Environ. Sci. Technol. 2008, 42, 7438–7443
Efficient Decomposition of Perfluorocarboxylic Acids and Alternative Fluorochemical Surfactants in Hot Water HISAO HORI,* YUMIKO NAGAOKA, MISAKO MURAYAMA, AND SHUZO KUTSUNA National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba West, 16-1 Onogawa, Tsukuba 305-8569, Japan
Received March 24, 2008. Revised manuscript received July 28, 2008. Accepted August 5, 2008.
Decomposition of C5-C9 perfluorocarboxylic acids (PFCAs) and perfluoroether carboxylic acids (alternatives to PFCA-based surfactants) in hot water in a sealed reactor was investigated. Although PFCAs showed almost no decomposition in hot water at 80 °C in the absence of persulfate (S2O82-), the addition of S2O82- to the reaction system led to efficient decomposition, even at this relatively low temperature. The major products in the aqueous and gas phases were F- ions and CO2, respectively, and short-chain PFCAs were also detected in the aqueous phase. For example, when an aqueous solution containing perfluorooctanoic acid (PFOA, 374 µM) and S2O82- (50.0 mM) was heated at 80 °C for 6 h, PFOA concentration in the aqueous phase fell below 1.52 µM (detection limit of HPLC with conductometric detection), and the yields of F- ions [i.e., (moles of F- formed) /(moles of fluorine content in initial PFOA)] and CO2 [i.e, (moles of CO2 formed) /(moles of carbon content in initial PFOA) ] were 77.5% and 70.2%, respectively. This method was also effective in decomposing perfluoroether carboxylic acids, such as CF3OC2F4OCF2COOH, CF3OC2F4OC2F4OCF2COOH, and C2F5OC2F4OCF2COOH, which are alternatives to PFCA-based surfactants, producing F- and CO2 with yields of 82.9-88.9% and 87.7-100%, respectively, after reactions at 80 °C for 6 h. In addition, the method was successfully used to decompose perfluorononanoic acid in a floorwaxsolution.WhenPFOAwastreatedatahighertemperature (150 °C), other decomposition reactions occurred: the formation of F- and CO2 was dramatically decreased, and 1Hperfluoroalkanes (CnF2n+1H, n ) 4-7) formed in large amounts. This result clearly indicates that treatment with hightemperature water was not suitable for the decomposition of PFCAs to F-: surprisingly, the relatively low temperature of 80 °C was preferable.
Introduction Perfluorocarboxylic acids (PFCAs) such as perfluorooctanoic acid (C7F15COOH, PFOA) have recently received much attention because they are ubiquitous environmental contaminants (1-4). After PFCAs were shown to be persistent and bioaccumulate in the environment, efforts to eliminate * Corresponding author phone: +81-298-61-8161; fax: +81-29861-8866; e-mail:
[email protected]. 7438
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them from facility emissions and products began (5). The development of techniques for decomposing waste PFCAs (especially in wastewater) to harmless species under mild conditions is important. If PFCAs could be decomposed to F- ions, then the well-established protocol for treatment of F- ions could be used: Ca2+ is added to the system to form environmentally harmless CaF2, which is a raw material for hydrofluoric acid. Thus, development of such a method would not only reduce the environmental impact of PFCAs but also provide a method for the recycling of a fluorine resource. In parallel with the effort to eliminate PFOA and other bioaccumulative PFCAs from facility emissions, there have been efforts to develop alternatives to these compounds. The main strategy for the design of alternatives has been to shorten the perfluoroalkyl moiety (6), because compounds with shorter chains are less bioaccumulative (7-9), although shorter-chain PFCAs are still persistent in the environment (10, 11). Perfluoroether carboxylic acids, in which ether linkages are inserted in the perfluoroalkyl chain so that the molecules contain only short perfluoroalkyl (eC4) groups, are representative alternatives to PFCA-based surfactants (12, 13). The measure to decompose these species at sites where they are emitted in large quantities would also be desirable for the purpose of fluorine recycling. However, conventional methods for wastewater treatment, such as the use of Fenton’s reagent (Fe2+ + H2O2) and H2O2 + UV light irradiation, are not applicable for these chemicals, because aqueous OH radicals are only slightly reactive toward PFCAs and perfluoroether carboxylic acids: this fact was demonstrated for trifluoroacetic acid (TFA) (14, 15), PFOA (16, 17), and a perfluoroether carboxylic acid, C2F5OC2F4OCF2COOH (18). We previously reported that a heteropolyacid, H3PW12O40, efficiently photocatalyzes the decomposition of aqueous PFCAs to F- and CO2 at room temperature in the presence of oxygen gas (16). We also showed that persulfate (S2O82-) speeds up the photochemical decomposition of aqueous PFCAs (19, 20); the photolysis of S2O82- at room temperature produces two sulfate radical anions (SO4 · -) with a quantum efficiency of 2 (21) (eq 1): S2O82- + hν f 2SO4·-
