Linear perfluoroalkyl carboxylate reduction dynamics with solvated electrons from ferrocyanide and sulfite

Abstract

The initial reduction dynamics of perfluoroalkyl substances (PFASs) by the hydrated electron, e_aq⁻, is a topic of great interest and importance due to the pervasive environmental presence of PFAS in soil and waters and the need to remediate the contamination. Understanding how PFAS behaves in water, including the potential effect of aggregation on the apparent PFAS reduction rate constant (k_PFAS) is, therefore, of paramount importance. In this publication we re-examine the proposition that submicellar aggregation decreases the apparent k_PFAS for sodium perfluorocarboxylates of varying chain lengths (NaPFxA, x = number of carbons in the PFAS backbone, ranging from 4 to 8) using transient absorption spectroscopy. We compare the dynamics for e_aq⁻ quenching by NaPFxA in aqueous solutions of ferrocyanide, Fe(CN)₆⁴⁻, and sulfite, SO₃²⁻. The results demonstrate that the apparent rate constant depends on the choice of e_aq⁻ precursor. We demonstrate that the ionic strength of the solution and the counterion of the PFxA salt both affect the measured rate constant of PFAS reduction by e_aq⁻. The results presented here help to better understand PFAS degradation by advanced reduction processes.

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Environmental significance

Advanced reduction processes (ARP) have proven to be quite effective at the destruction of PFAS and other contaminants in tainted waters. However, the processes underlying the reduction of PFAS by hydrated electrons (formed by the UV-excitation of appropriate ions capable of carrying out ARP, like sulfite) are still poorly understood; particularly, the early processes related to the initial reduction of PFAS. Until now, studies utilizing fast spectroscopic techniques like nanosecond transient absorption spectroscopy to elucidate the dynamics between the hydrated electron and PFAS have used ferrocyanide instead of sulfite as the electron donor for a variety of reasons with the assumption that the electron precursor has little effect on the reaction between the hydrated electron and PFAS. In this study we demonstrate that this assumption may not hold true and explore ionic strength effects, as well as counterion effects, to try to pinpoint the factors that make the fast reaction occurring in solutions of ferrocyanide and sulfite so vastly different. We argue that perfluoroalkyl carboxylates (PFxA) tend to aggregate in water at concentrations well below the critical micelle concentration. Furthermore, the concentration of sulfite needed to effectively carry out ARP for practical degradation of contaminated waters shifts the PFxA equilibrium towards aggregation. The implication is that the [PFxA] used in typical degradation studies found in the literature are primarily in some aggregated state. Therefore, the degradation models proposed thus far require revision to include the aggregation of PFxA and the role of aggregation in modulating reactivity with hydrated electrons.

Introduction

Groundwater contamination by poly- and perfluoroalkyl substances (PFAS), such as perfluorooctanoic acid (PFOA in the literature, PF8A here), has garnered significant attention due to their potential adverse health effects.¹ Historically, PFASs were used as particularly effective components in firefighting foams,² however their ubiquitous use now extends beyond fire suppression³ for their exceptional thermal and electrochemical stability arising from the strength of the C–F bond (e.g. ∼127 kcal mol⁻¹, 5.5 eV for C₂F₆).⁴ Not coincidentally, decontamination efforts aimed at removing PFAS from the environment have proven challenging due to these same chemical properties.⁵

UV-irradiation of solutions containing sulfite (SO₃²⁻) under alkaline conditions (pH > 8) efficiently degrades PFAS by producing hydrated electrons, e_aq⁻, that are photodetached from SO₃²⁻.^6–8 This process has become the prototype of so-called ‘Advanced Reduction Processes’ (ARP) for high energy reductive decontamination of tainted waters. The large standard reduction potential (−2.9 V)⁹ of the e_aq⁻ is nearly iso-energetic to the potentiometric biases required for the defluorination reaction of many perfluoroalkanes (e.g. and ).^10–14 However, most of the work reporting on the fast reaction dynamics of the reduction of PF8A by e_aq⁻ has been done in solutions of ferrocyanide (Fe(CN)₆⁴⁻) rather than SO₃²⁻.^15–18

Fe(CN)₆⁴⁻ is a well-studied precursor for generating solvated electrons via photodetachment.^19–23 It is an attractive alternative to SO₃²⁻ for modelling the behaviour of e_aq⁻ quenching by PFAS, particularly in the absence of Brønsted acids. Unlike SO₃²⁻ (pK_b = 6.8), Fe(CN)₆⁴⁻ is aprotic at pH > 4.3 and has a 100-fold larger molar extinction coefficient (ε) at the 254 nm excitation wavelength (SO₃²⁻ε_254nm ∼ 50 M⁻¹ cm⁻¹; Fe(CN)₆⁴⁻ε_254nm ∼ 5000 M⁻¹ cm⁻¹).^24–28 Ferrocyanide also has a high photodetachment quantum yield, even in the near UV.²³ In particular, the quantum yield of e_aq⁻ formation (Φ_e) from Fe(CN)₆⁴⁻ approaches unity as the excitation wavelength decreases (SO₃²⁻Φ_e ∼ 0.1 at 254 nm excitation, pH 8; Fe(CN)₆⁴⁻Φ_e ∼ 0.65 at 254 nm excitation, pH 8).²⁸

