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Novel phenyl-phosphate-based porous organic polymers for removal of pharmaceutical contaminants in water
Chemical Engineering Journal 379 (2020) 122290
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Novel phenyl-phosphate-based porous organic polymers for removal of pharmaceutical contaminants in water Seenu Ravi, Yongju Choi, Jong Kwon Choe
T
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Department of Civil and Environmental Engineering and Institute of Construction and Environmental Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
porous organic polymers with • Novel inbuilt phosphate groups (P-POP) were synthesized.
exhibited excellent adsorption • P-POPs capacity of pharmaceutical compounds.
equilibrium time was • Adsorption 50 min, much faster than other reported materials.
were easily regenerable and • P-POPs retained their adsorption capacity for > 5 cycles.
A R T I C LE I N FO
A B S T R A C T
Keywords: Porous organic polymers Pharmaceutical contaminants Phosphate-based polymer Adsorption Water treatment
A clean water system is a basic necessity for the survival of living organisms, and thus, addressing the removal of emerging contaminants from aquatic systems is a priority research topic to reinstate the ecosystem balance for a sustainable future. In this study, we have synthesized two types of novel phosphate based porous organic polymers (P-POP-1 and P-POP-2) by one-pot straightforward synthesis with a Friedel-Crafts reaction using diphenyl phosphate and 1,1,2,2-tetraphenylethylene as precursors for P-POP-1, and diphenyl phosphate and 1,3,5triphenylbenzene for P-POP-2. A series of material characterization using powder X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, N2 adsorptiondesorption, and thermal gravimetric analysis showed that P-POP-1 and P-POP-2 have porous structures with intact phosphates and show a specific surface area of 714 and 581 m2/g, and mesoporosity of 19.6% and 32.5%, respectively. Using caffeine, diclofenac, and carbamazepine as three target pharmaceutical compounds, P-POP-2 showed excellent maximum adsorption capacity (caffeine 301 mg/g, diclofenac 217 mg/g, and carbamazepine 248 mg/g). P-POP-2 also exhibited an adsorption equilibrium time of 50 min, which is shorter than those of other materials. Furthermore, P-POP-2 exhibited high pharmaceutical compound removal (> 95% for caffeine and carbamazepine; > 82% for diclofenac) in the presence of other competing cations and humic acid. The repeated use of P-POPs over 5 cycles showed < 10% deterioration in adsorption capacities. Results from this study show that phosphate-based porous organic polymer material could be a promising adsorbent for removing pharmaceutical compounds in water.
⁎
Corresponding author at: 35-402, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea. E-mail address: [email protected] (J.K. Choe).
https://doi.org/10.1016/j.cej.2019.122290 Received 30 April 2019; Received in revised form 15 July 2019; Accepted 17 July 2019 Available online 18 July 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
Chemical Engineering Journal 379 (2020) 122290
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1. Introduction
objectives of this study are 1) to synthesize and characterize P-POP adsorbents, 2) to evaluate and compare their adsorption capacities and kinetics for three model PPCPs, 3) to assess the influence of pH and other water constituents (i.e., cations, humic acid), and 4) to identify the sorption mechanisms of pharmaceutical compounds on the P-POPs. To the best of our knowledge, there is no existing report on a one-step strategy of P-POP synthesis for the removal of emerging water contaminants.
With the increasing production and use of synthetic chemicals, the contamination of water systems with these chemicals poses a significant risk to humanity and aquatic organisms. Some of these contaminants, often referred to as micropollutants, raise toxicological concerns even at a trace level [1,2]. Among them, pharmaceuticals and personal care products (PPCPs) such as antibiotics, anti-inflammatory drugs, and antiepileptic drugs are reported to be ubiquitously detected in surface water, sediments, and even in wild animals and fresh produce [3–5]. Studies suggest that domestic and industrial wastewater treatment plants are major sources that are discharging PPCPs into water systems, as conventional sewage treatment processes cannot efficiently treat them [6,7]. Therefore, it is imperative to develop effective remediation techniques to treat these pollutants and mitigate their threats to the environment. Several treatment strategies to date have been studied to treat PPCPs in water. These include advanced oxidation [8–10], photocatalytic degradation [11–14], membrane filtration [15–17], and adsorption [18,19]. Advanced oxidation and photocatalytic degradation use highly reactive species such as hydroxyl radicals to chemically destruct PPCPs and mineralize them, but they often require high energy demands or show low efficiency, especially when background organics are present. Undesired byproducts may also form during the chemical destruction [20,21]. The removal of PPCPs by membranes requires nanofiltration or reverse osmosis membranes because most PPCPs penetrate through larger pore size membranes. Adsorption has drawn great interest for PPCP removal because of its ease of operation and simplicity in the process design. The sorption of PPCPs to common adsorbent materials such as activated carbon [22,23], silica [24,25], and zeolite, as well as sewage sludge and agricultural soils, has been well studied. While these adsorbents can remove PPCPs from water, they often have low selectivity, especially for hydrophilic compounds, thus requiring frequent replacement and/or regeneration [26,27]. Other adsorbents, such as metal organic frameworks [28], metal oxides [29], and biopolymers [30,31], have also been synthesized and studied, but they often suffer from low chemical stability and are prone to degrade under an acidic or basic environment. Porous organic polymers (POPs) are novel porous materials with predictable network structures, which can be precisely assembled following the reticular chemistry principle. Having a high specific surface area in addition to their tunable and relatively uniform pore structure, POPs are receiving growing interest in several fields, including gas adsorption and separation [32,33], energy storage [34], and biomedical applications [35]. Specifically, the synthesis of hyper-cross-linked polymers via Friedel-Crafts reaction [36] enables adding specific functional groups to POPs. Combining uniform structure and high surface area with tailored functional sites, POPs can be a promising adsorbent for the selective removal of water contaminants such as PPCPs. The development of such polymers requires a careful design in which the functional group does not affect the heterogeneous nature of the adsorbent to ensure the structural stability over long-term use. A previous study has suggested that adsorbents with phosphate functional groups are effective for pharmaceutical pollutant removal [37]. Because phosphates are weakly acidic, we hypothesized that incorporating phosphate sites within the polymer framework should provide an excellent sorption property, especially for weakly basic pharmaceutical pollutants. Based on this hypothesis, the purpose of this study is to develop phosphate-based porous organic polymers and evaluate their applicability for the removal of pharmaceutical compounds in water. In this study, we have synthesized, for the first time, phosphate (-PO3OH)-based porous organic polymers (P-POP) using diphenyl phosphate as the main functional group precursor and two different aromatic hydrocarbons, 1,1,2,2-tetraphenylethylene and 1,3,5-triphenylbenzene, as linkers (Scheme 1). Using caffeine, diclofenac, and carbamazepine as model PPCP compounds, the specific
