Removal of perfluorinated carboxylates from washing wastewater of perfluorooctanesulfonyl fluoride using activated carbons and resins

Journal of Hazardous Materials 286 (2015) 136–143

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Removal of perfluorinated carboxylates from washing wastewater of perfluorooctanesulfonyl fluoride using activated carbons and resins Ziwen Du, Shubo Deng ∗ , Youguang Chen, Bin Wang, Jun Huang, Yujue Wang, Gang Yu State Key Joint Laboratory of Environment Simulation and Pollution Control, Beijing Key Laboratory for Control of Emerging Organic Contaminants, School of Environment, Tsinghua University, Beijing 100084, China

h i g h l i g h t s • • • • •

PFOSF washing wastewater contains high concentrations of perfluorinated carboxylates. Bamboo-derived activated carbon and resin IRA67 are suitable for PFCA removal. PFOA is preferentially adsorbed on BAC and IRA67 among different PFCAs. BAC exhibits stable removal of three PFCAs in the wide pH range. IRA67 is successfully regenerated and exhibits stable removal in adsorption cycles.

a r t i c l e

i n f o

Article history: Received 29 October 2014 Received in revised form 10 December 2014 Accepted 20 December 2014 Available online 24 December 2014 Keywords: PFOA PFOSF washing wastewater Competitive adsorption Activated carbon Resin

a b s t r a c t Perfluorooctanesulfonyl fluoride (PFOSF) washing wastewater contains high concentrations of perfluorinated carboxylates (PFCAs) including perfluorohexanoate (PFHxA, 0.10 mmol/L), perfluoroheptanoate (PFHpA, 0.11 mmol/L), and perfluorooctanoate (PFOA, 0.29 mmol/L). For the first time, we investigated the removal of these PFCAs from actual wastewater using the bamboo-derived activated carbon (BAC) and resin IRA67. Adsorption kinetics, effects of adsorbent dose, solution pH, and inorganic ions, as well as regeneration and reuse experiments were studied. The removal percents of three PFCAs by BAC and IRA67 followed the increasing order of PFHxA < PFHpA < PFOA, but the adsorption equilibrium time conformed to the reverse trend. PFCAs removal on IRA67 decreased with increasing pH, but BAC almost kept stable PFCAs removal at pH above 5.0. Among competitive adsorption of three PFCAs, PFOA was preferentially adsorbed on both BAC and IRA67. PFCAs removal from actual wastewater by BAC was higher than that in simulated solution, due to the presence of high concentration of inorganic ions in the wastewater. However, the co-existing organic compounds in wastewater significantly suppressed the adsorption of PFCAs. Both spent BAC and IRA67 were successfully regenerated by ethanol solution or NaCl/methanol mixture, and IRA67 showed the stable removal of PFCAs in five adsorption cycles. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Perfluorinated carboxylates (PFCAs) are a major class of perfluorinated compounds (PFCs) and have been widely applied in many industries, such as fluoropolymer manufacture, food packaging, and fabric protection [1,2]. Among these PFCAs, perfluorooctanoate (PFOA) is regarded as the most typical one and was added into reduction plan by United States Environmental Protection Agency in 2006 due to its toxicity, bioaccumulation, and widespread distribution in aquatic environments [1]. Recently, the short-chain

∗ Corresponding author. Tel.: +86 10 62792165; fax: +86 10 62794006. E-mail address: [email protected] (S. Deng). http://dx.doi.org/10.1016/j.jhazmat.2014.12.037 0304-3894/© 2014 Elsevier B.V. All rights reserved.

PFCAs, such as perfluorohexanoate (PFHxA) and perfluoroheptanoate (PFHpA) have also been found to be bioaccumulative and persistent during natural degradation processes although their environmental risk is less than PFOA [3,4]. High concentrations of PFCAs caused by direct discharge of wastewater have been detected in the waters near concentrated facilities involved in production or use of PFCAs [5–7]. Therefore, it is very important to remove PFCAs from industrial wastewater before being discharged into aquatic environments. Adsorption is considered as an efficient and preferred technique for the removal of PFCAs from wastewater [1,8]. Various adsorbents, such as resin [9,10], activated carbon (AC) [11,12], chitosan [13], and hydrotalcite [12] have been reported to be effective for PFOA removal. Most researchers used simulated wastewater for

