamentals of Membranes for Water Treatment,” in The Future of Desalination in Texas: Volume 2, Report Number 363, Texas Water Development Board, Austin, TX, pp. 137-154 (2004).

9) H.B.Park, B.D.Freeman, Z.Zhang, M.Sankir, J.E.McGrath, Highly Chlorine-Tolerant Polymers for Desalination, Angew. Chem. Int. Ed. (2008), 47, 6019-6024.

10) N. Hilal, H. Al-Zoubi, N. A. Darwish, A. W. Mohammad, M. Abu Arabi, A comprehensive review of nanofiltration membranes: Treatment, pretreatment, modelling, and atomic force microscopy, Desalination (2004), 170, 281-308.

11) A. Gorenflo, D. Velazquez-Padron, F.H. Frimmel, Nanofiltration of a German groundwater of high hardness and NOM content: performance and costs. Desalination (2002), 151, 253-265.

12) M.E.Williams, A Brief Review of Reverse Osmosis Membrane Technology,EET Corporation and Williams Engineering Services Company, Inc., Harriman, TN, 2003.

13) K. P. Lee, T. C. Arnot, D. Mattia, A Review of Reverse Osmosis Membrane Materials for Desalination – Development to Date and Future Potential. J. Membr. Sci. 370 (2011) 1-22.