2. Materials and methods 2.1. Chemicals Diphenyl phosphate (DPP), 1,1,2,2-tetraphenylethylene (TPE), 1,3,5-triphenylbenzene (TPB), iron chloride anhydrous, formaldehyde dimethyl acetal (FDA), 1,2-dichloroethane (DCE), caffeine, diclofenac sodium, carbamazepine, sodium nitrate, calcium nitrate, copper nitrate, and methanol were purchased from Sigma Aldrich (U.S.A). Sodium hydroxide, hydrochloric acid, and potassium nitrate were purchased from Acros Organics (U.S.A). Humic acid was purchased from Wako Chemicals, Japan. Magnesium nitrate was purchased from Daejung Chemicals, Korea. All these chemicals were analytical grade and used without further purification. 2.2. Synthesis of P-POP-1 and P-POP-2 For the synthesis of P-POP-1 polymer, DPP (1 g, 3.9 mmol) and TPE (0.66 g, 1.98 mmol) were dissolved in 100 mL of DCE at room temperature (21 ± 1.0 °C) in a 250 mL round bottom flask, and the solution was purged with nitrogen gas for 10–15 min. Then, FDA (2.4 g, 31.5 mmol) and anhydrous FeCl3 (3.86 g, 23.8 mmol) were added to the solution. After the mixture was refluxed overnight, it was cooled, filtered, and washed twice using 100 mL each of DCM, methanol, and water, successively. The solid product was further washed with DMF:MeOH (v/v) for 1 day in a Soxhlet extraction unit and then dried under vacuum for 10 h at 160 °C to obtain P-POP-1. The P-POP-2 synthesis followed a similar procedure; however, TPB (0.92 g, 3.0 mmol) was used instead of TPE. 2.3. Sorbent characterization Characterization of the P-POP-1 and P-POP-2 sorbents were done using the following techniques described in this section. The Fourier transform infrared (FTIR) spectra were collected using a VERTEX 80 V (Bruker, Germany) spectrometer with an MCT narrow band detector. Xray photoelectron spectroscopy (XPS) measurements were conducted on a Kratos AXIS-SUPRA spectrometer using a monochromatic Al Kα Xray source and a hemispherical analyzer (Kratos, UK) operated at 300 W. 13C and 31P cross polarization magic angle spinning nuclear magnetic resonance (CP/MAS NMR) spectra were recorded on a WB 500 MHz Bruker AVANCE III HD (Germany) spectrometer. The powder X-ray diffractograms (PXRD, Rigaku, Japan) of the porous polymers were analyzed using CuKα radiation (λ = 1.54 Å) at a scan rate of 0.2° per min with a 2θ scan range of 4–60°. N2 adsorption-desorption isotherms were measured at 77 K using a BELsorpMax (BEL, Japan). Prior to the analysis, the polymer samples were degassed for 10 h at 150 °C under a high-pump vacuum. Pore size distributions of the polymers were measured from the desorption branch using nonlocal density functional theory (NLDFT) methods. High-resolution transmission electron microscopy (HR-TEM) images of the polymer samples were collected using a JEM-3010 (JEOL, Japan) at an accelerating voltage of 300 kV. To prepare the sample specimen, the porous polymer materials were dispersed in methanol and then sonicated for approximately 10 min. Then, 4 µL of the solution was placed onto a copper grid, which was dried for 6 h prior to the analysis. The phosphorous content of each polymer sorbent was measured using inductively coupled plasma 2
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Scheme 1. Synthesis of phosphate-based porous organic polymers P-POP-1 and P-POP-2.
is given by Eq. (2)
optical emission spectroscopy (ICP-AES, Perkin Elmer Optima 8300DV, USA). The thermal stability of each polymer was analyzed via thermogravimetric analyses (TGA) using a SCINCO S-1000 thermal gravimeter (Japan).
log(Qe − Qt ) = log Qe − (K1/2.303) t
where Qe and Qt represent the adsorbed pharmaceutical compounds on P-POPs adsorbent (mg/g) at equilibrium and time t, respectively, and k1 is the rate constant (min−1). Similarly, the pseudo-second-order kinetic model is given by Eq. (3)
2.4. Adsorption experiment A series of adsorption experiments were conducted by preparing solutions of pharmaceutical compounds (i.e., caffeine, diclofenac, and carbamazepine) in deionized water (no buffer) at different initial concentrations ranging from 0 to 53.5 mg/L. To observe pH effect, the adsorption experiments were carried out at the pH range of 3–10, adjusted by adding aliquots of 0.01 M NaOH and/or HNO3 prior to adding P-POPs in the solution. The highest adsorption performance was observed at pH 5 for diclofenac as well as for caffeine and carbamazepine (as further discussed in Section 3.2), thus the solution pH of 5 was chosen for all other adsorption experiments. The sorbent (P-POPs) concentrations were 150 or 200 mg/L for caffeine and 50 or 100 mg/L for diclofenac and carbamazepine. The samples are equilibrated using orbital shaker at 150 rpm for 24 h for P-POP-1 and 3 h for P-POP-2 at room temperature. Then the solution aliquots were collected and separated through a 0.2 µm syringe filter, and the aqueous concentrations of pharmaceutical compounds were analyzed using UV–visible spectroscopy (HUMAS HS-3300, Korea) at a specific wavelength of 273, 286, and 276 nm for caffeine, carbamazepine, and diclofenac, respectively. The sorbed concentrations of pharmaceutical compounds were calculated using Eq. (1),
Qe =
[Ci − Ce] × V Mads
(2)
t / Qt = (1/ K2 Qe2) + (t / Qe )
(3)
where Qt (mg/g) represents the concentrations of adsorbed pharmaceutical compounds at time t (min), k2 (g/mg·min) is the rate constant, and Qe is the adsorption equilibrium capacity (mg/g). 3. Results and discussion 3.1. Characterization of P-POP-1 and P-POP-2 sorbents P-POPs were analyzed using a series of characterization techniques to confirm whether they have the targeted chemical functionality and structure. Fig. 1 shows FTIR spectra of precursor compounds (i.e., DPP, TPB, TPE) and the synthesized polymer materials (P-POP-1 and P-POP2). DPP spectra showed intense bands at 1279 cm−1, which corresponds to the P]O stretching frequency [38]. While slightly shifted, the bands were also observed at 1289 and 1274 cm−1 in P-POP-1 and PPOP-2, respectively. The peak at 1185 cm−1 in DPP is ascribed to the CO/P-O symmetric stretching frequency, which is also observed in the both P-POP-1 and P-POP-2 polymers, at slightly shifted peaks in1214 and 1200 cm−1 respectively. A sharp peak at 957 cm−1 in DPP ascribed , the P-O asymmetric stretching frequency, which for polymers P-POP-1 and P-POP-2 are shown at 949 and 944 cm−1, respectively. The multiple peaks between the range of 1400–1610 cm−1 are due to the C]C stretching frequency of benzene rings, which are consistently shown in both polymers as well as in all precursor compounds. The peaks at 3100 and 2900 cm−1 correspond to the CeH stretching frequency either symmetric or asymmetric from aromatic rings and aliphatic hydrocarbon (eCH2 from FDA) of monomers and polymer materials (Table 1). The fingerprint regions show several peaks and are majorly ascribed to CeH bending modes, which can also be consistently observed for all precursors and P-POPs as shown Table 1. The characteristic peak at 876 cm−1 of TPB also appeared in P-POP-2 at