SO₃²⁻ and Fe(CN)₆⁴⁻ are not unique in their ability to undergo photoinduced ionization to produce e_aq⁻. Other photochemically active anions also produce e_aq⁻ and have been used in ARP for the degradation of PFAS. Examples include ethylenediaminetetraacetate (EDTA⁴⁻),²⁹ ionic indole derivatives,³⁰ as well as iodide (I⁻) solutions that produce e_aq⁻ in both the absence and presence of SO₃²⁻.^31–34

We previously reported that the apparent rate constant measured for e_aq⁻ quenching by PF8A can be underestimated at high concentrations (above ∼1 mM) due to surfactant aggregation¹⁸ including micellization above ∼30 mM. Aggregation of PF8A results in a decrease of the monomeric quencher concentration and appears to reduce the apparent e_aq⁻ quenching rate constants.¹⁸

As mentioned above, the majority of transient studies probing PFAS reduction dynamics by the e_aq⁻ have been carried out in solutions of Fe(CN)₆⁴⁻ due to the reasons already outlined. This assumes that the initial reduction of PFAS by e_aq⁻ is independent of the source of the e_aq⁻. However, this assumption omits other factors that could potentially modulate e_aq⁻ reactivity with PFAS. Due to the differences in the photophysical properties of the various electron donors and the solution conditions required to optimize the degradation of contaminants by ARP, it is imperative to characterize the reduction dynamics of PFAS in each case. In this report, we challenge the supposition that the dynamics of ARP are independent of the e_aq⁻ donor, find clear differences between processes carried out in solution of Fe(CN)₆⁴⁻versus SO₃²⁻, and present evidence that suggest that these differences are due to differences in the aggregation state of the PFAS induced by the respective solution conditions. The evidence presented here suggests that these differences are, in part, due to ionic strength effects that affect not only the apparent quenching rate constants resulting from well-known screening affects, but also promote submicellar aggregation of PFAS at low concentrations. In addition to the transient absorption studies used to characterize the quenching rate constants, concentration-dependent UV-visible absorption spectra were measured and provide corroborating evidence of aggregation.

Experimental

All chemicals were used as received without further purification. Sodium sulfite (Na₂SO₃, 98%) was obtained from Alfa Aesar. Potassium ferrocyanide trihydrate was obtained from Sigma Aldrich and Alfa Aesar at different levels of purity (>99.95%, >99.5%, and >98.5%). Sodium perfluorooctanoate (NaPF8A, 95%) was obtained from Strem. Sodium perfluoroheptanoate (NaPF7A, 95%) and sodium perfluoropentanoate (NaPF5A, 95%) were obtained from Matrix Scientific. Sodium perfluorohexanoate (NaPF6A, 95%) and sodium perfluorobutanoate (NaPF4A, 95%) were obtained from Combi-Blocks, Inc.

Sample preparation

Stock solutions of Na₂SO₃ and K₄Fe(CN)₆ were freshly prepared before each analysis. Samples used for transient absorption experiments were prepared in a standard 1 cm pathlength quartz cuvette and consisted of either 10 mM Na₂SO₃ or 40 μM K₄Fe(CN)₆ in 40 mM borate buffer¹⁵ (pH ∼ 10) by the appropriate dilution of the stock solutions. Dissolved O₂ was removed by sparging with N₂ and samples were kept de-aerated by maintaining a dynamic headspace of N₂. Stock solutions of NaPFxA, where x is the number of carbons comprising the perfluoroalkyl backbone, were prepared at a concentration of either 80 mM or 500 mM.

UV-visible absorption

Steady-state UV-visible absorption spectra were obtained using a PerkinElmer Lambda 1050 UV-vis spectrometer. Spectra were collected between 190 nm and 400 nm either in neat water or aqueous 1 M NaCl solutions. No K₄Fe(CN)₆ or Na₂SO₃ were added in these experiments. Aliquots of the NaPFxA stock solutions were added to 2 mL neat water or 1 M NaCl and the absorbance at 230 nm was monitored.

Nanosecond transient absorption

Nanosecond transient absorption (nsTA) measurements were performed on an Edinburgh LP980K where the pump and probe were quasi-collinear. Samples were irradiated with a 254 nm laser pulse (pulse width < 7 ns) using an Ekspla NT340B optical parametric oscillator. Each transient shown represents the average of at least 5 shots. Sample solutions were continuously stirred at 800 rpm using a Quantum Northwest Luma 40 thermostatted cuvette sample holder while maintaining a dynamic headspace of N₂ during the acquisition of the e_aq⁻ lifetimes. All transient data obtained were analyzed using OriginPro software.

