doi.org/10.26434/chemrxiv.11634348.v1

Real-Time Degradation Dynamics of Hydrated Perfluoroalkyl Substances(PFASs) in the Presence of Excess ElectronsSharma Yamijala, Ravindra Shinde, Bryan Wong

Submitted date: 17/01/2020 • Posted date: 20/01/2020Licence: CC BY-NC-ND 4.0Citation information: Yamijala, Sharma; Shinde, Ravindra; Wong, Bryan (2020): Real-Time DegradationDynamics of Hydrated Perfluoroalkyl Substances (PFASs) in the Presence of Excess Electrons. ChemRxiv.Preprint. https://doi.org/10.26434/chemrxiv.11634348.v1

Perfluoroalkyl substances (PFASs) are synthetic chemicals that are harmful to both the environment andhuman health. Using self-interaction-corrected Born-Oppenheimer molecular dynamics simulations, weprovide the first real‐time assessment of PFAS degradation in the presence of excess electrons. In particular,we show that the initial phase of the degradation involves the transformation of an alkane-type C-C bond intoan alkene-type C=C bond in the PFAS molecule, which is initiated by the trans elimination of fluorine atomsbonded to these adjacent carbon atoms.

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Received 17th December 2019,

Accepted 00th January 2020

DOI: 10.1039/x0xx00000x

Real-Time Degradation Dynamics of Hydrated Perfluoroalkyl Substances (PFASs) in the Presence of Excess Electrons

Sharma S.R.K.C. Yamijala, Ravindra Shinde, and Bryan M. Wong*

Perfluoroalkyl substances (PFASs) are synthetic chemicals that are

harmful to both the environment and human health. Using self-

interaction-corrected Born-Oppenheimer molecular dynamics

simulations, we provide the first real‐time assessment of PFAS

degradation in the presence of excess electrons. In particular, we

show that the initial phase of the degradation involves the

transformation of an alkane-type C-C bond into an alkene-type C=C

bond in the PFAS molecule, which is initiated by the trans

elimination of fluorine atoms bonded to these adjacent carbon

atoms.

The strongest and most stable bond in organic chemistry is

the carbon-fluorine (C-F) bond.1 The high stability of this bond,

which has enabled several technological advancements in the

past century, now poses a serious health hazard.2–5 In the last

century, the industrial sector has harnessed the intrinsic

strength of the C-F bond to incorporate perfluoroalkyl

substances (PFASs) in a wide variety of consumer and industrial

goods.6–8 Specifically, PFASs are regularly used in the

electronics, automotive, and aviation industries;6,7 several

consumer goods, including non-stick cookware, clothing,

carpets, shampoos, cleaning agents, and adhesives contain

these persistent pollutants.8 As such, the wide-spread usage of

these anthropogenic compounds has contaminated water

resources in the US and many other countries.9–11 Most

importantly, the subsequent consumption of this contaminated

water leads to the bio-accumulation of these compounds in

various organisms, including humans.9,10,12–14 In all of these

examples, the intrinsic strength of the C-F bond in PFASs

prevents most organisms from dissociating these compounds

through natural means,15 which facilitates their bio-

accumulation and toxicity. Thus, the intrinsic strength of the C-

F bond poses a severe threat to many forms of life, making the

treatment of these contaminated water sources essential.

An efficient treatment often includes either direct in-situ

destruction or removal of PFASs from contaminated water (for

example, using sorption or nanofiltration) and their subsequent

destruction in the wastewater concentrate.16,17 Since

perfluoroalkyl compounds primarily contain C-C and C-F bonds,

the degradation of PFASs requires the dissociation of both of

these bonds. However, breaking a C-C bond is less effective than

C-F bond cleavage,16,17 since the former generates short-chain

perfluoroalkyl substances that are still found to be toxic.18,19

Thus, the primary challenge in the complete treatment of PFAS-

contaminated water is the efficient dissociation of the strong C-

F bonds in these pollutants.

