Microwave-hydrothermal decomposition of perfluorooctanoic acid in water by iron-activated persulfate oxidation

water research 44 (2010) 886–892

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Microwave-hydrothermal decomposition of perfluorooctanoic acid in water by iron-activated persulfate oxidation Yu-Chi Lee a, Shang-Lien Lo a,*, Pei-Te Chiueh a, Yau-Hsuan Liou a, Man-Li Chen b a b

Graduate Institute of Environmental Engineering, National Taiwan University, 71, Chou-Shan Rd., Taipei, Taiwan, R.O.C. Taipei Water Department, 131, Chang-Xing Street, Taipei, Taiwan, R.O.C.

article info

abstract

Article history:

The microwave-hydrothermal decomposition of persistent and bioaccumulative per-

Received 9 July 2009

fluorooctanoic acid (PFOA) in aqueous solution using persulfate activated by zero-valent

Received in revised form

iron (ZVI) at 60 and 90  C was examined. The results of laboratory study reveal that when

14 September 2009

PFOA is treated with 5 mM persulfate (PS) and ZVI at 90  C for 2 h, 67.6% of PFOA is

Accepted 23 September 2009

effectively decomposed to form shorter-chain perfluorinated carboxylic acids (PFCAs) and

Available online 9 October 2009

fluoride ions, with 22.5% defluorination efficiency. Introducing ZVI into the PFOA solution with PS addition will lead to synergetic effect that accelerates the PFOA decomposition

Keywords:

rate, and reduces the reaction time. ZVI not only decomposes PFOA, but also releases

Microwave

ferrous ions to lower the activation energy of PS while forming sulfate free radicals at

Perfluorooctanoic acid

a lower reaction temperature. The combined use of ZVI and persulfate will lead to signif-

PFOA

icant savings in energy consumption and reduction of process time. ª 2009 Published by Elsevier Ltd.

Persulfate Zero-valent iron Hydrothermal decomposition

1.

Introduction

Perfluorooctanoic acid (C7F15COOH, PFOA) and its derivatives have been found in wild animal (Giesy and Kannan, 2001; Kannan et al., 2002) and human blood serum (Yeung et al., 2006) that this has caused serious health concerns. PFCAs are extremely stable to chemical and thermal stress so that they are considered almost un-degradable in nature (Key et al., 1997), or by most commonly used treatments such as biological method, ozonation, or Fenton’s oxidation. PFOA and its derivatives are significantly important to the semiconductor industry because of their unique optical characteristics and acid-generating efficiency (Brooke et al., 2004). A method that uses mild conditions is urgently needed to degrade PFOA to * Corresponding author. Tel.: þ886 2 23625373; fax: þ886 2 23928821. E-mail address: [email protected] (S.-L. Lo). 0043-1354/$ – see front matter ª 2009 Published by Elsevier Ltd. doi:10.1016/j.watres.2009.09.055

harmless species due to its environmental persistence, bioaccumulation, and potential toxicity to humans. Results of previous studies indicated that the sonochemical and photochemical degradation processes are effective to decompose PFOAs (Cheng et al., 2008; Hori et al., 2004). Iron (II)/(III) has been reported to enhance the photochemical decomposition of PFCAs under UV–Visible light irradiation (220–460 nm) (Hori et al., 2007; Wang et al., 2008). Persulfate anion is a strong oxidizing agent with a redox potential of 2.0 V, and can be reduced to sulfate anions as shown below:

2  S2O2 8 þ 2e / 2SO4

(1)

water research 44 (2010) 886–892

In the microwave-induced persulfate oxidation, persulfate is activated thermally or chemically to form sulfate free radicals (House, 1962), as shown in Eq. (2). Sulfate radicals exhibit higher redox potential of 2.6 V, and are capable of decomposing most organic contaminants (Huang et al., 2005).

