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Role of the air-water interface in removing perfluoroalkyl acids from drinking water by activated carbon treatment
Journal of Hazardous Materials 386 (2020) 121981
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Role of the air-water interface in removing perfluoroalkyl acids from drinking water by activated carbon treatment
T
Pingping Menga,b, Xiangzhe Jianga, Bin Wanga, Jun Huanga, Yujue Wanga, Gang Yua, Ian T. Cousinsc,*, Shubo Denga,* a
State Key Joint Laboratory of Environment Simulation and Pollution Control, Beijing Key Laboratory for Emerging Organic Contaminants Control, School of Environment, Tsinghua University, Beijing, 100084, China b Department of Civil, Construction and Environmental Engineering, North Carolina State University, Raleigh, NC, 27695, United States c Department of Environmental Science and Analytical Chemistry (ACES), Stockholm University, SE-106 91, Stockholm, Sweden
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Editor: Xiaohong Guan
Contamination of drinking water by per- and polyfluoroalkyl substances (PFASs) is a worldwide problem. In this study, we for the first time revealed the role of the air-water interface in enhancing the removal of long-chain perfluoroalkyl carboxylic (PFCAs; CnF2n+1COOH, n ≥ 7) and perfluoroalkane sulfonic (PFSAs; CnF2n+1SO3H, n ≥ 6) acids, collectively termed as perfluoroalkyl acids (PFAAs), through combined aeration and adsorption on two kinds of activated carbon (AC). Aeration was shown to enhance the removal of long-chain PFAAs through adsorption at the air-water interface and subsequent adsorption of PFAA-enriched air bubbles to the AC. The removal of selected long-chain PFAAs was increased by 50–115 % with the assistance of aeration, depending on the perfluoroalkyl chain length. Aeration is more effective in enhancing long-chain PFAA removal as air-water interface adsorption increases with PFAA chain length due to higher surface activity. After removing adsorbed air bubbles by centrifugation, up to 80 % of the long-chain PFAAs were able to desorb from the sorbent, confirming the contribution of the air-water interface to the adsorption of PFAAs on AC. Aeration during AC treatment of water could enhance the removal of long-chain PFAAs, and improve the performance of AC during water treatment.
Keywords: PFAAs Air-water interface Activated carbon Aeration Adsorption
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Corresponding authors. E-mail addresses: [email protected] (I.T. Cousins), [email protected] (S. Deng).
https://doi.org/10.1016/j.jhazmat.2019.121981 Received 5 November 2019; Received in revised form 16 December 2019; Accepted 25 December 2019 Available online 26 December 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.
Journal of Hazardous Materials 386 (2020) 121981
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previous work first demonstrated the overlooked contribution of airwater interface to the PFAS adsorption on some non-porous adsorbents, by removing the air bubbles on the surface of adsorbents via vacuum degassing (Meng et al., 2014, 2017). However, the contribution of the air-water interface to PFAA adsorption on AC was not previously demonstrated, because of difficulties in eliminating the air bubbles from the porous structure of AC (Meng et al., 2014). Considering the wide application of AC in water treatment technologies, figuring out the influence of the air-water interface on the sorption performance of AC for PFAAs is of great importance. Based on the strong interfacial adsorption of long-chain PFAAs at the air-water interface, we hypothesize that if the solution is aerated during AC treatment, enhanced removal of the long-chain PFAAs will occur through concentration of the PFAAs at the air-water interface of air bubbles, and that the PFAA-enriched bubbles will be further adsorbed to the AC. This hypothesis was tested by investigating the influence of aeration on the adsorption of nine PFAAs with different perfluoroalkyl chain lengths and functional groups at environmentally realistic concentrations, using two kinds of AC with different particle sizes. The objectives of this study are to: 1) demonstrate enhanced PFAA removal when combining AC adsorption with aeration, 2) determine the contribution of aeration to the removal of different PFAAs, and 3) propose the mechanism of enhanced PFAA retention during AC treatment combined with aeration.
1. Introduction Per- and polyfluoroalkyl substances (PFASs) are a class of synthetic organic chemicals which have attracted worldwide attention as contaminants of high concern (Moody and Field, 1999; Wang et al., 2015a). Among PFASs, the subclasses that have received the most attention are the perfluoroalkyl carboxylic (PFCAs) and perfluoroalkane sulfonic (PFSAs) acids, collectively termed here as perfluoroalkyl acids (PFAAs). The concentrations of PFAAs in groundwater can reach μg/L or even mg/L levels at some contaminated sites, which can result contamination of drinking water with PFAAs (Moody and Field, 1999; Backe et al., 2013; Yin et al., 2017). In 2018, the Agency for Toxic Substances and Disease Registry (ATSDR) published the toxicological profile describing the toxicological properties of different PFAAs (Agency for Toxic Substances and Disease Registry, 2018). Since drinking water intake is shown to be an important source of PFAA exposure in our daily life, guidelines for safe PFAA levels in drinking water have been published in different countries (Livsmedelsverket, 2017; Mackay, 2018; USEPA, 2016). For example, the recommended guideline value for drinking water provided by the Swedish National Food Agency is 90 ng/L for the sum of PFASs (Livsmedelsverket, 2017), and the European Food Safety Agency (EFSA) provided preliminary tolerable weekly intakes (TWIs) for perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) of 13 ng/kg and 6 ng/kg bodyweight per week, respectively (Mackay, 2018). The need for efficient removal of PFASs, including PFAAs, from water is growing as a result of increasing regulatory attention and general concern regarding exposure to PFASs (Agency for Toxic Substances and Disease Registry, 2018; Livsmedelsverket, 2017; Mackay, 2018; USEPA, 2016). However, traditional water treatments demonstrated poor removal efficiency for PFAAs, including flocculation, coagulation, sand filtration, ozonation and so on (Appleman et al., 2014; Flores et al., 2013; Pan et al., 2016; Shivakoti et al., 2010). The concentrations of PFAAs in the effluent of drinking water treatment plants (DWTPs) can be even higher than those in the influent, due to PFAA desorption or degradation of unknown PFAA-precursors (Takagi et al., 2008, 2011). Comparatively, adsorption by activated carbon (AC) has been proven to be efficient in removing PFAAs from water (Kothawala et al., 2017; McCleaf et al., 2017; Qu et al., 2009). Nevertheless, the removal of PFAAs by AC is usually PFAA-dependent and co-contaminants-dependent (Appleman et al., 2014; Eschauzier et al., 2012; Liu et al., 2019). Short-chain PFAA (PFCAs with less than 7, and PFSAs with less than 6, fluorinated carbons) retention by AC is less satisfactory as a result of lower hydrophobicity and higher mobility in the aqueous phase (Sun et al., 2016; Du et al., 2015). Meanwhile, ACs tend to adsorb all the dissolved organic matters (DOMs) indiscriminately, resulting in a reduced life-time of AC when dealing with water containing high concentrations of DOMs (Appleman et al., 2013). Overall, improving the performance of AC for PFAA removal is of great significance in water treatments. The interfacial behavior of PFAAs at air-water interface is attracting more attention, which may influence their transport and removal in aquatic and terrestrial environment (Johansson et al., 2019; Brusseau, 2018; Brusseau et al., 2019; Schaefer et al., 2019; McMurdo et al., 2008). A recent study suggested that the air-water interface contributed up to 75 % of PFOA retention in an unsaturated porous column, since the air-water interface inside the column provide with more adsorption sites for PFOA (Lyu et al., 2018). And the relative magnitudes of PFAA adsorption at solid-water or air-water interface were compared using a quantitative-structure/property-relationship (QSPR) analysis, suggesting that the air-water interface adsorption was dependent on the total air-water interface area and the properties of solid-phase properties (Brusseau, 2019). Based on the adsorption of PFASs at air-water interfaces, technologies consisting of aeration and follow-up abstraction of generated aerosols or foams were developed for the removal of PFAAs (Meng et al., 2018; Ebersbach et al., 2016). Moreover, our