(1)
where Qe is the concentration of each pharmaceutical compound sorbed at equilibrium (mg/g), Ci is the initial aqueous concentration (mg/L), Ce represents the aqueous concentration at equilibrium (mg/L), V represents the aqueous solution volume (L), and Mads is the mass of the adsorbent (g). The kinetic experiments were also performed at pH 5. Over the experiment, the solution samples were collected at different time intervals of 5, 10, 15, 20, 30, 45, 60, 120, and 180 min. The observed experimental values were fitted to the pseudo-first-order and pseudosecond-order kinetic models, the pseudo-first-order kinetic rate model 3
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the diester functionality of the phosphorus element, suggesting the presence of a phosphate group in P-POPs. Other peaks at 40 ppm and −60 ppm are also possibly attributed to the resonance of phosphate groups in the polymer [39]. The powder XRD patterns of P-POP-1 and P-POP-2 are shown in Fig. 2a. Two peaks at 2θ of 10° and 22° show the phenyl-phenyl ring interaction of polymer materials. These peaks are relatively broad, suggesting that the polymer matrix is not homogeneously structured. It is possible that the phenyl rings in the polymer are connected at a different substitution position. For instance, one linkage may be connected between ortho positions of two phenyl rings, whereas other linkage may be connected between para positions. Combining FTIR, XPS, and NMR results with XRD results, the P-POPs are likely to be the polymers with the repeating units made up of aromatic hydrocarbons and phenyl phosphate but with an amorphous polymer matrix (as shown in Scheme 1). The permanent pore structure, surface area and pore volume of the polymers were assessed through N2-adsorption desorption isotherms (Fig. 2b). P-POP-1 exhibited type IV N2 adsorption isotherms with steep gas uptakes at relative pressures of 0.1–0.2, indicating that the polymer materials are microporous [40]. P-POP-2 also featured type-IV isotherms with steep nitrogen gas adsorption in the low-pressure region and moderate uptake in the high-pressure region (0.7–1.0) with a broad hysteresis loop arising from the mesoporous nature of these networks. Notably, the percentage of mesopores in the P-POP-2 is significantly higher (32.5%) than P-POP-1 (19.6%), as shown in the Table 2. Such mesoporosity is highly beneficial for easy interaction between the sorbent and the target compounds. The obtained surface area and pore volume of P-POP-1/P-POP-2 are 714/581 m2/g and 0.58/0.72 cm3/g, respectively. These structural details support the formation of P-POP with high surface area and intrinsic porosity. TEM micrographs of the pure polymer materials P-POP-1 and POP-2 are shown in Fig. S4. The two micrographs confirm the long-range order pore structure of polymer materials as discussed earlier in the N2 adsorption isotherm.
Fig. 1. FTIR spectra of P-POP-2, P-POP-1, 1,1,2,2-tetraphenylethylene (TPE), 1, 3,5-triphenyl benzene (TPB), and diphenyl phosphate (DPP) (in the order from top to bottom).
Table 1 FTIR vibrational modes of DPP, TPE, DPP, POP-1 and P-POP-2 materials. Functional groups
Wavenumber (cm−1) DPP
TPE
TPB
POP-1
POP-2
P]O (s) PeO/CeO (s) PeO (as) C]C (s)
1279 1185 957 1592 1487
– – – 1592 1492 1445
1289 1214 949 1606 1508
1294 1200 944 1592 1487
CeH (s, as)
3069
3084–3014
– – – 1596 1492 1445 1415 3085–3014
CeH bending modes
1038 781 751 682
1080 1034 761 699
2931 (s), 2855(as) 1085 762 684
2927(s), 2850 (as) 1074 871 756 688
1074 1028 876 752 692
3.2. Removal of caffeine, diclofenac and carbamazepine using P-POPs The sorption capacity of P-POPs is investigated using caffeine, diclofenac, and carbamazepine as model pharmaceutical compounds that have presumably high affinity with the weakly acidic phosphate functional group on these sorbents. The chemical structure and properties of these compounds are summarized in Table 3. First, the influence of the solution pH on adsorption capacity of PPOP-1 was investigated at a natural pH range of aqueous medium (i.e., 3–10), as shown in Fig. 3a. When the pH increased from 5 to 7, Fig. 3a shows that the pH did not result in any statistically significant change (p > 0.05) during adsorption for caffeine and carbamazepine. A slight decrease in adsorption performance was observed at pH > 8, possibly due to the higher concentrations of basic hydroxyl ions in water competing with pharmaceutical compounds for sorption to the acidic sites (phosphate groups) on the adsorbent. Overall, the adsorption performance for caffeine and carbamazepine are relatively consistent over the pH range of 5–8. On the other hand, the diclofenac sorption to the sorbent sharply decreased as the pH increased from 4 to 8. The pKa of diclofenac is 4.15, and it is possible that the protonated form of diclofenac has a higher affinity to phosphate groups on P-POP-1 than the
871 cm−1. FTIR results show that the synthesized P-POPs have characteristic peak intensities of the P]O, PeO, CeH and C]C of precursor compounds (DPP, TPB, and TPE), suggesting successful polymer formation. Furthermore, the chemical states of the major elements in the porous polymer (i.e., C, P, O) were also analyzed using XPS, as shown in Fig. S1. The intense binding energies of C, P, and O at 285 eV, 134 eV, and 532 eV, respectively, are attributed to the successful formation of PPOP-1 and P-POP-2 materials. The phosphorous content was analyzed through inductively coupled plasma atomic emission spectroscopy and shown that 1.51 and 1.67 mmol/g for P-POP-1 and P-POP-2, respectively (Table 2). The solid-state 13C and 31P CP-MAS NMR spectra of P-POPs are also shown in Figs. S2 and S3, respectively. The peaks at approximately 127 and 143 ppm are due to the resonance of aromatic carbons (Fig. S2). More importantly, a major peak −10 ppm in Fig. S3 of 31P NMR shows Table 2 Surface properties of polymer materials. Materials
SBET (m2/g)
V
P-POP-1 P-POP-2
714 581
0.58 0.72
tot
(cm3/g)
Mesoporosity (%)
Pore Size Distribution (nm)
Phosphorus Content (mmol/g)
19.6 32.5
0.9, 1.4 0.9, 1.4, 6.8
1.51 1.67
Vtot = total pore volume at 0.999 relative pressure. 4
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Fig. 2. (a) X-ray diffraction pattern of P-POP-1 and P-POP-2; (b) N2 adsorption–desorption isotherms curves measured at 77 K for P-POP-1 and P-POP-2, respectively (inset figure: Pore size distribution of P-POP materials).
