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Removal of perfluoroalkanesulfonic acids (PFSAs) from synthetic and natural groundwater by electrocoagulation
Chemosphere 248 (2020) 125951
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Removal of perfluoroalkanesulfonic acids (PFSAs) from synthetic and natural groundwater by electrocoagulation Jia Bao *, Wen-Jing Yu , Yang Liu **, Xin Wang , Zhi-Qun Liu , Yan-Fang Duan School of Science, Shenyang University of Technology, Shenyang, 110870, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
PFSAs in groundwater could be removed by the periodically reverse electrocoagulation. Orthogonal experiments were used to confirm the best conditions for the treatment. Optimal removal efficiencies of PFSAs in synthetic groundwater were up to 87.4%e100%. 59.0%e100% of removal efficiencies reached for PFSAs removal from natural groundwater.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 October 2019 Received in revised form 14 January 2020 Accepted 16 January 2020 Available online 22 January 2020
Severe contaminations of perfluoroalkanesulfonic acids (PFSAs) existed in the natural groundwater beneath a fluorochemical industrial park (FIP) in Fuxin of China. In the present study, systematic researches were performed to determine the best conditions of efficient treatment for 1 mg L1 of PFSAs in the synthetic groundwater samples with the periodically reverse electrocoagulation (PREC) using the Al eZn electrodes. Based upon the orthogonal experiments, the removal efficiencies of perfluorobutane sulfonate (PFBS), perfluorohexane sulfonate (PFHxS), and perfluorooctane sulfonate (PFOS) could reach 87.4%, 95.6%, and 100%, respectively, within the initial 10 min, under the optimal conditions of voltage at 12.0 V, pH at 7.0, and stirring speed at 400 rpm. In addition, the optimized PREC technique was further applied to remove the PFSA contaminations from the natural groundwater samples of the Fuxin FIP, subsequently generating the removal efficiencies of three target PFSA analytes in the range between 59.0% and 100% at 60 min. Moreover, the SEM-EDS analyses showed the hydroxide flocs formed during the process of PREC treatment had clear characteristics of floc aggregates, with the major constituents of O, Al, C, N, Zn, and F elements. As a result, long-chain PFHxS and PFOS tended to be eliminated completely from the natural groundwater by their absorptions on the AleZn hydroxide flocs, potentially because of their higher hydrophobicity compared with short-chain PFBS. © 2020 Elsevier Ltd. All rights reserved.
Handling Editor: E. Brillas Keywords: Perfluorobutane sulfonate (PFBS) Perfluorohexane sulfonate (PFHxS) Perfluorooctane sulfonate (PFOS) Periodically reverse electrocoagulation (PREC) Hydroxide flocs Groundwater
1. Introduction * Corresponding author. School of Science, Shenyang University of Technology, Shenliao West Road 111, Economic & Technological Development Zone, Shenyang, 110870, PR China. ** Corresponding author. School of Science, Shenyang University of Technology, Shenliao West Road 111, Economic & Technological Development Zone, Shenyang, 110870, PR China. E-mail addresses: [email protected] (J. Bao), [email protected] (Y. Liu). https://doi.org/10.1016/j.chemosphere.2020.125951 0045-6535/© 2020 Elsevier Ltd. All rights reserved.
Perfluoroalkyl substances (PFASs), consisting of perfluoroalkanesulfonic acids (PFSAs), perfluoroalkyl carboxylic acids (PFCAs), perfluoroalkyl phosphinic acids (PFPiAs), perfluoroether carboxylic and sulfonic acids (PFECAs and PFESAs), and their
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fluorotelomer-based precursors, have been widely manufactured and used as effective and efficient surfactants and surface protectors in a wide variety of domestic and industrial productions around the world, since their initial commercialization seventy years ago, including carpets, leather, paper, packaging, fabric, upholstery, aqueous film-forming foams (AFFFs), mining and oil well surfactants, alkaline cleaners, floor polishes, photographic film, denture cleaners, shampoos, and insecticide (OECD, 2018). It has been well-known that the enormously strong carbon-fluorine (CeF) bonds (115 kcal mol1) in the structures of perfluoroalkyl moiety contribute to the exclusive physio-chemical characteristics of PFASs, involving extraordinary resistance to both environmental and biological degradation, thermal and chemical stability against oxidation, photolysis, and hydrolysis reactions, as well as both hydrophobicity and oleophobicity (Kissa, 2001). Accordingly, the large-scale productions and widespread applications of PFASs have resulted in the ubiquitous distributions of these compounds in different biotic and abiotic matrices so far (Giesy and Kannan, 2001; Houde et al., 2006, 2011). Moreover, additional studies revealed that PFASs have bioaccumulative ability in diverse living organisms (Sedlak et al., 2017; Letcher et al., 2018) and various toxicities such as developmental toxicity, hepatotoxicity, and immunotoxicity (Lau et al., 2007; Gomis et al., 2018). Consequently, perfluorooctane sulfonate (PFOS), its salts and precursor, perfluorooctane sulfonyl fluoride (PFOSF), were added into Annex B of the Stockholm Convention on Persistent Organic Pollutants (POPs) in 2009, calling for restricted uses worldwide (UNEP, 2009). In the recent years, the Persistent Organic Pollutants Review Committee (POPRC) has also evaluated the proposals for potential inclusions of perfluorohexane sulfonate (PFHxS), perfluorooctanoic acid (PFOA), their salts and PFHxS- and PFOA-related compounds into the Stockholm Convention on POPs (UNEP, 2017, 2019). As an alternative for long-chain PFSAs (C 6), short-chain perfluorobutane sulfonate (PFBS) has been adopted for commercial manufacture, since the ban of PFOS production. However, the technical performance of short-chain PFBS are lower than that of long-chain PFHxS and PFOS, much larger quantities of PFBS have thus been employed to achieve a similar performance to PFHxS and PFOS (Lindstrom et al., 2011a). It is evident that short-chain PFASs are highly mobile in the water bodies, and their final degradation products are extremely persistent, hence a lack of proper water treatment techniques for short-chain alternates would bring about the never-ending existence of these contaminations in the aqueous environment (Brendel et al., 2018). Some further epidemiological studies have shown that populations expose to PFAS contaminations in the environment via several important pathways, such as daily intake of drinking water € lzer et al., 2008) and inhalation and dietary (Fromme et al., 2007; Ho of indoor dust (Strynar and Lindstrom, 2008), and the human exposure might lead to the increased levels of serum cholesterol, uric acid, liver enzymes, estradiol, and thyroid hormone (Nelson et al., 2010; Steenland et al., 2010; Lopez-Espinosa et al., 2012). Many evidences showed that PFAS contaminations could migrate into the shallow groundwater through the release of aqueous filmforming films (AFFFs) (Moody et al., 2003), the outflow of wastewater from fluorochemical production facilities (Hoffman et al., 2011), or the utilization of biosolids (Lindstrom et al., 2011b). As a result, the US Environmental Protection Agency (U.S.EPA) has issued a health advisory with the levels of 0.070 mg L1 for both PFOS and PFOA in drinking water originated from surface water and groundwater throughout the nation (U.S.EPA, 2016). In light of historical discharge of PFAS contaminations into local groundwater (ATSDR, 2005), the Minnesota Department of Health (MDH) recently established the health risk limits (HRLs) for both PFOS and PFOA in drinking water with even lower levels at 0.015 and