2. Materials and methods 2.1. Chemicals and materials Coal-based AC (AC100, AC350) of different particle sizes (100 μm or 350 μm) was purchased from Zhengzhou Yedao Environmental Protection Co. (China). The particle size, BET surface area and pore size distribution of the different types of AC are provided in Table S1 and Fig. S1. Tap water used for adsorption and aeration experiments was taken directly from the laboratory tap at Stockholm University, and the properties of the tap water are shown in Table S2. Milli Q-water was used for sample preparation and during analyzing procedures. Native and mass-labeled PFAAs (Table S3) were purchased from Wellington Laboratories Inc. 2.2. Adsorption and aeration experiments All adsorption experiments were conducted in a horizontal shaker with a shaking speed of 180 rpm (rpm) at around 22 °C (Fig. 1). The initial nominal concentration of the solution was set at around 1 μg/L per PFAA (measured concentrations are listed in Table S4) for all the experiments by spiking the tap water with multi-solute PFAA stock solution. The pH of the PFAA solutions was 8.5 ( ± 0.1). All the experiments were conducted in triplicate, and arithmetic mean values with error bars were used in figures. Adsorption experiments combined with aeration were conducted with 400 mL PFAA solutions in 1 L polypropylene bottles. An amount of 5 mg AC100 or 15 mg AC350 was added for adsorption (the dosage of AC350 was higher to maintain a similar PFAS removal), and continuous aeration was performed during the adsorption. Aeration was performed by using a titanium filter with an average pore diameter of 10 μm (Fig. 1), similar to that used in our previous study (Meng et al., 2018). A purified nitrogen system was used to maintain a steady air flow as well as to minimize the blank contamination in the air. The effects of aeration flow rate on the removal of PFAAs was investigated by adjusting the aeration flow rate from 0 mL/min to 250 mL/min. Unless otherwise specified, the aeration flow rate was controlled at 250 mL/ min. During the aeration, bottles were shaken intermittently by hand to remove the aerosol drops on the walls and caps of the bottles due to the bursting of air bubbles on the surface of the solution. For comparison, adsorption experiments without aeration were conducted under the 2
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Fig. 1. Setups for adsorption and adsorption combined with aeration experiments.
same conditions. To evaluate the atmospheric transmission of PFAAs during aeration, aeration experiments (without the addition of AC) with the presence or absence of intermittent shaking (to recover the aerosols) by hand were conducted. Samples were taken after adsorption and aeration for 24 h.
Waters), similar to a previous study (Johansson et al., 2019). Procedural blank samples were prepared throughout the sample preparation procedures to determine blank contamination.
2.3. Desorption experiments by centrifugation
The concentrations of PFAAs in the tap water varied from 0.18 to 5.43 ng/L (Table S5), which were much lower than the spiked PFAA concentrations. Procedural blank samples were set during all the experiments in case of blank contamination. The limit of detection (LOD) and the limit of quantification (LOQ) of the instrument were calculated as the mean value of Milli-Q samples (n = 10) plus three/ten times their standard deviation, respectively. And the recovery of PFAA during sample treatments was evaluated by recovery experiments, performed with a group (n = 10) of 30 mL tap water spiked with 1 μg/L per PFAA. After centrifuging for 5 min, 15 mL of the supernatant was taken and spiked with IS for SPE treatment before analysis. Briefly, negligible PFAA contamination was observed in the procedural blank samples, and the loss of PFAAs onto the container walls was less than 5 % (Fig. S4). The LOQ of different PFAAs varied from 0.02 ng/L to 0.92 ng/L, and the recovery of different PFAAs in the recovery experiments varied from 75 % to 95 %, depending on the perfluoroalkyl chain length. Further descriptions of method evaluation, including the LOD, LOQ, accuracy and precision of the method could be found in Table S5 and Table S6. In conclusion, the results of the method evaluation demonstrate the reliability of this study.
2.5. Method evaluation
After adsorption, or adsorption combined with aeration for 24 h, the bottles were shaken completely before sampling to homogenize the AC solution as well as to avoid the influence of aerosols. A volume of 15 mL AC solution was added into the centrifuge tubes in duplicate, and the samples were then divided into two groups. Group A samples were centrifuged for 15 min in a high-speed centrifuge with a speed of 5000 rpm, while Group B samples were settled by gravitation for 15 min as comparison. Supernatant was then taken for the analyzation. A schematic diagram for desorption by centrifugation is shown in Fig. S2. By comparing the PFAA removal efficiency in Group A and Group B samples, the desorption efficiency of PFAAs by centrifugation was calculated using Eq. (1):
DEPFAA =
RemovalA, PFAA − RemovalB, PFAA RemovalA, PFAA
(1)
where DEPFAA is the desorption efficiency of a single PFAA, RemovalA,PFAA is the removal efficiency of PFAAs in Group A samples, and RemovalB,PFAA is the removal efficiency of PFAAs in the corresponding Group B samples.
3. Results and discussion 2.4. Sample treatments and analysis 3.1. Enhanced PFAA adsorption on AC with the assistance of aeration A schematic diagram for sample treatments is shown in Fig. S3. After adsorption or adsorption combined with aeration for 24 h, the bottles were shaken completely before sampling to avoid the influence of aerosol drops on the walls and caps. Gravitational settling was used to separate the AC and supernatant samples were taken. Oasis Weakanion exchange (WAX) solid phase extraction (SPE) cartridges were used to load the samples. A detailed description of the SPE procedure and method description can be found in the SI that refers to a previous study (Lofstedt Gilljam et al., 2016; Benskin et al., 2012). The samples were finally analyzed by using a UPLC/MS/MS system (Xevo TQ-S,
The removal of total PFAAs was controlled below 50 % by using low AC dosage to better evaluate the influence of simultaneous aeration on the adsorption of PFAAs on AC. As illustrated in Fig. 2, the results are consistent with our hypothesis that increasing the air-water interface by aeration will increase PFAA removal significantly. Generally, the removal of total PFAAs by both AC increased by 50 % (Fig. 2b and 2d) with the assistance of aeration. The similar increased ratios of PFAA removal by AC100 and AC350 could be explained by their similar total surface areas (Brusseau, 2019), and therefore similar PFAA removal 3
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Fig. 2. Influence of aeration on the removal efficiency of PFAAs and increased ratio of removal of long-chain PFAAs on AC100 (a, b) and AC350 (c, d).
perfluorobutanesulfonic acid (PFBS), aeration showed no obvious influence on their adsorption onto the AC.