that P-POPs have much better adsorption performance than others. Having high qe values, even at relatively low Ce, suggest better applicability of P-POPs over other sorbent materials for the removal of trace-level pharmaceutical compounds.
deprotonated form. The above results suggest that high adsorption performance was observed at pH 5 for all three pharmaceutical compounds, therefore, all other adsorption experiments from here on were carried out at pH 5. The adsorption isotherms of P-POP-1 and P-POP-2 were obtained as shown in Fig. 3. Isotherm data were fit with Langmuir and Freundlich models. Fit parameters are summarized in Table 4; the correlation coefficients (R2) indicated that the Langmuir adsorption isotherm fit the data better than the Freundlich model. Based on the Langmuir model, the maximum adsorption capacities of the three pharmaceutical compounds were obtained for the P-POPs. P-POP-2 showed a higher adsorption capacity for all the three compounds (caffeine 301 mg/g, diclofenac 217 mg/g, and carbamazepine 248 mg/g) than P-POP-1 (caffeine 245 mg/g, diclofenac 166 mg/g, and carbamazepine 224 mg/ g). This is likely due to the high pore volume and slightly excess mesoporosity, making the phosphate functional group on P-POP-2 more available as adsorption sites. In addition, the P-POP-2 possess slightly more phosphate content (1.67 mmol/g) than P-POP-1 (1.51 mmol/g) as shown in Table 2. When the adsorption capacities of P-POPs are compared to other sorbent materials such as activated carbon, mesoporous silica, and clay minerals (Table 5), P-POP-2 showed the highest maximum adsorption capacity (qm) for caffeine and diclofenac. Additionally, the equilibrium adsorption capacity (qe) when Ce = 2 mg/L was also compared for a more consistent assessment; results showed
3.3. Adsorption kinetics Adsorption kinetics for caffeine, diclofenac, and carbamazepine were monitored for both P-POP-1 and P-POP-2 as shown in Fig. S5 and Fig. 4. Within 20 min of contact time, > 70% of pharmaceutical compounds were adsorbed to P-POP-1 (Fig. S5), whereas the P-POP-2 captured > 75% during the same time interval. Similarly, For P-POP-2, the equilibrium concentration has reached at the contact time of 50 min for all compounds, whereas > 120 min was necessary for caffeine to reach the equilibrium concentration for P-POP-1. P-POP-2 showed fastest equilibrium time (teq) among other materials we have compared (Table 5). The faster adsorption rate of P-POP-2 is probably due to its high surface property, including higher and well-defined mesoporosity compared to P-POP-1. The observed kinetic data were fitted to the pseudo-first-order and pseudo-second-order kinetic models for P-POP-1 (Fig. S6 & S7) and P-POP-2 (Fig. 4), and the corresponding fitted parameters are listed in Table 5. The correlation coefficients for the pseudo-second-order model were fairly higher than those for the pseudo-first-order model (Fig. 4). The rate constant (k2) of P-POP-2 for
Table 3 The chemical structure and properties of selected pharmaceutical contaminants. Compound
Use
Caffeine
Structure
Formulae
pKa [22,41]
log Kow [3,42,43]
Mol.wt
Molecular Size (nm) [44]
stimulant
C8H10N4O2
14
−0.23
194.19
0.98 × 0.87 × 0.56
Diclofenac
Arthritis
C14H10Cl2NNaO2
4.15
0.7
318.13
0.97 × 0.96
Carbamaze-pine
Anti-epileptic
C15H12N2O
13.9
2.45
236.09
1.2 × 0.92 × 0.58
5
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Fig. 3. (a) Effect of pH on sorption of caffeine, diclofenac, and carbamazepine on P-POP-1. Conditions: Initial sorbate concentration of 50 mg/L for caffeine and 15 mg/L for diclofenac and carbamazepine, Adsorbent dose of 200 mg/L for caffeine and 100 mg/L for diclofenac and carbamazepine, and reaction time of 24 h were used. (b–d) Langmuir and Freundlich adsorption isotherm of (b) caffeine, (c) diclofenac, and (d) carbamazepine for P-POP-1 and P-POP-2. Conditions: pH 5, initial sorbate concentration of 0–53.5 mg/L for caffeine, 0–13.5 ppm for diclofenac, and 0–20 mg/L for carbamazepine, adsorbent dose of 150 mg/L for caffeine, 50 mg/L for diclofenac and carbamazepine, and reaction time of 24 h were used.
adsorption selectivity for pharmaceutical compounds was assessed in the presence of 5–40 mg/L humic acid. P-POP-2 selectively removed > 95% of caffeine and carbamazepine, and > 88% of diclofenac from the solution. It was observed that the humic acid had a negligible effect on the adsorption performance of P-POP-2 for pharmaceutical compounds with basic groups. Similarly, the effect of other cations commonly presents in the aqueous solutions such as K+, Na+, Ca2+, Cu2+, and Mg2+ ions were tested. The influence of each cation was monitored at 20 mg/L concentration. Fig. 5b shows that K+ and Na+ had a small effect on the adsorption of pharmaceutical compounds, lowering its adsorption capacity by 5–8% on only the diclofenac adsorption, but the other ions had no significant effect on the adsorption of pharmaceutical compounds by P-POP-2.
Table 4 Adsorption isotherm data values of caffeine, diclofenac, and carbamazepine over P-POPs. Pharmaceutical contaminants
Caffeine Diclofenac Carbamazepine
Sorbents
P-POP-1 P-POP-2 P-POP-1 P-POP-2 P-POP-1 P-POP-2
Langmuir adsorption isotherm
Freundlich adsorption isotherm
qm (mg/ g)
KL (L/ mg)
R2
KF (mg/ g)
n
R2
245 301 166 217 224 248
1.43 1.09 4.38 6.08 7.69 6.72
0.989 0.984 0.975 0.974 0.951 0.942
118 139 58.80 71.50 92.84 87.16
3.65 3.22 4.59 4.34 4.73 5.03
0.730 0.769 0.812 0.814 0.732 0.738
3.5. Adsorption mechanism pharmaceutical compounds were considerably higher than that of PPOP-1 (Table 6), in line with the enhanced adsorption caused by larger mesoporosity of the P-POP-2 polymer matrix.