0.035 mg L1, respectively (MDH, 2018; 2019a). In addition, the HRLs for the other two PFSAs involving PFHxS and PFBS in drinking water were also issued lately by the MDH at 0.047 and 3 mg L1, respectively (MDH, 2017; 2019b). Fuxin fluorochemical industrial park (FIP) has been built in northeastern China since 2004, because of the local abundant resources of mineral fluorite (CaF2). Our previous study (Bao et al., 2019), focused on the groundwater beneath the FIP, determined that the dominant PFAS contaminants in regional groundwater, including PFOA, PFOS, PFHxS, and PFBS, reached the maximum concentrations up to 2.51, 0.403, 1.14, and 21.2 mg L1, respectively, all of which remarkably exceeded the updated HRLs from the MDH mentioned above. Furthermore, short-chain PFBS could enter the home-produced vegetables via irrigation with local groundwater. So far, few studies have been implemented on the efficient removal of PFAS contaminations from natural groundwater, especially short-chain PFASs, most of which concentrating on the treatment of groundwater PFOS and PFOA contaminations. For instance, Schaefer et al. (2015) applied the electrochemical treatment with commercially-produced Ti/RuO2 anode in a divided electrochemical cell for the decomposition of PFOS and PFOA in the AFFF-impacted groundwater from a former firefighter training area. In 2017, Schaefer et al. further used a nanocrystalline boron-doped diamond (BDD) anode for the electrochemical treatment of PFOS and PFOA in natural groundwater. Furthermore, Xiao et al. (2017) implemented on the removal of PFOS and PFOA contaminations from the AFFF-impacted groundwater by the sorption of biochars and activated carbon. Our recent study (Liu et al., 2018) adopted the periodically reverse electrocoagulation (PREC) with different electrode materials for the removal of PFOA contamination from the groundwater beneath the Fuxin FIP. Compared with the conventional EC technique, the PREC could efficiently eliminate the passivation generated from long-term use of single electrode (Pi et al., 2014). However, further studies would still be required to investigate the removal efficiencies of the PREC technique for the treatment of PFSA contaminations in the natural groundwater around the fluorochemical facilities. The objectives of the present study were to i) compare the treatment effects of the PREC technique on the removal of three PFSAs from the synthetic groundwater under different conditions, involving the voltage, the pH value, and the stirring speed, thereafter further confirm the optimal treatment parameters with the orthogonal experiments; ii) determine the removal efficiencies of all the PFSA contaminations from the natural groundwater using the optimized PREC technique; and iii) investigate the morphology features of the floc aggregates that generated during the process of PREC treatment based upon the additional analysis.
2. Materials and methods 2.1. Chemicals and reagents Native linear PFSAs including potassium PFBS, PFHxS, and PFOS were acquired from Wellington Laboratories (Guelph, Canada). HPLC-grade methanol and acetonitrile were obtained from Fluka (Steinheim, Germany). Sodium chloride (NaCl), sulfuric acid (H2SO4), sodium hydroxide (NaOH), and ammonium acetate (CH3COONH4) were purchased from Acros Organics (Geel, Belgium). Milli-Q water was cleaned using Waters Oasis HLB Plus (225 mg) cartridges (Milford, MA) to remove possible residue of PFSA contaminations. All reagents were used as received. Teflon in all the equipment and labware were avoided when possible to prevent any contamination.
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2.2. Cell construction and experiments As illustrated in our previous study (Liu et al., 2018), the PREC reactor was a 400 mL organic glass electrolytic cell with two parallel metal electrodes, and the size of each electrode was 60 40 2 mm independently. The whole electrodes were immersed in the synthetic PFSAs-containing groundwater vertically with constant stirring by a magnetic stirrer, and both electrodes were connected to a digital DC power supply, which provided the ranges of 0e3 A for current intensity and 0e30 V for voltage. The combination of aluminum (Al) and zinc (Zn) were adopted as the metal electrodes for the treatment of PFSA in the synthetic groundwater. Briefly, PFAS contaminants could be removed by the sorption of metal hydroxide that generated in-situ by electro-oxidation and in turn as the sacrificial anode, while the passivation coating was eliminated as the cathode mode (Lin et al., 2015). During the procedure of PREC treatment, all the initial concentration of three PFSA analytes, including PFBS, PFHxS, and PFOS, in the synthetic groundwater was set to 1 mg L1, voltage of processing was 9 V, space between both electrodes was 2 cm, stirring speed was 600 r min1, electrolyte concentration of NaCl was 1 g L1, and the reversing period was 10 s. 10 mL of synthetic solution was sampled at an interval of 10 min and then filtered with 0.22 mm glass fiber membrane for the analysis of PFSA concentrations. 2.3. Sample preparation and analysis In the present study, synthetic groundwater samples were prepared with mixed standard solutions of three PFSA analytes and several major inorganic salts in the Milli-Q water, while natural groundwater samples were collected from the shallow aquifer beneath the Fuxin FIP in July of 2019, and the concentrations of PFBS, PFHxS, and PFOS in the groundwater from Fuxin were determined as 31.0, 0.80, and 0.50 mg L1, respectively. All the synthetic and natural groundwater samples were pretreated with the modified method applied by Saito et al. (2004) to reduce the negative impact, which derived from diverse inorganic salts in the aqueous solutions, on the instrumental analysis. In detail, Waters Oasis HLB Plus cartridges were conditioned with 5 mL of methanol and 10 mL of Milli-Q water. Each sample of 10 mL aqueous solution was loaded onto the preconditioned cartridge at a flow rate of 1 mL min1, and then all the target analytes were eluted from the cartridges with 1 mL of methanol at a flow rate of 1 mL min1. Extracts of water samples were analyzed for three target PFSAs via high performance liquid chromatographyetandem mass spectrometry (HPLCeMS/MS). Chromatography was performed by an Agilent 1100 HPLC system (Palo Alto, CA). A 25 mL aliquot of extract was injected onto a 2.1 mm 100 mm (3.5 mm) Agilent Eclipse Plus C18 column (Palo Alto, CA) with 10 mM ammonium acetate and acetonitrile as mobile phases, initializing with 40% acetonitrile at a flow rate of 250 mL min1 and column temperature of 40 C. The gradient was increased to 90% acetonitrile at 9 min and then held for 2 min. In addition, an 8 min re-equilibration interval was run before each following sample. The HPLC system was interfaced to an Agilent 6410 Triple Quadrupole (QQQ) mass spectrometer (Santa Clara, CA) operated with electrospray ionization (ESI) in negative mode. Ions were monitored with a multiple reaction monitoring (MRM) mode. The gas temperature and ion spray voltage were maintained at 350 C and 4000 V. Ions were monitored with a multiple reaction monitoring (MRM) mode, and the MS/MS transition for each target analyte was described elsewhere (Bao et al., 2011). Finally, the flocs generated in the natural groundwater samples
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were filtered and freeze-dried after the PREC treatment, and the morphology features of the flocs were confirmed using the Scanning Electron Microscopy - Energy Dispersive Spectrometer (SEMEDS) system (Thermo Fisher Scientific, Waltham, MA). 2.4. Data analysis The removal efficiency of each PFSA (h) was calculated according to the following equation:
h¼
c0 ci 100% c0
c0 - The initial concentration of each PFSA, mg L1, ci - The concentration of each PFSA under different working conditions (voltage, pH, and stirring speed) at i time, mg L1. 3. Results and discussion 3.1. The effect of voltage Voltage is a commonly considered as a significant factor for the elimination of contaminants in the electrocoagulation process. Voltage could determine the amount of sacrifice electrodes dissolution, and affect the generation of metal hydroxide flocs as well as removal efficiency of contaminants consequently. The removal efficiencies of voltage with 6, 9, or 12 V for the synthetic PFSAscontaining groundwater were investigated in this study (Liu et al., 2018). The trends in removal efficiencies of PFBS, PFHxS, and PFOS concentrations by different voltages over time were shown in Fig. 1, illustrating that the removal efficiencies of three compounds increased with the growing of voltage. Remarkably, at the first 10 min, the removal efficiencies of three PFSA substances could reach 90%, when the voltage was set to 12 V. Nevertheless, the removal efficiencies of PFBS and PFHxS were unable to reach 70%, when the voltages were reduced to 6 or 9 V. In detail, the optimal removal efficiencies of PFBS, PFHxS, and PFOS were individually up to 97.2%, 98.4%, and 100% at 40 min, when the voltage was 12 V. According to the Faraday’s law, the increase of voltage might contribute to more cations dissolved from the sacrifice anode. The electrolytic reaction at the cathode increased the concentration of hydroxyl group (OH), enhancing the driving force of charged particles in the solution, and subsequently accelerating the formation of more metal hydroxide flocs to facilitate the adsorption of
Fig. 1. The effect on removal of PFSAs in synthetic groundwater with different voltages (AleZn electrodes, stirring speed ¼ 400 rpm, pH ¼ 7.0, reversing period ¼ 10 s).
