Photochemical decomposition of perfluorooctanoic acids in aqueous carbonate solution with UV irradiation

Chemical Engineering Journal 221 (2013) 258–263

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Photochemical decomposition of perfluorooctanoic acids in aqueous carbonate solution with UV irradiation Lan-Anh Phan Thi a, Huu-Tuan Do b, Yu-Chi Lee c, Shang-Lien Lo a,⇑ a

Graduate Institute of Environmental Engineering, National Taiwan University, 71 Chou-Shan Rd., Taipei, Taiwan, ROC Faculty of Environmental Sciences, College of Science, Vietnam National University, 334 Nguyen Trai Street, Thanh Xuan Dist., Ha Noi, Viet Nam c Taipei Water Department, 131 Chang-Xing Street, Taipei, Taiwan, ROC b

h i g h l i g h t s 

" Using CO3 under 254 nm UV emission, 400 W completely decomposed PFOA after 12 h. " After 12 h irradiation in H2O2/NaHCO3, 100% of PFOA decomposition has been observed. 

" Decomposing PFOA with CO3 is more favorable in a slightly alkaline solution. 

" The intermediates include PFCAs with shorter carbon chains and F .

a r t i c l e

i n f o

Article history: Received 22 October 2012 Received in revised form 26 January 2013 Accepted 28 January 2013 Available online 9 February 2013 Keywords: Perfluorooctane acid PFOA Photocatalysis Carbonate radical anion Photochemical decomposition

a b s t r a c t Perfluorooctanoic acid (PFOA) is a persisted organic pollutant and a common contaminant in wastewater because of its widespread occurrence in the environment and its ability to bioaccumulate. Recent studies indicate that PFOA is toxic and carcinogenic to animals such as rats, fishes, monkeys, and even humans. In this study, PFOA was decomposed in water using carbonate radical anions (CO 3 ) under 254 nm UV irradiation at 400 W. CO 3 is a strong oxidizing and selective radical, however it works efficiently in decomposing PFOA solution. In this study, the results showed that PFOA was decomposed 100% after 12 h by using a combination of UV irradiation and CO 3 , while under only UV irradiation, 52.1% of PFOA was decomposed. In addition, the decomposition of PFOA with CO 3 under UV irradiation was more favorable in a slightly alkaline (pH = 8.8) solution and sodium hydrogen carbonate (NaHCO3) 40 mM. Moreover, the intermediates included the shorter-chain perfluorinated carboxylic acids and fluorine ions. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Perfluorocarboxylic acids (CnF2n+1COOH, PFCAs) are fluorocarbon compounds derived from hydrocarbons by replacing hydrogen atoms with fluorine atoms. PFCAs and their derivatives are a set of anthropogenic fluorinated organic compounds with a wide range of applications [1]. The extremely strong carbon–fluorine bonds in their structure gives the material high resistance, making them important for various commercial and industrial applications such as surface treatment, surfactant, and fire retardant [2]. Perfluorooctanoic acid (PFOA) is an important chemical in this groups and a common contaminant in wastewater because of its widespread occurrence in the environment and its ability to bioaccumulate. In addition, recent studies indicate that these compounds are toxic and carcinogenic to animals such as rats, fishes, monkeys, and even

⇑ Corresponding author. Tel.: +886 2 2362 5373; fax: +886 2 2392 8821. E-mail address: [email protected] (S.-L. Lo). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.01.084

humans [3]. The compounds are listed as persisted organic pollutants (POPs) recently [4]. Many PFOA treatment techniques have recently been developed. These techniques are either photo-based approaches, such as photolysis, photochemical reaction, and photocatalysis [5–9], sonochemical treatments [10], microwave-hydrothermal treatments, [11,12] electrochemical treatment [13] or hybrid of electrolysis and photocatalysis [14]. The key factor in these treatment reactions is that one-electron oxidants such as persulfate [6,11,15] or photocatalysts such as heteropolyacid [5], TiO2 [7,9] or periodate [8] have been used to decompose PFCAs hydrothermally, under UV irradiation, or by sonification. The electron transfer from PFCAs to the excited species or free radicals is a key point in the decomposition of PFCAs; this electron transfer also facilitates decarboxylation. PFOA also was cleaved the C–C bond between the C7F15 and COOH in PFOA by direct electrochemical oxidation or photolysis to generate a C7F15 radicals and CO2 [13,14].