Possible adsorption mechanisms between adsorbent and aromatic adsorbate include van der Waals interactions, π-π dispersion interactions, hydrogen bonding, hydrophobic-hydrophobic mechanisms, and electrostatic interactions. Caffeine and carbamazepine exist as natural species at the pH range used in our study, suggesting electrostatic interaction is unlikely to be the dominant adsorption mechanism between P-POPs and these compounds. The pHpzc of P-POP-1 and P-POP-2 were measured to be 3.62 and 3.64, respectively (Fig. S8); both diclofenac (pKa = 4.25) and P-POP-1 have an overall negative charge at pH 5, so
3.4. Effect of competing water constituents on sorption of pharmaceutical compounds The influence of competing water constituents on adsorption performance of P-POPs are also investigated as shown in Fig. 5. First, the 6
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Table 5 Comparison of adsorption performance of P-POPs with sorbent materials reported in other literature (Bold indicates the best value in each column). Adsorbent
Caffeine
Diclofenac
Carbamazepine
qm (mg/g)
qe at Ce = 2 mg/L (mg/ g)a
teq (min)b
qm (mg/g)
qe at Ce = 2 mg/L (mg/ g)
teq (min)
qm (mg/g)
qe at Ce = 2 mg/L (mg/ g)
teq (min)
Ref
Carbon-PC Carbon xerogel Saponite SBA-15 Commercial PAC BA-smectite H3PO4 chemically activated carbon from peach stones Magnetic activated carbon P-POP-1
260 182 80.5 2.35 12.6 135 –
97 81 9 0.5 12 77.6 –
110 N/A 120 N/A N/A 1440 –
201 80 – – 25.35 – –
N/Ac 27 – – 25.11 – –
145 N/A – – N/A – –
335 – – – – – 241.6
N/Ac – – – – – 43
160 – – – – – 240
[44] [45] [46] [47] [48] [49] [37]
– 245
– 212
– 120
– 166
– 152
– 120
182.9 222
180 213
60 120
P-POP-2
301
266
50
217
189
50
246
225
50
[50] This work This work
a b c
Values of equilibrium adsorption capacity (qe) when equilibrium aqueous concentration (Ce) is 2 mg/L was taken from the data in each reference. Equilibrium time was based on adsorption kinetics data (teq ≥ t95%); listed as NA if adsorption kinetic data were not available. Adsorption isotherm in this study [44] did not have adsorption isotherm data below Ce = 20 mg/L.
Fig. 4. (a) Adsorption kinetics of caffeine, diclofenac, and carbamazepine on P-POP-2; Conditions: Initial sorbate concentration of 50 mg/L for caffeine and 15 mg/L for diclofenac and carbamazepine, adsorption dose of 200 mg/L for caffeine and 100 mg/L for diclofenac and carbamazepine, and pH = 5 were used. (b) kinetic data fitted with the pseudo-first-order kinetic model, and (c) kinetic data fitted with the pseudo-second-order kinetic model.
interactions between aromatics in adsorbent and pharmaceutical compounds may also contribute to their sorption. However, P-POPs showed much better adsorption performance compared to commercial powdered activated carbons (Table 5, [47]), in which π-π interactions are reported to play a dominant role. Thus, it is likely that other adsorption mechanisms may be more important for P-POPs. For our adsorbent, the less hydrophobic compound (i.e., caffeine) showed higher sorption capacity than the more hydrophobic compound (i.e., diclofenac), suggesting hydrophobic-hydrophobic mechanism contributes a minor role especially for caffeine and carbamazepine. For P-POPs, we speculate that hydrogen bonding between the phosphate site on the adsorbent and adsorbate plays a dominant role in adsorption (Scheme 2). Phosphate group (PO3OH) has four oxygen atoms connected with P atom, in which one oxygen is connected with a double bond and one oxygen is connected with a hydrogen atom, thus providing multidentate sites for hydrogen bonding. Pharmaceutical compounds (e.g., caffeine, carbamazepine, diclofenac) have multiple heteroatom (nitrogen) and carbene hydrogens that could interact with the phosphate unit via hydrogen bonding. However, it is possible that hydrophobic-hydrophobic mechanism or other mechanism can play more dominant roles at different solution conditions or when other pharmaceutical compounds sorb to P-POPs.
Table 6 Adsorption kinetics of caffeine, diclofenac sodium, and carbamazepine over PPOPs. Pharmaceutical contaminants
Caffeine Diclofenac Carbamazepine
Porous organic Polymer
P-POP-1 P-POP-2 P-POP-1 P-POP-2 P-POP-1 P-POP-2
Psuedo-1st-order model
Psuedo-2nd-order model
k1 (min−1)
R2
k2×10−3 (g mg−1 min−1)
R2
0.0401 0.0660 0.0403 0.0401 0.0407 0.0621
0.925 0.905 0.714 0.707 0.833 0.787
0.824 1.280 0.701 0.942 0.712 0.949
0.999 0.998 0.997 0.996 0.998 0.998
electrostatic attraction should not occur between the two. Minimal influence of other cations (e.g., Na+, Ca2+) on adsorption performance of the pharmaceutical (discussed in Section 3.4) also confirms this. However, adsorption capacity for diclofenac has sharply decreased as pH increases, possibly due to the electrostatic repulsion between P-POP surface and diclofenac inhibiting diclofenac sorption to the phosphate site. Since P-POPs has a benzene ring backbone, π-π dispersion 7
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Fig. 5. Effect of (a) humic acid and (b) cations on the adsorption of caffeine, diclofenac, and carbamazepine over P-POP-2; Conditions: Initial sorbate concentration of 50 mg/L for caffeine and 15 mg/L for diclofenac and carbamazepine, adsorbent dose of 200 mg/L for caffeine and 100 mg/L for diclofenac and carbamazepine, reaction time of 3 h, and pH 5 were used.
Scheme 2. Possible host guest hydrogen bonding interaction mechanism of a) caffeine, b) diclofenac, and c) carbamazepine with phosphate functional sites of PPOPs at pH 5.
that after 5 repeated cycles there was a slight decrease (< 10%) in the adsorption capacity of P-POP-2 (Fig. S10), suggesting that the sorbent has a good recyclability over extended periods of use. Furthermore, FTIR results (Fig. S11) showed that the P-POP polymers are highly stable even after the 24 h treatment with either the acidic (1 M HCl) or basic (1 M NaOH) aqueous solution. Similarly, the thermal stability of P-POPs was also analyzed using thermal gravimetric analysis (Fig. S12), and the results suggested that the polymers are stable up to 250 °C.
