2.1. Materials

Perfluorooctanoic acid (PFOA, C₈F₁₅HO₂, 95.5%) and perfluorooctane Sulfonate (PFOS, C₈F₁₇SO₃K, 98.0%) were supplied by Dr. Ehrenstorfer (Augsburg, Germany). Ammonium acetate (LC grade, 99%), fluoride standard (99.99%) and methanol (HPLC grade, 99.99%) were provided by Shanghai Anpel scientific instrument corporation. Sodium carbonate (Na₂CO₃), Sodium bicarbonate (NaHCO₃), Hydrogen chloride (HCl), sodium hydroxide (NaOH) and tert-butanol (t-BuOH) were purchased from Sinopharm Chemical Reagent Co., Ltd. All aqueous solutions were prepared by a Millipore Milli-Q system (resistance [18.2 MΩ]).

2.2. The samples and their irradiation

Four initial concentrations of PFOA and PFOS solutions in 100 mL polypropylene vessels were stored at 4°C before use. Samples pouched in high-density polyethylene (HDPE) bags were irradiated to 100–500 kGy at ambient temperatures by 1.8 MeV electron beams of up to 10 mA. The samples were placed about 30 cm away under the beam-scan horn.

2.3. Analytical methods

Concentrations of PFOA, PFOS and their degradation products were determined by liquid chromatography-tandem mass spectrometry (LC-MS/MS) using an Agilent 1260 LC chromatograph coupled to an Agilent 6460 mass spectrometer with electron spray ionization (ESI) interface and a heated nebulizer. HPLC separation was carried out by ZORBAX SB-C18 reversed-phase column (4.6×50 mm, 5μm, Agilent) and EC-C18 (3.0×100 mm, 2.7 μm, Agilent) at 40°C using a gradient composition of solvent A (methanol) and solvent B (water containing 2 mmol/L of ammonium acetate). The injection volume was 5 μL. The gradient (%A) was as follows: 0−0.75 min, a linear increase from 5% to 60%; 0.75−5 min, increase to 92% A; 5−5.1 min, further increase to 100% A; 5.1−8.5 min, being held at 100% A; 8.5−9.0 min, the mobile phase returning to the initial conditions; and 9−10 min, being held at 5% A. The flow rate was 0.4 mL/min. Quantification was performed using multiple reaction monitoring (MRM) of the transitions m/z 499→80.1 (PFOS) and m/z 413→368.9 (PFOA). Mass spectrometry full scanning analysis (m/z 100−550) was used to identify the intermediate products of PFOS and PFOA after irradiation. The instrument conditions were as follows: capillary voltage, 4.0 kV; drying gas flow, 3 L/min; drying gas temperature, 350°C; and source gas flow, 10 L/min; nebulizer pressure, 38 psi and nozzle voltage of 1 kV.

An ion-chromatograph system (Dionex ICS-1100, USA) was used to determine the concentration of F⁻. It consisted of an automatic sample injector (sample injection volume: 25μL), a degasser, a pump, a guard column (Dionex AG22, 4×50 mm, USA), a separation column (Dionex AS22, 4×250 mm, USA), a column oven (30°C), and a conductivity detector with a suppressor device. The mobile phase was composed of 4.5 mmol/L Na₂CO₃ and 1.4 mmol/L NaHCO₃, and the flow rate was set at 1.0 mL/min. The lowest detection limit of F⁻ was 0.02 mg/L.

3.1. Kinetic studies of PFOA and PFOS under electron beam radiolysis

PFOA and PFOS aqueous solutions in initial concentrations of 10–40 mg/L were irradiated to 100–500 kGy. As shown in Fig.1(a), the PFOA and PFOS contents decreased with increasing doses. At initial concentrations of 10, 20, 30 and 40 mg/L, the PFOA degradation rates at 500 kGy were 63.6%, 51.4%, 48.2% and 41.7%, respectively; and the PFOS degradation rates were 57.1%, 42.4%, 35.5% and 32.4%, respectively. The PFOA and PFOS concentration changing rate were plotted in natural logarithm versus the irradiation dose, and each of the data sets could be fitted linearly with R² > 0.90, suggesting that the EB degradation of PFOA and PFOS followed pseudo-first order kinetics at different initial concentrations. Also, as shown in Fig.1(b), certain amounts of fluoride ions were detected at different doses, implying that the EB-degradation of PFOA and PFOS was mainly through the defluorination.