In the scientific literature, a few studies have suggested

possible avenues for C-F bond dissociation in the presence of

additional electrons.20–22 However, the majority of these studies

carried out their analysis after the degradation has occurred,

and specific mechanistic details of PFAS degradation are less

understood. Although a few computational studies have

addressed possible degradation of PFASs with excess

electrons,20 these studies were limited to static (i.e., non-

dynamic or stationary) electronic structure methods. As such,

these prior approaches restrict our understanding of PFAS

degradation dynamics. Also, to the best of our knowledge, there

is little or no information on defluorination timescales in PFASs.

To bridge this significant knowledge gap, we carried out

extensive first-principles Born-Oppenheimer Molecular

Dynamics (BOMD) simulations on both perfluorooctanoic acid

(PFOA) and perfluorooctanesulfonic acid (PFOS) molecules (the

most common PFAS contaminants in the environment) in the

presence of explicit water molecules. While we find these

pollutants to be stable in neutral aqueous environments, our

results indicate that both of these molecules readily

defluorinate at ultrafast timescales (< 100 fs) in the presence of

excess electrons. By considering a variety of different initial

conditions, we unambiguously show that defluorination in

PFASs often leads to the formation of an intermediate with an

alkene type C=C bond, which is crucial in the degradation of

PFASs. In addition, we also show that the formation of an HF

Department of Chemical & Environmental Engineering, Materials Science & Engineering Program, and Department of Physics & Astronomy. University of California, Riverside, Riverside, CA 92521, United States. E-mail:

bryan.wong@ucr.edu; Web: http://www.bmwong-group.com † Electronic Supplementary Information (ESI) available: Computational details and supporting data. See DOI: 10.1039/x0xx00000x Self-interaction-corrected density functional theory calculations were supported by the U.S. Department of Energy, Office of Science, Early Career Research Program under Award No. DE-SC0016269. Born-Oppenheimer Molecular Dynamics simulations of PFAS contaminants were supported by the National Science Foundation under Grant No. CHE-1808242.

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molecule is a possible outcome of the BOMD simulations. As

such, this work provides detailed mechanistic insight and

presents the first real-time picture of degradation dynamics of

PFASs in the presence of excess electrons.

Inspired by recent works demonstrating that reduction

processes (i.e., the addition of electrons) can degrade PFASs,20–

22 we studied the degradation dynamics of both PFOA and PFOS

in the presence of excess electrons. To avoid any spurious self-

interaction effects, which are usually observed with semi-local

functionals such as PBE, we have included self-interaction

corrections in all our MD simulations (further details are given

in the Supplementary Information). To mimic the water

environment, we solvated each of these PFASs with 43 explicit

water molecules. Due to this explicit solvent environment, both

PFOA and PFOS lose their acidic proton to water molecules (as

they should) during the NVT equilibration. In figure 1, we

present the geometries of PFOA and PFOS molecules (top and

bottom panels, respectively) after a 1-ps-long NVE simulation

carried out with a different number of excess electrons. In the

left panel, we show our results with no additional charge. As

expected, both PFOA and PFOS were stable (i.e., no bond

dissociation) during the entire simulation time. In contrast, for

simulations containing excess electrons, we observed an

apparent degradation of these molecules. In the middle and

right panels of figure 1, we show our results with -1 and -2

electronic charges, respectively. Both PFOA and PFOS were

defluorinated in the presence of excess electrons, and we find

that the number of dissociated C-F bonds is proportional to the

number of additional electrons in the simulation box. We

verified the robustness of these results by simulating six

different initial conditions (i.e., positions and velocities) for both

PFOA and PFOS (i.e., an additional 12 NVE simulations), and

these results are shown in figures S1 and S2 in the

Supplementary Information. The initial conditions for all these

additional runs were obtained from a 10-ps-long NVT

equilibration run, and a C-F bond dissociation was observed in

all these simulations. Collectively, these results demonstrate

that “excess electrons” can dissociate the crucial C-F bonds in

perfluoroalkyl substances to facilitate their degradation.