Comparing the physicochemical properties of caffeine, carbamazepine, and diclofenac, the influence of adsorbate characteristics on the adsorption capacity of P-POPs were also investigated. Some factors that often affect sorption of adsorbate to adsorbent are molecular size, hydrophobicity (i.e., log(Kow)), and pKa, as well as types and structure of substituents on aromatic compounds [51,52]. For P-POPs, no noticeable trend between adsorption capacity and log(Kow) of the three pharmaceutical compounds was observed (Fig. S9), suggesting that hydrophobicity of adsorbates have minimal influence on their sorption to PPOPs. An inverse relationship between the molecular weight of pharmaceutical compounds and their adsorption capacities to the porous polymer adsorbents were observed for both P-POP-1 and P-POP-2 (Fig. S9), in which the smaller molecule (i.e., caffeine) had higher qm than the larger molecule (i.e., diclofenac). This is likely due to smaller molecules having easier and higher accessibility to all available sites (even ones within small micropores) on the adsorbent. Further studies with a larger number of pharmaceutical compounds are needed to more carefully identify the key characteristics that influence adsorption capacities of these pharmaceutical compounds on P-POPs.
4. Conclusions Novel sorbents, phosphate-based porous organic polymers P-POP-1 and P-POP-2, were synthesized for the removal of pharmaceutical compounds in water, and their adsorption performance was evaluated using three pharmaceutical compounds (i.e., caffeine, diclofenac, and carbamazepine). Overall, P-POPs exhibited excellent adsorption capacities compared to the other sorbent materials due to their uniform polymer structure containing phosphate group. P-POP-2 exhibited excellent textural properties with more mesopores than P-POP-1, thus showing higher adsorption capacity as well as faster equilibrium time. Fits with the isotherm model and kinetic model showed that the adsorption of pharmaceutical compounds on P-POPs follow a Langmuir isotherm with pseudo-second-order kinetics. The dominant adsorption mechanism is likely to be multidentate hydrogen bonding between the adsorbate and phosphate group on P-POPs. Both P-POP-1 and P-POP-2 showed high selectivity (> 98%) for the pharmaceutical compounds in
3.6. Adsorbent regeneration To verify the recyclability and stability of P-POP-2, repeated cycles of adsorption–regeneration tests were carried out. In each cycle, the experimental conditions for the adsorption step was kept the same. After each adsorption step, P-POP-2 was regenerated by washing with acidified ethanol followed by drying in a vacuum. It was demonstrated 8
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the presence of humic acids and other common cations (i.e., Na+, Mg2+, Ca2+) in water. They also showed high chemical stability over repeated use without deterioration in adsorption capacity. The results from this study show that phosphate-based porous organic polymer material could be a promising adsorbent for removing pharmaceutical compounds in water and wastewater. Additionally, coupling porous organic polymer with the target functional group explores a new possibility for selective physicochemical interaction not only for water treatment but also for other versatile industrial applications, including heterogeneous catalysis and energy applications.
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Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Novel phenyl-phosphate-based porous organic polymers for removal of pharmaceutical contaminants in water Seenu Ravi, Yongju Choi, Jong Kwon Choe
T
⁎
Department of Civil and Environmental Engineering and Institute of Construction and Environmental Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
porous organic polymers with • Novel inbuilt phosphate groups (P-POP) were synthesized.
exhibited excellent adsorption • P-POPs capacity of pharmaceutical compounds.
equilibrium time was • Adsorption 50 min, much faster than other reported materials.
were easily regenerable and • P-POPs retained their adsorption capacity for > 5 cycles.
A R T I C LE I N FO
A B S T R A C T
Keywords: Porous organic polymers Pharmaceutical contaminants Phosphate-based polymer Adsorption Water treatment
A clean water system is a basic necessity for the survival of living organisms, and thus, addressing the removal of emerging contaminants from aquatic systems is a priority research topic to reinstate the ecosystem balance for a sustainable future. In this study, we have synthesized two types of novel phosphate based porous organic polymers (P-POP-1 and P-POP-2) by one-pot straightforward synthesis with a Friedel-Crafts reaction using diphenyl phosphate and 1,1,2,2-tetraphenylethylene as precursors for P-POP-1, and diphenyl phosphate and 1,3,5triphenylbenzene for P-POP-2. A series of material characterization using powder X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, N2 adsorptiondesorption, and thermal gravimetric analysis showed that P-POP-1 and P-POP-2 have porous structures with intact phosphates and show a specific surface area of 714 and 581 m2/g, and mesoporosity of 19.6% and 32.5%, respectively. Using caffeine, diclofenac, and carbamazepine as three target pharmaceutical compounds, P-POP-2 showed excellent maximum adsorption capacity (caffeine 301 mg/g, diclofenac 217 mg/g, and carbamazepine 248 mg/g). P-POP-2 also exhibited an adsorption equilibrium time of 50 min, which is shorter than those of other materials. Furthermore, P-POP-2 exhibited high pharmaceutical compound removal (> 95% for caffeine and carbamazepine; > 82% for diclofenac) in the presence of other competing cations and humic acid. The repeated use of P-POPs over 5 cycles showed < 10% deterioration in adsorption capacities. Results from this study show that phosphate-based porous organic polymer material could be a promising adsorbent for removing pharmaceutical compounds in water.
⁎
Corresponding author at: 35-402, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea. E-mail address: [email protected] (J.K. Choe).
https://doi.org/10.1016/j.cej.2019.122290 Received 30 April 2019; Received in revised form 15 July 2019; Accepted 17 July 2019 Available online 18 July 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
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1. Introduction
objectives of this study are 1) to synthesize and characterize P-POP adsorbents, 2) to evaluate and compare their adsorption capacities and kinetics for three model PPCPs, 3) to assess the influence of pH and other water constituents (i.e., cations, humic acid), and 4) to identify the sorption mechanisms of pharmaceutical compounds on the P-POPs. To the best of our knowledge, there is no existing report on a one-step strategy of P-POP synthesis for the removal of emerging water contaminants.