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The carbonate radical anion (CO 3 ) is a secondary radical produced by the one-electron oxidation of carbonate or bicarbonate ions in an aqueous solution. It is a powerful oxidant with one-electron reduction potentials of 1.59 and 1.78 V at pHs of 12.5 and 7, respectively, [16] and a pKa < 0 [17]. The oxidation of carbonate or bicarbonate anions by the hydroxyl radical proceeds according to the following reactions [18]:   OH þ CO2 3 ! OH þ CO3

ð1Þ

OH þ HCO3 ! H2 O þ CO 3

ð2Þ

It is strongly electrophilic towards electron-rich compounds such as anilines, phenols, and sulfur-containing compounds [19]. It is hypothesized that carbonate radicals are important in reducing the persistence of chemical pollutants, especially electron-rich compounds, in natural waters. Carbonate radical anions play an important role in reacting with electron-rich compounds, such as aromatic compounds, and sulfur containing compounds by electron transfer [20]. Pétrier et al. [21] suggested that carbonate radical anion could migrate towards the bulk of the solution and induce the decomposition of the pollutants. In addition, PFCAs molecules have a hydrophobic group (perfluoroalkyl group) and a hydrophilic group (acid group) that behave like anionic surfactants in an aqueous solution. They are nonvolatile molecules, so the reaction in the cavitation bubbles could be excluded. Furthermore, it is suggested that PFOA cannot be decomposed by reactions with OH radicals based on preliminary experiments with Fenton reactions [22]. Therefore, it is possible that PFOA molecules can be  decomposed by CO 3 under HO /UV irradiation via the electron transfer mechanism. In this study, we attempted to decompose PFOA using carbonate radicals formed by reacting carbonate anions with HO under UV irradiation. The using CO 3 radical, which is available in sunlit water [20], can contribute to an oxidant in PFOA and other organic compounds oxidation processes. The objectives of this study are: (1) to investigate the viability of using carbonate radical anions to decompose PFOA; (2) to compare the efficiencies of decomposition and defluorination of PFOA in an aqueous solution under various decomposition conditions, such as different initial pH values, concentrations, or reaction time; and (3) to investigate the mechanisms of PFOA decomposition when using carbonate radical anions. 2. Materials and methods 2.1. Materials The perfluorooctanoic acid (PFOA, C7F15COOH, 96% purity) was from Aldrich, whereas the perfluoroheptanoic acid (PFHpA, C6F13 COOH, 98% purity), perfluoropentanoic acid (PFPeA, C4F9COOH, 97% purity), and perflurobutyric acid (PFBA, C3F7COOH, 99% purity) were from Alfa Aesar. The perfluorohexanoic acid (PFHxA, C5F11 COOH, 97% purity) and perfluoropropionic acid (PFPrA, C2F5COOH, 97% purity) were from Fluka, and the trifluoroacetic acid (TFA, CF3 COOH) was from Riedel–deHaen. The sodium carbonate (Na2CO3, 100% purity) was from Nacalai Tesque, the sodium bicarbonate (NaHCO3, 99.6% purity) was from J.T. Baker, and the hydrogen peroxide (H2O2, 30%) was from Wako. The fluorine standard was purchased from High-Purity Standards.