Next, we examined the defluorination timescales in both

PFOA and PFOS molecules with excess electrons. The initial

conditions for all our NVE runs were taken from NVT

calculations of neutral systems, with excess electrons

subsequently added to simulate the dynamics of

electrostatically charged environments. That is, the first frame

of all the NVE runs correspond to the neutral (i.e., uncharged)

geometries. For each of the dissociated C-F bonds shown in

figure 1, we studied the dissociation dynamics as a function of

time, which are presented in figure 2. Clearly, at the beginning

of all these simulations, the C-F bond distance is slightly above

1.3 Å, which is approximately the equilibrium C-F bond distance

in neutral PFASs. At the end of these simulations, the bond

distance increased to 3 Å, suggesting a complete dissociation of

these C-F bonds in the presence of excess electrons. However,

interestingly, in all these simulations, the C-F bond dissociation

occurred within the first 100 fs (see the insets of different

panels in figure 2). Here, it is essential to note that the

dissociated C-F bonds in the PFOA and PFOS molecules are

different, and even the dissociated C-F bonds in PFOA for the -1

and -2 electronic charges also differ (note the legends and top

panel of figure 2 for the various dissociated C-F bonds). Despite

these variations, we consistently observed an ultrafast

Figure 1: Geometries of hydrated PFOA (top panel) and PFOS (bottom panel) molecules after a 1 ps long NVE run with 0 (left), 1 (middle), and 2 (right) excess electrons. For both molecules, we observe defluorination in the presence of excess electrons. We verified the robustness of these results by simulating six different initial conditions for both PFOA and PFOS (i.e., an additional 12 NVE simulations), which are shown in figures S1 and S2 in the Supplementary Information.

Figure 2: Bond distance profiles of the dissociated C-F bonds for the structures shown in Figure 1. Results with -1 (middle panel) and -2 (bottom panel) electronic charges are presented for both PFOA (top left) and PFOS (top right) molecules. The insets in each panel show the variation in the C-F bond distances during the first 100 fs (= 0.1 ps) of the simulations. In all our simulations, we observed the dissociation of C-F bonds in less than 100 fs (i.e., an ultrafast dissociation). The legends in the figure designate the dissociated C-F bonds, and the corresponding atom numbering scheme is shown in the top panel. The structures in this panel were obtained after a 1 ps NVE simulation containing a -2 electronic charge (water molecules were omitted for clarity).

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degradation (i.e., < 100 fs) of PFASs in all these simulations.

Furthermore, we verified this ultrafast dissociation of C-F bonds

in all the other 12 NVE simulations (see the Supplementary

Information). Thus, we note that excess electrons not only

facilitate C-F bond dissociation in PFASs, but also initiate this

process on an ultrafast timescale.

In figure 3, we show the evolution of the spin-density as a

function of time for both PFOA and PFOS with -1 and -2

electronic charges. We note that the LUMO of neutral hydrated

PFOA and PFOS is localized entirely on the PFAS molecule (i.e.,

not on water); as such, the addition of an electron to this

composite system is expected to fill this unoccupied orbital on

PFAS. Indeed, as shown in the top panel of figure 3, the

additional charge (more precisely, spin) initially delocalizes on

the entire PFOA molecule; however, as the simulation proceeds,

the spin slowly accumulates at the site of dissociation. We also

obtained similar results with the PFOS molecule (see figure S3)

as well as in four additional NVE calculations performed with a

-1 electronic charge (not shown). These four calculations are

different from the 12 NVE calculations presented in the

Supplementary Information.

In a previous study on excess electrons,20 Su and co-workers

were unable to assign the initial site of the degradation process

through their electronic structure calculations. In all our

simulations, we also find that the site of dissociation is arbitrary.