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PFSAs (Liu et al., 2018). In addition, Fig. 1 also showed that the removal efficiencies of these contaminants were positively correlated to the chain length. Under the same conditions, the longer the carbon chain was, the higher the removal efficiency performed. Our previous study observed that hydrophobic interaction could play an important role for the removal of PFSAs during the process of electrocoagulation treatment (Liu et al., 2018). Since PFSA substances with longer CeF chain length tend to be more hydrophobic, PFOS was more easily adsorbed on flocs compared with PFHxS and PFBS. 3.2. The effect of pH The pH of aqueous solution is regarded as another factor for removing contaminants effectively during the electrocoagulation process. It has an effect on the occurrences of all the hydrolysis equilibria chiefly, and is quite sensitive to the addition of metallic cations, affecting on the formation of floc aggregates and the efficiency of decontamination (Bao et al., 2014). In the present study, the influence of pH values with 3.0, 5.0, 7.0, and 10.0 on the removal efficiencies of PFBS, PFHxS, and PFOS in the synthetic groundwater were estimated individually (Liu and Wu, 2019). As shown in Fig. 2, both acidic and neutral solutions were more favorable than the alkaline solution for the removal of PFSAs by electrocoagulation. Furthermore, PFBS and PFHxS could be removed efficiently from the neutral solution than the acidic solution. Moreover, the removal efficiencies of PFSAs gradually increased during the initial 5 min. After 10 min, the removal efficiencies of PFSAs were relatively stable with the increase of the time. During the process of PREC treatment with the pH of solution at 7.0, the removal efficiencies of 90.5%, 93.7%, and 100% were achieved for PFBS, PFHxS, and PFOS, respectively, indicating the removal of PFSAs were also correlated with the chain length under the same pH value. It was consistent with a previous study that the flocs of aluminum hydroxide were positively charged in the range of neutral pH, inducing the electrostatic attraction with the anionic PFSA species (Xiao et al., 2013). Consequently, the best pH value for eliminating PFSAs from aqueous solution could be fixed at 7.0. 3.3. The effect of stirring speed Stirring speed determines the collision contact of PFSA molecules with metal hydroxide flocs. The effect of stirring speed on the
Fig. 3. The effect on removal of PFSAs in synthetic groundwater with various stirring speeds (AleZn electrodes, voltage ¼ 9 V, pH ¼ 7.0, reversing period ¼ 10 s).
removal efficiencies of PFBS, PFHxS, and PFOS in the synthetic groundwater were also examined in this study, adopting the stirring speed in the range between 200 and 1000 rpm (Liu et al., 2018). As can be seen in Fig. 3, when the stirring speed was 400 rpm, the removal efficiencies of three target analytes were the best. In brief, PFSAs were mostly removed during the initial 5 min of the PREC process, and the optimal removal efficiencies of PFBS, PFHxS, and PFOS could reach 87.0%, 88.0%, and 99.7%, respectively. The results demonstrated that increasing the stirring speed in a certain range could improve the removal efficiencies of PFSA contaminants, but the removal efficiencies might be reduced when the stirring speed exceeded the range. As elucidated previously, the huge shear force produced at high stirring speed could destroy the sorption of PFSAs on hydroxide flocs (Liu et al., 2018). Furthermore, the removal efficiency of longer-chain PFSA was better than that of shorter-chain one. 3.4. Orthogonal experiments and analysis of the optimal parameters Based upon the single-factor conditional experiments, orthogonal experiments were implemented to optimize the process and determine the best combination for the PREC treatment of PFSA analytes in the synthetic groundwater. The influences of different voltages, pH values, and stirring speeds on the removal efficiencies of PFSAs were determined by the orthogonal experiments. The experimental factors were presented in Table 1 and the results of orthogonal text were shown in Table 2. According to Table 2, all the removal efficiencies of PFSAs followed the order of B > C > A (i.e. voltage > pH > stirring speed), which further confirmed that voltage was a significant factor affecting the removal efficiencies of three PFSA substances. As a result, the experimental results showed that the optimal combination for the PFSAs removal was A2B3C3,
Table 1 Factors and levels of the orthogonal text. Level number
Fig. 2. The effect on removal of PFSAs in synthetic groundwater with diverse pH (AleZn electrodes, voltage ¼ 9 V, stirring speed ¼ 400 rpm, reversing period ¼ 10 s).
1 2 3
Factor A
B
C
Stirring speed (rpm)
Voltage (V)
pH
200 400 600
6 9 12
3 5 7
J. Bao et al. / Chemosphere 248 (2020) 125951
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Table 2 Results from the orthogonal text. No.