Ionic strength correction

The observed decay rates obtained from monoexponential fits of the kinetic traces were corrected for the ionic strength (μ) of the solution,
where c_i is the concentration of ion i and Z_i is the charge of the ion, with each addition of PFAS according to^35–38
where k_obs is the observed e_aq⁻ decay rate, k_μ=0 is the e_aq⁻ decay rate at zero μ, Z_PFAS and Z_e are the charge on the PFAS and e_aq⁻, respectively, and a is a parameter related to the contact distance (r) between the interacting species; if r < 3 Å a = 1, whereas a = 3 for r > 3 Å.^38,39 The contact distance r between PFAS and e_aq⁻ is estimated as the sum of their hydrodynamic radii. The radii have been estimated from their diffusion coefficients, D_i, according to

The diffusion coefficient for e_aq⁻ is reported to be 2.5 × 10⁻⁵ cm² s⁻¹ (r = 1.5 Å),⁴⁰ and determined by us for PF8A to be 4 × 10⁻⁶ cm² s⁻¹ (r = 4 Å) using ¹⁹F DOSY NMR (see ESI, Fig. S1 and S2†). We, therefore, chose a = 3 to correct the observed decay rates (Fig. S3†).

Results and discussion

Irradiation of aqueous solutions containing either Na₂SO₃ or K₄Fe(CN)₆ with a 254 nm, <7 ns pulse produces a positive absorption on the microsecond timescale at 720 nm, which corresponds to the formation and decay of the e_aq⁻.^41–43 In 40 mM borax (pH ∼ 10), the lifetimes obtained for the e_aq⁻ produced from a 10 mM solution of Na₂SO₃ or a 40 μM solution of K₄Fe(CN)₆ in the absence of NaPFxA (τ_o) are 28 ± 6 μs and 11 ± 3 μs (Fig. 1), respectively.

Fig. 1 (A) Transient lifetime decay of the e_aq⁻ in 40 mM borate solutions of 40 μM K₄Fe(CN)₆ in the absence (black) and presence of 160 μM (red), 1.2 mM (blue), 5.6 mM (green), and 10 mM (purple) NaPF8A. (B) Modified Stern–Volmer plot of the e_aq⁻ lifetime rate as a function of the [NaPF8A]. (C) Transient lifetime decay of the e_aq⁻ in a 40 mM borate solution of 10 mM Na₂SO₃ in the absence (black) and presence of 180 μM (red), 1.8 mM (blue), and 5 mM (green) NaPF8A. (D) Modified Stern–Volmer plot of the e_aq⁻ lifetime data presented in panel (C).

e_aq⁻ quenching by NaPF8A in Fe(CN)₆⁴⁻ and SO₃²⁻

NaPF8A was added stepwise to aqueous borate solutions containing either 40 μM Fe(CN)₆⁴⁻ or 10 mM SO₃²⁻ and the transient decays at 720 nm were recorded upon excitation at 254 nm to probe the interaction of e_aq⁻ with NaPF8A. The transients obtained at 720 nm with each addition of NaPF8A were fit to a monoexponential decay function. In the presence of NaPF8A, an increase in the e_aq⁻ decay rate was noted in solutions of both SO₃²⁻ and Fe(CN)₆⁴⁻ (Fig. 1). The apparent bimolecular rate constant, k_NaPF8A, for the initial interaction of NaPF8A and the e_aq⁻ was determined from the decrease in the e_aq⁻ lifetime with increased [NaPF8A]. The rate expression corresponding to the diffusional quenching of e_aq⁻ by NaPF8A monomers resulting in the reduction of NaPF8A according to reaction 1,is:where k_o is the e_aq⁻ decay rate in the absence of NaPF8A, and corresponds to the reciprocal of the lifetime of the e_aq⁻ (k_o = τ_o⁻¹) in the absence of a quencher, and k_NaPF8A is the apparent rate constant for the reduction of NaPF8A according to reaction 1. The data are presented according to the Stern–Volmer formalismwhere τ_o has the same meaning given above and τ_NaPF8A is the lifetime of the e_aq⁻ in the presence of NaPF8A and assumes diffusional oxidative quenching of e_aq⁻ by NaPF8A according to eqn (1).

In Fe(CN)₆⁴⁻, the e_aq⁻ decay rate depends linearly on [NaPF8A] for [NaPF8A] < 1 mM with a k_NaPF8A of ∼3.3 × 10⁸ M⁻¹ s⁻¹. However, the e_aq⁻ decay rate becomes nonlinear for [NaPF8A] > 1 mM. These results are consistent with our previous results,¹⁸ which we attributed to the submicellar aggregation of NaPF8A. We will note that others have reported on the propensity of PF8A to form premicellar aggregates. For example, Sarmiento, et al.^44,45 invoked this process to explain changes in the partial molar volume in aqueous solutions of LiPF8A at submicellar concentrations.

