arenes for Fast and Efficient Removal of Organic Micropollutants from

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Porous Polycalix[4]arenes for Fast and Efficient Removal of Organic Micropollutants from Water Dinesh Shetty,† Ilma Jahovic,† Jesus Raya,φ Zouhair Asfari,‡ John-Carl Olsen,§ and Ali Trabolsi*,† †

New York University Abu Dhabi (NYUAD), Saadiyat Island, Abu Dhabi, United Arab Emirates CNRS/Université de Strasbourg, 1 rue Blaise Pascal, Strasbourg, France 67000 ‡ Laboratoire de Chimie Analytique et Sciences Séparatives, Institut Pluridisciplinaire Hubert Curien, 67087 Strasbourg CEDEX, France § Department of Chemistry, University of Rochester, RC Box 270216, Rochester, New York 14607-0216, United States φ

S Supporting Information *

ABSTRACT: Organic micropollutants are hazards to the environment and human health. Conventional technologies are often inefficient at removing them from wastewater. For example, commercial activated carbon (AC) exhibits slow uptake rates, limited capacities, and is costly to regenerate. Here, we report the utility of porous calix[4]arene-based materials, CalPn (n = 2−4), for water purification. Calixarenes are a common motif in supramolecular chemistry but have rarely been incorporated into extended, porous networks such as organic polymers. CalPn exhibit pollutant uptake rates (kobs) and adsorption capacities (qmax) that are among the highest reported. For example, the kobs of CalP4 for bisphenol A (BPA) is 2.12 mg/g·min, which is significantly higher (16 to 240 times) than kobs for ACs and 1.4 times higher than that of the most efficient material previously reported; the qmax of CalP4 for BPA is 403 mg/g. The CalPn polymers can be regenerated several times, with performance levels left undiminished, by a simple wash procedure that is less energy intensive than that required for ACs. These findings demonstrate the potential of calixarene-based materials for organic micropollutant removal. KEYWORDS: adsorption, bisphenol A, calix[n]arenes, micropollutants, porous organic polymers, propranolol, water purification

1. INTRODUCTION

adsorbent that is free of the drawbacks of ACs is an active area of research. Recently, due to their ability to adsorb micropollutants from water, polymeric materials that contain macrocycles with wellknown guest recognition properties have become a focus of research interest.13−18 The Dichtel group reported a breakthrough in this area, describing porous cyclodextrin-containing polymers that exhibits adsorption rate constants that are 15 to 200 times greater than those of other adsorbents, including commercial ones.15 Calix[n]arenes (n = 4, 6, 8) are a class of macrocycles,19−22 with guest recognition properties, that are easy to prepare from inexpensive starting materials such as phenols and aldehydes.23,24 They feature a hydrophobic cavity surrounded by polar and nonpolar rims that can be selectively functionalized to facilitate polymerization or to induce analyte selectivity. With these properties in mind, we decided to incorporate calixarenes into the backbones of porous covalent

Organic micropollutantsa diverse set of molecules that include pharmaceuticals, constituents of personal care products, steroid hormones, and pesticidespose serious threats to the environment and human health.1−5 Conventional technologies such as membrane filtration are not optimized for their removal from wastewater and often fail to lower their concentrations to safe levels. Sorption is one of the more effective methods for the purification of secondary effluent, and its efficiency varies with the physical properties of both the adsorbate (e.g., aromaticity vs aliphaticity, the presence of particular functional groups) and adsorbent (e.g., surface area, pore size, surface chemistry).6,7 Activated carbons (ACs) have been widely used for sorption; however, with ACs, the amount of adsorbent and the time needed for adequate pollutant removal are often impractical.8−10 Also, the efficiency of ACs decreases over time because of the saturation of adsorption sites, and this necessitates regeneration, which, for this type of material, is an expensive and energy-intensive process that requires temperatures in excess of 500 °C and does not fully restore performance.11,12 Consequently, development of an effective © XXXX American Chemical Society

Received: October 31, 2017 Accepted: December 18, 2017

A

DOI: 10.1021/acsami.7b16546 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