1 2 3 4 5 6 7 8 9 K1 K2 K3 R K1 ' K2 ' K3 ' R0 K100 K200 K300 R00 PFBS (Kn), PFHxS
Factor
Removal efficiency (%)
A
B
C
PFBS
PFHxS
PFOS
1 1 1 2 2 2 3 3 3 229.5 233.3 234.4 4.90 257.9 258.2 251.7 6.50 295.3 296.0 297.5 2.20 (Kn’), PFOS (Kn")
1 2 3 1 2 3 1 2 3 223.0 232.0 242.2 19.2 242.0 247.0 275.8 33.8 297.6 291.7 299.5 7.80
1 2 3 2 1 3 2 3 1 228.5 243.1 225.3 17.8 260.9 268.7 235.2 33.5 299.5 296.0 293.3 6.20
74.5 80.3 74.7 75.4 80.1 87.4 73.1 73.9 77.8
83.8 83.1 91.0 90.0 89.2 95.6 68.2 87.9 76.0
99.8 96.0 99.5 100 100 100 97.8 99.7 96.0
consisting of the voltage at 12.0 V, the pH at 7.0, and the stirring speed at 400 rpm. Under the optimal condition, the best removal efficiencies of PFBS, PFHxS, and PFOS contaminations from the synthetic groundwater could be up to 87.4%, 95.6%, and 100%, respectively. 3.5. Elimination of PFSAs from the contaminated natural groundwater In the present study, natural groundwater samples around the Fuxin FIP in China were employed to evaluate the application of PREC technique for real water bodies. Based upon the instrumental analysis, the groundwater level of PFBS was detected much higher than that of PFHxS and PFOS. In detail, the concentrations of PFBS, PFHxS, and PFOS leaped to 31.0, 0.80, and 0.50 mg L1, respectively, compared with those reported previously (Bao et al., 2019). Electrocoagulation experiments were conducted under the optimized conditions of treatment mentioned above to remove all the PFSA contaminations from the natural groundwater from Fuxin, and the removal efficiencies of PFBS, PFHxS, and PFOS in groundwater were illustrated in Fig. 4. In general, PFOS was completely eliminated in 30 min, and the removal efficiency of PFHxS went up over time, and reached 88.2% at 60 min eventually. However, the removal efficiency for PFBS increased rapidly to 50% in the initial 5 min, and then slowly reached 59.0% in 30 min before the trend of slight decreasing observed. The ultimate removal efficiencies followed an increasing order as PFOS > PFHxS > PFBS, which might be because that short-chain PFSAs are hydrophilic and difficult to be adsorbed on the flocs. For instance, the most commonly studied PFBS has a logKow (neutral form) ¼ 2.82e4.60, water solubility > 20 g L1, logKoa ¼ 6.0e6.7, pKa <1, and logKoc ¼ 2.7e3.6 (Ateia et al., 2019). Moreover, PFBS was one of the main products from the Fuxin FIP (Bao et al., 2011). As the precursor of PFBS, perfluorobutanesulfonyl fluoride (PFBSF) might be transformed to PFBS during the process of PREC treatment for the contaminated groundwater samples, though the transformation procedure is warranted to be further confirmed. These findings indicated the electrocoagulation technique was practicable to eliminate PFSA substances from the contaminated natural groundwater. However, compared with the
removal efficiencies for the synthetic aqueous solutions, those for the nature groundwater were slightly lower, which may be attributed to the negative effects originated from complicated ions in the real water bodies. In order to investigate the morphology features of the hydroxide flocs, supplementary SEM-EDS analyses were carried on the flocs that generated in-situ during the process of PREC treatment with AleZn electrodes for the PFSA contaminations in the natural groundwater samples. As shown in Fig. 5, the results of SEM analysis illustrate that the hydroxide flocs formed were not smooth, but presented pore structures and clear aggregate characteristics. Furthermore, the EDS analysis reflected that the major constituents of the flocs contained the elements of O (68.1%), Al (21.2%), C (4.16%), N (4.97%), Zn (3.43%), and F (3.29%), demonstrating that the PFSA contaminants in natural groundwater could be adsorbed on the AleZn hydroxide floc aggregates.
Fig. 4. Removal efficiencies for PFSAs from the natural groundwater with the optimal treating conditions (AleZn electrodes, voltage ¼ 12 V, stirring speed ¼ 400 rpm, pH ¼ 7.0, reversing period ¼ 10 s).
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Fig. 5. SED image and EDS analysis of flocs formed in the process of PREC treatment for the contaminated natural groundwater.
4. Conclusions
References
In summary, the PREC treatment represents a promising approach for the removal of PFSA substances from both the synthetic and natural groundwater. Systematic researches on PFSAs in the synthetic aqueous solutions were implemented under the conditions of treatment, including the voltage, pH value, and stirring speed. The optimal treatment conditions were set as the voltage at 12 V, the pH value at 7.0, and the stirring speed at 400 rpm, based upon the orthogonal experiments. Under such conditions, the removal efficiencies of PFBS, PFHxS, and PFOS in the synthetic aqueous solutions could be up to 87.4%, 95.6%, and 100%, respectively. On the other side, those of the three PFSAs reached 59.0%, 88.2%, and 100% individually in the natural groundwater. Generally, the removal efficiencies of PFSAs had a positive correlation with the chain length, i.e. the contaminants with longer chain length were more likely to be eliminated by the PREC treatment. Furthermore, additional results from the SEM-EDS analyses revealed the hydroxide flocs formed had visible characteristics of floc aggregates, and the PFSA contaminants could be removed from the natural groundwater by their absorptions on the flocs. As a result, the PREC technique would be applied to the removal of PFSA contaminations from natural water bodies and even various wastewaters.
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CRediT authorship contribution statement Jia Bao: Conceptualization, Data curation, Funding acquisition, Investigation, Project administration, Resources, Supervision, Writing - original draft, Writing - review & editing. Wen-Jing Yu: Data curation, Formal analysis, Investigation, Validation, Visualization, Writing - original draft. Yang Liu: Conceptualization, Methodology, Project administration, Resources, Supervision, Writing - original draft, Writing - review & editing. Xin Wang: Methodology, Project administration, Resources, Software, Supervision, Validation. Zhi-Qun Liu: Formal analysis, Software, Visualization. Yan-Fang Duan: Software, Visualization.
Acknowledgements This research was supported by the National Natural Science Foundation of China (No.21976124 and No.21507092) and the Natural Science Foundation of Liaoning Province of China (No.2019ZD-0217).
J. Bao et al. / Chemosphere 248 (2020) 125951 Alabama, USA. Environ. Sci. Technol. 45, 8015e8021. Liu, Y., Hu, X.M., Zhao, Y., Wang, J., Lu, M.X., Peng, F.H., Bao, J., 2018. Removal of perfluorooctanoic acid in simulated and natural waters with different electrode materials by electrocoagulation. Chemosphere 201, 303e309. Liu, N., Wu, Y., 2019. Removal of methylene blue by electrocoagulation: a study of the effect of operational parameters and mechanism. Ionics 25, 3953e3960. Lopez-Espinosa, M.J., Mondal, D., Armstrong, B., Bloom, M.S., Fletcher, T., 2012. Thyroid function and perfluoroalkyl acids in children living near a chemical plant. Environ. Health Perspect. 120, 1036e1041. Minnesota Department of Health, 2017. Toxicological summary for: perfluorobutane sulfonate (PFBS). https://www.health.state.mn.us/communities/environment/ risk/docs/guidance/gw/pfbssummary.pdf. Minnesota Department of Health, 2018. Toxicological summary for: perfluorooctanoate (PFOA). https://www.health.state.mn.us/communities/ environment/risk/docs/guidance/gw/pfoa.pdf. Minnesota Department of Health, 2019a. Toxicological summary for: perfluorooctane sulfonate (PFOS). https://www.health.state.mn.us/communities/ environment/risk/docs/guidance/gw/pfos2010.pdf. Minnesota Department of Health, 2019b. Toxicological summary for: perfluorohexane sulfonate (PFHxS). https://www.health.state.mn.us/communities/ environment/risk/docs/guidance/gw/pfhxs.pdf. Moody, C.A., Hebert, G.N., Strauss, S.H., Field, J.A., 2003. Occurrence and persistence of perfluorooctanesulfonate and other perfluorinated surfactants in groundwater at a fire-training area at Wurtsmith Air Force Base, Michigan, USA. J. Environ. Monit. 5, 341e345. Nelson, J.W., Hatch, E.E., Webster, T.F., 2010. Exposure to polyfluoroalkyl chemicals and cholesterol, body weight, and insulin resistance in the general U.S. population. Environ. Health Perspect. 118, 197e202. OECD, 2018. Toward a new comprehensive global database of per- and polyfluoroalkyl substances (PFASs). http://www.oecd.org/officialdocuments/ publicdisplaydocumentpdf/?cote¼ENV-JM-MONO(2018)7&doclanguage¼en. Pi, K.W., Xiao, Q., Zhang, H.Q., Xia, M., Gerson, A.R., 2014. Decolorization of synthetic methyl orange wastewater by electrocoagulation with periodic reversal of electrodes and optimization by RSM. Process Saf. Environ. Protect. 92, 796e806. Saito, N., Harada, K., Inoue, K., Sasaki, K., Yoshinaga, T., Koizumi, A., 2004. Perfluorooctanoate and perfluorooctane sulfonate concentrations in surface water in Japan. J. Occup. Health 46, 49e59. Schaefer, C.E., Andaya, C., Urtiaga, A., McKenzie, E.R., Higgins, C.P., 2015.