 S2O2 8 þ heat / SO4 

(2)

The decomposition of persulfate anion in the presence of a transitional metal activator (e.g., Fe2þ) leads to the formation of SO4. The overall reaction between persulfate and ferrous ions is depicted by Eq. (3) (House, 1962): 

3þ þ SO4 þ SO2 Fe2þ þ S2O2 8 / Fe 4 

(3)

Zero-valent iron (ZVI; Fe0) is an internal source of ferrous ions through the oxidation reaction of ZVI. Under both aerobic and anaerobic conditions, the initial product of ZVI is Fe2þ in accordance with the following equations (Furukawa et al., 2002):

application of ZVI as a source of ferrous ions to enhance the decomposition efficiency of PFOA with PS in the microwavehydrothermal treatment.

2.

Materials and methods

2.1.

Materials

Perfluorooctanoic acid (PFOA, C7F15COOH, 96%) was from Aldrich. Perfluoroheptanoic acid (PFHpA, C6F13COOH, 98%), perfluoropentanoic acid (PFPeA, C4F9COOH, 97%) and heptaflurobutyric acid (PFBA, C3F7COOH, 99%) were from Alfa Aesar. Undecafluorohexanoic acid (PFHeA, C5F11COOH, 97%), pentafluoropropionic acid (PFPrA, C2F5COOH, 97%) and 1,10-phenanthroline (98%) were purchased from Fluka. Trifluoroacetic acid (TFA, CF3COOH, 98%) and perfluorononanoic acid (PFNA, C8F17COOH, 97%) were purchased from Riedel-deHae¨n. Ferrous sulfate (FeSO4$7H2O) and sodium persulfate (Na2S2O8, 99.0%) were purchased from Aldrich. Zero-valent iron power (Fe0, >97%) was purchased from J.T. Baker.

2.2. Fe0 þ 1/2O2 þ H2O / Fe2þ þ 2OH

(4)

Fe0 þ 2H2O / Fe2þ þ 2 OH þ H2

(5)

The use of ZVI as a source of ferrous ion to activate the persulfate oxidation has been successfully applied for treating trichloroethylene contamination (Liang and Lai, 2007). The hydrothermal treatment usually requires that the temperature be maintained at 100–350  C (subcritical condition) or much greater than 350  C (supercritical condition). Introducing ZVI in sub- and super-critical water has been proposed to enhance the hydrothermal decomposition of perfluorooctane sulfonate (PFOS) (Hori et al., 2006). The conventional hydrothermal treatment has serious disadvantages of high energy costs, uneven heating, and slow heating rate; these disadvantages can be alleviated by using microwave (MW) heating that is homogeneous and fast to provide a rapid and cost-effective disposal of waste (Chou et al., 2009). The microwave-hydrothermal decomposition of PFOA at 90  C in water using persulfate is an effective and low energyconsuming method (Lee et al., 2009). An adequate amount of energy is provided by microwave treatment to cleave C-C bonds without causing depolymerisation (Adhikari et al., 2000). Hence, the microwave-hydrothermal treatment can lead to efficient energy conversion and significant savings (50%) in energy consumption (Jones et al., 2002). PS activated with radio waves has been applied for in-situ remediation of groundwater contaminants. However, the present study is the first successful demonstration in which MW irradiation enables effective degradation of PFOA at lower temperatures with PS and ZVI. Zero-valent iron has been confirmed to effectively decompose PFOS in the hydrothermal treatment (Hori et al., 2006). Additionally, it is inexpensive and environmental friendly, thus providing a cost-effective waste treatment option. The objective of this study is to explore the

887

Reaction procedures

The microwave-hydrothermal treatment was carried out in a microwave digestion system (Ethos Touch Control, Milestone, U.S.A.), which provided 800 W of microwave energy with a frequency of 2450 MHz at full power. The reaction was carried out at 60  C (45 W microwave power), 90  C (70 W microwave power), and 130  C (140 W microwave power), which approximately resulted in 15, 18 and 40 psi pressure, respectively, in the system. Samples (50 mL) with addition of various combinations of persulfate (5 mM) and zero-valent iron powder dosages (0.09–0.9 mmole) were held in 70-mL Teflon vessels with sleeve for the microwave treatment. The sample initial temperature was adjusted with 50% MW power level for about 3 min to reach a pre-determined reaction temperature; the reaction time was then zeroed to start the reaction until its completion. The treated samples were quickly cooled down by quenching in iced water. All degradation experiments were conducted with duplicate samples.

2.3.