(1)
The SO4 · - acts as a strong oxidant, with the decomposition of PFCAs proceeding via formation of PFCAs with chain lengths that are shorter than the initial chain lengths. During the reaction, all of the initial S2O82- is transformed to SO42-, for which there is a well-established treatment process. Although these photochemical techniques effectively decompose PFCAs, a simpler and more convenient reaction system in terms of equipment is desirable for large-scale treatment, although photochemical techniques are acceptable for the wastewater treatment processes such as disinfection (22). Because SO4 · - can be obtained from S2O82- not only photochemically but thermally (23), we investigated the decomposition of PFCAs and perfluoroether carboxylic acids by hot water in a simple sealed reactor; the reaction temperature was much lower than the temperatures used for traditional treatment using subcritical (∼300 °C) or supercritical (>374 °C) water (24), procedures that have been used to decompose hazardous compounds such as poly(chlorobiphenyl) (25) and perfluoroalkylsulfonates (26). We found that PFCAs and perfluoroether carboxylic acids could be effectively decomposed in hot water to F- and CO2 at a relatively low temperature (80 °C) in the presence of S2O82-; 10.1021/es800832p CCC: $40.75
2008 American Chemical Society
Published on Web 08/30/2008
during the process, S2O82- was transformed to SO42-. We also used this method to decompose perfluorononanoic acid (C8F17COOH; PFNA) contained in a floor wax solution to test the utility of the method for wastewater treatment. Furthermore, we examined the effect of temperature on the reactions by performing them at 150 °C. Using the results of these experiments, we point out the strengths and weaknesses of hot water treatment in the presence of S2O82- for PFCAs and perfluoroether carboxylic acids.
Experimental Section Reaction Procedures. Vendors and purities of materials we used are described in the Supporting Information. A stainless steel pressure-resistant reactor (35.1 mL volume) equipped with a thermocouple and a stainless steel screw cap was used. The screw cap was connected to a pressure gauge for measuring the pressure in the reactor and to a sampling port for analyzing gas products. A gold vessel (24.6 mL, 2.8 cm i.d.) was fitted into the reactor to eliminate the possibility of contamination from the reactor material (Note: The use of glass equipment should be avoided because it may cause heterogeneous reaction of the fluorinated compounds (27)). In a typical run, an aqueous (Milli-Q) solution (10 mL) containing a PFCA or a perfluoroether carboxylic acid (3.03-4.00 µmol; 303-400 µM) and K2S2O8 (0.10-0.50 mmol; 10.0-50.0 mM) was poured into the gold vessel. After the reactor was pressurized to 0.78-0.81 MPa with synthetic air, the reactor was sealed. The reactor was placed in an oven, and the reactor temperature was raised to the desired temperature (e.g., 80 °C) and held constant for a specified time (e.g., 6 h). Then the reactor was quickly cooled to room temperature with ice water. Control reactions were performed either in the absence of K2S2O8 or under an argon atmosphere. To transfer the gas, we connected the reactor to a sampling bag by means of a valve, and we analyzed the collected gas in the sampling bag by gas chromatography/mass spectrometry (GC/MS). After the reaction, the aqueous phase was analyzed by ion chromatography, ion-exclusion chromatography, and HPLC. Analytical Procedures. An ion-chromatography system (IC-2001, Tosoh Corp., Tokyo, Japan) consisting of an automatic sample injector (30-µL injection volume), a degasser, a pump, a guard column (TSKguard column Super IC-A, 4.6-mm i.d., 1.0-cm length, Tosoh Corp.), a separation column (TSKgel Super IC-Anion, 4.6-mm i.d., 15-cm length, Tosoh Corp.), a column oven (40 °C), and a conductivity detector with a suppressor device was used to measure the F- and SO42- concentrations. The mobile phase was an aqueous solution containing Na2B4O7 (6 mM), H3BO3 (15 mM), and NaHCO3 (0.2 mM), and the flow rate was 0.8 mL min-1. The limits of detection (LODs), which were calculated from a