MAS) 13C NMR spectroscopy (Figure S2). Absorption peaks in the range of 135−160 ppm correspond to the phenylene carbons of the macrocycle, whereas peaks near 130 ppm correspond to the phenylene carbons of the linkers. The peaks near 25 and 90 ppm correspond to the aliphatic methylene (−CH2−) carbons of the macrocycle and the ethylene (−C C−) carbons of the linkers, respectively. The microstructure and morphology of the polymers were characterized by microscopic techniques. Both SEM and TEM images show fused amorphous clumps; however, TEM images hint at the presence of pores (Figure S3). Powder X-ray diffraction (PXRD) patterns generated by these polymers (Figure S4) show a characteristic broad peak that suggests amorphous character. Thermogravimetric analysis (TGA) of the polymers shows high thermal stability over 500 °C (Figure S5). The porosities of the polymers were evaluated by N2 gas adsorption/desorption experiments. Prior to the measurements, the polymers were activated at 85 °C for 24 h to remove solvent and trapped gas molecules. The observed N2 adsorption/desorption isotherms (Figure 1c) can be classified as IUPAC type II with H4 type desorption hysteresis loops. The polymers have pore size distributions that lie mainly in the mesopore region. NLDFT average pore diameters are in the range of 62−97 Å, and cumulative pore volumes are in the range of 0.56−1.09 cm3 g−1 (Figures S6−S8). These data suggest complex polymeric networks that contain both micropores and mesopores and that swell with gas intake. The specific surface areas calculated using the Brunauer− Emmett−Teller (BET) equation were found to be 596, 630, and 759 m2 g−1 for CalP2, CalP3, and CalP4, respectively, with greater surface area corresponding to a greater number of acetylene groups in the aryl linker moiety. 2.2. Organic Micropollutant Adsorption Studies. We anticipated that the CalPn polymers, with their high stabilities, large surface areas, permanent porosities, π-bond-rich networks, and guest recognition moieties, would adsorb organic micropollutants efficiently from water. Using four representative pollutants, we tested the polymers’ adsorption properties and compared them to those of a representative AC (Norit row 0.8 supra), a commonly used commercial adsorbent. The micropollutant adsorption experiments were conducted under sonication to allow for the uniform dispersion of the superhydrophobic polymers in the solutions. First, we tested the removal of bisphenol A (BPA), a toxic component of plastics that can cause serious health problems33,34 and found that all of the polymers could remove BPA from solution (5 mg of adsorbent per 10 mL of a 0.1 mM aqueous pollutant solution). CalP4 adsorbed BPA the fastest, reaching ∼70% removal within 30 s and ∼80% removal by the time equilibrium had been reached after 15 min (Figures 2a and 3a). In contrast, CalP2 and CalP3 required 45 and 20 min, respectively, to reach equilibrium and removed 7 and 16% of the pollutant in the first 30 s of contact, respectively. The ultrafast adsorption demonstrated by CalP4 is likely the result of its higher surface area (759 m2 g−1 vs 596 for CalP2 and 630 for CalP3) and more extensive π-bond-rich network (10 triple bonds and 20 phenyl rings per repeated structural unit vs 6 or 8 triple bonds and 8 phenyl rings for CalP2 and CalP3, respectively), which allows for attractive hydrophobic interactions. The removal efficiency of CalP4 is greater than that of AC (∼25% removal in 30 s and >60 min required to reach equilibrium) and similar to the highly efficient cyclodextrin-based porous polymer PCDP (∼90% removal in 30 s and 10 min to reach

polymeric materials. Few prior studies have described the incorporation of calixarenes into polymer backbones.25−27 Here, we present the utility of three hyper-cross-linked πbond-rich porous organic polymers (POPs; CalPn, n = 2−4) that are based on calix[4]arene. We hypothesized that immobilizing calixarenes in the polymer backbones would enable the materials to be efficient adsorbents of organic micropollutants. Moreover, these polymers are embedded with alkynes, a functional group known to endow materials with high surface area and enhanced adsorption capabilities.21,28 As the result of dipolar interactions involving the rims of the calixarenes as well as nonpolar interactions involving their hydrophobic cavities and the constituent aromatic rings and alkynes of the polymers, the synthesized materials exhibited excellent adsorption efficiencies and fast adsorption kinetics for a range of micropollutants. For example, CalP4 outperformed commercial materials9,10,29−32 as well as the most efficient polymeric adsorbent previously reported.15 Furthermore, the CalPn polymers could be fully regenerated several times with a simple wash procedure. The findings presented here demonstrate the potential of porous polycalix[4]arenes as materials for efficient water purification.

