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The MIP adsorbents using perfluorooctanoic acid. (PFOA) as the template had good imprinting effects and could selectively remove PFOS from aqueous solution ...
Front. Environ. Sci. Engin. China 2009, 3(2): 171–177 DOI 10.1007/s11783-009-0017-4

RESEARCH ARTICLE

Selective sorption of perfluorooctane sulfonate on molecularly imprinted polymer adsorbents Shubo DENG, Danmeng SHUAI, Qiang YU, Jun HUANG, Gang YU (✉) Department of Environmental Science and Engineering, POPs Research Center, and State Key Joint Laboratory of Environment Simulation and Pollution Control, Tsinghua University, Beijing 100084, China

© Higher Education Press and Springer-Verlag 2009

Abstract Perfluorooctane sulfonate (PFOS), as a potential persistent organic pollutant, has been widely detected in water environments, and has become a great concern in recent years. PFOS is very stable and difficult to decompose using conventional techniques. Sorption may be an attractive method to remove it from water. In this study, the molecularly imprinted polymer (MIP) adsorbents were prepared through the polymerization of 4vinylpyridine under different preparation conditions in order to remove perfluorooctane sulfonate (PFOS) from water. The MIP adsorbents using perfluorooctanoic acid (PFOA) as the template had good imprinting effects and could selectively remove PFOS from aqueous solution. The sorption behaviors including sorption kinetics, isotherms, and effect of pH, salt, and competitive anions were investigated. Experimental results showed that the sorption of PFOS on the MIP adsorbents was very fast, pHdependent, and highly selective. The achieved fast sorption equilibrium within 1 h was attributed to the surface sorption on the fine adsorbents. The sorption isotherms showed that the sorption selectivity of PFOS on the MIP adsorbents decreased at high PFOS concentrations, which may be due to the double-layer sorption and the formation of PFOS micelles on the sorbent surface. The sorption of PFOS on the MIP adsorbents was mainly dominated by the electrostatic interaction between the protonated vinylpyridine on the adsorbent surface and the anionic PFOS. The prepared MIP adsorbents can potentially be applied in water and wastewater treatment for selective removal of PFOS. Keywords perfluorooctane sulfonate (PFOS), molecularly imprinted polymer (MIP) adsorbents, selective sorption, electrostatic interaction

Received December 23, 2008; accepted February 23, 2009 E-mail: [email protected]

1

Introduction

Perfluorochemicals have been used in many products for about 50 years. They are commonly used as surfactants for metal plating, fire-fighting foams, varnishes, vinyl polymerization, gasoline, and water repellents for leather, paper, and textiles [1–3]. Perfluorooctane sulfonate (PFOS) is of increasing scientific and regulatory interest because it is the end product of the degradation of most perfluorochemicals and has been detected in wastewater, surface water, sediment, and many wildlife species [4–6]. As PFOS is found to be globally distributed, environmentally persistent, bioaccumulative and potentially toxic, it has been proposed as a candidate of persistent organic pollutants (POPs) [7–9]. Additionally, PFOS is still manufactured and currently used in some fields such as semiconductor, metal plating, hydraulic fluids, and photographic industries although it has been banned in most countries [10]. Because PFOS has high water solubility and exhibits anionic properties in water, it can exist and easily transport in water environments. It has been detected in wastewater, surface water, groundwater, and even tap water throughout the world [9]. Many researchers found high concentrations of PFOS in the rivers near the PFCs-related factories [11,12], and the PFOS concentrations at mg/L level were detected in the river near a Canadian airport due to the accidental release of fire-fighting foam [13]. Industrial wastewater was regarded as a point source for PFOS entering natural waters [14]. Tang et al. [15] reported that the concentration of PFOS in the original wastewater generated from photolithographic processes of semiconductor manufacturing was up to 1650 mg/L. Therefore, it is necessary to develop effective techniques to remove PFOS from industrial wastewaters. In recent years, some technologies such as ultraviolet irradiation, ultrasonic irradiation under an argon atmosphere, and zerovalent iron in subcritical water have been