signal-to-noise (S/N) ratio of 3, were 0.74 and 2.62 µg L-1 for F- and SO42-, respectively. An ion-exclusion chromatograph system consisting of a guard column (TSKgel OApak-P, 7.8-mm i.d., 1.0-cm length, Tosoh Corp.), a separation column (TSKgel OApak-A, 7.8mm i.d., 30-cm length, Tosoh Corp.), a pump, a column oven (40 °C), and a conductivity detector was used to measure the concentrations of short-chain PFCAs (TFA to C3F7COOH). The mobile phase was phthalic acid (10 mM) at a flow rate of 0.6 mL min-1, and a typical sample injection volume was 5 µL. The LODs (S/N ) 3) (injected at 5 µL) were 0.27, 0.28, and 0.81 mg L-1 for TFA, C2F5COOH, and C3F7COOH, respectively. The concentrations of longer-chain PFCAs (C4F9COOH to PFNA) and perfluoroether carboxylic acids were measured with an HPLC system with conductometric detection (IC2001, Tosoh Corp.); the column was a Tosoh TSKgel SuperODS (4.6-mm i.d., 10-cm length × 2), and the mobile phase was a mixture of methanol and aqueous NaH2PO4 (20 mM,
adjusted to pH 3.0 with H3PO4) at several mixing ratios with a flow rate of 0.4 mL min-1. When the sample injection volume was 30 µL, the LODs (mg L-1, S/N ) 3) were as follows: 0.30 and 0.63 for C4F9COOH and C5F11COOH (mobile phase 55:45 v/v methanol/aqueous NaH2PO4), respectively; and 0.18, 0.15, 0.63, 0.57 0.34, 0.98, and 0.30 for C5F11COOH, C6F13COOH, PFOA, PFNA, CF3OC2F4OCF2COOH, CF3OC2F4OC2F4OCF2COOH, and C2F5OC2F4OCF2COOH (65:35 v/v methanol/ aqueous NaH2PO4), respectively. In our ion-chromatography system, S2O82- showed no peak, so the concentration of S2O82- in the aqueous phase was determined on the basis of the absorbance of S2O82around 1049 cm-1 in the attenuated total reflectance IR spectra. The system consisted of a FTIR spectrometer (MB106, Bomem, Que´bec, Canada) with a HgCdTe detector and a diamond cell (DuraSamplIR II, S. T. Japan, Tokyo, Japan); the LOD (S/N ) 3) of S2O82- was 211 mg L-1. The products in the gas phase were identified by GC/MS. We used two systems. One system consisted of an electronimpact ionization instrument (5973 Inert, Agilent Technologies, Palo Alto, CA) with a capillary column (Rx-1, 0.32-mm i.d., 60-m length), which was used for the analysis of 1Hperfluoroalkanes (CnF2n+1H, n ) 4-7). The column temperature was kept at 203 K for 2 min and then raised at 35 K min-1 to 273 K. After 10 min at 273 K, the column temperature was raised at 5 K min-1 to 373 K. The carrier gas was He, which was introduced in constant pressure mode at 2.00 psi. The sample gas (0.2 mL) was introduced into the GC/MS system in splitless mode, and the analyses were conducted in full-scan mode (m/z 10-500). The LODs (S/N ) 3) for C4F9H and C7F15H were 0.11 and 0.03 ppmv, respectively, and were based on the intensity at m/z 131 in both cases. The other system consisted of a GC (HP5890, HewlettPackard, Wilmington, DE) with a Poraplot Q column (0.32mm i.d., 25-m length, Chrompack, Bergen op Zoom, The Netherlands) and a MS (HP 5972A) and was used for analyzing small molecules such as CF4, CF3H, and CO2. The carrier gas was He. The oven temperature was held constant at 30 °C. The sample gas (30 µL) was introduced into the GC/MS system in splitless mode. The injector temperature was held constant at 120 °C, and the electron impact source was operated at 70 eV. The analyses were conducted in full-scan mode (m/z 1.2-200); the LODs (ppmv, S/N ) 3) for CF4 (based on the intensity at m/z 69), CF3H (m/z 51), and CO2 (m/z 44) were 0.25, 0.56, and 5.65, respectively. Wax Sample Treatment. The decomposition of PFNA contained in floor wax was examined as a model for PFCA decomposition in wastewater. The wax, containing 75.4 mg L-1 of PFNA (details for the quantification of PFNA in the wax are described in the Supporting Information), was the same as that used in our previous study (19). An aqueous solution (10 mL) containing the wax (0.20 g) and K2S2O8 (1.00 mmol; 100 mM) was introduced into the reactor, and the mixture was allowed to react for 6 h at 80 °C in the manner as described above. After reaction, the pressure was released, and the reaction mixture was centrifuged (3000 rpm, 10 min). An InertSep RP-1 cartridge (GL Sciences, Tokyo, Japan) was preconditioned with methanol (10 mL) and then with water (10 mL). The liquid phase (8.6 mL) obtained from centrifugation was diluted to 50 mL with water, and the diluted solution was passed through the cartridge. The PFNA was eluted with methanol (5 mL), and the solution was concentrated under argon to 1 mL. The concentrated solution was analyzed by HPLC, and PFNA was quantified.