efficiencies. Aeration was shown to accelerate the mass-transfer of atrazine in the solution, and it was proved that physical mixing displayed similar effects on atrazine mass transfer as aeration (Jia et al., 2006). Namely, the influence of aeration on PFAA mass-transfer could be counteracted by physical mixing, for which reason all the adsorption experiments were conducted in a horizontal shaker with a shaking speed of 180 rpm. Moreover, samples were taken after adsorption for 24 h, when adsorption equilibrium was close to be reached, indicating that the increased PFAA removal should not be attributed to accelerated PFAA adsorption kinetics. All the PFAAs displayed a higher removal when aeration occurred, except for three short-chain PFAAs. Specifically, aeration was demonstrated to be more efficient in enhancing the removal of long-chain PFAAs. PFCAs and PFSAs are both fluorinated surfactants with the ability to decrease the surface tension of water solution by accumulating at the air-water interface (Hill et al., 2018). The surface activity of PFCAs and PFSAs increases exponentially with their perfluoroalkyl chain length (Schaefer et al., 2019; Fernandez et al., 2016). Consequently, the contribution of the air-water interface to the adsorption of PFAAs onto AC is also dependent on their chain length (Fig. 2b and 2d). For example, the removal efficiencies of perfluorodecanoic acid (PFDA) by AC100 and AC350 were increased by 115 % and 75 %, respectively. For other long-chain PFAAs, such as perfluorononanoic acid (PFNA) and PFOS, aeration was able to increase their removal efficiencies by 65%–90%. The increased PFAA removal with the assistance of PFAA might be beneficial to improve the life-time of AC, as more water could be treated with the same dosage of AC, yet further evaluation at pilot- or full-scale is required. However, for short-chain PFAAs such as perfluoropentanoic acid (PFPeA), perfluorohexanoic acid (PFHxA) and
3.2. Influence of aeration flow rate on PFAA adsorption on AC100 To further verify the role of the air-water interface on PFAA adsorption on the AC, the influence of the aeration flow rate on the PFAA removal was investigated. By increasing the aeration flow rate from 100 mL/min to 250 mL/min, the removal of total PFAAs by AC100 increased from 25 % to 32 %, increasing by 28 %, suggesting that increasing the aeration flow rate is beneficial for PFAA removal (Fig. 3). Similarly, the influence of the aeration flow rate on the removal of PFAAs is also chain-length dependent. The removal of long-chain PFAAs was more strongly affected by the increasing aeration flow rate. For example, increasing the aeration flow rate enhanced the removal of PFDA (nperfluorocarbons = 9) more efficiently than PFHpA (nperfluorocarbons = 6). According to our previous study, air bubbles on the surface of some hydrophobic carbonaceous adsorbents displayed considerable adsorption capacity for PFOS (Meng et al., 2014), suggesting that the air-water interface provides ideal adsorption sites for compounds with high surface activity such as PFOS. The enhanced retention of PFAAs by AC in the presence of aeration is also consistent with the behavior of PFOA retention in some unsaturated columns, in which the adsorption at the air-water interface contributes up to 75 % of the PFOA retention (Lyu et al., 2018). Despite that the aforementioned column tests were designed to evaluate the PFOA migration potential in some vadose zones, instead of improving the PFOA removal, similar effects may be important to consider when designing AC columns for PFAS removal. AC adsorption beds are commonly used for 4
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Fig. 3. The influence of aeration flow rate on the removal efficiency of PFAAs by AC100.
Fig. 5. The relationship between the number of fluorinated carbons and the removal of PFAA by aeration.
the removal of PFAAs at the pilot- or full-scale, yet the role of the airwater interface in PFAA removal remains unclear. By running the AC columns under unsaturated condition, the retention of PFAAs by the AC could be potentially improved, with air-water interface providing with more adsorption sites. Accordingly, a later breakthrough of AC is anticipated, which could potentially prolong the life time of AC. Although increasing the aeration flow rate is beneficial for PFAA removal, the generation of aqueous aerosols was also observed as a result of the bursting of rising air bubbles, and bubble busting increased with aeration flow rate. Intermittent manual shaking was shown to recover all the formed aerosols from the walls and caps of the bottles even at the maximum aeration rate of 250 mL/min when maximum bubble bursting and aerosol ejection occurred (see below).
shaking was less than 2 %, indicating that the removal of PFAAs via the formation of aerosols could be neutralized by recovering the aerosols drops on the walls and caps into the solution. On the contrary, a high loss of long-chain PFAAs (up to about 60 % for PFDA) was observed after aeration without intermittent shaking. The removal of both PFCA and PFSA increased proportionally with the increasing number of fluorinated carbons (Fig. 5), as a result of increasing surface activity. Aeration alone showed no removal for the short-chain PFAAs, such as PFPeA, PFHxA and PFBS, which are more mobile in the aqueous environment and in urgent needs for efficient removal (Sun et al., 2016; Wang et al., 2015b). The removal of longchain PFAAs by aeration alone is consistent with a previous study (Ebersbach et al., 2016). Since PFAAs tend to accumulate at the surface of air bubbles (i.e. air-water interface) in the solution, aerosols enriched with PFAAs (at the air-water interface) will be formed when the air bubbles burst at the water surface, resulting in a decrease of the PFAA concentrations in water (Ebersbach et al., 2016). However, an effective collection of aerosol droplets formed from bubble bursting is not feasible, especially at full-scale water treatment facilities. For example, large aerosol particles may coagulate and drop directly back into the solution, while small aerosol particles may be rapidly transported away in the air. The sub-micrometer aerosols are especially likely to be transported long distances (Cochran et al., 2016; McInnes et al., 2013), suggesting that the water-to-air transport might be one of the explanations for the PFAAs long-range atmospheric transport (Johansson et al., 2019; McMurdo et al., 2008). The direct evaporation of PFAAs to the atmosphere during aeration was also investigated. The outlet air above the water surface in the adsorption experiments was made to pass through a liquid consisting of methanol and water (Fig. 1), which was used to fully absorb the PFAAs in the air. The PFAA concentrations in the outlet gas sink and the calculated mass ratio of PFAAs detected compared to the mass spiked in the adsorption/aeration reactor are listed in Table S7, suggesting that the transfer of PFAAs to the atmosphere was negligible (< 0.06 %). Due to the low pKa of PFAAs (Maimaiti et al., 2018), all the PFAAs in this study exist as anions in solution, with negligible volatility with an experimental water pH of 8.5 (Reth et al., 2011). Especially for the longchain PFAAs, it has been shown that the volatilization is negligible at pH > 2.5 (Johansson et al., 2017; Vierke et al., 2013). Based on the aforementioned discussion, the water-to-air gaseous transport of PFAAs into the atmosphere should have negligible contribution to the increased PFASs removal.
3.3. Atmospheric emission of PFAAs during aeration As mentioned above, aerosols were formed continuously during the aeration. To ensure that the increased removal of PFAAs was attributed to the increase of air-water interface adsorption, instead of by generating PFAA-enriched aerosols during the aeration, all the bottles were shaken upside down intermittently to recover the liquid aerosol droplets on the walls and caps. Control experiments were done by testing the removal of PFAAs by aeration alone (in the absence of AC), with the presence or absence of intermittent shaking by hand. According to Fig. 4, the loss of PFAAs after aeration for 24 h with intermittent
Fig. 4. The removal efficiency of PFAAs by aeration with the presence or absence of intermittent shaking by hand. 5
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Fig. 6. PFAA desorption efficiency by centrifugation for AC after adsorption with aeration (a) or adsorption alone (b).
is especially relevant for the remediation of PFAAs from water, revealing the underappreciated role of air-water adsorption in the AC treatment for long-chain PFAAs. Finally, it should be noted that the desorption treatment in this study is not able to regenerate the spent AC completely, considering that the PFAAs adsorbed on the solid-phase would not be desorbed via centrifuging. Therefore, the reutilization of the AC after adsorption and aeration was not discussed in this study, which usually requires incineration at extremely high temperature over 700 ℃ (Watanabe et al., 2018, 2016).