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adsorption experiments, and only few studies reported the removal of PFCAs from actual wastewater. Pattarawan [14] used granular activated carbon (GAC, Filtrasorb 400) and resins (XAD4, PFA300, and Dow Marathon A) to remove PFHxA, PFHpA, and PFOA from the effluent of industrial wastewater treatment plant associated with polytetrafluoroethylene manufacture. The concentrations of PFCAs were only ng/L level, and the commercial granular AC was found to be the worst adsorbent. Schaefer et al. [15] also reported that PFCs could easily penetrate the GAC beds in German wastewater treatment plants. It seems that commercial ACs are not applicable for the removal of PFCAs from wastewater. Recently, Jackson and Mabury [16] revealed a historical PFCAs source by electrochemical fluorination (ECF) of alkyl sulfonyl fluorides and suggested that the major contribution of PFOA in the environment was related to the production of perfluorooctanesulfonyl fluoride (PFOSF). PFOSF is a raw material for producing perfluorooctane sulphonate (PFOS) and PFOS-related chemicals, such as N-alkyl substituted perfluorooctane sulfonamides and N-alkyl substituted perfluorooctane sulfonamidoethanol-type substances [17]. About 220–240 t of PFOSF is produced annually in China to meet the domestic and international demand [18]. Octane sulfonyl chlorine reacts with anhydrous hydrogen fluoride to produce PFOSF by the classical ECF method [16,18]. Since PFOSF is insoluble in water and heavier than octane sulfonyl chlorine, the crude PFOSF product can be separated from reactor bottom [19], and requires water washing to remove soluble impurities. The washing wastewater contains high concentrations of PFHxA, PFHpA, and PFOA, and it can cause serious pollution for receiving waters. By now, the treatment of this wastewater has not drawn the attention of enterprise and researchers. To the best of our knowledge, this study is the first time to report the property of wastewater from PFOSF production and investigate the removal of different PFCAs from this actual wastewater using different adsorbents. The optimal bamboo-derived activated carbon (BAC) [20] and anion-exchange resin IRA67 [10] were used to remove PFHxA, PFHpA, and PFOA via different adsorption mechanisms, and the adsorption behavior, regeneration, and reuse efficiency were studied in detail. To fully understand the competitive adsorption of different PFCAs, their removal percents were compared in both simulated solution and actual wastewater.

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or synthetic PFCAs solution. In the preliminary test for adsorbent selection, 20 mg of different adsorbents were severally added into actual wastewater to test their efficiencies for PFOA removal. Effect of pH on the PFCAs removal was studied with 0.2 g/L BAC and 0.1 g/L IRA67 in actual wastewater at pH ranging from 2.0 to 9.0, and no pH adjustment was conducted during the adsorption process due to the little pH change (<±0.2). In the adsorption kinetic experiments, 0.2 g/L BAC and 0.1 g/L IRA67 were added in actual wastewater at pH 4.0. The effect of adsorbent dose on PFCAs removal was conducted with different dose of BAC and IRA67 in actual wastewater at pH 4.0. In order to be consistent with the actual wastewater, the same concentrations of PFCAs were adopted to prepare synthetic PFCAs solution for the experiments of competitive adsorption. In the investigation of competitive adsorption of different PFCAs, 20 mg of BAC or 8 mg of IRA67 was added into 100 mL of 0.10 mmol/L PFHxA solution, or 0.11 mmol/L PFHpA solution or 0.29 mmol/L PFOA solution. For comparison, BAC or IRA67 was also added into 100 mL of synthetic solution containing the three PFCAs at the above concentrations. According to the concentrations of all salts and PFCAs in the actual wastewater, the synthetic solution was prepared and adjusted to 4.0 ± 0.1 using NaOH and HCl, followed by adding 0.2 g/L BAC or 0.1 g/L IRA67. The effect of inorganic ions was studied with the above mixed PFCAs solution in the presence of 0.59 mmol/L NaCl, 0.10 mmol/L KCl, 0.61 mmol/L Na2 SO4 , 0.04 mmol/L NaNO3 , 0.70 mmol/L CaCl2 , 1.31 mmol/L NaF, or 0.96 mmol/L MgCl2 .