2. RESULTS AND DISCUSSION 2.1. Synthesis and Characterization of CalPn (n = 2− 4). The polymers were synthesized from a tetrabromoderivative of calix[4]arene (1) and three different acetylenecontaining linkers (2, 3, and 4) by Sonogashira−Hagihara coupling reactions (Figure 1a).27 The success of cross-coupling

Figure 1. (a) Synthetic route to porous calix[4]arene polymers CalPn (n = 2−4) by Sonogashira−Hagihara cross-coupling. (b) Schematic view of the network structure of CalP4, the best-performing polycalix[4]arene. (c) N2 uptake isotherms corresponding to CalP2, CalP3, and CalP4 at 197 K; calculated surface areas were 596, 630, and 759 m2 g−1, respectively.

was first revealed by FTIR spectra (Figure S1), which show no stretching vibrations consistent with acetylene −C−H or bromocalix[4]arene −C−Br bonds, but do exhibit an absorption band near 2250 cm−1 and a broad band near 3320 cm−1, which correspond to alkyne −CC− stretching and calix[4]arene phenolic −O−H stretching, respectively. The molecular structures of the polymers were characterized by solid-state cross-polarization magic angle spinning (CP/ B


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was deduced from the Langmuir isotherm was 1.97 (Figure 3b), which is of the same order of magnitude as those values reported for the inclusion complexes of free calix[4]arene and para-substituted phenols (log K = 2.93 in ethanol solvent).35 The similarity of these values is consistent with complexation between the phenolic groups of BPA and the calixarenes of CalP4. Also, the maximum adsorption quantity (qmax) of CalP4 was found to be 403 mg/g, a value much higher than those of previously reported adsorbents13−15,36−38 (Figure 3b) and likely results from additional binding of BPA to sites within the network that lie outside the cavities of the calixarenes. Though the conformations of the calix[4]arene units in the main chains of the polymers no doubt play an important role, we believe that total pollutant adsorption is due to the additive effects of the macrocyclic cavities, the π-surfaces of the polymer, and the pores of the network. For polymeric chain growth to occur in the first place, the calixarenes must assume partial cone conformation, and this conformation is likely predominant in the product; however, we cannot rule-out the presence of alternative 1,2 and 1,3 conformations in these hyper-crosslinked networks. Propranolol, a drug used to treat high blood pressure, is another significant micropollutant that conventional wastewater treatment fails to adequately remove.39 CalP4 adsorbed propranolol with a speed similar to the one observed for BPA: ∼82% within 30 s of contact and 91% at 20 min, when equilibrium had been reached (Figures 2b and 3a). In contrast, CalP2 and CalP3 removed only 28 and 36% of the propranolol, respectively, after 30 s of contact. Kinetic measurement of batch-adsorption demonstrated that CalP4 adsorbs propanolol more quickly (kobs = 1.75 g/mg·min) than CalP2, CalP3, and AC (Figure S10 and Table 1). The observed rate constant for CalP4 is 16 times greater than that of AC and significantly higher than those of other reported polymers.15 By the Langmuir adsorption isotherm measurement, we found for the adsorption of propanolol by CalP4 K of 13 697 M−1 (Figure 3c), which is comparable to the value observed for complexation between napthalene and free calix[4]arene [K = (1.37 ± 0.02) × 104 M−1].40 This similarity of binding constants suggests that the napthalene of propranolol binds within the calixarene cavities of the polymer. The qmax for the adsorption of propanolol by CalP4 was determined to be 257 mg/g, which is highest compared to any previous reports.15 2.3. Polymer Regeneration and General Usability. Regeneration of CalP4 was achieved by soaking the pollutantloaded polymer in ethanol at room temperature for 6 h. The polymer was centrifuged and dried before another cycle of adsorption was checked. This simple procedure is much easier than the energy-intensive process used to regenerate ACs. Ethanol washings allowed CalP4 to be reused multiple times with no significant reduction in adsorption capacity (Figure 3d). The generality of CalPn micropollutant adsorption was further assessed by measuring the adsorption of two other compounds: 1-naphthylamine (1-NA), a pollutant known to cause cancer,41 and 2-naphthol (2-NO), a representative of naphthol-containing pollutants. Adsorption studies involving these compounds were performed under the same conditions as those with BPA and propranolol (5 mg of polymer/10 mL of 0.1 mM pollutant solution). Time-dependent adsorption curves reveal excellent performance by CalP4 (∼97% removal for 1NA and ∼68% removal for 2-NO in 30 s) vs the other adsorbents (Figures 2c,d and 3a). With CalP4, the adsorption equilibrium was reached in 10 min with 1-NA and in 45 min