3.4. Desorption of PFAAs by removing the adsorbed bubbles on AC Based on the aforementioned discussion, a considerable fraction of the PFAAs was adsorbed at the surface of the air bubbles during aeration, which were then adsorbed onto the AC. The stability of such airwater interfaces was important to evaluate the treatability of this method, for which reason the bottles after adsorption or adsorption combined with aeration were additionally settled for different time before sampling. As shown in Fig. S6, the removal of PFOS by adsorption combined with aeration remained stable after settling for 24 h, suggesting that the air-water interface adsorption in this study could exist for at least 24 h, making sure that no adsorbed PFAAs would be released back into the solution during water treatment. To quantify the contribution of air-water interface adsorption to the adsorption of PFAAs on AC, desorption experiments were conducted. After adsorption or adsorption combined with aeration for 24 h, the spent AC solutions were centrifuged in a high-speed centrifuge, which was proven to be effective in removing adsorbed air bubbles from the adsorbents (LaFratta et al., 2004). Control experiments showed negligible losses of PFAAs during centrifugation (Fig. S5). As shown in Fig. 6, for the AC after adsorption combined with aeration, removing adsorbed air bubbles by centrifugation resulted in a desorption efficiency of 48%–80% for the long-chain PFAAs, in agreement with Fig. 1b, where aeration was shown to improve the removal of long-chain PFAAs by 50%–120%. For the long-chain PFAAs, aeration showed a larger influence on their adsorption, and the desorption efficiency by removing the air-water interface via high-speed centrifuging was correspondingly higher. Specifically, the desorption efficiency of PFAA increased proportionally with the increasing number of fluorinated carbons in PFCA and PFSA, comparable to the removal efficiency of PFAAs by aeration alone (the slope of the lines in Fig. 5 and Fig. 6a are similar). Surprisingly, even for the AC after adsorption alone (without aeration), the long-chain PFAAs displayed a desorption efficiency of around 28 % to 35 %, suggesting that the air-water interface played a considerable role in the adsorption of long-chain PFAAs on AC under routine operation. The desorption efficiency of PFAAs is higher for the adsorption combined with aeration scenario, which might be explained by a larger number of PFAA-enriched air bubbles with the assistance of aeration. Notably, large error bars were observed in Fig. 6, especially for the short-chain PFAAs and in the adsorption scenario, which might be explained by their lower removal efficiency and smaller contribution of air-water interface, respectively. Especially in the adsorption scenario, where the solution was not as saturated with air as in the adsorption combined with aeration scenario, the magnitude of PFAA adsorption at air-water interface was supposed to decrease with the decrease of total area of air-water interface in the system (Brusseau, 2019). This finding
3.5. Adsorption mechanism of PFAAs on AC Figuring out the adsorption mechanism of PFAAs on AC is important to improve the performance of AC during water treatment. For the most cases, hydrophobic interaction is considered to be the major adsorption mechanism for long-chain PFAAs on AC (Chen et al., 2017; Deng et al., 2015; Wang et al., 2015c). According to the elemental composition and surface functional groups characterization results (Table S8), the amount of hydrophilic groups on AC100 (0.47 mmol/g of acidic groups) was smaller than some other AC (Du et al., 2016), and C (85 %) was the dominant element, suggesting that the surface of AC100 was mostly hydrophobic. Accordingly, the perfluoroalkyl chain of PFAAs may interact directly with the hydrophobic surface of AC (Du et al., 2014; Yu et al., 2009), as shown in Fig. 7. The electrostatic interaction between the PFAAs and AC could be ignored in this study, as the AC was negatively charged under the experimental condition, based on the characterization of ζ potential of AC100 and AC 350 (Fig. S7). The formation of micelles is also not applicable in the present study given the low water concentrations that were used, since the critical micelle concentration (CMC) are many orders of magnitude higher than the water concentrations of PFAAs used in these experiments (Johnson et al., 2007). Most importantly, this present study revealed an important overlooked adsorption mechanism for long-chain PFAAs on AC by air-water interface adsorption and subsequent contribution to the solid-phase adsorption onto the AC. It was observed that the removal of long-chain PFAAs from solution was more significant when aeration occurred, compared to non-aerated adsorption treatments (Fig. 2). By desorbing the PFAAs adsorbed at the air-water interface via high-speed centrifuging, the long-chain PFAAs showed a removal efficiency of up to 80 % for the AC after adsorption combined with aeration (Fig. 6a), further proving the important contribution of air-water interface adsorption. Even for the AC from the adsorption experiments in the absence of aeration, removing the adsorbed PFAA-enriched air bubbles resulted in a desorption efficiency up to 35 % (Fig. 6b), suggesting that air-water interface adsorption is ubiquitous for PFAA adsorption under routine treatments, as 6
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Fig. 7. Schematic description of the adsorption mechanism of PFAAs on AC with the contribution of air-water interface adsorption.
CRediT authorship contribution statement
schematically described in Fig. 7. Similar elevated PFAA removal was observed in an unsaturated adsorption column, in which the air-water interface provided the PFAA with more adsorption sites (Lyu et al., 2018). In fact, the adsorption at air-water interface has been demonstrated to be one of the primary retention for PFAAs in the subsurface system under environmental realistic conditions (Brusseau, 2018; Brusseau et al., 2019). Therefore, it is expected that air-water interface contribute greatly to the adsorption on AC. Based on the surface activity of PFAAs, it is proposed that the hydrophobic perfluoroalkyl chain of adsorbed PFAAs at air-water interface will point into the air, with the hydrophilic anionic group remaining in the water (Fig. 7) (Ebersbach et al., 2016; Meng et al., 2017). Similar binding mode of PFAAs at the air-water interface in the presence of aeration was proposed in a recent study, in the investigation of direct water-to-air transfer of PFAAs (Ebersbach et al., 2016). Such highly ordered binding mode of fluorinated surfactants at air-water interface was verified by molecular dynamic simulations (Zhang et al., 2014). Finally, it should be noted that limited by the characterization of air-water interface during the dynamic adsorption experiments, and the transfer of PFAA-enriched air bubbles to the AC still needs further investigation.
Pingping Meng: Writing - original draft, Methodology. Xiangzhe Jiang: Validation. Bin Wang: Resources. Jun Huang: Resources. Yujue Wang: Resources. Gang Yu: Resources. Ian T. Cousins: Writing - review & editing, Writing - review & editing. Shubo Deng: Methodology, Supervision, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements We thank the National Natural Science Foundation of China (Project no. 21577074), and Vetenskapsradet (VR), the Swedish Research Council Formas, for financial support. Pingping Meng also thanks Oskar Sandblom, Nikola Radoman and Bo Sha for their laboratory support. Appendix A. Supplementary data
4. Conclusions and implications Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2019.121981.
To the best of our knowledge, this is the first paper reporting that aeration could be used to enhance the removal of long-chain PFAAs during AC treatments of drinking water at environmentally relevant levels. Combining aeration with AC adsorption increases the removal of PFAAs, and the preferential adsorption of PFAAs at air-water interfaces is useful for designing unsaturated GAC columns for enhanced PFAA removal, although further evaluation is needed at the pilot and fullscale. More attention should be paid in particular to the stability of adsorbed air bubbles on AC in water treatment plants even without aeration, since breaking air-water interfaces under severe conditions may release the PFAAs back into the water. Finally, follow-up experiments conducted at a range of PFAA bulk water concentrations are recommended to further evaluate these treatment technologies.