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used to decompose PFOS in solutions [16–18]. However, they all require specific conditions and high energy consumption. Some physical technologies including membrane separation and sorption were also reported to remove PFOS from aqueous solution. It was found that the commercial reverse osmosis and nanofiltration membranes could efficiently separate PFOS from wastewater [19]. Activated carbon also seems to be an effective method to remove PFOS from water [20], but some scientists found that PFOS could quickly penetrate the carbon filter in water and wastewater treatment plants [21]. Our previous study also showed that the sorption of PFOS on the granular activated carbon was very slow, while its sorption on powder activated carbon was fast and effective [22]. Many anionic compounds and colloids in water and wastewater will compete with anionic PFOS and decrease the sorption capacity of the adsorbents for PFOS in the sorption process. Especially, the concentrations of the competitive compounds are normally much higher than those of the target adsorbates. Therefore, selective sorption is attractive to remove PFOS from water in actual application. Molecular imprinting is a useful technique to construct specific sites for the target compounds in the preparation of molecularly imprinted polymer (MIP). After the template is removed from the resulting polymer matrices, the binding sites having the size and shape for the template are generated, and thus the MIP adsorbent can effectively recognize specific adsorbates in the sorption process [23]. Although the MIP adsorbents have been successfully used to remove many pollutants such as αestradiol and bisphenol A from water [24,25], the removal of PFOS using the MIP adsorbents PFOS has hardly been reported. In our previous paper, we have successfully prepared the chitosan-based MIP adsorbent for selective removal of PFOS from water [26]. In this study, the efficient MIP adsorbents were prepared by the conventional monomer polymerization, namely, the copolymerization of a crosslinking agent with the complex formed from PFOA and 4-vinylpyridine. The prepared MIP adsorbents with the sorption selectivity were used to remove PFOS from aqueous solution. The sorption behaviors including sorption kinetics, sorption isotherms, effect of solution pH, and sorption selectivity were investigated and the possible sorption mechanism was discussed.

including 4-vinylpyridine (4-VP), ethyleneglycol dimethacrylate (EGDMA), trimethylolpropane triacrylate (TRIA), 2,2-azobis isobutyronitrile (AIBN), 2,4-dichlorophenoxy acetic acid (2,4-D), and acetone were of reagent grade. 2.2

MIP adsorbent preparation

An amount of template (PFOS or PFOA) and 4-VP was added into a 50-mL glass vessel containing methanol or acetone/methanol solution, and then shaken for 10 min. The crosslinker (EGDMA or TRIA) and initiator (AIBN) were added into the flask and the content was bubbled with nitrogen for 15 min to remove oxygen, and then heated at 65 °C for 24 h to obtain the polymer. The obtained polymer was ground into powders less than 74 μm in diameter, and then washed with methanol in an ultrasonic cleaner for 20 min to remove the template. The washing process was repeated several times until no template was detected in methanol. The so-obtained powder was filtrated and vacuum-dried before use. The corresponding nonimprinted polymer (NIP) adsorbents were prepared in the same procedure except in the absence of the template and no washing process. 2.3

Sorption experiments

Batch sorption experiments were conducted in 100-mL flasks, each of which contained 20 mL of 200 mg/L of PFOS solution. 0.02 g of MIP/NIP adsorbents was added to a flask and shaken at 120 rpm in a thermostatic shaker at 25°C for 4 h. The investigation on the effect of solution pH on PFOS sorption was conducted at the initial solution pH from 2 to 11, and the solution pH values were not controlled throughout the experiment as they were relatively constant. In the sorption kinetic experiments, the solution pH was adjusted to 5.1 using 1 mmol/L of NaH2PO4 buffer solution, and the samples were taken at predetermined time intervals. The sorption isotherms were carried out at pH 5.1 with the PFOS concentrations ranging from 25 to 500 mg/L. In the experiments of salt effect on the sorption, the concentrations of NaCl were in the range of 1–1000 mmol/L. In the selectivity experiments, the initial solution pH was 5.1 and the concentrations of 2,4-D were 0.226, 0.452, and 2.26 mmol/L, respectively. Because the buffer solution of NaH2PO4 was used, the solution pH was constant during the sorption process.