Results and Discussion PFOA Decomposition in Hot Water. The time course of the reaction of aqueous PFOA and S2O82- (134 molar excess) at 80 °C under pressurized air in a sealed reactor is shown in Figure 1. The amount of PFOA decreased over time, and FVOL. 42, NO. 19, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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SO4·- + PFOA f SO42- + PFOA·+
(2)
When the reaction was carried out in the presence of S2O82- under argon (entry 3), both the amount of PFOA decomposed and the amounts of F-, CO2, and shorter-chain PFCAs formed were similar to the amounts for reactions under air (entry 1), which indicates that the oxygen gas did not play a substantial role. The fate of PFOA · + after the electron transfer reaction with SO4 · - is not clear. However, on the basis of reported results (28, 29), it is possible to propose a mechanism. The first bond to be cleaved is the C-C bond between the C7F15 and COOH groups. In water, the resulting C7F15 radicals may react with water or OH to form a thermally unstable alcohol C7F15OH, which undergoes HF elimination to form C6F13COF (28). Hydrolysis of this acid fluoride (29) results in the formation of the one-CF2unit-shortened species C6F13COOH. The effect of the initial amount of S2O82- on the pseudofirst-order rate constant for the PFOA decomposition is shown in Figure 2. As expected, the rate constant increased when the initial amount of S2O82- was increased. However, the slope of the curve decreased as the initial amount of S2O82was increased. SO4 · - radical anions reportedly react with S2O82- to form SO42- and S2O8 · - under acidic conditions (eq 3) (21): SO4·- + S2O82- f SO42- + S2O8·-
FIGURE 1. Time course of PFOA decomposition in hot water containing S2O82-: moles of (A) PFOA, CO2, and F- and (B) short-chain PFCAs. After 6 h of reaction, 0.13 µmol of C2F5COOH was also detected. An aqueous solution (10 mL) containing S2O82- (0.50 mmol; 50.0 mM) and PFOA (3.74 µmol; 374 µM) was warmed at 80 °C under synthetic air (0.78 MPa) for 0.5-6 h. and CO2 were detected as major products in the aqueous and gas phases, respectively (Figure 1A). The decomposition of PFOA followed pseudo-first-order kinetics with a rate constant of 1.36 h-1. After 6 h, PFOA had disappeared from the HPLC chromatogram (LOD ) 1.52 × 10-8 mol), and the amount of F- in the reaction solution was 43.5 µmol. The Fyield [(moles of F- formed) /(moles of fluorine content in initial PFOA, i.e., moles of initial PFOA × 15)] was 77.5% (Table 1, entry 1); and the CO2 yield [(moles of CO2 formed) /(moles of carbon content in initial PFOA, i.e., moles of initial PFOA × 8)] was 70.2%. We also detected shorter-chain PFCAs as minor products in the aqueous phase (Figure 1B), as was the case for the photochemical decomposition of PFOA induced by S2O82- (19). The formation of C6F13COOH, a oneCF2-unit-shortened species of PFOA, was followed by formation of even shorter species; C5F11COOH, C4F9COOH, C3F7COOH, and C2F5COOH. After 6 h of the reaction, all the shorter-chain PFCAs detected in the aqueous phase were species with less than 6 carbons, which are not bioaccumulative (8, 9). After 6 h of reaction, the total recovery of fluorine (i.e., the molar ratio of total fluorine content in Fand short-chain PFCAs formed to total fluorine in PFOA before reaction) was 88.1%. Total recovery of carbon (i.e., the molar ratio of total carbon content in CO2 and shortchain PFCAs formed to total carbon in PFOA before reaction) was 81.4%. In the gas phase, no CF4 and 1H-perfluoroalkanes (e.g., C7F15H, C4F9H, CF3H) were detected. We determined the reaction products for various combinations of S2O82- and reaction atmosphere (Table 1). When the aqueous solution of PFOA was warmed at 80 °C in the absence of S2O82-, virtually no reaction occurred; no F- and CO2 formed (entry 2; compare with entry 1). Hence, SO4 · clearly reacted with PFOA (eq 2): 7440