2.3. Regeneration and reuse experiments Twenty five milligrams of BAC or ten milligrams of IRA67 was added into 100 mL of actual wastewater at pH 4.0. After 48 h adsorption, the spent adsorbents were filtered by a filter with 0.45 ␮m nylon membrane, and then placed into 30 mL of various regeneration agents, followed by shaking at a thermostatic shaker at 170 rpm for 12 h. The concentrations of PFCAs in the solution after regeneration process were measured to calculate the regeneration percents of spent adsorbents. After the regeneration by optimal regeneration agents (50% ethanol solution at 45 ◦ C for BAC and 1% NaCl/methanol (30/70, v/v) mixture at 25 ◦ C for IRA67), the adsorbents were reused in the next adsorption cycle.

2. Materials and methods 2.1. Chemicals and materials

2.4. Analytical methods

Perfluorohexanoic acid, perfluoroheptanoic acid, and perfluorooctanoic acid were purchased from Tokyo Kasei Kogyo (Tokyo, Japan), and their values of water solubility are 260, 118, and 3.4 g/L, respectively. Anion-exchange resin Amberlite IRA67 (geltype polyamine, 0.30–1.2 mm) and non-ionic exchange resins XAD4 (macroporous styrene, 0.25–0.85 mm), and XAD7HP (macroporous aliphatic acrylic polymer, 0.25–0.85 mm) were purchased from Sigma–Aldrich (St. Loius, MO, USA). Coal-based activated carbon (CAC, pHpzc 7.5, 0.60–0.85 mm) was obtained from Jingke Activated Carbon Co., (Tangshan, China). Bamboo-derived activated carbon (BAC, pHpzc 3.2, 0.60–0.85 mm) was prepared by our reported method [20]. The properties of BAC and IRA67 are listed in Table S1. Other chemicals were analytical grade. PFOSF washing wastewater was obtained from a Chinese PFOSF manufacturer in Yingcheng, Hubei, China.

All inorganic anions in wastewater including fluoride, chloride, sulfate, and nitrate were measured using an ion chromatograph (ICS-1000, Dionex, USA), while sodium, magnesium, potassium, and calcium cations were analyzed by an inductively coupled plasma atomic emission spectroscopy (ICP-AES, Thermo IRIS Intrepid II XSP, USA). Total organic carbon (TOC) of wastewater was determined by a TOC–VCHP analyzer (Shimadzu, Japan). A high performance liquid chromatography–tandem mass spectrometry (HPLC–MS/MS) was used to measure the PFCs in wastewater samples after 500-fold dilution with an UltiMate 3000HPLC (Dionex) equipped with an API 3200 triple quadrupole mass spectrometer (AB SCIEX, Canada). The PFCAs in wastewater were also analyzed using a LC-10ADvp HPLC with a CDD-10Avp conductivity detector (HPLC–CDD, Shimadzu). Before determination, the sample solution was filtered with a 0.22 ␮m nylon membrane, and about 1 mL of solution was collected after 5 mL of filtrate to eliminate the influence of membrane sorption (recovery >99%). Since the concentrations of PFCAs obtained by HPLC–CDD were consistent with those by HPLC–MS/MS, HPLC–CDD was adopted to determine the concentrations of PFCAs in the subsequent adsorption samples. The analytical procedure was described in our previous studies [21,22].

2.2. Adsorption experiments All batch adsorption experiments were conducted at 170 rpm and 25 ◦ C in an orbital shaker for 48 h with adsorbents in 250mL polypropylene flasks containing 100 mL of actual wastewater

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Table 1 Primary characteristics of the PFOSF washing wastewater.

3. Results and discussion

while hydrophobic interaction should be dominant for the adsorption of PFOA on BAC [20]. Although electrostatic attraction would occur between the positively charged nitrogen-containing groups on CAC and anionic PFOA, hydrophobic interaction should play a more important role in PFOA adsorption on CAC. Since the specific surface areas of CAC (1000 m2 /g), XAD4 (725 m2 /g), and XAD7HP (380 m2 /g) are much lower than that of BAC (2450 m2 /g), the BAC exhibited higher PFOA removal. Based on the removal percents of PFOA, the BAC and IRA67 were selected as the suitable adsorbents to remove PFCAs from PFOSF washing wastewater in the following experiments.