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Electrochemical treatment of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) in groundwater impacted by aqueous film forming foams (AFFFs). J. Hazard Mater. 295, 170e175. Schaefer, C.E., Andaya, C., Burant, A., Condee, C.W., Urtiaga, A., Strathmann, T.J., Higgins, C.P., 2017. Electrochemical treatment of perfluorooctanoic acid and perfluorooctane sulfonate: insights into mechanisms and application to groundwater treatment. Chem. Eng. J. 317, 424e432. Sedlak, M.D., Benskin, J.P., Wong, A., Grace, R., Greig, D.J., 2017. Per- and polyfluoroalkyl substances (PFASs) in San Francisco Bay wildlife: temporal trends, exposure pathways, and notable presence of precursor compounds. Chemosphere 185, 1217e1226. Steenland, K., Tinker, S., Shankar, A., Ducatman, A., 2010. Association of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) with uric acid among adults with elevated community exposure to PFOA. Environ. Health Perspect. 118, 229e233. Strynar, M.J., Lindstrom, A.B., 2008. Perfluorinated compounds in house dust from Ohio and North Carolina, USA. Environ. Sci. Technol. 42, 3751e3756. U.S. Environmental Protection Agency, 2016. Fact Sheet: PFOA and PFOS drinking water health advisories. https://www.epa.gov/sites/production/files/2016-06/ documents/drinkingwaterhealthadvisories_pfoa_pfos_updated_5.31.16.pdf. UNEP, 2009. Stockholm convention on persistent organic pollutants (POPs). http:// chm.pops.int/Portals/0/Repository/convention_text/UNEP-POPS-COPCONVTEXT-FULL.English.PDF. UNEP, 2017. Risk management evaluation on pentadecafluorooctanoic acid (CAS No 335-67-1, PFOA, perfluorooctanoic acid), its salts and PFOA-related compounds. http://chm.pops.int/TheConvention/POPsReviewCommittee/Meetings/ POPRC13/Overview/tabid/5965/Default.aspx. UNEP, 2019. Draft risk management evaluation perfluorohexane sulfonic acid (PFHxS), its salts and PFHxS-related compounds. http://chm.pops.int/ TheConvention/POPsReviewCommittee/Meetings/POPRC15/Overview/tabid/ 8052/Default.aspx. Xiao, F., Simcik, M.F., Gulliver, J.S., 2013. Mechanisms for removal of perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA) from drinking water by conventional and enhanced coagulation. Water Res. 47, 49e56. Xiao, X., Ulrich, B.A., Chen, B., Higgins, C.P., 2017. Sorption of poly- and perfluoroalkyl substances (PFASs) relevant to aqueous film-forming foam (AFFF)impacted groundwater by biochars and activated carbon. Environ. Sci. Technol. 51, 6342e6351.
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Removal of perfluoroalkanesulfonic acids (PFSAs) from synthetic and natural groundwater by electrocoagulation Jia Bao *, Wen-Jing Yu , Yang Liu **, Xin Wang , Zhi-Qun Liu , Yan-Fang Duan School of Science, Shenyang University of Technology, Shenyang, 110870, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
PFSAs in groundwater could be removed by the periodically reverse electrocoagulation. Orthogonal experiments were used to confirm the best conditions for the treatment. Optimal removal efficiencies of PFSAs in synthetic groundwater were up to 87.4%e100%. 59.0%e100% of removal efficiencies reached for PFSAs removal from natural groundwater.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 October 2019 Received in revised form 14 January 2020 Accepted 16 January 2020 Available online 22 January 2020
Severe contaminations of perfluoroalkanesulfonic acids (PFSAs) existed in the natural groundwater beneath a fluorochemical industrial park (FIP) in Fuxin of China. In the present study, systematic researches were performed to determine the best conditions of efficient treatment for 1 mg L1 of PFSAs in the synthetic groundwater samples with the periodically reverse electrocoagulation (PREC) using the Al eZn electrodes. Based upon the orthogonal experiments, the removal efficiencies of perfluorobutane sulfonate (PFBS), perfluorohexane sulfonate (PFHxS), and perfluorooctane sulfonate (PFOS) could reach 87.4%, 95.6%, and 100%, respectively, within the initial 10 min, under the optimal conditions of voltage at 12.0 V, pH at 7.0, and stirring speed at 400 rpm. In addition, the optimized PREC technique was further applied to remove the PFSA contaminations from the natural groundwater samples of the Fuxin FIP, subsequently generating the removal efficiencies of three target PFSA analytes in the range between 59.0% and 100% at 60 min. Moreover, the SEM-EDS analyses showed the hydroxide flocs formed during the process of PREC treatment had clear characteristics of floc aggregates, with the major constituents of O, Al, C, N, Zn, and F elements. As a result, long-chain PFHxS and PFOS tended to be eliminated completely from the natural groundwater by their absorptions on the AleZn hydroxide flocs, potentially because of their higher hydrophobicity compared with short-chain PFBS. © 2020 Elsevier Ltd. All rights reserved.
Handling Editor: E. Brillas Keywords: Perfluorobutane sulfonate (PFBS) Perfluorohexane sulfonate (PFHxS) Perfluorooctane sulfonate (PFOS) Periodically reverse electrocoagulation (PREC) Hydroxide flocs Groundwater
1. Introduction * Corresponding author. School of Science, Shenyang University of Technology, Shenliao West Road 111, Economic & Technological Development Zone, Shenyang, 110870, PR China. ** Corresponding author. School of Science, Shenyang University of Technology, Shenliao West Road 111, Economic & Technological Development Zone, Shenyang, 110870, PR China. E-mail addresses: [email protected] (J. Bao), [email protected] (Y. Liu). https://doi.org/10.1016/j.chemosphere.2020.125951 0045-6535/© 2020 Elsevier Ltd. All rights reserved.