Analytical procedures

PFCAs were analyzed by using a high-performance liquid chromatography (HPLC) (DIONEX, UltiMate 3000, U.S.A.) equipped with conductivity detector and anion self-regenerating suppressor (ASRS 300, U.S.A.). A detailed analytical method is described in the Supplementary Material section. The PFCAs, which are C2–C9 perfluoroalkyl groups (TFAPFNA), can be quantified in sole chromatogram (Figure S-1). The limits of detection (LODs) using 50-mL samples, based on a signal-to-noise (S/N) ratio of 3, were 0.14 mg/L for PFBA, 0.11 mg/L for PFPeA, 0.13 mg/L for PFHeA, 0.15 mg/L for PFHpA, 0.18 mg/L for PFOA, and 0.28 mg/L for PFNA. Concentrations of short-chain PFCAs (TFA and PFPrA) were measured with an ion-chromatograph system (DIONEX, ICS3000) consisting of an automatic sample injector, a degasser, a pump, a guard column (Ion Pac AS4A Guard Column, DIONEX), a separation column (Ion Pac AS4A Analytical Column,

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water research 44 (2010) 886–892

DIONEX), and a conductivity detector with a suppressor device. The mobile phase was an aqueous solution containing Na2B4O7 (5 mM) at a flow rate of 2 mL/min. The LODs (S/N ¼ 3, injected at 50 mL) were 0.054 and 0.079 mg/L for TFA and PFPrA, respectively. An ion-chromatograph system was used to measure the F and SO2 4 concentrations. The mobile phase was an aqueous solution containing NaHCO3 (1.7 mM), Na2CO3 (1.8 mM), and the flow rate was 2 mL/min. The LODs (S/N ¼ 3, injected at 50 mL) were 0.024 and 0.089 mg/L for F and SO2 4 , respectively. Ferrous ion was measured colorimetrically with 1,10-phenanthroline at a wavelength of 510 nm using a GCB CINTRA 20 spectrophotometer, which the method is adapted from Standard Methods for the Examination of Water and Wastewater (APHA et al., 1992).

3.

Results and discussion

3.1.

Reaction temperature and dosage

The temperature is considered as the most important parameter to influence the reaction rate for the decomposition reactions except those with zero or small activation energy; it is also a major factor to affect the operating cost. Sulfate radical anion (SO4) that oxidizes PFOA was generated by pyrolysis of persulfate. The effects of temperature on PFOA decomposition with and without PS are shown in Fig. 1. The aqueous solution of PFOA (240.7 mM) irradiated with 140 W of microwave (without PS) at 130  C for 8 h exhibits a low decomposition efficiency (3.1%), indicating the high thermal and chemical stability of PFOA. At 25  C, the 5 mM PS in the PFOA solution without MW irradiation will decompose 8.7% of is an PFOA, after 8 h. This observation suggests that S2O2 8 efficient oxidant for degrading PFOA, even at a lower temperature. With addition of 5 mM PS under MW irradiation at 60 and 90  C, the efficiency of PFOA decomposition is improved by increasing temperature. After 4 hours, PFOA decomposition efficiencies with addition of 20/1 molar ratios of PS/PFOA are

34.5% and 62.6% for 60  C and 90  C, respectively. The PFOA decomposition reaction can be simulated with pseudo firstorder kinetics, and the rate constants (k) are 0.18 h1 at 60  C and 0.48 h1 at 90  C (Table 1, entries 1 and 2). With addition of 5 mM PS under MW irradiation at 130  C, 15.1% of PFOA in aqueous solution is decomposed rapidly within 30 min. Afterward, the decomposition efficiency increases only slightly with longer reaction time. Higher temperature accelerates the PFOA decomposition rate, but an extremely high temperature will level off the reaction rate because a large quantity of radical oxidants is released rapidly to consume most of the remaining persulfate ions (Lee et al., 2009). The effects of different persulfate doses on the PFOA decomposition rates are shown in Table 1 (entries 2–4). The PFOA decomposition efficiency increases when initial PS concentrations vary from 1 to 10 mM. The PFOA decomposition rate constants (k) at 90  C were 0.10, 0.48, and 0.74 h1 for the PS/PFOA molar ratios of 4/1, 20/1 and 40/1, respectively. The PFOA decomposition rate is 4.8 times higher at 5 mM and 7.4 times higher at 10 mM than at 1 mM. Higher PS concentrations lead to more available sulfate radicals to degrade PFOA at higher rates.