However, our spin-density results provide additional reasoning

for Su and co-workers’ observation. First, since the excess

electron initially delocalizes over the entire PFAS molecule, all

the C-F bonds are weakened and have an almost equal

probability for dissociation. The specific C-F bond that

dissociates during the simulation is thus arbitrary and is most

possibly triggered by interactions between the contaminant

and the solvent (here, PFAS and water). Next, although we

mentioned that the site of dissociation is random, we never

observed the dissociation of fluorine atoms attached to the

terminal (primary) carbon atoms in any of our sixteen NVE

simulations. This facile dissociation of secondary C-F bonds is

due to their lower bond dissociation energy compared to the

primary C-F bonds, as noted in a few earlier studies.23,24

Contrary to the addition of a single electron, we find that the

spin-density can evolve in two different ways with the addition

of multiple electrons, as shown in the middle and bottom panels

of figure 3. We refer to these different regimes as a trans-type

(middle panel) or cis-type elimination (bottom panel) based on

how the fluorine atoms have dissociated from the PFAS

backbone structure. In both cases, initially, the spin-density of

each additional electron is localized at different parts of the

PFAS (i.e., either towards the head or tail group), introducing a

net spin-polarization in these compounds. However, as the

simulation proceeds, this spin-polarization is either retained

(cis-type) or completely suppressed (trans-type) depending on

whether the dissociated fluorine atoms were bonded to

adjacent carbon atoms or otherwise. As further demonstrated

below, the spin-polarization quickly (< 100 fs) decays when

fluorine atoms dissociate from adjacent carbon atoms, due to

the formation of a double bond between these carbon atoms

(in other cases, the spin polarization is retained).

In figure 4, we present dynamical plots of atoms exhibiting

maximum changes in the Mulliken charge and spin during the

initial stages of the simulations (i.e., during the C-F bond

dissociation). Primarily, the charge profiles of both PFOA and

PFOS with 2 excess electrons (figures 4a and 4c) show that the

fluorine atoms captured these excess charges during the

Figure 3: Evolution of spin-density as a function of time for both PFOA and PFOS with excess electrons. PFOA with a -1 (-2) electronic charge is shown in the top (middle) panel, and PFOS with a -2 charge is shown in the bottom panel. The spin density of PFOS with a -1 electronic charge (not shown for brevity) is similar to the top PFOA panel (see figure S3). An iso-value of 0.001, 0.00001, and 0.001 eÅ-3 was used for the top, middle, and bottom panels, respectively.

Figure 4: Atoms exhibiting the largest changes in Mulliken charges (A, C) and spin (B, D) during the initial stages of the simulation (i.e., during C-F bond dissociation). The left (right) panel depicts results for a PFOA (PFOS) molecule with two excess electrons. In both cases, after the C-F bond dissociation, negative charges were localized on the fluorine atoms. For only the cis-type elimination (see text for details), spins were localized on the carbon atoms (panel D). In the inset of panel A (B), the charge (spin) on the dissociated fluorine atoms are shown until 500 fs. In the inset of panel D, changes in the spin during the first 10 fs of the simulation are depicted. The atom numbering scheme is presented in the top panel.

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simulation. As such, there are no significant differences in the

charge profiles for both cis and trans-type eliminations. Even in

the simulations with a -1 electronic charge, we find that the

excess charge is primarily localized on the fluorine atoms (see

figure S4). We note that in simulations with a -1 electronic

charge (see figure S4), the carbon atoms retain the electronic

spin. In contrast to the charge profiles, the difference between

the cis and trans-type eliminations manifests strongly in the

spin profiles. In the cis-type elimination (figure 4d), opposite

spins were localized on different carbon atoms (C1 and C18); in

contrast, for the trans-type elimination, no atom is spin-

polarized at the end of the simulation, which leads to the

formation of a C=C bond along the PFAS backbone. Although the

total residual spin is zero in both cis and trans-type eliminations,

the cis-type elimination produces an antiferromagnetic

intermediate, and a trans-type elimination results in a non-

magnetic intermediate. This behaviour was confirmed in all the

16 NVE simulations performed with 2 excess electrons.

Since we observed a trans-type elimination in most of our

simulations (12 out of 16), we suggest that this intermediate

could be central to the degradation of perfluoroalkyl

substances. A few recent experiments further support this

finding. For example, Liu et al. have shown that fluorine atoms

attached to a C=C bond (i.e., sp2 carbon atoms) dissociate more

readily compared to fluorine atoms attached to a C-C bond (sp3

carbon atoms).23 Since most of our simulations with excess

electrons resulted in a trans-type elimination with the

formation of a C=C bond (which is known to be more susceptible

to defluorination), we infer that this trans-type intermediate

plays a crucial role in PFAS defluorination, which could explain

the PFAS degradation (with excess electrons) observed by Su et

al.20 Apart from the trans-type intermediate, we also found that

an HF molecule was formed during our simulations (see figure

S5 and the Supplementary Information for further details).