Szajdzinska-Pietek and Gebicki⁴⁶ reported radiolysis data that also exhibited two distinct linear regimes in the dependence of the e_aq⁻ decay rate as a function of the concentration of ammonium perfluorooctanoate, NH₄PF8A, in water at [NH₄PF8A] between 5 mM and 40 mM. In their work, they reported a discontinuity that occurs near the critical micelle concentration (∼30 mM).

We previously developed a kinetic model for the e_aq⁻ decay rate dependence on [NaPF8A]¹⁸ that included e_aq⁻ quenching terms by both NaPF8A monomers according to reaction 4 and aggregates assuming that the only configuration of the aggregates are dimers according to reaction 5.

The fits reported were restricted to [NaPF8A] < 2 mM to isolate the dynamic concentration range in which monomers and dimers are presumed to dominate the population of NaPF8A. Although adequate fits of the data < 1 mM NaPF8A were obtained using the model, it was stressed that the PF8A aggregation is likely complex and involves a wide range of aggregation states. We, therefore, will refrain from invoking the dimerization model here and, instead, report k_NaPF8A based on linear fits over regions in which the concentration dependence is nearly linear (e.g. [NaPF8A] < 1 mM, 1 mM < [NaPF8A] < 10 mM, and [NaPF8A] > 10 mM). From this point on, [NaPF8A] refers to the total concentration of NaPF8A rather than distinguishing between monomers and aggregates; as a result, the reported k_NaPF8A are likely lower than the actual values for the bimolecular rate constants.

A linear fit of the data in 40 mM borate at [NaPF8A] < 1 mM yields a k_NaPF8A = (2.0 ± 0.9) × 10⁸ M⁻¹ s⁻¹, which is approximately a factor of 3 smaller, but on the same order of magnitude as the result previously reported at concentrations below 200 μM in neat water, (7.1 ± 0.6) × 10⁸ M⁻¹ s⁻¹.¹⁷ The reaction conditions here differ from those of our previous reports due to the presence of borate, which buffered the sample solutions at pH 10. Repeating the experiment where aliquots of NaPF8A were added to a solution of Fe(CN)₆⁴⁻ in neat water (Fig. S3B†) yields a k_NaPF8A = (5.5 ± 1.6) × 10⁸ M⁻¹ s⁻¹ at [NaPF8A] < 1 mM, which is in excellent agreement with our previous results.^17,18 The reason for the near 3-fold decrease in k_NaPF8A in borate is not readily apparent and outside the scope of this study.

Linear fits at 1 mM < [NaPF8A] < 10 mM yield a k_NaPF8A of (5.9 ± 2.0) × 10⁷ M⁻¹ s⁻¹, which is in agreement with previous results reported by us¹⁸ and by Szajdzinska-Pietek and Gebicki⁴⁶ (e.g., 5.1 × 10⁷ M⁻¹ s⁻¹) for [NH₄PF8A] < 30 mM, but still about five times greater than the (1.7 ± 0.5) × 10⁷ M⁻¹ s⁻¹ value reported by Huang, et al.¹⁶ Linear fits of the data at [NaPF8A] > 10 mM yield a k_NaPF8A of (2.4 ± 0.7) × 10⁷ M⁻¹ s⁻¹, which is within experimental error of the aforementioned results reported by Huang et al.¹⁶ and in good agreement with the 1.3 × 10⁷ M⁻¹ s⁻¹ value reported by Szajdzinska-Pietek and Gebicki⁴⁶ for [NH₄PF8A] > 30 mM, above its critical micelle concentration, CMC.

The reduction of PFxA by e_aq⁻ is believed to proceed by one of three mechanisms: the associative (reaction 6), concerted dissociative (reaction 7), or stepwise dissociative mechanism (reaction 8). Due to experimental limitations, the dominant mechanism driving PFAS reduction and defluorination has yet to be positively identified. As such, a good deal of theoretical work has been dedicated to disentangling these fast initial reactions.^12,47–51

RCF₂COO⁻ + e_aq⁻ → RCF₂(COO⁻)⁻ (6)

RCF₂COO⁻ + e_aq⁻ → RC˙FCOO⁻ + F⁻ (7)

RCF₂COO⁻ + e_aq⁻ → [R(CF₂)⁻COO⁻] (8a)

[R(CF₂)⁻COO⁻] → RC˙FCOO⁻ + F⁻ (8b)

Daily and Minakata⁴⁸ used density functional theory to calculate the one electron reduction potentials corresponding to reactions 6–8 for PF4A, PF6A, and PF8A among other organic compounds. From the theoretical reduction potentials the authors then calculated the predicted rate constants for reduction and found that for PF8A the associative and concerted dissociative mechanisms are expected to proceed with a rate constant of 5.8 × 10⁸ M⁻¹ s⁻¹ and 1.7 × 10⁷ M⁻¹ s⁻¹, respectively. These predicted values are in excellent agreement with our results in neat aqueous solutions of Fe(CN)₆⁴⁻ at [NaPF8A] < 1 mM and >10 mM, respectively, and in very good agreement with solutions of Fe(CN)₆⁴⁻ in borax (Table 1) in the same concentration ranges.