3.2.

Effects of iron



Before applying ZVI, the adsorption kinetics of PFOA on ZVI and Fe2O3 was examined. After 12 h of contact time, only 2% of PFOA is adsorbed on ZVI and 0.3% of PFOA is adsorbed on Fe2O3. Hence, the removal of PFOA due to adsorption effect is negligible. The influence of ZVI dosages on PFOA decomposition under MW irradiation at 90  C is shown in Fig. 2. With the addition of only ZVI, the PFOA decomposition efficiency increases with increasing ZVI doses. Further increase in ZVI doses will not significantly enhance the PFOA decomposition efficiency. In contrast, the PFOA decomposition efficiency with PS/ZVI is much higher than with ZVI only. In the presence of 5 mM PS and various ZVI doses, the PFOA decomposition efficiency increases slightly with increasing ZVI dose at low dosage levels (1.8–7.2 mM). However, further increase in ZVI doses (14.4–18 mM) results in a considerable reduction in

PFOA Remaining (%)

100% 80%

Table 1 – The pseudo first-order rate constants (k) on PFOA decomposition experiments.

60%

Entry Temp. ( C) PS (mM) ZVI (mM) Pseudo first-order kineticsa

40% With PS at 25°C With PS at 90°C MW only at 130°C

20% 0%

0

2

4

With PS at 60°C With PS at 130°C

6

8

Reaction Time (h) Fig. 1 – Comparison on decomposition of PFOA (240.7 mM) at different temperature levels with persulfate (5 mM) under microwave irradiation. With persulfate at 25 8C is at room temperature without microwave irradiation.

1 2 3 4 5 6

60 90 90 90 90 90

5.0 5.0 1.0 10.0 0 5.0

0 0 0 0 3.6 3.6

k (h1)

Rb

0.18 0.48 0.10 0.74 0.09 0.88

0.99 0.99 0.98 0.98 0.96 0.97

a Decomposition rate constant of PFOA was calculated for the initial period of 1 h. b R: linear correlation coefficient.

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water research 44 (2010) 886–892

PFOA+ZVI

PFOA Remaining (%)

100

PFOA+ZVI+PS

80 60 40 20 0

1.8

3.6

7.2 ZVI(mM)

14.4

18

Fig. 2 – The effects of ZVI dosage on PFOA (240.7 mM) decomposition with PS or without PS (5 mM) after 2 h of microwave irradiation at 90 8C.

the PFOA decomposition efficiency. This implies that high ZVI doses may release a large quantity of Fe2þ ions that compete with the SO4/PFOA interaction to cause a lower decomposition efficiency. Therefore, ZVI of 3.6 mM (0.18 mmol) was chosen as the catalyst dosage in subsequent experiments. The effects of applying ZVI on PFOA (240.7 mM) decomposition under MW irradiation at 90  C are shown in Fig. 3. The addition of ZVI to the PFOA solution, without PS at 90  C for 2 h will decompose 11.1% of PFOA confirming that ZVI is capable of decomposing PFCAs in the hydrothermal condition (Hori et al., 2006). The major reactions that cause effective PFOA decomposition include the chemical reaction between ZVI and H2O to produce H2 gas, and the reduction of PFOA to shortchain PFCAs and fluoride ions (Matheson and Tratnydk, 1994). Adding 3.6 mM ZVI to the PFOA solution with 5 mM PS, the decomposition efficiency of PFOA increases considerably under MW irradiation at 90  C. In the presence of PS and ZVI, the PFOA decomposition reaction will accelerate to reach the equilibrium concentration within a shorter reaction time. After one hour of MW irradiation, the PFOA decomposition efficiencies are 58.5% for PS with ZVI addition, 38.4% for PS 

PFOA Remaining (%)

100% 80%

ZVI at 90°C PS at 90°C

60%

PS + ZVI at 90°C

40% 20% 0%

0

2

4

6

8

Reaction Time (h) Fig. 3 – The effects of applying ZVI on PFOA (240.7 mM) decomposition under MW irradiation at 90 8C.