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Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Role of the air-water interface in removing perfluoroalkyl acids from drinking water by activated carbon treatment
T
Pingping Menga,b, Xiangzhe Jianga, Bin Wanga, Jun Huanga, Yujue Wanga, Gang Yua, Ian T. Cousinsc,*, Shubo Denga,* a
State Key Joint Laboratory of Environment Simulation and Pollution Control, Beijing Key Laboratory for Emerging Organic Contaminants Control, School of Environment, Tsinghua University, Beijing, 100084, China b Department of Civil, Construction and Environmental Engineering, North Carolina State University, Raleigh, NC, 27695, United States c Department of Environmental Science and Analytical Chemistry (ACES), Stockholm University, SE-106 91, Stockholm, Sweden
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Editor: Xiaohong Guan
Contamination of drinking water by per- and polyfluoroalkyl substances (PFASs) is a worldwide problem. In this study, we for the first time revealed the role of the air-water interface in enhancing the removal of long-chain perfluoroalkyl carboxylic (PFCAs; CnF2n+1COOH, n ≥ 7) and perfluoroalkane sulfonic (PFSAs; CnF2n+1SO3H, n ≥ 6) acids, collectively termed as perfluoroalkyl acids (PFAAs), through combined aeration and adsorption on two kinds of activated carbon (AC). Aeration was shown to enhance the removal of long-chain PFAAs through adsorption at the air-water interface and subsequent adsorption of PFAA-enriched air bubbles to the AC. The removal of selected long-chain PFAAs was increased by 50–115 % with the assistance of aeration, depending on the perfluoroalkyl chain length. Aeration is more effective in enhancing long-chain PFAA removal as air-water interface adsorption increases with PFAA chain length due to higher surface activity. After removing adsorbed air bubbles by centrifugation, up to 80 % of the long-chain PFAAs were able to desorb from the sorbent, confirming the contribution of the air-water interface to the adsorption of PFAAs on AC. Aeration during AC treatment of water could enhance the removal of long-chain PFAAs, and improve the performance of AC during water treatment.
Keywords: PFAAs Air-water interface Activated carbon Aeration Adsorption
⁎
Corresponding authors. E-mail addresses: [email protected] (I.T. Cousins), [email protected] (S. Deng).
https://doi.org/10.1016/j.jhazmat.2019.121981 Received 5 November 2019; Received in revised form 16 December 2019; Accepted 25 December 2019 Available online 26 December 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.
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previous work first demonstrated the overlooked contribution of airwater interface to the PFAS adsorption on some non-porous adsorbents, by removing the air bubbles on the surface of adsorbents via vacuum degassing (Meng et al., 2014, 2017). However, the contribution of the air-water interface to PFAA adsorption on AC was not previously demonstrated, because of difficulties in eliminating the air bubbles from the porous structure of AC (Meng et al., 2014). Considering the wide application of AC in water treatment technologies, figuring out the influence of the air-water interface on the sorption performance of AC for PFAAs is of great importance. Based on the strong interfacial adsorption of long-chain PFAAs at the air-water interface, we hypothesize that if the solution is aerated during AC treatment, enhanced removal of the long-chain PFAAs will occur through concentration of the PFAAs at the air-water interface of air bubbles, and that the PFAA-enriched bubbles will be further adsorbed to the AC. This hypothesis was tested by investigating the influence of aeration on the adsorption of nine PFAAs with different perfluoroalkyl chain lengths and functional groups at environmentally realistic concentrations, using two kinds of AC with different particle sizes. The objectives of this study are to: 1) demonstrate enhanced PFAA removal when combining AC adsorption with aeration, 2) determine the contribution of aeration to the removal of different PFAAs, and 3) propose the mechanism of enhanced PFAA retention during AC treatment combined with aeration.
1. Introduction Per- and polyfluoroalkyl substances (PFASs) are a class of synthetic organic chemicals which have attracted worldwide attention as contaminants of high concern (Moody and Field, 1999; Wang et al., 2015a). Among PFASs, the subclasses that have received the most attention are the perfluoroalkyl carboxylic (PFCAs) and perfluoroalkane sulfonic (PFSAs) acids, collectively termed here as perfluoroalkyl acids (PFAAs). The concentrations of PFAAs in groundwater can reach μg/L or even mg/L levels at some contaminated sites, which can result contamination of drinking water with PFAAs (Moody and Field, 1999; Backe et al., 2013; Yin et al., 2017). In 2018, the Agency for Toxic Substances and Disease Registry (ATSDR) published the toxicological profile describing the toxicological properties of different PFAAs (Agency for Toxic Substances and Disease Registry, 2018). Since drinking water intake is shown to be an important source of PFAA exposure in our daily life, guidelines for safe PFAA levels in drinking water have been published in different countries (Livsmedelsverket, 2017; Mackay, 2018; USEPA, 2016). For example, the recommended guideline value for drinking water provided by the Swedish National Food Agency is 90 ng/L for the sum of PFASs (Livsmedelsverket, 2017), and the European Food Safety Agency (EFSA) provided preliminary tolerable weekly intakes (TWIs) for perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) of 13 ng/kg and 6 ng/kg bodyweight per week, respectively (Mackay, 2018). The need for efficient removal of PFASs, including PFAAs, from water is growing as a result of increasing regulatory attention and general concern regarding exposure to PFASs (Agency for Toxic Substances and Disease Registry, 2018; Livsmedelsverket, 2017; Mackay, 2018; USEPA, 2016). However, traditional water treatments demonstrated poor removal efficiency for PFAAs, including flocculation, coagulation, sand filtration, ozonation and so on (Appleman et al., 2014; Flores et al., 2013; Pan et al., 2016; Shivakoti et al., 2010). The concentrations of PFAAs in the effluent of drinking water treatment plants (DWTPs) can be even higher than those in the influent, due to PFAA desorption or degradation of unknown PFAA-precursors (Takagi et al., 2008, 2011). Comparatively, adsorption by activated carbon (AC) has been proven to be efficient in removing PFAAs from water (Kothawala et al., 2017; McCleaf et al., 2017; Qu et al., 2009). Nevertheless, the removal of PFAAs by AC is usually PFAA-dependent and co-contaminants-dependent (Appleman et al., 2014; Eschauzier et al., 2012; Liu et al., 2019). Short-chain PFAA (PFCAs with less than 7, and PFSAs with less than 6, fluorinated carbons) retention by AC is less satisfactory as a result of lower hydrophobicity and higher mobility in the aqueous phase (Sun et al., 2016; Du et al., 2015). Meanwhile, ACs tend to adsorb all the dissolved organic matters (DOMs) indiscriminately, resulting in a reduced life-time of AC when dealing with water containing high concentrations of DOMs (Appleman et al., 2013). Overall, improving the performance of AC for PFAA removal is of great significance in water treatments. The interfacial behavior of PFAAs at air-water interface is attracting more attention, which may influence their transport and removal in aquatic and terrestrial environment (Johansson et al., 2019; Brusseau, 2018; Brusseau et al., 2019; Schaefer et al., 2019; McMurdo et al., 2008). A recent study suggested that the air-water interface contributed up to 75 % of PFOA retention in an unsaturated porous column, since the air-water interface inside the column provide with more adsorption sites for PFOA (Lyu et al., 2018). And the relative magnitudes of PFAA adsorption at solid-water or air-water interface were compared using a quantitative-structure/property-relationship (QSPR) analysis, suggesting that the air-water interface adsorption was dependent on the total air-water interface area and the properties of solid-phase properties (Brusseau, 2019). Based on the adsorption of PFASs at air-water interfaces, technologies consisting of aeration and follow-up abstraction of generated aerosols or foams were developed for the removal of PFAAs (Meng et al., 2018; Ebersbach et al., 2016). Moreover, our