Table 1 Summary of k_NaPFxA obtained in aqueous solutions of ferrocyanide and sulfite

40 μM Fe(CN)6^4−	k NaPFxA (M^−1 s^−1)			Literature	Ref.
	[NaPFxA] < 1 mM	1 mM < [NaPFxA] < 10 mM	[NaPFxA] > 10 mM	[NaPFxA] > 5 mM
NaPF8A	(2.0 ± 0.9) × 10^8	(5.9 ± 2.0) × 10^7	(2.4 ± 0.7) × 10^7	(1.7 ± 0.5) × 10^7	16
NaPF7A	(7.7 ± 2.0) × 10^7	(2.5 ± 0.4) × 10^7	(1.9 ± 0.4) × 10^7
NaPF6A	(9.8 ± 2.0) × 10^7	(3.3 ± 0.3) × 10^7	(2.4 ± 0.3) × 10^7
NaPF5A	(6.7 ± 1.6) × 10^7	(1.5 ± 0.3) × 10^7	(1.3 ± 0.1) × 10^7
NaPF4A	(1.0 ± 0.2) × 10^7	(3.7 ± 0.2) × 10^7	(2.5 ± 0.3) × 10^7	(7.1 ± 0.3) × 10^6, (1.3 ± 0.1) × 10^7	15 and 16

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10 mM SO3^2−	k NaPFxA (M^−1 s^−1)
NaPF8A	(5.6 ± 0.2) × 10^7
NaPF7A	(2.0 ± 0.1) × 10^7
NaPF6A	(2.9 ± 0.2) × 10^7
NaPF5A	(1.0 ± 0.1) × 10^7
NaPF4A	(2.0 ± 0.2) × 10^7

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We note that a recent study has disputed the nonlinearity observed by us for e_aq⁻ quenching in solutions of Fe(CN)₆⁴⁻ at PFAS concentrations < 1 mM.¹⁵ In order to verify that our observed trend is not due to impurities in our commercially obtained K₄Fe(CN)₆, we performed the same experiment where aliquots of NaPF8A were incrementally added to aqueous solutions (in the absence of borate) of K₄Fe(CN)₆ obtained from two different suppliers and three different degrees of purity. We found the trend and results, particularly at [NaPF8A] < 1 mM, to be virtually indistinguishable between the three data sets (Fig. S3B†).

Interestingly, the interactions between e_aq⁻ and NaPF8A are quite different in 10 mM SO₃²⁻ solutions compared to Fe(CN)₆⁴⁻ over similar [NaPF8A]. Upon addition of NaPF8A to 10 mM SO₃²⁻, negligible e_aq⁻ lifetime quenching was observed below 200 μM. At [NaPF8A] > 200 μM, however, the e_aq⁻ decay rate increases linearly with [NaPF8A] up to 10 mM and has a k_NaPF8A of (5.6 ± 0.2) × 10⁷ M⁻¹ s⁻¹, which correlates well with the values obtained here for the k_NaPF8A in 1 mM < [NaPF8A] < 10 mM solutions of Fe(CN)₆⁴⁻. A nonlinear dependence of the e_aq⁻ decay rate on [NaPF8A] is also observed in solutions of 1 mM KI (Fig. S4†) in neat water; however, the dependence of the e_aq⁻ decay rate on [NaPF8A] becomes increasing linear when the concentration of KI is increased. The k_NaPF8A between 1 mM < [NaPF8A] < 10 mM was also observed to decrease from 5.9 × 10⁷ M⁻¹ s⁻¹ in 1 mM KI to 3.5 × 10⁷ M⁻¹ s⁻¹ in 10 mM KI. Moreover, a significant difference in the k_NaPF8A obtained between 1 mM < [NaPF8A] < 10 mM was noted when comparing results in 10 mM KI and 80 μM indole to 40 μM Fe(CN)₆⁴⁻ and 10 mM SO₃²⁻.