Figure 2. Percent adsorption of (a) bisphenol A, (b) propanolol HCl, (c) 1-naphthylamine, and (d) 2-naphthol over time from 0.1 mM solutions by 0.5 mg mL−1 samples of CalPn and AC. Insets display the chemical structures of each pollutant.

Figure 3. Adsorption efficiencies, thermodynamic parameters, and regeneration capabilities of tested adsorbents. (a) Percent pollutant removal efficiencies, measured at equilibrium, by CalPn polymers and AC. (b) Langmuir isotherm of BPA adsorption by CalP4. The association constant (K) determined by Langmuir model fitting was found to be 94 L/mol, and the maximum adsorption quantity (qmax) was found to be 403 mg/g, when high concentrations (up to 1 mM) of pollutants were used. (c) Percentages of BPA and propranolol removed by CalP4 after three consecutive regenerations. (d) Langmuir isotherm of propranolol adsorption by CalP4. At high pollutant concentration (up to 1 mM), K was 13 697 L/mol, and qmax was 257 mg/g as determined by the Langmuir model.

equilibrium).15 From the batch-adsorption kinetic measurement, we determined that CalP4 adsorbs BPA with a pseudosecond-order rate constant (kobs) of 2.12 mg/g·min, which is 33-fold higher than that of CalP2 and 21-fold higher than that of CalP3 (Figure S9 and Table 1). It is also 16 times higher than the kobs of AC (Norit row 0.8 supra) and 1.4 times higher than that of P-CDP, the most efficient adsorbent of BPA previously reported.15 The log of the formation constant that C


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ACS Applied Materials & Interfaces Table 1. Representative Organic Micropollutants Uptake Rates by Different Adsorbentsa bisphenol A

propranolol HCl

1-naphthylamine

2-naphthol

sorbent

kobs (g/mg·min)

R2

teq (min)

kobs (g/mg·min)

R2

teq (min)

kobs (g/mg·min)

R2

teq (min)

kobs (g/mg·min)

R2

teq (min)

AC CalP2 CalP3 CalP4

0.1290 0.0648 0.1019 2.1244

1.0 0.97 1.0 1.0

>60 45 20 15

0.1382 0.0476 0.0310 1.7515

1.0 0.92 0.98 1.0

>60 >60 60 30

0.2404 0.0891 0.2725 9.6624

1.0 1.0 1.0 1.0

>60 >60 >60 10

0.3548 0.0587 0.3685 3.8206

1.0 0.9 1.0 1.0

>60 >60 >60 45

a Apparent second-order adsorbent uptake rate constants, kobs, calculated for 0.1 mM solutions of bisphenol A, propranolol HCl, 1-naphthylamine, and 2-naphthol and using 0.5 mg/mL samples of adsorbent; correlation coefficients (R2) obtained by fitting a pseudo-second-order kinetic model to the data, and the times required (min) for each adsorbent to attain equilibrium uptake (teq).

t /qt = t /qe + 1/(kobsqe 2)

with 2-NO. The pseudo-second-order rate constants (kobs) for adsorption of 1-NA and 2-NO by CalP4 were 9.66 and 3.82 g/ mg·min, respectively (Figures S11 and S12, Table 1). These values are 40 and 24 times higher than those corresponding to AC adsorption of 1-NA and 2-NO.