With the increasing production and use of synthetic chemicals, the contamination of water systems with these chemicals poses a significant risk to humanity and aquatic organisms. Some of these contaminants, often referred to as micropollutants, raise toxicological concerns even at a trace level [1,2]. Among them, pharmaceuticals and personal care products (PPCPs) such as antibiotics, anti-inflammatory drugs, and antiepileptic drugs are reported to be ubiquitously detected in surface water, sediments, and even in wild animals and fresh produce [3–5]. Studies suggest that domestic and industrial wastewater treatment plants are major sources that are discharging PPCPs into water systems, as conventional sewage treatment processes cannot efficiently treat them [6,7]. Therefore, it is imperative to develop effective remediation techniques to treat these pollutants and mitigate their threats to the environment. Several treatment strategies to date have been studied to treat PPCPs in water. These include advanced oxidation [8–10], photocatalytic degradation [11–14], membrane filtration [15–17], and adsorption [18,19]. Advanced oxidation and photocatalytic degradation use highly reactive species such as hydroxyl radicals to chemically destruct PPCPs and mineralize them, but they often require high energy demands or show low efficiency, especially when background organics are present. Undesired byproducts may also form during the chemical destruction [20,21]. The removal of PPCPs by membranes requires nanofiltration or reverse osmosis membranes because most PPCPs penetrate through larger pore size membranes. Adsorption has drawn great interest for PPCP removal because of its ease of operation and simplicity in the process design. The sorption of PPCPs to common adsorbent materials such as activated carbon [22,23], silica [24,25], and zeolite, as well as sewage sludge and agricultural soils, has been well studied. While these adsorbents can remove PPCPs from water, they often have low selectivity, especially for hydrophilic compounds, thus requiring frequent replacement and/or regeneration [26,27]. Other adsorbents, such as metal organic frameworks [28], metal oxides [29], and biopolymers [30,31], have also been synthesized and studied, but they often suffer from low chemical stability and are prone to degrade under an acidic or basic environment. Porous organic polymers (POPs) are novel porous materials with predictable network structures, which can be precisely assembled following the reticular chemistry principle. Having a high specific surface area in addition to their tunable and relatively uniform pore structure, POPs are receiving growing interest in several fields, including gas adsorption and separation [32,33], energy storage [34], and biomedical applications [35]. Specifically, the synthesis of hyper-cross-linked polymers via Friedel-Crafts reaction [36] enables adding specific functional groups to POPs. Combining uniform structure and high surface area with tailored functional sites, POPs can be a promising adsorbent for the selective removal of water contaminants such as PPCPs. The development of such polymers requires a careful design in which the functional group does not affect the heterogeneous nature of the adsorbent to ensure the structural stability over long-term use. A previous study has suggested that adsorbents with phosphate functional groups are effective for pharmaceutical pollutant removal [37]. Because phosphates are weakly acidic, we hypothesized that incorporating phosphate sites within the polymer framework should provide an excellent sorption property, especially for weakly basic pharmaceutical pollutants. Based on this hypothesis, the purpose of this study is to develop phosphate-based porous organic polymers and evaluate their applicability for the removal of pharmaceutical compounds in water. In this study, we have synthesized, for the first time, phosphate (-PO3OH)-based porous organic polymers (P-POP) using diphenyl phosphate as the main functional group precursor and two different aromatic hydrocarbons, 1,1,2,2-tetraphenylethylene and 1,3,5-triphenylbenzene, as linkers (Scheme 1). Using caffeine, diclofenac, and carbamazepine as model PPCP compounds, the specific
2. Materials and methods 2.1. Chemicals Diphenyl phosphate (DPP), 1,1,2,2-tetraphenylethylene (TPE), 1,3,5-triphenylbenzene (TPB), iron chloride anhydrous, formaldehyde dimethyl acetal (FDA), 1,2-dichloroethane (DCE), caffeine, diclofenac sodium, carbamazepine, sodium nitrate, calcium nitrate, copper nitrate, and methanol were purchased from Sigma Aldrich (U.S.A). Sodium hydroxide, hydrochloric acid, and potassium nitrate were purchased from Acros Organics (U.S.A). Humic acid was purchased from Wako Chemicals, Japan. Magnesium nitrate was purchased from Daejung Chemicals, Korea. All these chemicals were analytical grade and used without further purification. 2.2. Synthesis of P-POP-1 and P-POP-2 For the synthesis of P-POP-1 polymer, DPP (1 g, 3.9 mmol) and TPE (0.66 g, 1.98 mmol) were dissolved in 100 mL of DCE at room temperature (21 ± 1.0 °C) in a 250 mL round bottom flask, and the solution was purged with nitrogen gas for 10–15 min. Then, FDA (2.4 g, 31.5 mmol) and anhydrous FeCl3 (3.86 g, 23.8 mmol) were added to the solution. After the mixture was refluxed overnight, it was cooled, filtered, and washed twice using 100 mL each of DCM, methanol, and water, successively. The solid product was further washed with DMF:MeOH (v/v) for 1 day in a Soxhlet extraction unit and then dried under vacuum for 10 h at 160 °C to obtain P-POP-1. The P-POP-2 synthesis followed a similar procedure; however, TPB (0.92 g, 3.0 mmol) was used instead of TPE. 2.3. Sorbent characterization Characterization of the P-POP-1 and P-POP-2 sorbents were done using the following techniques described in this section. The Fourier transform infrared (FTIR) spectra were collected using a VERTEX 80 V (Bruker, Germany) spectrometer with an MCT narrow band detector. Xray photoelectron spectroscopy (XPS) measurements were conducted on a Kratos AXIS-SUPRA spectrometer using a monochromatic Al Kα Xray source and a hemispherical analyzer (Kratos, UK) operated at 300 W. 13C and 31P cross polarization magic angle spinning nuclear magnetic resonance (CP/MAS NMR) spectra were recorded on a WB 500 MHz Bruker AVANCE III HD (Germany) spectrometer. The powder X-ray diffractograms (PXRD, Rigaku, Japan) of the porous polymers were analyzed using CuKα radiation (λ = 1.54 Å) at a scan rate of 0.2° per min with a 2θ scan range of 4–60°. N2 adsorption-desorption isotherms were measured at 77 K using a BELsorpMax (BEL, Japan). Prior to the analysis, the polymer samples were degassed for 10 h at 150 °C under a high-pump vacuum. Pore size distributions of the polymers were measured from the desorption branch using nonlocal density functional theory (NLDFT) methods. High-resolution transmission electron microscopy (HR-TEM) images of the polymer samples were collected using a JEM-3010 (JEOL, Japan) at an accelerating voltage of 300 kV. To prepare the sample specimen, the porous polymer materials were dispersed in methanol and then sonicated for approximately 10 min. Then, 4 µL of the solution was placed onto a copper grid, which was dried for 6 h prior to the analysis. The phosphorous content of each polymer sorbent was measured using inductively coupled plasma 2
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Scheme 1. Synthesis of phosphate-based porous organic polymers P-POP-1 and P-POP-2.