only, and 9.0% for ZVI only. Simulated with the pseudo-first order kinetics; the rate constants (k) for PFOA decomposition are 0.10 h1 for ZVI system (kZVI), 0.48 h1 for PS system (kPS), and 0.88 h1 for ZVI/PS system (kZVI/PS) for decomposing PFOA (Table 1,entries 2, 5 and 6). The kZVI/PS value is greater than the sum of kZVI and kPS, reflecting a conspicuous synergetic effect. This implies that ZVI not only decomposes PFOA, but also releases ferrous ions to accelerate the formation of sulfate radicals and shorten the reaction time. In addition, Fig. 4 shows the concentration of SO2 4 formed during decomposition of PFOA with PS or PS/ZVI under MW irradiation at 90  C. The addition of ZVI and PS in the PFOA solution accelerates the formation of sulfate ion causing a rapid accumulation of a large quantity of SO2 4 during the initial first one hour. Both curves are consistent with the PFOA decomposition trend with PS or with PS/ZVI at 90  C as shown in Fig. 3. This phenomenon indicates that adding ZVI leads to a faster conversion of persulfate to cause a higher efficiency for the subsequent decomposition of PFOA. Comparison of the decomposition of PFOA in the presence of PS at different temperatures with or without ZVI is shown in Fig. 5. At 25  C, no significant difference in the decomposition efficiency is observed for the cases with or without ZVI. On the contrary, the decomposition efficiency of PFOA with ZVI/PS at 60  C is accelerated significantly; it is similar to that for PFOA with PS at 90  C. The activation energy of the reaction involving the thermal rupture of the O–O bond (Eq. (2)) was reported to be 33.5 Kcal/mole (House, 1962). Hence, the system temperature has to be raised high enough to provide the activation energy required for the production of sulfate radicals. When sufficient quantities of Fe2þ are present to serve as electron donors, the activation energy can be reduced to 12.1 Kcal/mole (Kolthoff et al., 1951; House, 1962). Persulfate anions can also be catalytically decomposed to form sulfate free radicals at a lower temperature in accordance with Eq. (3).

3.3.

Effects of ferrous ion

Ferrous ions serve as activators in the acceleration of sulfate free radical formation in persulfate oxidation reaction. The amount of Fe2þ was monitored during decomposition of PFOA to evaluate the influence of ferrous-activated PS oxidation on the destruction of PFOA in PS/ZVI treatment (Fig. 6). In the PS/ ZVI system at 25  C where the concentration of Fe2þ is very low (0.05 mM), and there is no significant difference between the PFOA decomposition efficiency in the PS and ZVI/PS system during the reaction process (Fig. 5). This suggests that insufficient Fe2þ is released at 25  C that it is not catalytically destructive to the formation of sulfate free radicals during PFOA decomposition. At 90  C, the amount of Fe2þ in the PS/ ZVI system increases rapidly to reach the maximum level during the initial first 2 hour. Afterward, the quantity of Fe2þ decreases sharply and then remains almost unchanged after 2 h. This observation corresponds well with the PFOA decomposition curve with PS/ZVI at 90  C as shown in Fig. 3. The PFOA destruction reaction is almost completed within 2 h; the decomposition efficiency only slightly increases when the reaction time is longer than 2 h. Increasing the concentration of Fe2þ will speed up the reaction rate shown in Eq. (3) and the formation of SO4 that reacts immediately with PFOA. 

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water research 44 (2010) 886–892

1.8

Conc. of Ferrous ion (mM)

Conc. of sulfate ion (mM)

10.0 8.0

PS at 90°C PS + ZVI at 90°C

6.0 4.0 2.0 0.0

0

2

4

6

1.6

PS/ZVI at 25°C

1.4

PS/ZVI at 60°C

1.2

PS/ZVI at 90°C

1 0.8 0.6 0.4 0.2 0

8

0

2

4

Reaction Time (h)

6

8

Reaction Time (h)

Fig. 4 – Comparison on the concentration of SO2L 4 formed in presence of persulfate (5 mM) during decomposition of PFOA at 90 8C with ZVI or without ZVI.