2

Materials and methods

2.4

2.1

Materials

After the sorption experiments, the mixture was filtrated through a 0.22-μm nylon membrane filter. The concentrations of PFOS were determined using high performance liquid chromatography (HPLC) (LC-10ADvp, Agilent Technologies, USA) with a CDD-6A conductivity detector. The TC-C18 column (4.6  250 mm) from Agilent Technologies was adopted and the mixture of methanol

Perfluorooctane sulfonate (PFOS) in the forms of acid and potassium salt was purchased from Tokyo Kasei Kogyo (Japan), and perfluorooctanoic acid (PFOA) was from Acros Company, Belgium. HPLC-grade methanol was purchased from Fisher Chemical (USA). Other chemicals

PFOS determination

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and 0.02 mol/L of NaH2PO4 (70/30 for PFOS, 65/35 for PFOA, v/v) was used as the mobile phase at the flow rate of 1.5 mL/min. The sample volume injected was 20 μL. In this study, the detection limits for PFOS and PFOA were about 1 mg/L and 0.7 mg/L, respectively. The sorption amount was calculated according to the different concentrations of PFOS or PFOA before and after sorption.

3

Results and discussion

3.1

MIP preparation and comparison

As shown in Table 1, seven MIP adsorbents were prepared using the monomer polymerization approach with different templates, crosslinkers, solvents, and molar ratios of template /monomer /crosslinker. The removal rates of PFOS using the seven prepared MIP and NIP adsorbents are shown in Fig. 1. It can be found that the effects of concentration of template, solvent, solution pH, and crosslinker on the sorption capacity of the prepared adsorbents were pronounced. The adsorbents A, B, and G had higher sorption capacities for PFOS, which may be due to the stronger electrostatic interaction between the monomer and templates. Because perfluorooctane sulfonic acid and perfluorooctanoic acid were used as the templates, the nitrogen atoms in 4-VP were protonated at low solution pH, and thus more PFOS anions were adsorbed on 4-VP via electrostatic interaction. It is noticeable that the MIP adsorbents using PFOS as the template had little difference in sorption capacity for PFOS compared with that of the NIP adsorbents, indicating the bad imprinting effect during the preparation. Although both the MIP adsorbents A and G had higher sorption capacity, the sorption selectivity was not satisfactory. Fortunately, the MIP adsorbents prepared using PFOA as the template had higher sorption capacity for PFOS compared to that of the NIP adsorbents. The good imprinting result was possibly attributed to the higher water solubility of PFOA than that of PFOS as well as the electrostatic interaction at low pH. The effective MIP adsorbent B was used in the following sorption experiments.

Fig. 1 PFOS removal rates using the MIP and NIP adsorbents prepared under different conditions (Sorption conditions: adsorbent dose = 1 g/L; PFOS concentration = 200 mg/L; pH = 5.1; sorption time = 4 h)

3.2

Sorption kinetics

Figure 2 shows the sorption kinetics of PFOS on the MIP and NIP adsorbents. It can be found that the sorption of PFOS on the MIP adsorbent was very fast and the equilibrium was almost achieved after 5 min. Because the MIP adsorbents used in this study were ground into the powder below 74 μm and the adsorbents were nonporous materials, the sorption mainly occurred on the surface of particles, resulting in the fast sorption. It was reported that the sorption of PFOS on the porous adsorbents including the granular activated carbon and resin was very slow, and the sorption equilibrium could be achieved after over 168 h [22]. Because PFOS is a surfactant with hydrophobic and oleophobic property, it is difficult to diffuse into the porous adsorbents and the intraparticle diffusion is very slow. For the MIP adsorbents, the PFOS molecules can recognize the imprinted sites and adsorb on the sites very quickly. It also can be seen that the removal rates of PFOS on the MIP were much higher than those on the NIP, implying the selective sorption of PFOS on the MIP adsorbents.

Table 1 Preparation conditions for the MIP and NIP adsorbents adsorbent

template concentration /mmol$L–1

crosslinker

molar ratio of template/monomer/ crosslinker

solvent

A

PFOSa), 80

TRIA

1/4/20

methanol

B

PFOA, 80

TRIA

1/4/20

methanol

C

PFOS, 80

EGDMA

1/4/20

water/acetone = 1∶4

D

PFOS, 40

EGDMA

1/4/20

water/acetone = 1∶4

E

PFOS, 20

EGDMA

1/4/20

water/acetone = 1∶4

F

PFOS, 10

TRIA

1/1/4

water/methanol = 1∶6

Gb)

PFOS, 10

TRIA

1/1/4

water/methanol = 1∶6

Notes: a) PFOS here is perfluorooctane sulfonic acid, while other PFOS denotes perfluorooctane sulfonate potassium salt; b) the mixture of PFOS and vinylpyridine was adjusted to pH 3.