Effect of NaPF8A on the absorption spectrum of e_aq⁻

We recorded the e_aq⁻ spectrum in Fe(CN)₆⁴⁻ and SO₃²⁻ solutions both with and without 4 mM NaPF8A (Fig. 2). The e_aq⁻ spectrum is unaffected by the presence of NaPF8A suggesting that the solvent properties are unperturbed. It is known that the transition energy of e_aq⁻ depends on the solvent and is, therefore, sensitive to the differences in its local environment.^52–54 These differences manifest in a shift of the wavelength of the e_aq⁻ absorption maximum. For example, excitation of aqueous Triton tx-165 at concentrations above its CMC results in the formation of two distinct populations of e_aq⁻ evidenced by the presence of two absorption maxima located at 720 nm and 630 nm that respectively correspond to e_aq⁻ in water and in the less polar micellar interior.⁵⁴ Here, the e_aq⁻ displays an absorption maximum at 719 nm and 717 nm in solutions of Fe(CN)₆⁴⁻ and SO₃²⁻, respectively, in the absence of NaPF8A. In the presence of 4 mM NaPF8A, no shifts were observed for the e_aq⁻ absorption maxima in solutions of Fe(CN)₆⁴⁻ and SO₃²⁻ suggesting that perturbations to the physicochemical properties of the solvent are negligible up to 4 mM NaPF8A.

Fig. 2 Transient absorption spectra of the solvated electron at 200 ns after excitation in the absence (black) and presence (red) of 4 mM NaPF8A in 40 mM borate solutions of (A) 40 μM Fe(CN)₆⁴⁻ and (B) 10 mM SO₃²⁻.

Salt effect on the NaPF8A reduction rate constant

Increased salt concentration and ionic strength are known to promote aggregation and lower CMC values.^55,56 To determine the effect of ionic strength on the measured rates constants, we incrementally added aliquots of NaPF8A into solutions of Fe(CN)₆⁴⁻ and SO₃²⁻ containing 1 M NaCl (Fig. 3). In solutions of Fe(CN)₆⁴⁻, the presence of 1 M NaCl results in a more pronounced deviation from a linear dependence of the e_aq⁻ decay rate with [NaPF8A] and the k_NaPF8A values at [NaPF8A] < 1 mM and 1 mM < [NaPF8A] < 10 mM are (1.3 ± 0.3) × 10⁸ M⁻¹ s⁻¹ and (4.1 ± 0.5) × 10⁷ M⁻¹ s⁻¹, respectively. The k_NaPF8A obtained at [NaPF8A] < 1 mM in the absence and presence of 1 M NaCl are found to agree within experimental error. However, at 1 mM < [NaPF8A] < 10 mM, addition of NaCl induces a nearly two-fold decrease in the k_NaPF8A. Interestingly, the addition of 1 M NaCl to solutions of SO₃²⁻ resulted in a nearly negligible decrease in k_NaPF8A to (4.2 ± 0.8) × 10⁷ M⁻¹ s⁻¹ (Fig. 3).

Fig. 3 Modified Stern–Volmer plots of the e_aq⁻ lifetime dependence on [NaPF8A] in the absence (black) and presence of (red) 1 M NaCl in 40 mM borate solutions of (A) 40 μM Fe(CN)₆⁴⁻ and (B) 10 mM SO₃²⁻.

There is less of an effect from adding NaCl to the 10 mM SO₃²⁻ because it has a higher initial ionic strength than that of the 40 μM Fe(CN)₆⁴⁻ so the former is less susceptible to additional increases in the ionic strength. Increasing the NaCl concentration promotes NaPF8A aggregation but the effect is reduced in sulfite solution because the ionic strength is already high. This interpretation is corroborated by the trend observed in varying concentrations of KI with NaPF8A (Fig. S4†) in which the non-linearity of the data decreases with increasing [KI] and, as a result, the solution ionic strength (vide supra). The results obtained at high [NaCl] lend further support to this proposal. As noted earlier, the critical micelle concentration is known to decrease with increasing ionic strength.^55,56 Analogously, we believe that the interactions between PFAS monomers increase with increasing ionic strength shifting the equilibrium from monomers to aggregates.