Fig. 6 – Comparison on the concentration of Fe2D formed during decomposition of PFOA with ZVI (3.6 mM) and PS (5 mM) at 25 8C, 60 8C and 90 8C.

However, the fast reaction between SO4 and excess Fe2þ may consume SO4, causing a lower decomposition efficiency of the organic contaminant as shown in Eq. (6) (Liang et al., 2004). Thus, when the reaction time reaches 2 h, Fe2þ as high as 1.61 mM may be produced to compete with PFOA by reacting with SO4.

concentrations than those with shorter chains. Additionally, the reaction times to reach the maximum concentration are 2 h for PFHpA (C6F13COOH), 4 h for PFHeA (C5F11COOH), 6 h for PFPeA (C4F9COOH), and 8 h for PFBA (C3F7COOH). These results







Fe2þ þ SO4 / Fe3þ þ SO2 4 

a 80

40 30 20 10

Intermediate and end products

b

2

4

6

8

60 50

Conc. (µM)

100%

PFOA Remaining (%)

0

Reaction Time (h)

Fig.7a shows the production of various intermediate species with 5 mM PS at 90  C during 8 h of MW irradiation. The species with longer chains generally have higher maximum

80% 60% 40%

PFHpA

PFHeA

PFBA

PFPrA

PFPeA

40 30 20 10

20% 0%

PFPeA

50

0

3.4.

PFHeA PFPrA

60

Conc. (µM)

On the other hand, in the PS/ZVI system at 60  C, ZVI releases significant quantities of Fe2þ continuously, resulting in approximately 0.6 mM Fe2þ in the treated solution. The steady release of Fe2þ enhances the PFOA decomposition efficiency. As shown in Fig. 5, the PFOA decomposition efficiency in the PS/ZVI system at 60  C increases significantly with longer reaction time during the initial 6 h of MW irradiation.

PFHpA PFBA

70

(6)

0

PS at 25°C

PS+ZVI at 25°C

PS+ZVI at 60°C

PS at 90°C

2

4

6

0

0

2

4

6

8

Reaction Time (h) 8

Reaction Time (h) Fig. 5 – Comparison the decomposition of PFOA in presence of PS (5 mM) at different temperature with or without ZVI.

Fig. 7 – Concentrations of various intermediates formed at various reaction times by degradation of PFOA (240.7 mM) under MW irradiation at 90 8C with (a) 5 mM PS and (b) with PS/ZVI.

water research 44 (2010) 886–892

Defluorination efficiency

30%

Percentage (%)

The loss of the carbon recovery

20%

10%

0%

0

2

4

6

8

Reaction Time (h) Fig. 8 – Comparison of the defluorinaition and the loss of the carbon recovery for PFOA with persulfate (5 mM) at 90 8C.

indicate that the initial intermediates formed are longer-chain species; they degrade stepwise into shorter-chain compounds. The Fig.7b shows the production of various intermediate species with 5 mM PS and ZVI at 90  C during 8 h of MW irradiation. The reaction times to reach the maximum concentration are 1 h for PFHpA and 2 h for PFHeA, PFPeA and PFBA. After the maximum concentration, concentrations of these intermediates decrease slightly with longer MW irradiation time. The intermediates degrade faster with ZVI than without ZVI during the PS oxidation process. Based on the detailed reaction pathway of mechanism proposed by the authors earlier (Lee et al., 2009), the mechanism accounting for the above observation is briefly described as follows: long-chain PFCAs are decomposed stepwise to form short-chain PFCAs. Sulfate radical anions (SO 4 ) in acidic water oxidize PFOA to form alkyl radical, i.e. perfluorinated radicals (CnF2nþ1). These unstable radicals may react in water to form unstable perfluorinated alcohols (CnF2nþ1OH), which undergo HF elimination to form Cn1F2n1COF (Nohara et al., 2001). Moreover, Cn1F2n1COFs further undergo hydrolysis, resulting in the formation of one-CF2-unit-shortened PFCA (Cn1F2n1COOH) (De Bruyn et al., 1995). PFHpA is first formed 



Defluorinaition (%)

