breaking the c-f bond

Breaking the C-F Bond: Solving the PFAS Water Pollution Issues

By Aman De Silva

PFASs (perfluoroalkyl substances and polyfluoroalkyl substances) are man-made molecules, which were used in many consumer products such as non-stick cookware, water-repellent clothing, stain resistant fabrics and carpets and some cosmetics since the 1940s. Due to their overuse, the chemicals now populate water systems and drinking water in America, the UK and many other developed nations which have produced these chemicals in the past. The molecules contain many Carbon-Fluorine (C-F) bonds; a bond which is recognised as one of the strongest bonds in organic chemistry. Consequently, the strong molecules are incredibly hard to break down and do not biodegrade, therefore polluting water for prolonged periods of time.

Due to the abundance of the strong molecules, they have been found to bioaccumulate in many species, including humans. They are toxic, and laboratory experiments in 2018 showed PFOA (perfluorooctanoic acid) and PFOS (perfluorooctanesulfonic acid), the most abundant and most studied PFASs, can cause negative and harmful immunological, liver, kidney and reproductive effects on animals. In some rare cases with low PFAS concentration in the water, the purification process can use processes which do not break apart the PFAS molecules, and instead use filtration membranes. However, in the case of high concentrations of PFAS such as carbon/resin regeneration waste, a chemical process must be used to degrade the large molecules, making new processes such as these have immense implications for water purification technology.

C-F bonds are extremely strong, and can have an average bond disassociation energy (BDE) of 544kJ/mol, meaning a large amount of energy is required to break down the bond. For comparison, the C-C bond in Ethane has a BDE of 369kJ/mol, and the C-H bond in methane has a BDE of 431kJ/mol. Breaking a C-F bond is vital in the degradation of PFASs, as should we attempt to break the C-C bonds in the carbon chain, a series of shorter fluorocarbons will be produces, which are also pollutants and very hard to get rid of. Another feature of the C-F bond which makes it so strong is that the difference in the electronegativity (ΔAEN - the tendency of an atom to attract a shared pair of electrons towards itself.) of Carbon and Fluorine is 1.5. This means C-F is classified as a very polar covalent bond, as 0.5< Difference in Electronegativities <1.6. Although not as strong as an ionic bond, the partial, attractive charges of the atoms explain why this is such a strong bond, and why it is so hard to break.

On March 11th 2020, Engineers at UC Riverside published their findings on PFAS degradation using computer modelling. They ran simulations on both PFOS and PFOA, finding that the molecules instantly lost the Fluorine atoms in the presence of excess electrons. They theorised a treatment scenario, where the electrons could be ultraviolet-generated electrons, They theorised a treatment scenario, where the electrons could be ultraviolet-generated electrons, provided by subjecting metal to ultraviolet radiation and harnessing the photoelectric effect to isolate electrons in solution (eaq-s). The photoelectric effect is where light is shined on the surface if a metal, and as a result, electrons are ejected from the surface. This computer modelling research linked to similar work done a year earlier, but with a real scenario and tangible reactants. The aim was to defluorinate the compounds using two mechanisms, a chain shortening reaction to create more manageable organic molecules, and a Hydrogen/ Fluorine Exchange, where the compound is reduced by the UV generated electrons.

In the H/F exchange (Reaction A), Fluorine atoms are replaced with hydrogen, in the reaction: CnF2n+1-COO- + H+ + 2e- --> CnF2nH-COO-+ F-

The initial reaction, which shortens the molecules into more manageable substances (Decarboxylation triggered HF elimination – Reaction B), removes the negatively charged carboxylate functional group (COO-), to then add a hydroxyl ion (OH) from a water molecule. Then, a hydrogen fluoride is eliminated from the molecule. After this, the newly formed acyl fluoride (CFO) molecule is hydrolysed (broken apart using water), and one of the products is another hydrogen fluoride molecule. This leads to a new final molecule, of the same homogenous series, but with the carbon chain length being one shorter. The final products are a series of defluorinated short chain carboxylate ions, and carbon dioxide, which are less toxic. These molecules are safer as their half-lives inside animals can be twice as short, meaning they have far less potential to bioaccumulate and harm those who consume them. They can also be more easily removed from water by more common water treatment processes such as sorption.

Currently, the science and resources being invested into removing PFAS from water is immense, and the new technologies and mechanisms being designed are impressive and solve an issue that affects all of us. PFAS are found in tap water, ground water, and the seas in and around the UK, and studies in America have shown 98% of people have PFASs in their blood, most commonly perfluorooctanesulfonic acid. These toxic chemicals affect all of us, and put our health at risk. This new method of reducing PFASs in water supplies with UV generated electrons has worldwide implications for water purification.

References

ATSDR – CDC. (April 25th 2019). Per- and Polyfluoroalkyl Substances (PFAS) and Your Health. Agency for Toxic Substances and Disease Registry: Atlanta, GA. Retrieved 30/05/2020 from https://www.atsdr. cdc.gov/pfas/pfas-exposure.html Lange’s Handbook of Chemistry - Properties of Atoms, Radicals, And Bonds. (1999). TABLE 4.11 Bond Dissociation Energies. McGraw-Hill Education: New York City, NY. Michael J. Bentel, Yaochun Yu, Lihua Xu, Zhong Li, Bryan M. Wong, Yujie Men, and Jinyong Liu. (March 15th 2019). Defluorination of Per- and Polyfluoroalkyl Substances (PFASs) with Hydrated Electrons: Structural Dependence and Implications to PFAS Remediation and Management. Environ. Sci. Technol. : Washington, D.C. Retrieved from https://pubs.acs.org/doi/10.1021/acs.est.8b06648# Sharma S. R. K. C. Yamijala, Ravindra Shinde and Bryan M. Wong. (January 21st 2020). Real-time degradation dynamics of hydrated per- and polyfluoroalkyl substances (PFASs) in the presence of excess electrons. Phys. Chem. Chem. Phys. – Royal Society of Chemistry: London, United Kingdom. Retrieved 27/05/2020 from https://pubs. rsc.org/en/content/pdf/article/2020/cp/c9cp06797c University of California - Riverside. (March 11th 2020). Pollution: A possible end to ‘forever’ chemicals: Excess electrons could help break the strong chemical bonds in products that contaminate water supplies. ScienceDaily. Retrieved May 27, 2020 from www.sciencedaily.com/ releases/2020/03/200311123318.htm