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exhibited anionic property in the experimental pH. Therefore, the electrostatic interaction played an important role when the pH was below 5.8. It should be pointed out that the removal rate of PFOS on the MIP adsorbents was over 50% even at pH 9, suggesting that other interactions such as hydrogen bonding and hydrophobic interaction were also involved in the sorption process as the electrostatic interaction disappeared. 3.4

Fig. 2 Sorption kinetics of PFOS on the MIP and NIP adsorbents at pH 5.1

3.3

Effect of pH

Solution pH is an important parameter influencing the sorption of adsorbates on the adsorbents. It not only influences the properties of the adsorbent surface, but also affects the adsorbate speciation in solution. Figure 3 shows the effect of solution pH on PFOS removal using the MIP and NIP adsorbents. For both the MIP and NIP adsorbents, the removal rates decreased with increasing pH, and almost all PFOS was removed at pH 3. Their difference in the removal rates became obvious with increasing pH, showing the selective removal of PFOS on the MIP adsorbents. It can be found that the removal rates of PFOS decreased quickly at pH below 5.1, and then decreased gradually at pH above 5.1, which was associated with the pKa of the functional groups on the adsorbent. The pKa of pyridine is about 5.8 [27], indicating that the surface of the adsorbents is positive at pH below 5.8. As the pKa of sulfonic group in PFOS is about – 3.27 [28], the PFOS

Sorption isotherms

The sorption isotherms of PFOS on the MIP and NIP adsorbents are shown in Fig. 4. It can be seen that the sorption amount of MIP for PFOS was much higher than that of the NIP adsorbents at the same equilibrium concentration of PFOS, suggesting the good imprinted effect of the MIP adsorbents. It is interesting to find that the sorption amount of PFOS on the MIP adsorbents increased with increasing equilibrium concentrations of PFOS, and especially it rose quickly when the equilibrium concentration was approaching 0.4 mmol/L, indicating that the multilayer sorption occurred in the sorption process. When the Langmuir model was used to describe these data, the correlation coefficient (R2) was 0.81, indicating that the sorption of PFOS on the MIP adsorbents was not the monolayer sorption.

Fig. 4 Sorption isotherms of PFOS on the MIP and NIP adsorbents at pH 5.1

Fig. 3 Effect of solution pH on PFOS removal using the MIP and NIP adsorbents

For the MIP adsorbents, the sorption amount increased quickly at low PFOS concentrations, which may be due to the electrostatic interaction between the MIP and PFOS. Some 4-vinylpyridines were polymerized on the MIP adsorbents, and the nitrogen atoms in the polymers could be protonated at low pH, showing the positive charge. The anionic sulfonate head in PFOS then adsorbed on the protonated groups through the electrostatic attraction, making the hydrophobic tail of the PFOS molecule go toward water phase, as schematically shown in Fig. 5(a).

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concentrations used in this study are far below the CMC value, almost no micelles can form in solution. However, it is possible to form some hemi-micelles on the adsorbent surface when the PFOS concentrations are in the range of 0.01–0.001 of the CMC [30]. Moreover, the micelles may also form on the adsorbents after a large number of PFOS molecules adsorb on the adsorbents, where the PFOS concentrations are likely much higher than that in solution. Based on the above discussion, the possibly formed micelles are proposed in Fig. 5(c). 3.5

Effect of ionic strength

NaCl was used as the background electrolyte to study the effect of ionic strength on PFOS removal by the MIP and NIP adsorbents. As shown in Fig. 6, the removal rates increased with increasing concentrations of NaCl, and almost all PFOS were removed when the concentration of NaCl was up to 1 mol/L. The difference of sorption amount between the MIP and NIP decreased with increasing concentrations of NaCl. When NaCl was present in solution, the electric double layer on the adsorbent surface was compressed and the zeta potential decreased, resulting in the decrease of electrostatic attraction between the adsorbents and PFOS. At the same time, the salting-out effect also played an important role in increasing the sorption amount of MIP adsorbents for PFOS. High salt concentrations significantly decreased the water solubility of PFOS [28], resulting in the sorption tendency of PFOS onto the adsorbent surface. Because the overall effect was that sorption amount increased with increasing concentrations of NaCl, the salting-out effect should be more obvious than the electrostatic interaction. Our previous study showed that the sorption of PFOS on the chitosanbased MIP adsorbents increased with increasing NaCl concentration from 50 to 500 mmol/L and kept almost constant at salt concentrations below 50 mmol/L [26]. Higgins and Luthy [31] found that the sorption of PFOS