30%

20%

by dissociation of two fluorine of PFOA, leading to the formation of C6F13COOH and other shorter-chain PFCAs. PHFeA, PFPeA, and PFBA are formed successively by losing the CF2 unit. From the reaction with sulfate radicals, PFCAs become mineralized, leading to the ultimate formation of CO2 and F (Vecitis et al., 2009). The formation of F was measured to monitor the extent of PFOA mineralization and defluorination efficiencies. After 8 h of reaction time with PS under MW irradiation, 61.4% of PFOA is decomposed to from 715.8 mM of fluoride ions. Therefore, the defluorination efficiency, expressed as (moles of F formed)/ (moles of fluorine content in initial PFOA), is about 20.3%. The molar ratio of fluorine in the recovered end products including F, short chain PFCAs and the un-reacted PFOA to the total fluorine in the original PFOA is around 94.5–100.3%. Hence, from the mass balance, the fluorine found in the end products accounts for all fluorine originally contained in PFOA. Besides, the carbon recovery has been calculated based on the molar ratio of the sum of total carbon in the short-chain PFCAs formed and that in the un-reacted PFOA to the total carbon in the PFOA before irradiation. A carbon recovery of less than 100% indicates that major carbon compounds other than PFCAs (i.e. CO2) are produced. Comparison of the defluorination and the loss of carbon based on the quantity of PFOA recovered with PS at 90  C is shown in Fig. 8. The defluorination and the loss of carbon in the recovered PFOA curves show a similar trend and indicate similar formation rates for F and CO2 The defluorination of PFOA (240.7 mM) in the presence of PS at 90  C, with or without ZVI, is shown in Fig. 9. In the PS/ZVI system with MW irradiation at 90  C, 67.6% of PFOA is decomposed rapidly at 2 h with the defluorination efficiency reaching 22.5%. The decomposition and defluorination efficiencies increase slowly afterwards with longer reaction time; they reach 73.1% and 24.3%, respectively, at the end of 8 h. Comparison of the decomposition of PFOA in the PS and the ZVI/PS systems shows that ZVI obviously accelerates the PFOA decomposition rate, which is accompanied by a higher defluorination efficiency in a shorter reaction time. In the PS/ZVI system, the total recovery of fluorine (74.6–81.1%) is much lower than in the PS system, indicating that other forms of fluorine-containing compounds are also produced but were not measured in this study. When significant quantities of Fe2þ/Fe3þ are present, the short-chain PFCAs may form iron–PFCAs complexes (Wang et al., 2008; Hori et al., 2007) to cause a lower fluorine recovery. This may also explain the phenomenon that concentrations of intermediates in the PS/ZVI system are much lower than in the PS system (Fig. 7a and b).

PS at 90°C

10%

0%

891

PS + ZVI at 90°C

0

2

4

6

8

Reaction Time (h) Fig. 9 – Comparison on the defluorination of PFOA in presence of persulfate (5 mM) under MW irradiation at 90 8C with or without ZVI.

4.

Conclusions

For temperatures ranging from 25 to 90  C, and more persulfate additions improve the decomposition efficiency of perfluorooctanoic acid in aqueous solutions under MW irradiation. The presence of trace amount of ZVI will induce the formation of sulfate free radicals from persulfate. It increases the PFOA decomposition rate constant from 0.48 h1 to 0.88 h1, and the corresponding defluorination efficiency

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water research 44 (2010) 886–892

from 16.4% to 22.5% after 2 h of reaction time. ZVI releases Fe2þ ions that reduce the PS activation energy to form sulfate free radicals at a lower operating temperature. The introduction of ZVI into the PFOA solution with PS addition will lead to a synergetic effect that accelerates the PFOA decomposition rate and reduces the reaction time. The combined use of ZVI and PS leads to more efficient decomposition of PFOA at lower reaction temperature within a shorter reaction time. Comparing with conventional hydrothermal treatments, the microwave-hydrothermal treatment with ZVI/PS is a more rapid and energy-efficient method for destruction of PFCAs.

Acknowledgement This study was funded by the National Science Council (NSC) of Taiwan under project number of NSC 96-2221-E-002-007.We also thank Taipei Water Department (TWD) for providing the IC instrumentation used for the sample analyses.

Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.watres.2009.09.055.

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