to the kinetic data, where qt is the quantity of adsorbed pollutant at time t (min), qe is the adsorbed amount at equilibrium, and kobs is the second-order rate constant (g/mg·min).42 4.2. Thermodynamic Studies of Pollutant Adsorption to CalP4. The adsorbent (CalP4, 2.5 mg) was introduced to a 20 mL vial containing 5 mL of pollutant stock solution (concentration ranging from 0.3 to 1 mM), and the suspension was sonicated to reach equilibrium (15 min for bisphenol A and 20 min for propranolol). The suspension was then filtered on a hydrophilic PTFE syringe filter (0.2 μm), and the amount of pollutant in the filtrate was determined by UV−vis spectroscopy. A Langmuir adsorption isotherm43,44 was obtained by using the equation: 1/qe = 1/qmax,e + 1/qmax,e · Kc, where qe (mg/g) is the quantity of pollutant adsorbed by the polymer at equilibrium, qmax,e (mg/g) is the maximum capacity of pollutant adsorption by an adsorbent at equilibrium, c (mol/L) is the concentration of pollutant at equilibrium, and K (M−1) is the equilibrium constant for the pollutant uptake. 4.3. CalP4 Regeneration. A total of 10 mg of CalP4 was introduced to a 20 mL glass vial containing bisphenol A or propranolol stock solutions (20 mL, 0.1 mM). The mixture was sonicated at RT (15 min for bisphenol A and 20 min for propranolol) and centrifuged for 15 min (4000 rpm). The concentration of the pollutant in the filtrate was determined by UV−vis spectroscopy. The regeneration of CalP4 was achieved by soaking the polymer in EtOH (10 mL × 2) for 6 h and centrifuging it. After drying, the polymer’s adsorption performance was tested two more times. The filtrates from the ethanol washes were concentrated under vacuum, and the recovered adsorbate was weighed. The adsorbate was redissolved in water, and this aqueous sample was analyzed by UV−vis spectroscopy.

3. CONCLUSION We have developed three calix[4]arene-based porous polymers and tested their abilities to remove various organic micropollutants from water. Of the three, CalP4, which was constructed from a tetraalkynylpyrene and tetrabromocalix[4]arene, exhibited the fastest adsorption kinetics and the highest adsorption capacity. CalP4 also outperformed activated carbon and was comparable in performance to the most efficient polymer previously reported. The macrocycles of the CalPn polymers, as well as their π-electron rich networks, provide effective adsorption sites for the pollutants. Furthermore, these polymers can be regenerated by simple treatment with ethanol at room temperature and maintain their adsorption efficiencies for multiple cycles. Their efficiencies and ease of use demonstrate the advantages of incorporating the calixarene macrocycle within functional materials. We are currently investigating other modifications of the calixarene-based design presented here to optimize the materials for water purification. 4. EXPERIMENTAL METHODS

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4.1. Kinetic Studies of Micropollutant Adsorption. In a typical experiment, 10 mg of adsorbent was added to a 20 mL solution of pollutant, and the mixture was sonicated to promote uniform dispersion. Aliquots (2 mL) of the mixture were removed via syringe at regular intervals for up to 15 min after sonication. The aliquots were immediately filtered through hydrophilic PTFE syringe filters (0.2 μm). UV−vis spectroscopy was used to determine the residual concentrations of the pollutants using calibration curves based on the molar extinction coefficients (ε, M−1 cm−1) for bisphenol A (3691 at λmax = 276 nm), 2-naphthol (5794 at λmax = 273 nm), 1-naphthylamine (5014 at λmax = 305 nm), and propranolol hydrochloride (5106 at λmax = 290 nm). The percentage of pollutant removal was determined by using the equation [(C0 − Ct)/C0] × 100, where C0 (mmol/L) is the concentration of the pollutant stock solution (before addition of polymer), and Ct (mmol/L) is the residual concentration of the pollutant after polymer treatment and filtration. We determined the amount of pollutant adsorbed by using the equation: qt = [(C0 − Ct)Mw]/m, where qt (mg/g) is the quantity of adsorbed pollutant per 1 g of adsorbent at a given time t (min), C0 (mmol/L) is the initial concentration before polymer treatment, Ct (mmol/L) is the residual concentration of pollutant after treatment with polymer and filtration of the solution, m (g) is the amount of adsorbent used, and Mw (g/mol) is the molecular weight of the tested pollutant. The rate of uptake of each polymer was calculated by fitting Ho and McKay’s pseudo-second-order adsorption model42

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b16546. Detailed experimental procedures, full characterization data, and results of kinetic studies on adsorption (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ali Trabolsi: 0000-0001-7494-7887 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by New York University Abu Dhabi. The authors thank NYUAD for their support for the research program. The authors also thank the NYUAD Core Technology Platforms and Ms. Khulood Alawadi for the 3D drawings. D


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