Effect of PF8A counterion on e_aq⁻ quenching

Counterions are known to affect surfactant aggregation.^57,58 For example, Lunkenheimer, et al. demonstrated a decrease in the CMC of various PF8A salts following the series Li⁺ > Na⁺ > K⁺ > NH4⁺ > Rb⁺ > Cs⁺.^57,58 We, therefore, investigated whether there were differences between Na⁺ and NH₄⁺ cations on the e_aq⁻ quenching behavior of PF8A in Fe(CN)₆⁴⁻ and SO₃²⁻ solutions. Results of NH₄PF8A and NaPF8A quenching using Fe(CN)₆⁴⁻ and SO₃²⁻ as precursor are shown in Fig. 4. In Fe(CN)₆⁴⁻ solutions the dependence of the e_aq⁻ decay rate was lower for NaPF8A than for NH₄PF8A at 1 mM < [XPF8A] < 10 mM which is consistent with more aggregation in the former. We found that the k_XPF8A (where X is either Na⁺ or NH₄⁺) was 3–4 times greater for NH₄PF8A, (2.0 ± 0.1) × 10⁸ M⁻¹ s⁻¹ than for NaPF8A, (5.9 ± 0.2) × 10⁷ M⁻¹ s⁻¹. We believe the difference in the k_XPF8A observed between Na⁺ and NH₄⁺ are related to differences in the interaction strength between the carboxylate head group of PF8A and the counter ion. We should note that the radius of hydration differ between Na⁺ and NH₄⁺ which may also contribute to the differences in the observed trends;⁵⁸ however, the nature of the contribution is uncertain at this time. Interestingly, in solutions of Fe(CN)₆⁴⁻ the dependence of the e_aq⁻ decay rate on [NH₄PF8A] displays a larger deviation from linearity in the presence of 1 M NaCl (Fig. S5†) likely due to cation exchange in excess Na⁺. In solutions of SO₃²⁻, we found that the k_XPF8A was similarly larger by a factor of 3–4 for NH₄PF8A, (1.8 ± 0.1) × 10⁸ M⁻¹ s⁻¹ compared to NaPF8A, (5.6 ± 0.2) × 10⁷ M⁻¹ s⁻¹.

Fig. 4 Modified Stern–Volmer plots of the e_aq⁻ lifetime dependence on the [NaPF8A] (black) and [NH₄PF8A] (red) in 40 mM borate solutions of (A) 40 μM Fe(CN)₆⁴⁻ and (B) 10 mM SO₃²⁻.

e_aq⁻ quenching by NaPFxA

In addition to NaPF8A, we also examined NaPFxAs of other chain lengths, where x corresponds to the number of carbons comprising the PFAS backbone (x = 8, 7, 6, 5, and 4). In each case we found a nonlinear dependence of the e_aq⁻ decay rate on [NaPFxA] in Fe(CN)₆⁴⁻ solutions (Fig. 5). Although the data are nonlinear in Fe(CN)₆⁴⁻, suggesting aggregation with increasing [NaPFxA] in all cases, we will direct our attention to the k_NaPFxA at 1 mM < [NaPFxA] < 10 mM for comparison to the data obtained in SO₃²⁻. In both Fe(CN)₆⁴⁻ and SO₃²⁻ a general decreasing trend is observed for the k_NaPFxA in the range 1 mM < [NaPFxA] < 10 mM with decreasing chain length, which is in good agreement with the trend that has been reported in the literature for the initial reduction of PFxA.¹⁶ We will note that although the k_NaPF8A obtained by us in Fe(CN)₆⁴⁻ (6.6 × 10⁷ M⁻¹ s⁻¹) and SO₃²⁻ (5.6 × 10⁷ M⁻¹ s⁻¹) at [NaPF8A] > 1 mM are 3–4 times greater than the 1.7 × 10⁷ M⁻¹ s⁻¹ value reported by Huang, et al.¹⁶ they are in very good agreement with the 5.1 × 10⁷ M⁻¹ s⁻¹ value reported by Szajdzinska-Pietek and Gebicki⁴⁶ in the concentration range of 5 mM < [NH₄PF8A] < 30 mM. We have previously¹⁸ pointed out that the rate constant reported by Huang, et al.¹⁶ is in very good agreement with the 1.3 × 10⁷ M⁻¹ s⁻¹ rate constant reported by Szajdzinska-Pietek and Gebicki⁴⁶ at [NH₄PF8A] above the CMC ∼ 30 mM.

Fig. 5 Modified Stern–Volmer plots of the e_aq⁻ lifetime dependence on the [NaPFxA] in 40 mM borate solutions of (A) 40 μM Fe(CN)₆⁴⁻ and (B) 10 mM SO₃²⁻.

Steady-state absorption spectrum of NaPFxA

A number of experimental techniques have traditionally been used to monitor aggregation in solution, particularly vis-à-vis surfactant aggregation and micelle formation, including surface tensiometry, conductivity, steady-state absorbance and fluorescence spectroscopy. In the case of absorbance and fluorescence spectroscopy, due to the low molar extinction and fluorescence quantum yields of many surfactant molecules, surfactant aggregation is typically estimated with the help of solvatochromic reporters or hydrophobic chromophores that preferentially partition within the hydrophobic core of micelles. However, despite the low molar extinction of many surfactants, we believe that deviations from linearity of the absorbance of the surfactants in water may be observed and would be indicative of aggregation (Fig. S6†).⁵⁹

As a demonstration that surfactant absorption can be used to identify aggregation, steady-state spectra were measured for aqueous solutions with increasing amounts of NaPFxA up to 40 mM. At low concentrations of NaPF8A (<100 μM), the spectrum obtained is relatively featureless at wavelengths > 200 nm (Fig. 6). A shoulder centered at ∼210 nm emerges at ∼100 μM NaPF8A and increases in prominence with increasing [NaPF8A]. At [NaPF8A] > 2 mM, another shoulder is observed centered at ∼255 nm; at much higher concentrations another feature is observed at ∼288 nm. A small deviation from linearity in the steady-state absorption is observed between 5 mM and 40 mM (as shown in the inset of Fig. 6). The first derivative plot of the absorbance at 230 nm with respect to the [NaPF8A] displays an inflection point between 30 mM and 38 mM; this is consistent with the reported CMC values between 33 mM and 36 mM for NaPF8A.^60–66 When adding NH₄PF8A to neat water up to 10 mM, we observed only a feature at 210 nm up to 10 mM.