In conclusion, we have presented the first real-time,

dynamical study of PFAS degradation with excess electrons to

elucidate defluorination timescales and mechanisms. Using

several large-scale ab initio trajectory calculations (16 different

NVE simulations), we show that excess electrons enable the

dissociation of the strongest C-F bonds in these contaminants,

which occurs on an ultrafast timescale (< 100 fs). Moreover,

these large-scale simulations provide new mechanistic insight

into PFAS degradation processes that manifest themselves

strongly as spin and charge fluctuations as a function of time. In

particular, we have shown that a trans-type C-F elimination

(which leads to the formation of a C=C bond along the PFAS

backbone) is more probable compared to a cis-type elimination.

Since fluorine atoms attached to a C=C bond are more readily

dissociated, this trans-type intermediate could play a central

role in accelerating further PFAS defluorination processes. In

addition, our dynamics simulations also show the spontaneous

formation of an HF molecule, which could be another significant

intermediate/product of the PFAS degradation process. Taken

together, these first-principles-based molecular dynamics show

that (1) the use of excess electrons can be a viable approach for

breaking the strongest C-F bonds in PFAS contaminants, and (2)

the application of dynamical computational techniques can

shed essential mechanistic insight (such as degradation

timescales and short-lived intermediates) that cannot be readily

gleaned from static electronic structure studies of PFAS or other

aqueous contaminants.

Conflicts of interest

There are no conflicts to declare.

Notes and references

1 D. O’Hagan, Chem. Soc. Rev., 2008, 37, 308–319.

2 E. M. Sunderland, X. C. Hu, C. Dassuncao, A. K. Tokranov, C. C. Wagner and J. G. Allen,

J. Expo. Sci. Environ. Epidemiol., 2019, 29, 131–147.

3 G. Goldenman, M. Fernandes, M. Holland, T. Tugran, A. Nordin, C. Schoumacher and

A. McNeill, The cost of inaction: A socioeconomic analysis of environmental and health

impacts linked to exposure to PFAS, Nordic Council of Ministers, 2019.

4 https://pfas-1.itrcweb.org.

5 P. Grandjean and R. Clapp, New Solut., 2015, 25, 147–163.

6 E. Kissa, Fluorinated Surfactants and Repellents, Second Edition, CRC Press, 2001.

7 B. Ameduri and H. Sawada, Fluorinated Polymers: Volume 2: Applications, Royal

Society of Chemistry, 2016.

8 M. Kotthoff, J. Müller, H. Jürling, M. Schlummer and D. Fiedler, Environ. Sci. Pollut.

Res. Int., 2015, 22, 14546–14559.

9 R. Bossi, M. Dam and F. F. Rigét, Chemosphere, 2015, 129, 164–169.

10 A. Cordner, V. Y. De La Rosa, L. A. Schaider, R. A. Rudel, L. Richter and P. Brown, J.

Expo. Sci. Environ. Epidemiol., 2019, 29, 157–171.

11 J. S. Boone, C. Vigo, T. Boone, C. Byrne, J. Ferrario, R. Benson, J. Donohue, J. E.

Simmons, D. W. Kolpin, E. T. Furlong and S. T. Glassmeyer, Sci. Total Environ., 2019,

653, 359–369.

12 K. Kannan, Environ. Chem., 2011, 8, 333.

13 K. Kannan, S. Corsolini, J. Falandysz, G. Fillmann, K. S. Kumar, B. G. Loganathan, M. A.

Mohd, J. Olivero, N. Van Wouwe, J. H. Yang and K. M. Aldoust, Environ. Sci. Technol.,

2004, 38, 4489–4495.

14 L. W. Y. Yeung, M. K. So, G. Jiang, S. Taniyasu, N. Yamashita, M. Song, Y. Wu, J. Li, J. P.

Giesy, K. S. Guruge and P. K. S. Lam, Environ. Sci. Technol., 2006, 40, 715–720.

15 J. S.-C. Liou, B. Szostek, C. M. DeRito and E. L. Madsen, Chemosphere, 2010, 80, 176–

183.