2. Materials and methods 2.1. Chemicals and materials Coal-based AC (AC100, AC350) of different particle sizes (100 μm or 350 μm) was purchased from Zhengzhou Yedao Environmental Protection Co. (China). The particle size, BET surface area and pore size distribution of the different types of AC are provided in Table S1 and Fig. S1. Tap water used for adsorption and aeration experiments was taken directly from the laboratory tap at Stockholm University, and the properties of the tap water are shown in Table S2. Milli Q-water was used for sample preparation and during analyzing procedures. Native and mass-labeled PFAAs (Table S3) were purchased from Wellington Laboratories Inc. 2.2. Adsorption and aeration experiments All adsorption experiments were conducted in a horizontal shaker with a shaking speed of 180 rpm (rpm) at around 22 °C (Fig. 1). The initial nominal concentration of the solution was set at around 1 μg/L per PFAA (measured concentrations are listed in Table S4) for all the experiments by spiking the tap water with multi-solute PFAA stock solution. The pH of the PFAA solutions was 8.5 ( ± 0.1). All the experiments were conducted in triplicate, and arithmetic mean values with error bars were used in figures. Adsorption experiments combined with aeration were conducted with 400 mL PFAA solutions in 1 L polypropylene bottles. An amount of 5 mg AC100 or 15 mg AC350 was added for adsorption (the dosage of AC350 was higher to maintain a similar PFAS removal), and continuous aeration was performed during the adsorption. Aeration was performed by using a titanium filter with an average pore diameter of 10 μm (Fig. 1), similar to that used in our previous study (Meng et al., 2018). A purified nitrogen system was used to maintain a steady air flow as well as to minimize the blank contamination in the air. The effects of aeration flow rate on the removal of PFAAs was investigated by adjusting the aeration flow rate from 0 mL/min to 250 mL/min. Unless otherwise specified, the aeration flow rate was controlled at 250 mL/ min. During the aeration, bottles were shaken intermittently by hand to remove the aerosol drops on the walls and caps of the bottles due to the bursting of air bubbles on the surface of the solution. For comparison, adsorption experiments without aeration were conducted under the 2
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Fig. 1. Setups for adsorption and adsorption combined with aeration experiments.
same conditions. To evaluate the atmospheric transmission of PFAAs during aeration, aeration experiments (without the addition of AC) with the presence or absence of intermittent shaking (to recover the aerosols) by hand were conducted. Samples were taken after adsorption and aeration for 24 h.
Waters), similar to a previous study (Johansson et al., 2019). Procedural blank samples were prepared throughout the sample preparation procedures to determine blank contamination.
2.3. Desorption experiments by centrifugation
The concentrations of PFAAs in the tap water varied from 0.18 to 5.43 ng/L (Table S5), which were much lower than the spiked PFAA concentrations. Procedural blank samples were set during all the experiments in case of blank contamination. The limit of detection (LOD) and the limit of quantification (LOQ) of the instrument were calculated as the mean value of Milli-Q samples (n = 10) plus three/ten times their standard deviation, respectively. And the recovery of PFAA during sample treatments was evaluated by recovery experiments, performed with a group (n = 10) of 30 mL tap water spiked with 1 μg/L per PFAA. After centrifuging for 5 min, 15 mL of the supernatant was taken and spiked with IS for SPE treatment before analysis. Briefly, negligible PFAA contamination was observed in the procedural blank samples, and the loss of PFAAs onto the container walls was less than 5 % (Fig. S4). The LOQ of different PFAAs varied from 0.02 ng/L to 0.92 ng/L, and the recovery of different PFAAs in the recovery experiments varied from 75 % to 95 %, depending on the perfluoroalkyl chain length. Further descriptions of method evaluation, including the LOD, LOQ, accuracy and precision of the method could be found in Table S5 and Table S6. In conclusion, the results of the method evaluation demonstrate the reliability of this study.
2.5. Method evaluation
After adsorption, or adsorption combined with aeration for 24 h, the bottles were shaken completely before sampling to homogenize the AC solution as well as to avoid the influence of aerosols. A volume of 15 mL AC solution was added into the centrifuge tubes in duplicate, and the samples were then divided into two groups. Group A samples were centrifuged for 15 min in a high-speed centrifuge with a speed of 5000 rpm, while Group B samples were settled by gravitation for 15 min as comparison. Supernatant was then taken for the analyzation. A schematic diagram for desorption by centrifugation is shown in Fig. S2. By comparing the PFAA removal efficiency in Group A and Group B samples, the desorption efficiency of PFAAs by centrifugation was calculated using Eq. (1):
DEPFAA =
RemovalA, PFAA − RemovalB, PFAA RemovalA, PFAA
(1)
where DEPFAA is the desorption efficiency of a single PFAA, RemovalA,PFAA is the removal efficiency of PFAAs in Group A samples, and RemovalB,PFAA is the removal efficiency of PFAAs in the corresponding Group B samples.
3. Results and discussion 2.4. Sample treatments and analysis 3.1. Enhanced PFAA adsorption on AC with the assistance of aeration A schematic diagram for sample treatments is shown in Fig. S3. After adsorption or adsorption combined with aeration for 24 h, the bottles were shaken completely before sampling to avoid the influence of aerosol drops on the walls and caps. Gravitational settling was used to separate the AC and supernatant samples were taken. Oasis Weakanion exchange (WAX) solid phase extraction (SPE) cartridges were used to load the samples. A detailed description of the SPE procedure and method description can be found in the SI that refers to a previous study (Lofstedt Gilljam et al., 2016; Benskin et al., 2012). The samples were finally analyzed by using a UPLC/MS/MS system (Xevo TQ-S,
The removal of total PFAAs was controlled below 50 % by using low AC dosage to better evaluate the influence of simultaneous aeration on the adsorption of PFAAs on AC. As illustrated in Fig. 2, the results are consistent with our hypothesis that increasing the air-water interface by aeration will increase PFAA removal significantly. Generally, the removal of total PFAAs by both AC increased by 50 % (Fig. 2b and 2d) with the assistance of aeration. The similar increased ratios of PFAA removal by AC100 and AC350 could be explained by their similar total surface areas (Brusseau, 2019), and therefore similar PFAA removal 3
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Fig. 2. Influence of aeration on the removal efficiency of PFAAs and increased ratio of removal of long-chain PFAAs on AC100 (a, b) and AC350 (c, d).
perfluorobutanesulfonic acid (PFBS), aeration showed no obvious influence on their adsorption onto the AC.