Perfluoroalkyl substances (PFASs), consisting of perfluoroalkanesulfonic acids (PFSAs), perfluoroalkyl carboxylic acids (PFCAs), perfluoroalkyl phosphinic acids (PFPiAs), perfluoroether carboxylic and sulfonic acids (PFECAs and PFESAs), and their
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fluorotelomer-based precursors, have been widely manufactured and used as effective and efficient surfactants and surface protectors in a wide variety of domestic and industrial productions around the world, since their initial commercialization seventy years ago, including carpets, leather, paper, packaging, fabric, upholstery, aqueous film-forming foams (AFFFs), mining and oil well surfactants, alkaline cleaners, floor polishes, photographic film, denture cleaners, shampoos, and insecticide (OECD, 2018). It has been well-known that the enormously strong carbon-fluorine (CeF) bonds (115 kcal mol1) in the structures of perfluoroalkyl moiety contribute to the exclusive physio-chemical characteristics of PFASs, involving extraordinary resistance to both environmental and biological degradation, thermal and chemical stability against oxidation, photolysis, and hydrolysis reactions, as well as both hydrophobicity and oleophobicity (Kissa, 2001). Accordingly, the large-scale productions and widespread applications of PFASs have resulted in the ubiquitous distributions of these compounds in different biotic and abiotic matrices so far (Giesy and Kannan, 2001; Houde et al., 2006, 2011). Moreover, additional studies revealed that PFASs have bioaccumulative ability in diverse living organisms (Sedlak et al., 2017; Letcher et al., 2018) and various toxicities such as developmental toxicity, hepatotoxicity, and immunotoxicity (Lau et al., 2007; Gomis et al., 2018). Consequently, perfluorooctane sulfonate (PFOS), its salts and precursor, perfluorooctane sulfonyl fluoride (PFOSF), were added into Annex B of the Stockholm Convention on Persistent Organic Pollutants (POPs) in 2009, calling for restricted uses worldwide (UNEP, 2009). In the recent years, the Persistent Organic Pollutants Review Committee (POPRC) has also evaluated the proposals for potential inclusions of perfluorohexane sulfonate (PFHxS), perfluorooctanoic acid (PFOA), their salts and PFHxS- and PFOA-related compounds into the Stockholm Convention on POPs (UNEP, 2017, 2019). As an alternative for long-chain PFSAs (C 6), short-chain perfluorobutane sulfonate (PFBS) has been adopted for commercial manufacture, since the ban of PFOS production. However, the technical performance of short-chain PFBS are lower than that of long-chain PFHxS and PFOS, much larger quantities of PFBS have thus been employed to achieve a similar performance to PFHxS and PFOS (Lindstrom et al., 2011a). It is evident that short-chain PFASs are highly mobile in the water bodies, and their final degradation products are extremely persistent, hence a lack of proper water treatment techniques for short-chain alternates would bring about the never-ending existence of these contaminations in the aqueous environment (Brendel et al., 2018). Some further epidemiological studies have shown that populations expose to PFAS contaminations in the environment via several important pathways, such as daily intake of drinking water € lzer et al., 2008) and inhalation and dietary (Fromme et al., 2007; Ho of indoor dust (Strynar and Lindstrom, 2008), and the human exposure might lead to the increased levels of serum cholesterol, uric acid, liver enzymes, estradiol, and thyroid hormone (Nelson et al., 2010; Steenland et al., 2010; Lopez-Espinosa et al., 2012). Many evidences showed that PFAS contaminations could migrate into the shallow groundwater through the release of aqueous filmforming films (AFFFs) (Moody et al., 2003), the outflow of wastewater from fluorochemical production facilities (Hoffman et al., 2011), or the utilization of biosolids (Lindstrom et al., 2011b). As a result, the US Environmental Protection Agency (U.S.EPA) has issued a health advisory with the levels of 0.070 mg L1 for both PFOS and PFOA in drinking water originated from surface water and groundwater throughout the nation (U.S.EPA, 2016). In light of historical discharge of PFAS contaminations into local groundwater (ATSDR, 2005), the Minnesota Department of Health (MDH) recently established the health risk limits (HRLs) for both PFOS and PFOA in drinking water with even lower levels at 0.015 and
0.035 mg L1, respectively (MDH, 2018; 2019a). In addition, the HRLs for the other two PFSAs involving PFHxS and PFBS in drinking water were also issued lately by the MDH at 0.047 and 3 mg L1, respectively (MDH, 2017; 2019b). Fuxin fluorochemical industrial park (FIP) has been built in northeastern China since 2004, because of the local abundant resources of mineral fluorite (CaF2). Our previous study (Bao et al., 2019), focused on the groundwater beneath the FIP, determined that the dominant PFAS contaminants in regional groundwater, including PFOA, PFOS, PFHxS, and PFBS, reached the maximum concentrations up to 2.51, 0.403, 1.14, and 21.2 mg L1, respectively, all of which remarkably exceeded the updated HRLs from the MDH mentioned above. Furthermore, short-chain PFBS could enter the home-produced vegetables via irrigation with local groundwater. So far, few studies have been implemented on the efficient removal of PFAS contaminations from natural groundwater, especially short-chain PFASs, most of which concentrating on the treatment of groundwater PFOS and PFOA contaminations. For instance, Schaefer et al. (2015) applied the electrochemical treatment with commercially-produced Ti/RuO2 anode in a divided electrochemical cell for the decomposition of PFOS and PFOA in the AFFF-impacted groundwater from a former firefighter training area. In 2017, Schaefer et al. further used a nanocrystalline boron-doped diamond (BDD) anode for the electrochemical treatment of PFOS and PFOA in natural groundwater. Furthermore, Xiao et al. (2017) implemented on the removal of PFOS and PFOA contaminations from the AFFF-impacted groundwater by the sorption of biochars and activated carbon. Our recent study (Liu et al., 2018) adopted the periodically reverse electrocoagulation (PREC) with different electrode materials for the removal of PFOA contamination from the groundwater beneath the Fuxin FIP. Compared with the conventional EC technique, the PREC could efficiently eliminate the passivation generated from long-term use of single electrode (Pi et al., 2014). However, further studies would still be required to investigate the removal efficiencies of the PREC technique for the treatment of PFSA contaminations in the natural groundwater around the fluorochemical facilities. The objectives of the present study were to i) compare the treatment effects of the PREC technique on the removal of three PFSAs from the synthetic groundwater under different conditions, involving the voltage, the pH value, and the stirring speed, thereafter further confirm the optimal treatment parameters with the orthogonal experiments; ii) determine the removal efficiencies of all the PFSA contaminations from the natural groundwater using the optimized PREC technique; and iii) investigate the morphology features of the floc aggregates that generated during the process of PREC treatment based upon the additional analysis.
2. Materials and methods 2.1. Chemicals and reagents Native linear PFSAs including potassium PFBS, PFHxS, and PFOS were acquired from Wellington Laboratories (Guelph, Canada). HPLC-grade methanol and acetonitrile were obtained from Fluka (Steinheim, Germany). Sodium chloride (NaCl), sulfuric acid (H2SO4), sodium hydroxide (NaOH), and ammonium acetate (CH3COONH4) were purchased from Acros Organics (Geel, Belgium). Milli-Q water was cleaned using Waters Oasis HLB Plus (225 mg) cartridges (Milford, MA) to remove possible residue of PFSA contaminations. All reagents were used as received. Teflon in all the equipment and labware were avoided when possible to prevent any contamination.