3.1. Wastewater characteristics

3.3. Adsorption kinetics of PFCAs on BAC and resin IRA67

The primary characteristics of PFOSF washing wastewater are shown in Table 1, and the HPLC–MS/MS chromatograms of PFCs are illustrated in Fig. S1. The concentrations of PFHxA, PFHpA, and PFOA in the wastewater are high up to 0.10, 0.11, and 0.29 mmol/L, respectively. The high concentrations of PFCAs in the wastewater might be produced from the hydrolysis reaction of perfluoroacyl fluorides generated from electrochemical oxidation of the perfluorocarbon chain during ECF process [16,23]. Wendling [24] reported an average of 0.09% PFOA in PFOSF-based products, supporting its high concentration in PFOSF washing wastewater in our study. In contrast, PFOS concentration in the wastewater is only 1.01 ␮mol/L, much lower than the three PFCAs. In addition, the wastewater contains high concentrations of F− , Cl− , SO4 2− , Mg2+ , Na+ , and Ca2+ , probably originated from the chemical reactions in the production of PFOSF and local tap water [25]. The wastewater TOC value (63.2 mg/L) is mainly attributed to the PFCAs (equivalent to 44.3 mg/L of TOC), and other reaction by-products may also have the contribution.

Fig. 2 shows the sorption kinetics of PFHxA, PFHpA, and PFOA on BAC and IRA67, and their removal percents are less than 80% (Fig. S2). The adsorption equilibria of three PFCAs on the IRA67 was almost achieved within 33.5 h. In contrast, the equilibrium time of PFHxA, PFHpA, and PFOA adsorbed on the BAC was obviously different from each other. PFOA required more than 33.5 h to reach the sorption equilibrium on BAC, while the equilibrium time of PFHxA and PFHpA was only about 12.5 h, which might be related with their molecular sizes and diffusion in BAC. PFHxA and PFHpA molecules with smaller sizes are more likely to possess weaker steric effect and faster diffusion in the porous BAC, leading to shorter sorption equilibrium time [8]. To further explore the difference in sorption kinetics of PFCAs on BAC and IRA67, the pseudo-second-order equation was adopted to describe the kinetic data, and the modeling results are shown in Table S2. PFOA initial sorption rates (v0 ) on both BAC and IRA67 are higher than PFHpA and PFHxA, possibly attributed to the readily accessible pore structure of both adsorbents and competitive adsorption of co-existing PFCAs. The BAC has lots of pores

PFCs concentration (mmol/L)

pH

TOC (mg/L)

Concentration of inorganic ions (mmol/L) Anions

PFHxA PFHpA PFOA PFOS

0.1 0.11 0.29 1.01 × 10−3

4.0

63.2

2−

SO4 NO3 − F− Cl−

Cations 0.61 0.04 1.31 1.04

K+ Mg2+ Na+ Ca2+

0.1 0.96 0.59 0.7

3.2. PFOA removal from wastewater by different adsorbents PFOS and PFOA are commonly adopted to represent the typical PFCs in the adsorption studies. According to the recent review paper [8], anion-exchange resins show high adsorption capacity for both PFOS and PFOA, while commercial ACs exhibit relatively low adsorption capacity. Our previous study shows that the BAC with enlarged pore structure had the adsorbed amount of 1.15 mmol/g for PFOA at pH 5, much higher than other reported ACs [20]. Xiao et al. [26] and Senevirathna et al. [27] reported that the moderately polar and non-polar resins had higher PFCs removal than ACs and anion-exchange resins. Therefore, different ACs and resins were selected to remove PFCAs from the wastewater, and PFOA removal by these adsorbents is shown in Fig. 1. The resin IRA67 and BAC exhibited higher removal percents for PFOA than other adsorbents. Numerous amine groups on resin surface are responsible for PFOA adsorption via anion-exchange mechanism [10],

Fig. 1. PFOA removal from actual wastewater by different activated carbons and resins (sorption conditions: 20 mg of adsorbents in 100 mL of wastewater at pH 4.0 for 48 h).

Fig. 2. Sorption kinetics of three PFCAs on the BAC (a) and resin IRA67 (b) and modeling using the pseudo-second-order equation (sorption conditions: 0.2 g/L BAC or 0.1 g/L IRA67 in wastewater at pH 4.0 for different adsorption time).