experiments; the solution concentration was adjusted accordingly. Thirty percent hydrogen peroxide, which produced hydroxyl radicals under UV light, was prepared by diluting 100 times before used. Hydroxyl radicals were used to produce carbonate radicals via Eq. (2) in this study. The NaHCO3 solution was prepared at a concentration of 0.1 N and stored at 4 °C. The polypropylene equipment used for every step in the experiment procedures will prevent instrumental analysis from PTFE coating. The photochemical experimental setup is shown in Fig. 1. A 2 L closed double-layered glass reactor with a low-pressure mercury vapor quartz lamp (254 nm, 400 W, Philips, Holland) was placed in a closed cabinet. The light intensity was 120000 lux at the outer surface of the quartz tube measured with a LX-101 LUX Meter. The reaction temperature was controlled at 25 °C by a circulating water bath (B204, Firstek Scientific Co. Ltd., Taiwan). The H2O2 and NaHCO3 solutions were mixed well with PFOA solution by a magnetic stirrer. The samples were removed at various intervals and filtered by Millipore syringe filters with a 0.22 lm pore size (Millipore, Ireland) to analyze the concentration of PFOA and other byproducts.

2.3. Sample analysis The concentrations of PFOA and the products from its decomposition were measured using high-performance liquid chromatography (HPLC) (Dionex, UltiMate 3000, USA) equipped with a conductivity detector (ED-50, Dionex, USA) and an anion selfregenerating suppressor (ASRS 300 2-mm, USA). The PFOA and other PFCAs (C3–C8) were extracted by a 150  2.1 mm, 3.5 lm column (AcclaimÒ Polar Advantage II, C18, Dionex, USA) maintained at 30 °C. The ternary solution that included 70:30 (v/v) acetonitrile/Milli-Q water (solution A), Milli-Q water (solution B) and 9 mM NaOH and 100 mM H3BO3 in Milli-Q water (solution C) was employed as an eluent at a flow rate of 0.3 mL/min. The gradient was operated at 20% solution A, 40% solution B and 40% solution C for the initial 5 min. For the next 15 min, it was operated at 20–60% solution A, 40–0% solution B and 40% solution C. Finally, solution A was maintained at 60% and solution B at 0% for the last 15–20 min. All calibration curves for PFCAs were linear over the 0.5–50 ppm range. The decomposition ratios of PFOA were calculated according to the following equation:

Degradation ð%Þ ¼

C0  C  100 C0

A stock solution of 1000 ppm PFOA was prepared by diluting the chemicals into the desired volume of Milli-Q water. This solution was stored in a 4 °C refrigerator and used for all following

ð3Þ

where C is the concentration of PFOA (ppm) and C0 is the initial concentration of PFOA (ppm). The other byproducts of PFCA were calculated based on the external calibration curves. An ion-chromatograph system (Dionex, ICS-3000, USA) consisted of an automatic sample injector, a degasser, a pump, a guard column (Ion Pac AS4A guard column, Dionex, USA), a separation column (Ion Pac AS4A analytical column, Dionex, USA), and a conductivity detector with a suppressor device was used to measure the F concentration. The mobile phase was an aqueous solution containing NaHCO3 (1.7 mM) and Na2CO3 (1.8 mM) at a flow rate of 2 mL/min. The defluorination ratios were calculated according to the following equation:

Defluorination ð%Þ ¼ 2.2. Experimental procedures

259

CF  100 C 0  SF 

ð4Þ

where C F  is the concentration of the fluoride ions (ppm), C0 is the initial concentration of PFOA (ppm) and SF  is the stoichiometric factor of the fluoride ions (15 for PFOA).

260

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Fig. 1. The photochemical experiment setup schematic.

3. Results and discussion 3.1. Decomposition of PFOA The adsorption of PFOA in the glass reactor was eliminated by conducting controlled experiments at 25 °C for 2 days. The amount of PFOA decreased to 3.2% and 4.2% in the solution with 50 ppm PFOA and the solution with 50 ppm PFOA, 40 mM NaHCO3, and 0.075% H2O2, respectively. The decomposition efficiencies of using direct photolysis and using UV irradiation on the solution with H2O2 were compared with the efficiency of the solution containing aqueous carbonate radical ions. The decomposition efficiencies under different conditions are shown in Fig. 2. When the solution

100 PFOA/H2O2/UV

PFOA/UV

PFOA/NaHCO3/H2O2/UV

C/C0 (%)