efficiencies. Aeration was shown to accelerate the mass-transfer of atrazine in the solution, and it was proved that physical mixing displayed similar effects on atrazine mass transfer as aeration (Jia et al., 2006). Namely, the influence of aeration on PFAA mass-transfer could be counteracted by physical mixing, for which reason all the adsorption experiments were conducted in a horizontal shaker with a shaking speed of 180 rpm. Moreover, samples were taken after adsorption for 24 h, when adsorption equilibrium was close to be reached, indicating that the increased PFAA removal should not be attributed to accelerated PFAA adsorption kinetics. All the PFAAs displayed a higher removal when aeration occurred, except for three short-chain PFAAs. Specifically, aeration was demonstrated to be more efficient in enhancing the removal of long-chain PFAAs. PFCAs and PFSAs are both fluorinated surfactants with the ability to decrease the surface tension of water solution by accumulating at the air-water interface (Hill et al., 2018). The surface activity of PFCAs and PFSAs increases exponentially with their perfluoroalkyl chain length (Schaefer et al., 2019; Fernandez et al., 2016). Consequently, the contribution of the air-water interface to the adsorption of PFAAs onto AC is also dependent on their chain length (Fig. 2b and 2d). For example, the removal efficiencies of perfluorodecanoic acid (PFDA) by AC100 and AC350 were increased by 115 % and 75 %, respectively. For other long-chain PFAAs, such as perfluorononanoic acid (PFNA) and PFOS, aeration was able to increase their removal efficiencies by 65%–90%. The increased PFAA removal with the assistance of PFAA might be beneficial to improve the life-time of AC, as more water could be treated with the same dosage of AC, yet further evaluation at pilot- or full-scale is required. However, for short-chain PFAAs such as perfluoropentanoic acid (PFPeA), perfluorohexanoic acid (PFHxA) and
3.2. Influence of aeration flow rate on PFAA adsorption on AC100 To further verify the role of the air-water interface on PFAA adsorption on the AC, the influence of the aeration flow rate on the PFAA removal was investigated. By increasing the aeration flow rate from 100 mL/min to 250 mL/min, the removal of total PFAAs by AC100 increased from 25 % to 32 %, increasing by 28 %, suggesting that increasing the aeration flow rate is beneficial for PFAA removal (Fig. 3). Similarly, the influence of the aeration flow rate on the removal of PFAAs is also chain-length dependent. The removal of long-chain PFAAs was more strongly affected by the increasing aeration flow rate. For example, increasing the aeration flow rate enhanced the removal of PFDA (nperfluorocarbons = 9) more efficiently than PFHpA (nperfluorocarbons = 6). According to our previous study, air bubbles on the surface of some hydrophobic carbonaceous adsorbents displayed considerable adsorption capacity for PFOS (Meng et al., 2014), suggesting that the air-water interface provides ideal adsorption sites for compounds with high surface activity such as PFOS. The enhanced retention of PFAAs by AC in the presence of aeration is also consistent with the behavior of PFOA retention in some unsaturated columns, in which the adsorption at the air-water interface contributes up to 75 % of the PFOA retention (Lyu et al., 2018). Despite that the aforementioned column tests were designed to evaluate the PFOA migration potential in some vadose zones, instead of improving the PFOA removal, similar effects may be important to consider when designing AC columns for PFAS removal. AC adsorption beds are commonly used for 4
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Fig. 3. The influence of aeration flow rate on the removal efficiency of PFAAs by AC100.
Fig. 5. The relationship between the number of fluorinated carbons and the removal of PFAA by aeration.
the removal of PFAAs at the pilot- or full-scale, yet the role of the airwater interface in PFAA removal remains unclear. By running the AC columns under unsaturated condition, the retention of PFAAs by the AC could be potentially improved, with air-water interface providing with more adsorption sites. Accordingly, a later breakthrough of AC is anticipated, which could potentially prolong the life time of AC. Although increasing the aeration flow rate is beneficial for PFAA removal, the generation of aqueous aerosols was also observed as a result of the bursting of rising air bubbles, and bubble busting increased with aeration flow rate. Intermittent manual shaking was shown to recover all the formed aerosols from the walls and caps of the bottles even at the maximum aeration rate of 250 mL/min when maximum bubble bursting and aerosol ejection occurred (see below).
shaking was less than 2 %, indicating that the removal of PFAAs via the formation of aerosols could be neutralized by recovering the aerosols drops on the walls and caps into the solution. On the contrary, a high loss of long-chain PFAAs (up to about 60 % for PFDA) was observed after aeration without intermittent shaking. The removal of both PFCA and PFSA increased proportionally with the increasing number of fluorinated carbons (Fig. 5), as a result of increasing surface activity. Aeration alone showed no removal for the short-chain PFAAs, such as PFPeA, PFHxA and PFBS, which are more mobile in the aqueous environment and in urgent needs for efficient removal (Sun et al., 2016; Wang et al., 2015b). The removal of longchain PFAAs by aeration alone is consistent with a previous study (Ebersbach et al., 2016). Since PFAAs tend to accumulate at the surface of air bubbles (i.e. air-water interface) in the solution, aerosols enriched with PFAAs (at the air-water interface) will be formed when the air bubbles burst at the water surface, resulting in a decrease of the PFAA concentrations in water (Ebersbach et al., 2016). However, an effective collection of aerosol droplets formed from bubble bursting is not feasible, especially at full-scale water treatment facilities. For example, large aerosol particles may coagulate and drop directly back into the solution, while small aerosol particles may be rapidly transported away in the air. The sub-micrometer aerosols are especially likely to be transported long distances (Cochran et al., 2016; McInnes et al., 2013), suggesting that the water-to-air transport might be one of the explanations for the PFAAs long-range atmospheric transport (Johansson et al., 2019; McMurdo et al., 2008). The direct evaporation of PFAAs to the atmosphere during aeration was also investigated. The outlet air above the water surface in the adsorption experiments was made to pass through a liquid consisting of methanol and water (Fig. 1), which was used to fully absorb the PFAAs in the air. The PFAA concentrations in the outlet gas sink and the calculated mass ratio of PFAAs detected compared to the mass spiked in the adsorption/aeration reactor are listed in Table S7, suggesting that the transfer of PFAAs to the atmosphere was negligible (< 0.06 %). Due to the low pKa of PFAAs (Maimaiti et al., 2018), all the PFAAs in this study exist as anions in solution, with negligible volatility with an experimental water pH of 8.5 (Reth et al., 2011). Especially for the longchain PFAAs, it has been shown that the volatilization is negligible at pH > 2.5 (Johansson et al., 2017; Vierke et al., 2013). Based on the aforementioned discussion, the water-to-air gaseous transport of PFAAs into the atmosphere should have negligible contribution to the increased PFASs removal.
3.3. Atmospheric emission of PFAAs during aeration As mentioned above, aerosols were formed continuously during the aeration. To ensure that the increased removal of PFAAs was attributed to the increase of air-water interface adsorption, instead of by generating PFAA-enriched aerosols during the aeration, all the bottles were shaken upside down intermittently to recover the liquid aerosol droplets on the walls and caps. Control experiments were done by testing the removal of PFAAs by aeration alone (in the absence of AC), with the presence or absence of intermittent shaking by hand. According to Fig. 4, the loss of PFAAs after aeration for 24 h with intermittent
Fig. 4. The removal efficiency of PFAAs by aeration with the presence or absence of intermittent shaking by hand. 5
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Fig. 6. PFAA desorption efficiency by centrifugation for AC after adsorption with aeration (a) or adsorption alone (b).
is especially relevant for the remediation of PFAAs from water, revealing the underappreciated role of air-water adsorption in the AC treatment for long-chain PFAAs. Finally, it should be noted that the desorption treatment in this study is not able to regenerate the spent AC completely, considering that the PFAAs adsorbed on the solid-phase would not be desorbed via centrifuging. Therefore, the reutilization of the AC after adsorption and aeration was not discussed in this study, which usually requires incineration at extremely high temperature over 700 ℃ (Watanabe et al., 2018, 2016).