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2.2. Cell construction and experiments As illustrated in our previous study (Liu et al., 2018), the PREC reactor was a 400 mL organic glass electrolytic cell with two parallel metal electrodes, and the size of each electrode was 60 40 2 mm independently. The whole electrodes were immersed in the synthetic PFSAs-containing groundwater vertically with constant stirring by a magnetic stirrer, and both electrodes were connected to a digital DC power supply, which provided the ranges of 0e3 A for current intensity and 0e30 V for voltage. The combination of aluminum (Al) and zinc (Zn) were adopted as the metal electrodes for the treatment of PFSA in the synthetic groundwater. Briefly, PFAS contaminants could be removed by the sorption of metal hydroxide that generated in-situ by electro-oxidation and in turn as the sacrificial anode, while the passivation coating was eliminated as the cathode mode (Lin et al., 2015). During the procedure of PREC treatment, all the initial concentration of three PFSA analytes, including PFBS, PFHxS, and PFOS, in the synthetic groundwater was set to 1 mg L1, voltage of processing was 9 V, space between both electrodes was 2 cm, stirring speed was 600 r min1, electrolyte concentration of NaCl was 1 g L1, and the reversing period was 10 s. 10 mL of synthetic solution was sampled at an interval of 10 min and then filtered with 0.22 mm glass fiber membrane for the analysis of PFSA concentrations. 2.3. Sample preparation and analysis In the present study, synthetic groundwater samples were prepared with mixed standard solutions of three PFSA analytes and several major inorganic salts in the Milli-Q water, while natural groundwater samples were collected from the shallow aquifer beneath the Fuxin FIP in July of 2019, and the concentrations of PFBS, PFHxS, and PFOS in the groundwater from Fuxin were determined as 31.0, 0.80, and 0.50 mg L1, respectively. All the synthetic and natural groundwater samples were pretreated with the modified method applied by Saito et al. (2004) to reduce the negative impact, which derived from diverse inorganic salts in the aqueous solutions, on the instrumental analysis. In detail, Waters Oasis HLB Plus cartridges were conditioned with 5 mL of methanol and 10 mL of Milli-Q water. Each sample of 10 mL aqueous solution was loaded onto the preconditioned cartridge at a flow rate of 1 mL min1, and then all the target analytes were eluted from the cartridges with 1 mL of methanol at a flow rate of 1 mL min1. Extracts of water samples were analyzed for three target PFSAs via high performance liquid chromatographyetandem mass spectrometry (HPLCeMS/MS). Chromatography was performed by an Agilent 1100 HPLC system (Palo Alto, CA). A 25 mL aliquot of extract was injected onto a 2.1 mm 100 mm (3.5 mm) Agilent Eclipse Plus C18 column (Palo Alto, CA) with 10 mM ammonium acetate and acetonitrile as mobile phases, initializing with 40% acetonitrile at a flow rate of 250 mL min1 and column temperature of 40 C. The gradient was increased to 90% acetonitrile at 9 min and then held for 2 min. In addition, an 8 min re-equilibration interval was run before each following sample. The HPLC system was interfaced to an Agilent 6410 Triple Quadrupole (QQQ) mass spectrometer (Santa Clara, CA) operated with electrospray ionization (ESI) in negative mode. Ions were monitored with a multiple reaction monitoring (MRM) mode. The gas temperature and ion spray voltage were maintained at 350 C and 4000 V. Ions were monitored with a multiple reaction monitoring (MRM) mode, and the MS/MS transition for each target analyte was described elsewhere (Bao et al., 2011). Finally, the flocs generated in the natural groundwater samples
3
were filtered and freeze-dried after the PREC treatment, and the morphology features of the flocs were confirmed using the Scanning Electron Microscopy - Energy Dispersive Spectrometer (SEMEDS) system (Thermo Fisher Scientific, Waltham, MA). 2.4. Data analysis The removal efficiency of each PFSA (h) was calculated according to the following equation:
h¼
c0 ci 100% c0
c0 - The initial concentration of each PFSA, mg L1, ci - The concentration of each PFSA under different working conditions (voltage, pH, and stirring speed) at i time, mg L1. 3. Results and discussion 3.1. The effect of voltage Voltage is a commonly considered as a significant factor for the elimination of contaminants in the electrocoagulation process. Voltage could determine the amount of sacrifice electrodes dissolution, and affect the generation of metal hydroxide flocs as well as removal efficiency of contaminants consequently. The removal efficiencies of voltage with 6, 9, or 12 V for the synthetic PFSAscontaining groundwater were investigated in this study (Liu et al., 2018). The trends in removal efficiencies of PFBS, PFHxS, and PFOS concentrations by different voltages over time were shown in Fig. 1, illustrating that the removal efficiencies of three compounds increased with the growing of voltage. Remarkably, at the first 10 min, the removal efficiencies of three PFSA substances could reach 90%, when the voltage was set to 12 V. Nevertheless, the removal efficiencies of PFBS and PFHxS were unable to reach 70%, when the voltages were reduced to 6 or 9 V. In detail, the optimal removal efficiencies of PFBS, PFHxS, and PFOS were individually up to 97.2%, 98.4%, and 100% at 40 min, when the voltage was 12 V. According to the Faraday’s law, the increase of voltage might contribute to more cations dissolved from the sacrifice anode. The electrolytic reaction at the cathode increased the concentration of hydroxyl group (OH), enhancing the driving force of charged particles in the solution, and subsequently accelerating the formation of more metal hydroxide flocs to facilitate the adsorption of
Fig. 1. The effect on removal of PFSAs in synthetic groundwater with different voltages (AleZn electrodes, stirring speed ¼ 400 rpm, pH ¼ 7.0, reversing period ¼ 10 s).
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PFSAs (Liu et al., 2018). In addition, Fig. 1 also showed that the removal efficiencies of these contaminants were positively correlated to the chain length. Under the same conditions, the longer the carbon chain was, the higher the removal efficiency performed. Our previous study observed that hydrophobic interaction could play an important role for the removal of PFSAs during the process of electrocoagulation treatment (Liu et al., 2018). Since PFSA substances with longer CeF chain length tend to be more hydrophobic, PFOS was more easily adsorbed on flocs compared with PFHxS and PFBS. 3.2. The effect of pH The pH of aqueous solution is regarded as another factor for removing contaminants effectively during the electrocoagulation process. It has an effect on the occurrences of all the hydrolysis equilibria chiefly, and is quite sensitive to the addition of metallic cations, affecting on the formation of floc aggregates and the efficiency of decontamination (Bao et al., 2014). In the present study, the influence of pH values with 3.0, 5.0, 7.0, and 10.0 on the removal efficiencies of PFBS, PFHxS, and PFOS in the synthetic groundwater were estimated individually (Liu and Wu, 2019). As shown in Fig. 2, both acidic and neutral solutions were more favorable than the alkaline solution for the removal of PFSAs by electrocoagulation. Furthermore, PFBS and PFHxS could be removed efficiently from the neutral solution than the acidic solution. Moreover, the removal efficiencies of PFSAs gradually increased during the initial 5 min. After 10 min, the removal efficiencies of PFSAs were relatively stable with the increase of the time. During the process of PREC treatment with the pH of solution at 7.0, the removal efficiencies of 90.5%, 93.7%, and 100% were achieved for PFBS, PFHxS, and PFOS, respectively, indicating the removal of PFSAs were also correlated with the chain length under the same pH value. It was consistent with a previous study that the flocs of aluminum hydroxide were positively charged in the range of neutral pH, inducing the electrostatic attraction with the anionic PFSA species (Xiao et al., 2013). Consequently, the best pH value for eliminating PFSAs from aqueous solution could be fixed at 7.0. 3.3. The effect of stirring speed Stirring speed determines the collision contact of PFSA molecules with metal hydroxide flocs. The effect of stirring speed on the
Fig. 3. The effect on removal of PFSAs in synthetic groundwater with various stirring speeds (AleZn electrodes, voltage ¼ 9 V, pH ¼ 7.0, reversing period ¼ 10 s).
removal efficiencies of PFBS, PFHxS, and PFOS in the synthetic groundwater were also examined in this study, adopting the stirring speed in the range between 200 and 1000 rpm (Liu et al., 2018). As can be seen in Fig. 3, when the stirring speed was 400 rpm, the removal efficiencies of three target analytes were the best. In brief, PFSAs were mostly removed during the initial 5 min of the PREC process, and the optimal removal efficiencies of PFBS, PFHxS, and PFOS could reach 87.0%, 88.0%, and 99.7%, respectively. The results demonstrated that increasing the stirring speed in a certain range could improve the removal efficiencies of PFSA contaminants, but the removal efficiencies might be reduced when the stirring speed exceeded the range. As elucidated previously, the huge shear force produced at high stirring speed could destroy the sorption of PFSAs on hydroxide flocs (Liu et al., 2018). Furthermore, the removal efficiency of longer-chain PFSA was better than that of shorter-chain one. 3.4. Orthogonal experiments and analysis of the optimal parameters Based upon the single-factor conditional experiments, orthogonal experiments were implemented to optimize the process and determine the best combination for the PREC treatment of PFSA analytes in the synthetic groundwater. The influences of different voltages, pH values, and stirring speeds on the removal efficiencies of PFSAs were determined by the orthogonal experiments. The experimental factors were presented in Table 1 and the results of orthogonal text were shown in Table 2. According to Table 2, all the removal efficiencies of PFSAs followed the order of B > C > A (i.e. voltage > pH > stirring speed), which further confirmed that voltage was a significant factor affecting the removal efficiencies of three PFSA substances. As a result, the experimental results showed that the optimal combination for the PFSAs removal was A2B3C3,
Table 1 Factors and levels of the orthogonal text. Level number
Fig. 2. The effect on removal of PFSAs in synthetic groundwater with diverse pH (AleZn electrodes, voltage ¼ 9 V, stirring speed ¼ 400 rpm, reversing period ¼ 10 s).