80

60

40

with 50 ppm PFOA was decomposed with direct photolysis under 254 nm irradiation for 12 h, the decomposition efficiency of PFOA reached 52.1%. However, the solution with 0.075% H2O2 had a lower decomposition efficiency of PFOA under direct photolysis, only 32.3%. The results confirmed that hydroxyl radicals in aqueous solutions react poorly with PFOA [5]. Because PFOA are nonvolatile molecules, they were decomposed mainly in the bulk of the solution, where OH radical is barely available [22]. While, CO 3 could migrate towards the bulk of the solution and induce the decomposition of the pollutants [21]. In the solution with hydrogen peroxide, the UV irradiation was applied to hydrogen peroxide and PFOA. Further, photolysis of PFOA can produce electron that reacts with H2O2 and OH radical. This could be a major reason for lower decomposition efficiency in presence of H2O2. Therefore, the amount of PFOA decomposed is lower than that in the solution containing PFOA only. When bicarbonate ions (NaHCO3, 40 mM) were added into the solution with H2O2, the PFOA decomposed 100% under UV irradiation. In this solution, the defluorination efficiency reached 82.3% after 12 h; however, the PFOA solution had a defluorination efficiency of only 38.3%, and 27.9% in photolysis of PFOA in 0.075% H2O2 as shown in Fig. 3. This result implies that NaHCO3 act as an efficient oxidant for PFOA degradation when combined with the effects of H2O2 and UV irradiation. 3.2. Effect of pH

20

0

0

2

4

6

Reaction time (h)

8

10

12

Fig. 2. Comparison on decomposition of PFOA (50 ppm) at different conditions: ( ) photolysis PFOA in H2O2 0.075%; ( ) photolysis PFOA and ( ) photolysis PFOA in H2O2 0.075% and NaHCO3 40 mM.

The initial pH value of the solution with 0.075% H2O2 and 40 mM NaHCO3 was adjusted using 1 N HCl and/or 1 N NaOH to compare to the effect of pH on the decomposition efficiency of PFOA. This work proved the effect of pH of the carbonate radical acid on the decomposition of PFOA. The results of the decomposition efficiency depending on the pH are shown in Fig 4. After 12 h of irradiation, the decomposition efficiencies of PFOA at pH values

261

L.-A. Phan Thi et al. / Chemical Engineering Journal 221 (2013) 258–263 90 PFOA/UV

70

PFOA/H2O2/UV

60

No.

Initial pH

Final pH

Rate constant (h1)

R2

50

1 2 3

11 8.8 4.09

11.3 8.2 3.67

0.076 0.37 0.27

0.99 0.99 0.98

40 30 20 10 0

0

2

4

6

8

10

12

Reaction time (h)

3.3. Effect of bicarbonate ion concentration

Fig. 3. Comparison on defluorination of PFOA (50 ppm) at different conditions: ( ) photolysis of PFOA in H2O2 0.075%; ( ) photolysis of PFOA and ( ) photolysis PFOA in H2O2 0.075% and NaHCO3 40 mM.

Decomposition efficiency (%)

120

Defluorination efficiency (%)

100

Efficiency (%)

solution with a pH of 11, the pH value increased slightly after 12 h. The OH formed through the decay of CO 3 in a highly alkaline solution Eq. (6) results in the increasing pH.

80 60

The effects of bicarbonate ion concentration on the decomposition and defluorination of PFOA were examined by varying the concentration of NaHCO3 from 5 mM to 50 mM. Fig. 5 shows the decomposition and defluorination efficiencies of PFOA at various initial concentrations of NaHCO3. The efficiencies increased with NaHCO3 concentrations from 5 mM to 40 mM. However, no conspicuous additional increase in the PFOA decomposition rate was observed at the highest NaHCO3 concentration; this indicates that a sufficient concentration of bicarbonate ions is present for PFOA decomposition. In addition, a high concentration of bicarbonate ions would result in a solution with high pH (as shown in Table 2), causing the decay of CO 3 . 3.4. Decomposition products of PFOA