16 N. B. Saleh, A. Khalid, Y. Tian, C. Ayres, I. V Sabaraya, J. Pietari, D. Hanigan, I.

Chowdhury and O. G. Apul, Environ. Sci. Water Res. Technol., 2019, 5, 198–208.

17 K. H. Kucharzyk, R. Darlington, M. Benotti, R. Deeb and E. Hawley, Novel treatment

technologies for PFAS compounds: A critical review, Academic Press, 2017, vol. 204.

18 National Toxicology Program, , DOI:10.22427/ntp-tox-97.

19 National Toxicology Program, , DOI:10.22427/ntp-tox-96.

20 Y. Su, U. Rao, C. M. Khor, M. G. Jensen, L. M. Teesch, B. M. Wong, D. M. Cwiertny and

D. Jassby, ACS Appl. Mater. Interfaces, 2019, 11, 33913–33922.

21 S. Huang and P. R. Jaffé, Environ. Sci. Technol., 2019, 53, 11410–11419.

22 R. K. Singh, S. Fernando, S. F. Baygi, N. Multari, S. M. Thagard and T. M. Holsen,

Environ. Sci. Technol., 2019, 53, 2731–2738.

23 J. Liu, D. J. Van Hoomissen, T. Liu, A. Maizel, X. Huo, S. R. Fernández, C. Ren, X. Xiao,

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Environ. Sci. Technol. Lett., 2019, 6, 624–629.

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S1

Supporting Information

Real-Time Degradation Dynamics of Hydrated

Perfluoroalkyl Substances (PFASs) in the Presence of

Excess Electrons

Sharma S.R.K.C. Yamijala, Ravindra Shinde, and Bryan M. Wong*

Department of Chemical & Environmental Engineering, Materials Science & Engineering Program, and

Department of Physics & Astronomy

University of California, Riverside, Riverside, CA 92521, United States

*Corresponding author. E-mail: bryan.wong@ucr.edu. Web: http://www.bmwong-group.com

Contents

p. S2: Figure S1: Final geometries of hydrated PFOA molecules obtained with six different initial BOMD conditions.

p. S3: Figure S2: Final geometries of hydrated PFOS molecules obtained with six different initial BOMD conditions.

p. S4: Figure S3: Evolution of the spin-density in a PFOS molecule with one excess electron during an NVE simulation.

p. S5: Figure S4: Atoms exhibiting the largest changes in Mulliken charges and spin during the initial stages of the simulation (i.e., during C-F bond dissociation).

p. S6: Figure S5: Formation of an HF molecule in the NVE simulations of (a) PFOA and (b) PFOS with a -2 electronic charge.

p. S7: Computational details.

S2

Figure S1. Final geometries of hydrated PFOA molecules obtained with six different initial BOMD conditions (positions and velocities). All NVE simulations were performed with 2 excess electrons, and all simulations were propagated for 150 fs or longer. For each of the different initial conditions, we observe a defluorination in the presence of excess electrons. All of these six initial conditions were obtained from an NVT simulation with a separation of 500-1000 fs between each initial condition.

S3

Figure S2. Final geometries of hydrated PFOS molecules obtained with six different initial BOMD conditions (positions and velocities). All NVE simulations were performed with 2 excess electrons, and all simulations were propagated for 150 fs or longer. For each of the different initial conditions, we observe a defluorination in the presence of excess electrons. All of these six initial conditions were obtained from an NVT simulation with a separation of 500-1000 fs between each initial condition.

S4

Figure S3. Evolution of the spin-density in a hydrated PFOS molecule with one excess electron during an NVE simulation.

S5

Figure S4. Atoms exhibiting the largest changes in Mulliken charges (left) and spin (right) during the initial stages of the simulation (i.e., during C-F bond dissociation). The top (bottom) panel depicts results for a PFOA (PFOS) molecule with one excess electron. In both cases, after the C-F bond dissociation, negative charges (left panels) were localized on the fluorine atoms, and spins (right panels) were localized on the carbon atoms. In the inset of the left panels, we show the atom numbering scheme used in our simulations.