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in the range of 2–4 nm, which are favorable for the diffusion and adsorption of PFOA with a molecular length of 1.2 nm [20]. IRA67 is a polyacrylic gel resin, which can easily adsorb water in aqueous solution and exhibit open pore structure, allowing the easy diffusion of PFOA. Additionally, the concentration of PFOA in the wastewater is much higher than PFHxA and PFHpA, and more adsorbed PFOA molecules are likely to occupy the sorption sites on adsorbents prior to the sorption of PFHxA and PFHpA. As a consequence, the short-chain PFCAs reached the equilibrium quickly, but their initial sorption rates were much lower. The equilibrium adsorbed amounts (qe ) of PFOA on both adsorbents were much higher than those of PFHpA and PFHxA (Table S1). It has been widely reported that adsorption capacities of PFCAs on various adsorbents increased with increasing C F chain length [21,28–30]. Although the BAC contains some functional groups, its surfaces without functional groups still exhibit hydrophobic property. The PFCAs with longer C F chain have stronger hydrophobicity, and thus, they are more easily adsorbed onto the hydrophobic portion of BAC through hydrophobic interaction. The higher adsorbed amount of PFOA on IRA67 may be attributed to its high concentration in the wastewater. 3.4. Effect of adsorbent dose Fig. 3(a) shows the effect of BAC dose on the removal of three PFCAs from wastewater. The removal percents of PFOA increased from 33.2% to 96.6% when BAC doses increased from 0.1 to 0.5 g/L, and kept stable at the dose above 0.7 g/L. The removal of PFHxA and PFHpA increased slowly at the initial increase of BAC dose. Especially for PFHxA, its removal percent was only 8.8% at the BAC dose of 0.3 g/L, while PFOA removal reached 88.4%, indicating the selective adsorption of PFOA at low adsorbent dose. It is evident that the competitive adsorption of PFOA, PFHxA, and PFHpA occurred during the adsorption process, and the long-chain PFCAs

(a)

(b)

Fig. 3. Effect of BAC (a) and IRA67 (b) dose on the removal of PFCAs from wastewater (sorption conditions: 0.05–1.5 g/L adsorbent in wastewater at pH 4.0 for 48 h).

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were preferentially adsorbed on BAC due to the strong hydrophobic interaction. Although BAC was not very effective for PFHxA removal at low BAC dose, PFHxA removal still exceeded 90% at the BAC dose of 1.9 g/L. The trend of PFCAs removal by IRA67 with varying dose was quite similar to that by BAC, but there was no stable and low removal of PFHxA at the low resin dose (Fig. 3(b)). Since anion exchange is mainly responsible for the sorption of PFCAs on IRA67, the removal percents of three PFCAs all increased with increasing resin dose. It is strange that their removal percents still follow the increasing order of PFHxA < PFHpA < PFOA, indicating that intraparticle diffusion is not a limiting factor, and the formation of hemi-micelles and micelles may be the dominant reason for the high removal of long-chain PFCAs. As shown in Fig. 3, it requires less dose of IRA67 than BAC to achieve the same removal of three PFCAs from the wastewater. In order to simultaneously remove over 90% of the three PFCAs from the wastewater, 1.9 g/L BAC or 1.5 g/L IRA67 should be added. Although BAC dose is a little higher than IRA67, BAC has the advantages of low adsorbent cost and high selective adsorption of PFOA at low dose. The sorption isotherms of three PFCAs on BAC and IRA67 were calculated and shown in Fig. S3, and the fitting results by the Langmuir and Freundlich equations are presented in Table S3. The sorption isotherms of PFCAs on both adsorbents were fitted better by the Langmuir model. Only the sorption isotherm of PFOA on the BAC was described better by the Freundlich model (R2 = 0.93) than the Langmuir model (R2 = 0.86), indicating the possible multilayer adsorption of PFOA on BAC. The maximum sorption capacity (qm ) of three PFCAs on both adsorbents followed the increasing order of PFHxA < PFHpA < PFOA. The IRA67 exhibited higher qm values for three PFCAs than BAC. For example, the adsorption capacity of IRA67 for PFOA was 2.82 mmol/g, much higher than 1.03 mmol/g

(a)

(b)

Fig. 4. Effect of wastewater pH on PFCA removal by BAC (a) and IRA67 (b) (sorption conditions: 0.2 g/L BAC or 0.1 g/L IRA67 in wastewater at pH ranging from 2.0 to 9.0 for 48 h).