1 2 3
Factor A
B
C
Stirring speed (rpm)
Voltage (V)
pH
200 400 600
6 9 12
3 5 7
J. Bao et al. / Chemosphere 248 (2020) 125951
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Table 2 Results from the orthogonal text. No.
1 2 3 4 5 6 7 8 9 K1 K2 K3 R K1 ' K2 ' K3 ' R0 K100 K200 K300 R00 PFBS (Kn), PFHxS
Factor
Removal efficiency (%)
A
B
C
PFBS
PFHxS
PFOS
1 1 1 2 2 2 3 3 3 229.5 233.3 234.4 4.90 257.9 258.2 251.7 6.50 295.3 296.0 297.5 2.20 (Kn’), PFOS (Kn")
1 2 3 1 2 3 1 2 3 223.0 232.0 242.2 19.2 242.0 247.0 275.8 33.8 297.6 291.7 299.5 7.80
1 2 3 2 1 3 2 3 1 228.5 243.1 225.3 17.8 260.9 268.7 235.2 33.5 299.5 296.0 293.3 6.20
74.5 80.3 74.7 75.4 80.1 87.4 73.1 73.9 77.8
83.8 83.1 91.0 90.0 89.2 95.6 68.2 87.9 76.0
99.8 96.0 99.5 100 100 100 97.8 99.7 96.0
consisting of the voltage at 12.0 V, the pH at 7.0, and the stirring speed at 400 rpm. Under the optimal condition, the best removal efficiencies of PFBS, PFHxS, and PFOS contaminations from the synthetic groundwater could be up to 87.4%, 95.6%, and 100%, respectively. 3.5. Elimination of PFSAs from the contaminated natural groundwater In the present study, natural groundwater samples around the Fuxin FIP in China were employed to evaluate the application of PREC technique for real water bodies. Based upon the instrumental analysis, the groundwater level of PFBS was detected much higher than that of PFHxS and PFOS. In detail, the concentrations of PFBS, PFHxS, and PFOS leaped to 31.0, 0.80, and 0.50 mg L1, respectively, compared with those reported previously (Bao et al., 2019). Electrocoagulation experiments were conducted under the optimized conditions of treatment mentioned above to remove all the PFSA contaminations from the natural groundwater from Fuxin, and the removal efficiencies of PFBS, PFHxS, and PFOS in groundwater were illustrated in Fig. 4. In general, PFOS was completely eliminated in 30 min, and the removal efficiency of PFHxS went up over time, and reached 88.2% at 60 min eventually. However, the removal efficiency for PFBS increased rapidly to 50% in the initial 5 min, and then slowly reached 59.0% in 30 min before the trend of slight decreasing observed. The ultimate removal efficiencies followed an increasing order as PFOS > PFHxS > PFBS, which might be because that short-chain PFSAs are hydrophilic and difficult to be adsorbed on the flocs. For instance, the most commonly studied PFBS has a logKow (neutral form) ¼ 2.82e4.60, water solubility > 20 g L1, logKoa ¼ 6.0e6.7, pKa <1, and logKoc ¼ 2.7e3.6 (Ateia et al., 2019). Moreover, PFBS was one of the main products from the Fuxin FIP (Bao et al., 2011). As the precursor of PFBS, perfluorobutanesulfonyl fluoride (PFBSF) might be transformed to PFBS during the process of PREC treatment for the contaminated groundwater samples, though the transformation procedure is warranted to be further confirmed. These findings indicated the electrocoagulation technique was practicable to eliminate PFSA substances from the contaminated natural groundwater. However, compared with the
removal efficiencies for the synthetic aqueous solutions, those for the nature groundwater were slightly lower, which may be attributed to the negative effects originated from complicated ions in the real water bodies. In order to investigate the morphology features of the hydroxide flocs, supplementary SEM-EDS analyses were carried on the flocs that generated in-situ during the process of PREC treatment with AleZn electrodes for the PFSA contaminations in the natural groundwater samples. As shown in Fig. 5, the results of SEM analysis illustrate that the hydroxide flocs formed were not smooth, but presented pore structures and clear aggregate characteristics. Furthermore, the EDS analysis reflected that the major constituents of the flocs contained the elements of O (68.1%), Al (21.2%), C (4.16%), N (4.97%), Zn (3.43%), and F (3.29%), demonstrating that the PFSA contaminants in natural groundwater could be adsorbed on the AleZn hydroxide floc aggregates.
Fig. 4. Removal efficiencies for PFSAs from the natural groundwater with the optimal treating conditions (AleZn electrodes, voltage ¼ 12 V, stirring speed ¼ 400 rpm, pH ¼ 7.0, reversing period ¼ 10 s).
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Fig. 5. SED image and EDS analysis of flocs formed in the process of PREC treatment for the contaminated natural groundwater.
4. Conclusions
References
In summary, the PREC treatment represents a promising approach for the removal of PFSA substances from both the synthetic and natural groundwater. Systematic researches on PFSAs in the synthetic aqueous solutions were implemented under the conditions of treatment, including the voltage, pH value, and stirring speed. The optimal treatment conditions were set as the voltage at 12 V, the pH value at 7.0, and the stirring speed at 400 rpm, based upon the orthogonal experiments. Under such conditions, the removal efficiencies of PFBS, PFHxS, and PFOS in the synthetic aqueous solutions could be up to 87.4%, 95.6%, and 100%, respectively. On the other side, those of the three PFSAs reached 59.0%, 88.2%, and 100% individually in the natural groundwater. Generally, the removal efficiencies of PFSAs had a positive correlation with the chain length, i.e. the contaminants with longer chain length were more likely to be eliminated by the PREC treatment. Furthermore, additional results from the SEM-EDS analyses revealed the hydroxide flocs formed had visible characteristics of floc aggregates, and the PFSA contaminants could be removed from the natural groundwater by their absorptions on the flocs. As a result, the PREC technique would be applied to the removal of PFSA contaminations from natural water bodies and even various wastewaters.
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CRediT authorship contribution statement Jia Bao: Conceptualization, Data curation, Funding acquisition, Investigation, Project administration, Resources, Supervision, Writing - original draft, Writing - review & editing. Wen-Jing Yu: Data curation, Formal analysis, Investigation, Validation, Visualization, Writing - original draft. Yang Liu: Conceptualization, Methodology, Project administration, Resources, Supervision, Writing - original draft, Writing - review & editing. Xin Wang: Methodology, Project administration, Resources, Software, Supervision, Validation. Zhi-Qun Liu: Formal analysis, Software, Visualization. Yan-Fang Duan: Software, Visualization.
Acknowledgements This research was supported by the National Natural Science Foundation of China (No.21976124 and No.21507092) and the Natural Science Foundation of Liaoning Province of China (No.2019ZD-0217).
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