S6

Figure S5. Formation of an HF molecule (bond length ~0.99 Å, circled in pink) in the NVE simulations of (a) PFOA and (b) PFOS with a -2 electronic charge. In both simulations, an H-F bond is formed after 2 ps. In panel (a), the formation of a C=C bond (~1.29 Å, circled in black) can also be seen.

Apart from the trans-type intermediate discussed in the main text, we also found that an HF

molecule was formed during our simulations (see Figure S5). Specifically, one of the dissociated

fluorine atoms from the PFAS molecule combines with a proton from the solvent (which originates

from PFOA/PFOS releasing its carboxylic/sulfonic acid proton into the solvent) to form an HF

molecule. In addition, we observed the formation of an HF molecule in our other NVE simulations as

well; as such, we suggest that an HF molecule could also be a significant intermediate or end product of

the PFAS degradation process.

S7

Computational Details

All of the Born-Oppenheimer Molecular Dynamics (BOMD) simulations were performed in a

microcanonical (NVE) ensemble using the Quick-Step method as implemented in the CP2K software

package.1 The BOMD equations of motion were integrated with a 0.5 fs time step, and initial velocities

and coordinates for all NVE runs were obtained by first running an NVT simulation at 300 K. For the

NVT simulations, we used the Nosé−Hoover thermostat of the chain length three. In all the NVT and

NVE runs, Grimme’s D3 dispersion2 correction was employed. To calculate the electronic energies and

gradients at each nuclear step, we used density functional theory with a self-interaction corrected PBE3

exchange-correlation functional. Specifically, we have included a self-interaction correction (SIC) for

all orbitals in our BOMD simulations using the average density SIC, as implemented in the CP2K

package.4 Following earlier work,5 we tuned the scaling parameters ‘a’ (=0.2) and ‘b’ (=0.25) to

reproduce the vertical electron affinities of PFOA and PFOS obtained with a non-empirically tuned

range-separated LC-BLYP functional (which typically gives energies that match experiment or high-

level wavefunction-based benchmarks6,7). To solve the self-consistent Kohn-Sham equations, we used

the molecularly optimized double-zeta quality (DZVP) basis-sets,8 which are compatible with the

employed Goedecker−Teter−Hutter (GTH) pseudopotentials.9,10 For the auxiliary plane-wave (PW)

basis used in the Gaussian-and-Plane-Waves method of CP2K,11 we used 600 Ry for the PW energy

cutoff and 60 Ry for the reference grid cutoff.

Calculations with Other Exchange-Correlation Functionals

Since prior studies on hydrated electrons have shown that the results are not sensitive to the

choice of semi-local functional (particularly PBE vs. BLYP),12,13 we omitted additional calculations

with the BLYP functional. However, to verify the robustness of our results, we performed additional

calculations with the B3LYP hybrid functional and found qualitatively similar results for some of our

initial studies. Due to the immense computational cost of hybrid functionals, we did not pursue

S8

additional BOMD calculations with the B3LYP functional, but we anticipate the results to be

qualitatively similar to our self-interaction corrected PBE calculations.

Tuning the Scaling Parameters

Following earlier studies,5 we tuned the scaling parameters ‘a’ and ‘b’ to reproduce the vertical

electron affinities of PFOA and PFOS, which we obtained with the non-empirically tuned range-

separated LC-BLYP functional. We found that the values of a = 0.2 and b = 0.25 reproduce these

benchmark electron affinities, which we subsequently used in our calculations. We also performed

additional calculations with a = 0.2 and b = 0.0, as used in prior studies;4 however, even with these

different parameters, we observed PFAS defluorination in the presence of excess charges, indicating the

robustness of our results.

Number of Explicit Water Molecules Considered in the Present Study

To mimic the surrounding water environment, we solvated each of these PFAS species with 43

explicit water molecules that were treated quantum mechanically. It is important to note that earlier

studies have already demonstrated that 31 water molecules are sufficient to reproduce the quantum

mechanical behavior of a hydrated electron.4 Considering the number of MD simulations performed in

this work and the substantial computational demand for each of these ab initio MD calculations, the

present choice of 43 water molecules was optimal for capturing all the necessary PFAS degradation

dynamics without sacrificing accuracy.

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