Fig. 1Full-screen View

Degradation kinetics (a) and fluoride ion release (b) of PFOA and PFOS in initial contents of 10–40 mg/L irradiated to 100–500 kGy

3.2. Effect of initial pH on PFOA and PFOS degradation

Initial pH is an important factor in the degradation processes, as hydroxyl radicals, hydrated electron and hydrogen radical may react with OH⁻ and H⁺ in aqueous solution. PFOA and PFOS aqueous solutions (20 mg/L) in initial pH 3–13 were irradiated to 100–500 kGy. The degradation (Fig. 2a) and the fluoride release (Fig. 2b) increased with the pH value. At pH13 and 500 kGy, the degradation rates of PFOA and PFOS at were 88.1% and 63.4%, respectively; and the concentration of fluoride ions increased rapidly to 239.5 μmol/L and 352.6 μmol/L with the corresponding defluorination percentage of 37.5% and 51.8%, respectively. Decomposition of PFOS is more difficult than that of PFOA, as oxidation of sulphonate to sulfate is more difficult. However, the generated fluoride ions concentrations of PFOS were slightly higher than those of PFOA. This can be attributed to hydrogen radical, which can be inverted into hydrated electron in alkaline condition, and the loss of hydrated electron in acidic condition, as shown in Eqs. (2) and (3). Thus, the solutions in alkaline conditions may be helpful to the degradation of PFOA and PFOS.

Fig. 2Full-screen View

Effects of initial pH on the decomposition (a) and fluoride release (b) of EB- irradiated PFOA (20 mg/L) and PFOS (20 mg/L)

3.3. Effect of radical scavengers on PFOA and PFOS degradation

In order to investigate which active species played the leading role in degradation of PFOA and PFOS, the aqueous solutions (20 mg/L) was saturated by N₂, O₂ and 0.1 mol/L tert-butanol with N₂. In N₂-saturated solution, hydroxyl radicals, hydrated electron and hydrogen radical were all existed and should be considered in the reactions process. It is well known that the reducing radicals hydrated electron and hydrogen radical can react with O₂ through Eqs. (4) and (5), and turn into activated radicals O₂^{− •} and HO₂^•. Hydroxyl radicals were the main reactive species in O₂ saturated solution. However, both hydrated electron and hydrogen radical existed in N₂-saturated solution containing 0.1 mol/L tert-butanol, because hydroxyl radicals was scavenged by tert-butanol, as shown in Eq. (6).

Fig. 3 shows the degradation and fluoride release of PFOA and PFOS under the different conditions. At 500 kGy, the degradation rates were higher in N₂-saturated solutions, being 81.6% for PFOA and 71.3% for PFOS, and in N₂-saturated solution containing 0.1 mol/L tert-butanol, being 87.4% for PFOA and 76.9% for PFOS; while the degradation rates were low in O₂-situated solutions, being just 11.0% for PFOA and 8.7% for PFOS. For PFOA, the higher concentrations of fluoride ions were observed in N₂-situated solution (200.3 μmol/L) and N₂-saturated solution containing 0.1mol/L tert-butanol (232.9 μmol/L) at 500 kGy, while it reached 27.5 μmol/L in O₂-saturated solution. Similarly, the formation of fluoride ions of PFOS showed the same trends as that of PFOA. Lower degradation rates and fluoride ions concentrations of PFOA and PFOS were observed in O₂-saturated solution indicated that hydroxyl radicals was not favorable for degrading PFOA and PFOS under electron beam irradiation. Consequently, higher degradation rates and concentrations of fluoride ions occurred in N₂-saturated solution and N₂-saturated solution containing 0.1 mol/L tert-butanol. Due to the strong reductive property and the relatively higher concentration of hydrated electron ^[41-43], we considered that hydrated electron, and hydrogen radical especially, played key roles in electron beam degradation of PFOA and PFOS.

Fig. 3Full-screen View

Effects of scavenger on the decomposition (a) and fluoride release (b) of EB-irradiated PFOA (20 mg/L) and PFOS (20 mg/L)

Furthermore, the quantum yield of hydrated electron increases greatly in alkaline solutions as the transformation of hydrogen atoms into hydrated electrons, i.e. Eq. (2). As the results show, the PFOA and PFOS degradation rates and concentrations of fluoride ions are higher in alkaline condition. Thus, the effect of pH on PFOA and PFOS decomposition further implies that hydrated electrons play the key role in the decomposition of PFOA and PFOS.