40 20 0

4.09

8.8

pH value

11

Fig. 4. Effect of initial pH value on the decomposition of PFOA (50 ppm) after 12 h of UV irradiation and H2O2 (0.075%)/NaHCO3 (40 mM).

of 4.09, 8.8, and 11 were 97%, 100%, and 82.4%, respectively and the defluorination efficiencies were 72.1%, 82.3%, and 65%, respectively. When PFOA is under direct photolysis, an acidic condition was found more favorable for photodecomposition [23]. However, in an environment with carbonate radicals, a slightly basic environment is the best to decompose PFOA. At a pH of 8.8, the pseudo-first-order rate constant for PFOA decomposition is the highest (0.37 h1) compared to 0.076 h1 and 0.27 h1 at pH values of 11 and pH 4.09, respectively. Czapski et al. reported that the rate of formation of CO 3 decreases strongly with decreasing pH, while the radical–radical reaction rates do not change. Therefore, the amount of CO 3 formed at a pH of 4.09 is less than the amount formed at a pH of 8.8. However, the decay of CO 3 increases with the alkalinity, according to the following equations [16,24,25]:  2 CO 3 þ CO3 ! CO2 þ CO4 H2 O

   CO 3 þ CO3 ! 2CO2 þ HO2 þ OH

ð5Þ ð6Þ

Table 1 shows the change in pH values for reactions in different pH conditions. In the solutions with the initial pH of 4.09 and 8.8, the pH value of the solutions decreased slightly. As more anionic fluoride ions were released into the solutions, the pH value of the solutions would decrease during photolysis. However, in the

The decomposition products of PFOA in the H2O2/NaHCO3 (30 mM) solution after 12 h of 254 nm UV irradiation are shown in Fig. 6. The solution with 50 ppm (120 lM) of PFOA released 27.5 ppm (1447 lM) of F ions. If the PFOA was completely mineralized, 15 F ions would form per one molecule of PFOA. However, only 80.4% of defluorination was observed in this experiment for 99.4% of degraded PFOA. After 12 h of irradiation, PFOA did not completely mineralize into F– ions and carbon dioxide. PFHpA, which has shorter carbon chain lengths, was detected after only 0.5 h of irradiation; PFHxA and PFPeA were detected after 1 h, whereas PFBA and PFPrA were detected after 2 h. The amounts of PFCAs detected in irradiated solutions were 20.7 lM (PFHpA), 15.4 lM (PFHxA), 5.9 lM (PFPeA), 5.0 lM (PFBA) and 3.2 lM (PFPrA) after 8 h of irradiation. The highest amounts of PFHpA and PFHxA were detected after 8 and 12 h of irradiation,

Defluorination (12 h)

Decomposition (12 h)

100

Decomposition/Defluorination (%)

Defluorination efficiency (%)

Table 1 The pseudo-first-order constants and pH value differences of PFOA decomposition with H2O2 (0.075%)/NaHCO3 (30 mM) and UV 254 nm irradiation at different initial pH values.

PFOA/NaHCO3/H2O2/UV

80

80

60

40

20

0 5

10

20

30

40

50

NaHCO3 conc. (mM) Fig. 5. Comparison the decomposition and defluorination of PFOA with different NaHCO3 concentrations.

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Table 2 The pseudo-first-order constants and pH value differences of PFOA decomposition with H2O2 (0.075%)/NaHCO3 and UV 254 nm irradiation at different initial NaHCO3 concentrations. No.

NaHCO3 conc. (mM)

Initial pH

Final pH

Rate constant (h1)

R2

1 2 3 4 5 6

5 10 20 30 40 50

8.4 8.58 8.61 8.8 8.96 9.06

8.18 8.21 8.2 8.2 8.3 8.32

0.19 0.223 0.32 0.37 0.4 0.39

0.98 0.99 0.99 0.99 0.98 0.99

act with water to form alcohols (C7F15OH). Unstable perfluorinated alcohols will then undergo HF elimination to form (C6F13COF) [5]. The hydrolysis of acid fluoride forms PFCAs with one less CF2 unit and produces one CO2 molecule and two fluorine ions. The reactions proceed according to the following equations:  2  CO 3 þ C7 F15 COO ! CO3 þ C7 F15 COO