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(3)
The decrease in the slope in Figure 2 at higher S2O82concentrations indicates that the reaction of SO4 · - with S2O82(eq 3) cannot be ignored at higher S2O82- concentrations. In our reaction system, SO4 · - radical anions formed by thermolysis of S2O82- and then reacted with PFOA. In such a system, we expected SO42- to form by one-electron transfer from PFOA to SO4 · - (eq 2). Therefore, we determined the time course of S2O82- consumption and SO42- formation in the aqueous phase during the PFOA decomposition (Figure 3). As expected, S2O82- was consumed and SO42- accumulated in the aqueous phase while PFOA decomposed to F- and CO2. During the reaction, the total sulfur content [(moles of S2O82- × 2) + moles of SO42-] was almost constant at ∼1.0 mmol, and this value was the same as the initial sulfur content in S2O82-. This result indicates that S2O82- and SO42- were the only stable sulfur species present in the aqueous phase. Application to Other PFCAs, Perfluoroether Carboxylic Acids, and a Wax Solution. We applied our method to C5-C9 PFCAs other than PFOA at a constant reaction time of 6 h at 80 °C. The results are summarized in Table 2, together with the data obtained in the absence of S2O82-. Without S2O82-, almost all (97.0-99.7%) of the initial amounts of the PFCAs remained in the aqueous phase, and no F- and CO2 formed (entries 2, 4, 6, and 8). In contrast, in the presence of S2O82-, the PFCAs decomposed efficiently: the amounts remaining after the reaction were below the LODs, and large amounts of F- and CO2 were formed, in yields of 75.7-89.7% and 74.7-82.3%, respectively (entries 1, 3, 5, and 7). These results clearly indicate the effectiveness of S2O82- for the decomposition of these PFCAs. We also applied this method to some perfluoroether carboxylic acids: CF3OC2F4OCF2COOH, CF3OC2F4OC2F4OCF2COOH, and C2F5OC2F4OCF2COOH. As shown in Table 3, in the absence of S2O82-, almost all (98.4-100%) of the initial amounts of the perfluoroether carboxylic acids remained in the aqueous phase, and almost no F- and CO2 were formed (Table 3, entries 2, 4, and 6). In contrast, in the presence of S2O82-, only 0-1.1% of the substrates remained in the aqueous phase, and the yields of F- and CO2 were 82.9-88.9% and 87.7-100%, respectively (entries 1, 3, and 5). Therefore, S2O82- led to efficient decomposition not only for PFCAs but also for these perfluoroether carboxylic acids.
TABLE 1. Product Distribution for Various Reaction Conditionsa atmosphereb
S2O82-
remaining PFOA (µmol) [%]c
F- (µmol) [yield, %]d
CO2 (µmol) [yield, %]e
1
presentf
air
n.d.g [0]
43.5 [77.5]
21.0 [70.2]
2
none
air
3.73 [99.7]
n.d.g [0]
n.d.g [0]
43.9 [78.3]
20.8 [69.5]
3
f
present
argon
g
n.d. [0]
short-chain PFCAs (µmol) C5F11COOH (0.15), C4F9COOH (0.21), C3F7COOH (0.25), C2F5COOH (0.13) n.d.g C5F11COOH (0.03), C4F9COOH (0.17), C3F7COOH (trace,