In order to investigate the optimal EB degradation condition of PFOA and PFOS, the aqueous solutions (20 mg/L) were saturated by N₂ and adjusted to pH 13. The results are shown in Fig. 3. At 500 kGy, the degradation rates of PFOA and PFOS in nitrogen atmosphere were 95.7% and 85.9%, and their fluoride ion concentrations increased rapidly to 324.6 and 416.9 μmol/L, respectively, indicating that PFOA and PFOS in an anoxic alkaline solution are efficiently decomposed by EB irradiation.

3.4. Mechanism of PFOA and PFOS degradation by irradiation

PFOA, PFOS and their degradation products in the aqueous solutions were identified using LC-MS/MS method. The results are shown in Fig. 4. C₇F₁₄HCOOH, C₆F₁₂HCOOH, C₅F₁₀HCOOH, C₄F₉COOH, C₃F₇COOH, C₂F₅COOH, C₇F₁₄HSO₃⁻, C₆F₁₂HSO₃⁻, C₅F₁₁SO₃⁻, C₃F₇SO₃⁻, C₂F₅SO₃⁻ and other intermediate products appeared after irradiation.

Fig. 4Full-screen View

MS of the intermediate products in PFOA (a) and PFOS (b) degradation by EB-generated e_aq^- radicals in aqueous solution

3.4.1. Mechanism of PFOA degradation by irradiation

Based on these results, a mechanism for PFOA degradation by EB irradiation is proposed in Fig. 5(a). Under nitrogen atmosphere, the hydrated electrons attack PFOA, induce the fluoride elimination and react with hydrogen radical to yield the C₆F₁₃CHFCOOH radical via Eqs. (7−9) ^[45,46]. The electrophilic radicals are attacked by hydrated electrons to cause an intramolecular defluorination and react further with hydrogen radical to yield C₆F₁₃CH₂COOH, via Eqs. (10–12) ^[47,48]. The generated C₆F₁₃CH₂COOH can transform to C₆F₁₃COOH, via Eqs. (13) and (14) ^[46]. Other short-chain perfluorocarboxylic acid such as PFHxA, PFPA and PFBA are formed successively by similar reaction.

Fig. 5Full-screen View

Mechanism for decomposition of (a) PFOA and (b) PFOS by electron beam irradiation

$e_{\text{aq}}^{-}+{\text{C}}_7{\text{F}}_{15}{\text{COO}}^{-}\to {\text{C}}_7{\text{F}}_{15}\text{COO}{\cdot }^{2-},$ (7) ${\text{C}}_7{\text{F}}_{15}\text{COO}{\cdot }^{2-}\to {\text{C}}_6{\text{F}}_{13}\text{C}\cdot {\text{FCOO}}^{-}+{\text{F}}^{-},$ (8) ${\text{C}}_6{\text{F}}_{13}\text{C}\cdot {\text{FCOO}}^{-}+\text{H}\cdot \to {\text{C}}_6{\text{F}}_{13}{\text{CHFCOO}}^{-}$ (9) ${\text{C}}_6{\text{F}}_{13}{\text{CHFCOO}}^{-}+e_{\text{aq}}^{-}\to {\text{C}}_6{\text{F}}_{13}\text{CHFCOO}{\cdot }^{2-},$ (10) ${\text{C}}_6{\text{F}}_{13}\text{CHFCOO}{\cdot }^{2-}\to {\text{C}}_6{\text{F}}_{13}\text{C}\cdot {\text{HCOO}}^{-}+{\text{F}}^{-},$ (11) ${\text{C}}_6{\text{F}}_{13}\text{C}\cdot {\text{HCOO}}^{-}+\text{H}\cdot \to {\text{C}}_6{\text{F}}_{13}{\text{CH}}_2{\text{COO}}^{-},$ (12) ${\text{C}}_6{\text{F}}_{13}{\text{CH}}_2\text{COOH}\to {\text{C}}_6{\text{F}}_{13}\cdot +{}_{\cdot }{}^{\cdot }\text{C}{\text{H}}_2+\text{COOH}\cdot ,$ (13) ${\text{C}}_6{\text{F}}_{13}\cdot +\text{COOH}\cdot \to {\text{C}}_6{\text{F}}_{13}\text{COOH}\cdot$ (14)