ð11Þ

C7 F15 COO ! CO2 þ C7 F15

ð12Þ

C7 F15 þ H2 O ! C7 F15 OH þ H

ð13Þ

HF

25

C7 F15 OH ! C6 F13 COF

PFHpA PFHxA

Intermediates (µM)

HF

C6 F13 COF þ H2 O ! C6 F13 COOH

PFPeA

20

ð14Þ ð15Þ

PFBA PFPrA

4. Conclusion

15

ð7Þ

Perfluorocarboxylic acids were directly photodegraded by 254 nm, 400 W UV irradiation. However, the decomposition and defluorination efficiencies of PFOA solutions were greatly increased with the addition of carbonate radical reagents compared to solution without the radical reagents. After examining the photochemical decomposition of PFOA in aqueous solutions with a carbonate radical oxidant, it was found that PFOA was degraded to 52.1% after 12 h of direct irradiation. However, PFOA degraded to 95.7% after 8 h with the addition of NaHCO3 and H2O2; it degraded completely after 12 h. The decomposition of PFOA with CO 3 and UV irradiation is more favorable in a slightly alkaline (pH = 8.8) condition because the concentration of CO 3 formed was low in acidic conditions; in highly alkaline conditions, the concentration of CO 3 could decay quickly by second-order reactions. The highest decomposition and defluorination efficiencies of PFOA was observed in the solution with an initial concentration of 40 mM NaHCO3. The PFCAs with shorter carbon chain lengths were detected in irradiated solutions after 0.5 h. PFHpA was detected at 0.5 h, PFHxA and PFPeA were detected at 1 h and PFBA and PFPrA were detected at 2 h. After 8 h, the amount of PFHpA detected decreased; however, the amount of other PFCAs with shorter chains increased with the reaction time. These results demonstrate that PFHpA and other short-chain PFCAs decompose stepwise into shorter-chain compounds, F ions and CO2. Contributing an oxidant in PFOA decomposition process, CO 3 will be investigated its role in wastewater, because in sunlit water, carbonate radical will be formed. Therefore, they can contribute to an oxidant in PFOA and other organic compounds oxidation processes. Applying bicarbonate solution in ultrasonication to decompose PFOA is currently being investigated in our laboratory.

ð8Þ

Acknowledgment

10 5 0

0

5

10 Reaction time (h)

15

20

Fig. 6. Concentration of intermediates formed at various reaction times by degradation of PFOA (50 ppm) under UV 254 nm in H2O2 (0.075%)/NaHCO3 (30 mM) solution.

respectively. The concentration of PFHpA and PFHxA decreased gradually with the reaction time, but the concentration of other PFCAs with shorter chain lengths increased with the reaction time. In Fig. 3, the amount of fluorine ions can be seen to increase with reaction time, demonstrating that PFHpA and other shorter chain PFCAs decompose stepwise into shorter-chain compounds and F ions. 3.5. Decomposition mechanism The pH of experimental solution is high for complete dissociation of PFOA because of the pKa value of PFOA < 0.5 [26]. Therefore PFOA firstly has been photodecomposed under UV irradiation. Unstable radicals including C7 H15 and COOH were formed by UV irradiation via the following equations: hm

C7 F15 COOH ! C7 F15 COOH þ e hm

C7 F15 COOH ! C7 F15 þ COOH

The decomposition of PFOA starts from terminal carboxylic groups, as proposed by other researchers [6]. Under UV irradiation, carbonate radical anions were produced according to Eqs. (8) and (9): hm

H2 O2 ! 2OH OH þ HCO3 ! H2 O þ CO 3

ð9Þ ð10Þ

Other while, PFOA is ionized and existed as an anionic compound (C7F15COO) in solution. As a powerful oxidant, carbonate radical anions oxidize electron-rich compounds [19]. In PFOA solutions, carbonate radicals can oxidize the perfluorinated carboxyl anions to form perfluorinated alkyl radicals (C7 H15 ), which can re-

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