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(a)

they would be protonated at pH < 9.5, and the anionic PFCAs can be adsorbed via anion exchange. With increasing solution pH, the number of protonated amine groups decreased and some amine groups lost the ability of anion exchange for PFCAs [10], leading to the decrease of PFCAs removal. It is noteworthy that BAC kept relatively high removal of PFOA and would be more suitable for the removal of PFOA from wastewater at high pH. 3.6. Effect of inorganic ions

(b)

Fig. 5. Effect of inorganic ions on PFCA removal from simulated and actual wastewater by BAC (a) and IRA67 (b) (sorption conditions: 0.2 g/L BAC or 0.1 g/L IRA67 in wastewater at pH 4.0 for 48 h).

on BAC. For comparison, adsorption capacities of different reported ACs and resins for PFOA are shown in Table S4. The BAC used in this study showed a much higher adsorption capacity for PFOA than other GACs (qm = 0.29–0.37 mmol/g), even almost the same as the best powered activated carbon (qm = 1.05 mmol/g). The adsorption capacity of IRA67 for PFOA in actual wastewater in this study was very close to the reported value (2.92 mmol/g) on AI400 anionexchange resin for PFOA removal from simulated solution, and still higher than all ACs and non-ion resins. Evidently, both BAC and IRA67 exhibited high PFOA removal from the actual wastewater. 3.5. Effect of wastewater pH The crude product of PFOSF is commonly washed until the washing wastewater is close to neutral pH, and thus, the pH values of washing water are dependent on washing times. Fig. 4 presents the effect of wastewater pH on the removal of PFCAs by BAC and IRA67. The removal percents of PFHpA and PFOA by BAC decreased rapidly when pH increased from 2.0 to 4.0, but became stable at pH above 5.0 (Fig. 4(a)). In contrast, the PFHxA removal was less than 10% and decreased insignificantly in the pH range studied. Since the pKa values of PFHxA, PFHpA, and PFOA are −0.16, −0.15, and −2.0, respectively [21,31], they should exist as anions at pH above 2.0. The point of zero charge of the BAC was at pH 3.2 [20], and thereby the BAC should be positively charged at pH < 3.2 and negatively charged at pH > 3.2. With increasing pH, the electrostatic attraction between PFCAs and BAC at pH < 3.2 decreased, resulting in the significant decrease of PFCAs removal. At pH > 5.0, electrostatic repulsion prevented the adsorption of PFCAs on BAC, while hydrophobic interaction should play a dominant role in the sorption process. In comparison, the removal of three PFCAs by IRA67 decreased gradually with increasing wastewater pH from 2.0 to 9.0. Since the amine groups on IRA67 has a pKa of 9.5 [32],

Fig. 5 illustrates the effect of various ions on PFCAs removal by BAC and IRA67. The addition of salts in wastewater enhanced the removal of PFCAs by BAC, and the enhancement was affected by salt type and concentration (Fig. 5(a)). The effect of inorganic salts on the removal of PFCAs followed the increasing order of NaNO3 (0.04 mmol/L) < KCl (0.10 mmol/L) < NaCl (0.59 mmol/L) < Na2 SO4 (0.61 mmol/L) < NaF (1.31 mmol/L) < MgCl2 (0.96 mmol/L) < CaCl2 (0.70 mmol/L). The removal percents of PFCAs in NaNO3 solution were close to those in the blank solution (no salts), while the removal percents of three PFCAs in CaCl2 and MgCl2 solutions were significantly enhanced. The addition of salts can compress the electrical double layer of adsorbents [33,34], and decrease the electrostatic repulsion between BAC and PFCAs anions, resulting in the increase of PFCAs removal. The divalent cations, such as Ca2+ and Mg2+ could neutralize the surface negative charges of adsorbents and form bridges between negatively charged groups and PFC anions [35–37], making for high PFCAs removal. In addition, PFCAs solubility may decrease in the presence of salts, making them separated from bulk solution and adsorb onto the adsorbents, namely salting-out effect [38,39]. When BAC was used to remove PFCAs from the simulated wastewater containing the mixed above salts (the same salts in actual wastewater), the removal percents of PFCAs further increased. In comparison with the actual wastewater, the removal percents of three PFCAs in simulated wastewater are higher, suggesting that other organic matters except for PFCAs in the actual wastewater suppressed the removal of PFCAs. For example, the PFOSF washing wastewater is very likely to contain partially fluorinated carboxylic acids [23]. The effect of salts on the removal of PFCAs by IRA67 is shown in Fig. 5(b). Most salts had slight impact on the removal of PFCAs, and only Na2 SO4 significantly decreased the removal percents of PFCAs, indicating the strong competitive adsorption of divalent anions with PFCAs. It has been revealed in our previous study that SO4 2– can compete and interfere with the sorption of PFOS on the anion-exchange resins [10]. It is interesting that the removal of PFCAs from the simulated and actual wastewater containing the mixed salts was very close to that in the blank solution, and the suppression of SO4 2– was not observed. The possible reason is that the high concentrations of salts in the wastewater reduced the solubility of PFCAs via salt-out effect and promoted the formation of PFCAs micelles, offsetting the decrease of PFCAs removal by SO4 2– . Pan et al. [40] reported that the solubility of PFOS was 307 mg/L in 5 mmol/L CaCl2 solution, about half of the value in the pure water. The IRA67 removed a little more PFOA from the simulated solution in comparison with the actual wastewater, indicating little influence of organic matters on the PFCAs removal by IRA67. 3.7. Competitive adsorption Co-existing compounds significantly affect the adsorption of target compounds on adsorbents [41]. To study the competitive adsorption, the removal percents of PFHxA, PFHpA, and PFOA in the single and mixed PFCAs solutions by BAC and IRA67 were compared (Fig. 6). Both BAC and IRA67 in the single-solute system exhibited substantially higher removal of PFCAs than these in the mixed three-solute system. When three PFCAs coexisted in