3.4. Desorption of PFAAs by removing the adsorbed bubbles on AC Based on the aforementioned discussion, a considerable fraction of the PFAAs was adsorbed at the surface of the air bubbles during aeration, which were then adsorbed onto the AC. The stability of such airwater interfaces was important to evaluate the treatability of this method, for which reason the bottles after adsorption or adsorption combined with aeration were additionally settled for different time before sampling. As shown in Fig. S6, the removal of PFOS by adsorption combined with aeration remained stable after settling for 24 h, suggesting that the air-water interface adsorption in this study could exist for at least 24 h, making sure that no adsorbed PFAAs would be released back into the solution during water treatment. To quantify the contribution of air-water interface adsorption to the adsorption of PFAAs on AC, desorption experiments were conducted. After adsorption or adsorption combined with aeration for 24 h, the spent AC solutions were centrifuged in a high-speed centrifuge, which was proven to be effective in removing adsorbed air bubbles from the adsorbents (LaFratta et al., 2004). Control experiments showed negligible losses of PFAAs during centrifugation (Fig. S5). As shown in Fig. 6, for the AC after adsorption combined with aeration, removing adsorbed air bubbles by centrifugation resulted in a desorption efficiency of 48%–80% for the long-chain PFAAs, in agreement with Fig. 1b, where aeration was shown to improve the removal of long-chain PFAAs by 50%–120%. For the long-chain PFAAs, aeration showed a larger influence on their adsorption, and the desorption efficiency by removing the air-water interface via high-speed centrifuging was correspondingly higher. Specifically, the desorption efficiency of PFAA increased proportionally with the increasing number of fluorinated carbons in PFCA and PFSA, comparable to the removal efficiency of PFAAs by aeration alone (the slope of the lines in Fig. 5 and Fig. 6a are similar). Surprisingly, even for the AC after adsorption alone (without aeration), the long-chain PFAAs displayed a desorption efficiency of around 28 % to 35 %, suggesting that the air-water interface played a considerable role in the adsorption of long-chain PFAAs on AC under routine operation. The desorption efficiency of PFAAs is higher for the adsorption combined with aeration scenario, which might be explained by a larger number of PFAA-enriched air bubbles with the assistance of aeration. Notably, large error bars were observed in Fig. 6, especially for the short-chain PFAAs and in the adsorption scenario, which might be explained by their lower removal efficiency and smaller contribution of air-water interface, respectively. Especially in the adsorption scenario, where the solution was not as saturated with air as in the adsorption combined with aeration scenario, the magnitude of PFAA adsorption at air-water interface was supposed to decrease with the decrease of total area of air-water interface in the system (Brusseau, 2019). This finding
3.5. Adsorption mechanism of PFAAs on AC Figuring out the adsorption mechanism of PFAAs on AC is important to improve the performance of AC during water treatment. For the most cases, hydrophobic interaction is considered to be the major adsorption mechanism for long-chain PFAAs on AC (Chen et al., 2017; Deng et al., 2015; Wang et al., 2015c). According to the elemental composition and surface functional groups characterization results (Table S8), the amount of hydrophilic groups on AC100 (0.47 mmol/g of acidic groups) was smaller than some other AC (Du et al., 2016), and C (85 %) was the dominant element, suggesting that the surface of AC100 was mostly hydrophobic. Accordingly, the perfluoroalkyl chain of PFAAs may interact directly with the hydrophobic surface of AC (Du et al., 2014; Yu et al., 2009), as shown in Fig. 7. The electrostatic interaction between the PFAAs and AC could be ignored in this study, as the AC was negatively charged under the experimental condition, based on the characterization of ζ potential of AC100 and AC 350 (Fig. S7). The formation of micelles is also not applicable in the present study given the low water concentrations that were used, since the critical micelle concentration (CMC) are many orders of magnitude higher than the water concentrations of PFAAs used in these experiments (Johnson et al., 2007). Most importantly, this present study revealed an important overlooked adsorption mechanism for long-chain PFAAs on AC by air-water interface adsorption and subsequent contribution to the solid-phase adsorption onto the AC. It was observed that the removal of long-chain PFAAs from solution was more significant when aeration occurred, compared to non-aerated adsorption treatments (Fig. 2). By desorbing the PFAAs adsorbed at the air-water interface via high-speed centrifuging, the long-chain PFAAs showed a removal efficiency of up to 80 % for the AC after adsorption combined with aeration (Fig. 6a), further proving the important contribution of air-water interface adsorption. Even for the AC from the adsorption experiments in the absence of aeration, removing the adsorbed PFAA-enriched air bubbles resulted in a desorption efficiency up to 35 % (Fig. 6b), suggesting that air-water interface adsorption is ubiquitous for PFAA adsorption under routine treatments, as 6
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Fig. 7. Schematic description of the adsorption mechanism of PFAAs on AC with the contribution of air-water interface adsorption.
CRediT authorship contribution statement
schematically described in Fig. 7. Similar elevated PFAA removal was observed in an unsaturated adsorption column, in which the air-water interface provided the PFAA with more adsorption sites (Lyu et al., 2018). In fact, the adsorption at air-water interface has been demonstrated to be one of the primary retention for PFAAs in the subsurface system under environmental realistic conditions (Brusseau, 2018; Brusseau et al., 2019). Therefore, it is expected that air-water interface contribute greatly to the adsorption on AC. Based on the surface activity of PFAAs, it is proposed that the hydrophobic perfluoroalkyl chain of adsorbed PFAAs at air-water interface will point into the air, with the hydrophilic anionic group remaining in the water (Fig. 7) (Ebersbach et al., 2016; Meng et al., 2017). Similar binding mode of PFAAs at the air-water interface in the presence of aeration was proposed in a recent study, in the investigation of direct water-to-air transfer of PFAAs (Ebersbach et al., 2016). Such highly ordered binding mode of fluorinated surfactants at air-water interface was verified by molecular dynamic simulations (Zhang et al., 2014). Finally, it should be noted that limited by the characterization of air-water interface during the dynamic adsorption experiments, and the transfer of PFAA-enriched air bubbles to the AC still needs further investigation.
Pingping Meng: Writing - original draft, Methodology. Xiangzhe Jiang: Validation. Bin Wang: Resources. Jun Huang: Resources. Yujue Wang: Resources. Gang Yu: Resources. Ian T. Cousins: Writing - review & editing, Writing - review & editing. Shubo Deng: Methodology, Supervision, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements We thank the National Natural Science Foundation of China (Project no. 21577074), and Vetenskapsradet (VR), the Swedish Research Council Formas, for financial support. Pingping Meng also thanks Oskar Sandblom, Nikola Radoman and Bo Sha for their laboratory support. Appendix A. Supplementary data
4. Conclusions and implications Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2019.121981.
To the best of our knowledge, this is the first paper reporting that aeration could be used to enhance the removal of long-chain PFAAs during AC treatments of drinking water at environmentally relevant levels. Combining aeration with AC adsorption increases the removal of PFAAs, and the preferential adsorption of PFAAs at air-water interfaces is useful for designing unsaturated GAC columns for enhanced PFAA removal, although further evaluation is needed at the pilot and fullscale. More attention should be paid in particular to the stability of adsorbed air bubbles on AC in water treatment plants even without aeration, since breaking air-water interfaces under severe conditions may release the PFAAs back into the water. Finally, follow-up experiments conducted at a range of PFAA bulk water concentrations are recommended to further evaluate these treatment technologies.
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