Fig. 6 (A) Steady-state absorbance of aqueous solutions (in the absence of borate, Fe(CN)₆⁴⁻, and SO₃²⁻) at concentrations of NaPF8A increasing from 40 μM to 40 mM. (B) First derivative of the steady-state absorbance at 230 nm as a function of the concentration of NaPF8A between 5 mM and 40 mM. The dotted red line is the CMC value reported for NaPF8A in ref. 60. (C) Steady-state absorbance at 230 nm as a function of the concentration of NaPF8A (black) and NH₄PF8A (red) up to 10 mM; the inset shows the first-derivative of the same data between 0 mM and 2 mM. (D) Steady-state absorbance at 230 nm as a function of the concentration of NaPF8A (black), NaPF7A (red), NaPF6A (blue), NaPF5A (green), and NaPF4A (purple) up to 10 mM; the inset shows the first-derivative of the same data between 0 mM and 2 mM.

Results similar to those for NaPF8A were observed for other NaPFxAs up to 10 mM. In all cases, a prominent inflection point in the steady-state absorbance data of aqueous NaPFxA solutions was observed at [NaPFxA] < 0.5 mM, well below the respective CMCs, which we attribute to the formation of pre-micellar aggregates (vide infra).^18,44,45 It is interesting that this inflection feature occurs at nearly the same concentration for each NaPFxA when the CMCs are very much chain length dependent and vary in magnitude by a considerable amount suggesting that premicellar aggregation is less influenced by the chain length and dominated by some other property.^60,66 It is also noteworthy that, although absent in the transient absorption data in solutions of SO₃²⁻, deviations from linearity in the observed e_aq⁻ decay rates as a function of [NaPFxA] in solutions of Fe(CN)₆⁴⁻ occur at nearly the same concentrations lending more weight to the argument that the trends observed in the transient absorption data in solutions of Fe(CN)₆⁴⁻ are due to aggregation effects. We believe that the latter is not coincidental and that the physical process inducing the nonlinearity in the steady-state absorbance is the same process that underly the trend in the transient absorption data in solutions of Fe(CN)₆⁴⁻; namely, submicellar aggregation of NaPFxA.

Conclusions

Although the biomolecular rate constants for the quenching of the e_aq⁻ by PFAS were previously assumed to not depend on the identity of the e_aq⁻ photodetachment precursor, in actuality the measurement of the reaction kinetics are complicated by several factors. Here, nanosecond transient absorption spectroscopy was used to measure apparent rate constants, k_NaPFxA, for NaPFxA quenching of the e_aq⁻ with ferrocyanide and sulfite precursors in water. The results herein demonstrated that ionic strength and cation identity effects carry significant ramifications for PFAS reactivity with the e_aq⁻ due to their influence on submicellar surfactant aggregation.

We found that the k_NaPF8A for e_aq⁻ quenching by NaPFxA is not constant over the quencher concentration range for ferrocyanide. Additional studies were carried out to demonstrate the aggregation of NaPFxA, even below the CMC. Ionic strength is known to not only influence the measured rate constants for e_aq⁻ due to electrostatic screening, which we accounted for, but it can also influence aggregation, as shown here. The results of other measurements further corroborate our interpretation that aggregation explains the anomalous, concentration-dependent rate constant values. The e_aq⁻ decay rate was also found to vary when the PFxA counterion was changed in a manner consistent with aggregation. Steady-state UV-visible spectra of NaPFxA without ferrocyanide or sulfite (or borate) deviate significantly from the Beer–Lambert law well below their CMC, which is yet another indication of submicellar aggregation of PFxA. The results here reinforce our previous interpretation that PFxA exhibit strong aggregation tendencies, which should be taken into account to get accurate rate constants when measuring solvated electron quenching kinetics.

Author contributions

WAM: conceptualization, methodology, investigation, formal analysis, writing – original draft. JAR: investigation, formal analysis, writing – review & editing. BLC: investigation, formal analysis, writing – review & editing. AE: supervision, writing – review & editing. JCO: supervision, writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This material is based upon work supported by the Office of Naval Research through the U.S. Naval Research Laboratory (N0001422WX00033).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2279639 contains the supplementary crystallographic data for the crystal structure of NaPF8A in Fig. S7 in the supplemental. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3va00223c

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