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solution, the removal percents of PFHxA, PFHpA, and PFOA by IRA67 decreased by 79.8%, 67.9%, and 28.1%, respectively, while these values were 63.4%, 70.0%, and 35.0% by BAC. The similar decreasing trend was also observed in the actual wastewater. These results suggest that three PFCAs have the same adsorption sites on the BAC and IRA67, and the long-chain PFCAs are more preferential to be adsorbed on the adsorbents than the short-chain ones in the competitive adsorption process, which is consistent with the results obtained from the effect of adsorbent dose (Fig. 3). The removal percents of three PFCAs by BAC and IRA67 decreased in the order of PFOA > PFHpA > PFHxA in both systems, consistent with their sequence of C F chain length. For BAC, the long-chain PFCAs are more easily adsorbed via hydrophobic interaction, while the resin may adsorb the long-chain PFCAs through anion exchange and the formation of micells. It is notable that BAC rarely removed PFHxA and PFHpA, but kept high removal for PFOA in the mixed PFCAs system, indicating the highly selective removal of PFOA from wastewater. It is also interesting that the removal of three PFCAs was enhanced in actual wastewater, attributed to the effect of co-existing inorganic ions mentioned above. 3.8. Regeneration and reuse of adsorbents In our previous studies, it was discovered that the PFOSadsorbed IRA67 and BAC could be successfully regenerated by organic solvents with salt solution and ethanol solution, respectively [10,20]. According to this result, different ethanol solutions

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at 45 ◦ C were used to regenerate the spent BAC after adsorption of PFCAs in actual wastewater, whereas, the spent IRA67 was regenerated using the mixture of NaCl/NaOH and methanol/ethanol solution. It can be seen that 50% ethanol at 45 ◦ C and 1% NaCl in 70% methanol solution are best for the regeneration of the spent BAC and IRA67, respectively (Fig. S4). Fig. 7 shows the removal of PFCAs from actual wastewater by BAC and IRA67 in the five adsorption cycles. The removal percents of PFOA on the BAC decreased dramatically in the first two cycles from 78.4% to 68.5%, and decline slowly in the following cycles (Fig. 7(a)). The similar trend was also observed for PFHxA and PFHpA. This spent BAC regenerated by 50% ethanol at 45 ◦ C was found to maintain the stable removal of PFOS from the synthetic solution in our previous study [20]. The gradual decrease of PFCAs removal by BAC in reuse cycles in this study may be attributed to the adsorption of co-existing other organic compounds on BAC, and they occupied the sorption sites and could not be desorbed in the regeneration process. For IRA67, the removal percents of three PFCAs only decreased a little and kept relatively stable in the five cycles (Fig. 7(b)), showing a good reusability for this wastewater treatment. It is noteworthy that the concentrated PFCAs in the regeneration solutions could be further treated to obtain purified PFCAs products using the reported method [42]. It is true that the organic solvents used in the regeneration process are expensive,

(a)

(a)

(b) (b)

Fig. 6. Removal of PFCAs by BAC (a) and IRA67 (b) in the single and mixed PFCA solutions, as well as the actual wastewater (sorption conditions: 0.2 g/L BAC or 0.08 g/L IRA67 in solution at pH 4.0 for 48 h).