Fig. 5 Schematic diagram illustrating the PFOS sorption models on the MIP adsorbents. (a) Hemi-micelle (b) double-layer sorption (c) micelle formation

When all the sorption sites were covered with PFOS, the hemi-micelles formed on the adsorbent surface. With further increasing of PFOS concentrations, other PFOS molecules then had the chance to adsorb on the adsorbed PFOS via hydrophobic interaction, resulting in the doublelayer sorption, as shown in Fig. 5(b). The PFOS molecules would form micelles on the adsorbent surface at high PFOS concentrations, which may be the reason for the significant increase of sorption amount of PFOS at the high PFOS equilibrium concentrations shown in Fig. 4. The critical micelle concentration (CMC) value for PFOS should be about 4573 mg/L [29]. Because the PFOS

Fig. 6 Effect of ionic NaCl on the sorption of PFOS onto the MIP and NIP adsorbents

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onto the sediments did not change with increasing concentrations of NaCl from 8 to 110 mmol/L. Obviously, the effect of ionic strength on the PFOS sorption is complex. All PFOS was removed when the NaCl concentration was increased to 1000 mmol/L. Because the PFOS concentration of 200 mg/L used in this study exceeded the PFOS solubility under such conditions (about 20 mg/L), some insoluble PFOS may be physically adsorbed onto the adsorbents, resulting in no difference in sorption amount between the MIP and NIP adsorbents at high salt concentrations. 3.6

Sorption selectivity

In order to investigate the sorption selectivity for PFOS using the MIP adsorbents, 2,4-D was used as the competitive anion in the sorption of PFOS. As shown in Fig. 7, when the concentrations of 2,4-D increased from 0 to 2.26 mmol/L, the removal rates of PFOS only slightly decreased, indicating the high sorption selectivity of the prepared MIP adsorbents for PFOS. Although both PFOS and 2,4-D are anionic pollutants, their molecular structure are different. 2,4-D has a benzene ring in the molecular structure, which makes it difficult to approach the sorption sites. In our previous study, the high selective sorption of PFOS on the chitosan-based adsorbents was obtained at the presence of some anions such as 2,4-D, perfluorooctanoic acid, sodium dodecyl benzene sulfonate, sodium pentachlorophenate, or phenol [26]. The good selectivity of PFOS on the MIP adsorbents was dominated not only by molecule size but also by the interactions between functional groups.

Fig. 7 Effect of 2,4-D on the sorption of PFOS onto the MIP and NIP adsorbents

4

Conclusions

The MIP adsorbents were successfully prepared through the monomer polymerization and used to selectively

remove PFOS from aqueous solution. The electrostatic interaction between the protonated nitrogen on the polymer surface and PFOS played an important role in the sorption process. The result of sorption behavior showed that the sorption of PFOS was very fast and the equilibrium was achieved within 1 h; the sorption amount decreased with increasing solution pH and increased with increasing ionic strength; the sorption isotherm of PFOS on the MIP adsorbent indicated that complex sorption occurred, and the hemi-micelles, double-layer sorption, and micelles may exist on the adsorbent surface. The MIP adsorbents had high selectivity for PFOS in the presence of 2,4-D, and the sorption of PFOS on the MIP adsorbents decreased little when the 2,4-D concentration was up to 2.26 mmol/L. In comparison with the NIP, the sorption amount of PFOS on the MIP adsorbents increased by over one-fold at the equilibrium concentration below 0.25 mmol/L according to the sorption isotherms. Because the concentrations of PFOS used in this study were significantly higher than most environmentally relevant concentrations, the effectiveness of the MIP sorption for PFOS at low concentrations needs to be further investigated. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 50608045), the special fund of State Key Joint Laboratory of Environment Simulation and Pollution (Grant No. 08Z04ESPCT), and the National Outstanding Youth Foundation of China (Grant No. 50625823). The authors also thank the Laboratory Found of Tsinghua University for the analytical work.

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