Fig. 7. PFCA removal by BAC (a) and IRA67 (b) in five successive adsorption cycles (sorption conditions: 0.25 g/L BAC or 0.1 g/L IRA67 in actual wastewater at pH 4.0 for 48 h).

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but methanol/ethanol can be recovered by distillation and reused to decrease the treatment cost in actual application. 4. Conclusions The PFOSF washing wastewater contains high concentrations of PFHxA, PFHpA, and PFOA, and BAC and IRA67 were found to be effective for the removal of these PFCAs. It required 0.3 g/L BAC or 0.2 g/L IRA67 to achieve over 88% removal of PFOA from the wastewater, while 1.9 g/L BAC or 1.5 g/L IRA67 was needed to obtain over 90% removal of all three PFCAs. Divalent cations in the wastewater caused the significant increase of PFCAs adsorption on BAC, while the presence of SO4 2− significantly suppressed the removal of PFCAs on IRA67. The co-existing other organic compounds in the wastewater were unfavorable for the adsorption of PFCAs on BAC, but had no effect on their adsorption on IRA67. The BAC exhibited the high selective adsorption of PFOA and stable removal of three PFCAs in the wide pH range, but its reusability was not satisfactory due to the decrease of PFCAs removal. Therefore, the BAC should be more suitable for the recovery of PFOA from actual wastewater, especially for PFCAs removal at high pH. In contrast, the IRA67 has the advantages of low dose and stable removal in the adsorption cycles. The PFCAs can be concentrated by both adsorbents and may be further purified from the regeneration solution to meet the reuse quality. The selection of suitable adsorbents for the treatment of PFOSF washing wastewater should consider adsorbent characteristics and treatment requirement. Removal of these PFCAs from PFOSF washing wastewater using adsorption technology is an effective and feasible method to control the source contamination of PFCs. Acknowledgements We thank the National High-Tech Research and Development Program of China (project no. 2013AA06A3), the National Nature Science Foundation of China (project no. 21177070), and Collaborative Innovation Center for Regional Environmental Quality for financial support. Additionally, the analytical work was supported by the Laboratory Fund of Tsinghua University. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2014. 12.037. References [1] S. Fujii, S. Tanaka, N.P. Hong Lien, Y. Qiu, C. Polprasert, New POPs in the water environment: distribution, bioaccumulation and treatment of perfluorinated compounds—a review paper, J. Water Supply Res. Technol. 56 (2007) 313. [2] K. Prevedouros, I.T. Cousins, R.C. Buck, S.H. Korzeniowski, Sources, fate and transport of perfluorocarboxylates, Environ. Sci. Technol. 40 (2005) 32–44. [3] M. Houde, A.O. De Silva, D.C.G. Muir, R.J. Letcher, Monitoring of perfluorinated compounds in aquatic biota: an updated review, Environ. Sci. Technol. 45 (2011) 7962–7973. [4] J.M. Conder, R.A. Hoke, W.D. Wolf, M.H. Russell, R.C. Buck, Are PFCAs bioaccumulative? A critical review and comparison with regulatory criteria and persistent lipophilic compounds, Environ. Sci. Technol. 42 (2008) 995–1003. [5] X. Dauchy, V. Boiteux, C. Rosin, J.F. Munoz, Relationship between industrial discharges and contamination of raw water resources by perfluorinated compounds. Part I: case study of a fluoropolymer manufacturing plant, Bull. Environ. Contam. Toxicol. 89 (2012) 525–530. [6] P. Wang, Y. Lu, T. Wang, Y. Fu, Z. Zhu, S. Liu, S. Xie, Y. Xiao, J.P. Giesy, Occurrence and transport of 17 perfluoroalkyl acids in 12 coastal rivers in south Bohai coastal region of China with concentrated fluoropolymer facilities, Environ. Pollut. 190 (2014) 115–122. [7] J.P. Benskin, L.W.Y. Yeung, N. Yamashita, S. Taniyasu, P.K.S. Lam, J.W. Martin, Perfluorinated acid isomer profiling in water and quantitative assessment of manufacturing source, Environ. Sci. Technol. 44 (2010) 9049–9054.

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