is given by Eq. (2)
optical emission spectroscopy (ICP-AES, Perkin Elmer Optima 8300DV, USA). The thermal stability of each polymer was analyzed via thermogravimetric analyses (TGA) using a SCINCO S-1000 thermal gravimeter (Japan).
log(Qe − Qt ) = log Qe − (K1/2.303) t
where Qe and Qt represent the adsorbed pharmaceutical compounds on P-POPs adsorbent (mg/g) at equilibrium and time t, respectively, and k1 is the rate constant (min−1). Similarly, the pseudo-second-order kinetic model is given by Eq. (3)
2.4. Adsorption experiment A series of adsorption experiments were conducted by preparing solutions of pharmaceutical compounds (i.e., caffeine, diclofenac, and carbamazepine) in deionized water (no buffer) at different initial concentrations ranging from 0 to 53.5 mg/L. To observe pH effect, the adsorption experiments were carried out at the pH range of 3–10, adjusted by adding aliquots of 0.01 M NaOH and/or HNO3 prior to adding P-POPs in the solution. The highest adsorption performance was observed at pH 5 for diclofenac as well as for caffeine and carbamazepine (as further discussed in Section 3.2), thus the solution pH of 5 was chosen for all other adsorption experiments. The sorbent (P-POPs) concentrations were 150 or 200 mg/L for caffeine and 50 or 100 mg/L for diclofenac and carbamazepine. The samples are equilibrated using orbital shaker at 150 rpm for 24 h for P-POP-1 and 3 h for P-POP-2 at room temperature. Then the solution aliquots were collected and separated through a 0.2 µm syringe filter, and the aqueous concentrations of pharmaceutical compounds were analyzed using UV–visible spectroscopy (HUMAS HS-3300, Korea) at a specific wavelength of 273, 286, and 276 nm for caffeine, carbamazepine, and diclofenac, respectively. The sorbed concentrations of pharmaceutical compounds were calculated using Eq. (1),
Qe =
[Ci − Ce] × V Mads
(2)
t / Qt = (1/ K2 Qe2) + (t / Qe )
(3)
where Qt (mg/g) represents the concentrations of adsorbed pharmaceutical compounds at time t (min), k2 (g/mg·min) is the rate constant, and Qe is the adsorption equilibrium capacity (mg/g). 3. Results and discussion 3.1. Characterization of P-POP-1 and P-POP-2 sorbents P-POPs were analyzed using a series of characterization techniques to confirm whether they have the targeted chemical functionality and structure. Fig. 1 shows FTIR spectra of precursor compounds (i.e., DPP, TPB, TPE) and the synthesized polymer materials (P-POP-1 and P-POP2). DPP spectra showed intense bands at 1279 cm−1, which corresponds to the P]O stretching frequency [38]. While slightly shifted, the bands were also observed at 1289 and 1274 cm−1 in P-POP-1 and PPOP-2, respectively. The peak at 1185 cm−1 in DPP is ascribed to the CO/P-O symmetric stretching frequency, which is also observed in the both P-POP-1 and P-POP-2 polymers, at slightly shifted peaks in1214 and 1200 cm−1 respectively. A sharp peak at 957 cm−1 in DPP ascribed , the P-O asymmetric stretching frequency, which for polymers P-POP-1 and P-POP-2 are shown at 949 and 944 cm−1, respectively. The multiple peaks between the range of 1400–1610 cm−1 are due to the C]C stretching frequency of benzene rings, which are consistently shown in both polymers as well as in all precursor compounds. The peaks at 3100 and 2900 cm−1 correspond to the CeH stretching frequency either symmetric or asymmetric from aromatic rings and aliphatic hydrocarbon (eCH2 from FDA) of monomers and polymer materials (Table 1). The fingerprint regions show several peaks and are majorly ascribed to CeH bending modes, which can also be consistently observed for all precursors and P-POPs as shown Table 1. The characteristic peak at 876 cm−1 of TPB also appeared in P-POP-2 at
(1)
where Qe is the concentration of each pharmaceutical compound sorbed at equilibrium (mg/g), Ci is the initial aqueous concentration (mg/L), Ce represents the aqueous concentration at equilibrium (mg/L), V represents the aqueous solution volume (L), and Mads is the mass of the adsorbent (g). The kinetic experiments were also performed at pH 5. Over the experiment, the solution samples were collected at different time intervals of 5, 10, 15, 20, 30, 45, 60, 120, and 180 min. The observed experimental values were fitted to the pseudo-first-order and pseudosecond-order kinetic models, the pseudo-first-order kinetic rate model 3
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the diester functionality of the phosphorus element, suggesting the presence of a phosphate group in P-POPs. Other peaks at 40 ppm and −60 ppm are also possibly attributed to the resonance of phosphate groups in the polymer [39]. The powder XRD patterns of P-POP-1 and P-POP-2 are shown in Fig. 2a. Two peaks at 2θ of 10° and 22° show the phenyl-phenyl ring interaction of polymer materials. These peaks are relatively broad, suggesting that the polymer matrix is not homogeneously structured. It is possible that the phenyl rings in the polymer are connected at a different substitution position. For instance, one linkage may be connected between ortho positions of two phenyl rings, whereas other linkage may be connected between para positions. Combining FTIR, XPS, and NMR results with XRD results, the P-POPs are likely to be the polymers with the repeating units made up of aromatic hydrocarbons and phenyl phosphate but with an amorphous polymer matrix (as shown in Scheme 1). The permanent pore structure, surface area and pore volume of the polymers were assessed through N2-adsorption desorption isotherms (Fig. 2b). P-POP-1 exhibited type IV N2 adsorption isotherms with steep gas uptakes at relative pressures of 0.1–0.2, indicating that the polymer materials are microporous [40]. P-POP-2 also featured type-IV isotherms with steep nitrogen gas adsorption in the low-pressure region and moderate uptake in the high-pressure region (0.7–1.0) with a broad hysteresis loop arising from the mesoporous nature of these networks. Notably, the percentage of mesopores in the P-POP-2 is significantly higher (32.5%) than P-POP-1 (19.6%), as shown in the Table 2. Such mesoporosity is highly beneficial for easy interaction between the sorbent and the target compounds. The obtained surface area and pore volume of P-POP-1/P-POP-2 are 714/581 m2/g and 0.58/0.72 cm3/g, respectively. These structural details support the formation of P-POP with high surface area and intrinsic porosity. TEM micrographs of the pure polymer materials P-POP-1 and POP-2 are shown in Fig. S4. The two micrographs confirm the long-range order pore structure of polymer materials as discussed earlier in the N2 adsorption isotherm.
Fig. 1. FTIR spectra of P-POP-2, P-POP-1, 1,1,2,2-tetraphenylethylene (TPE), 1, 3,5-triphenyl benzene (TPB), and diphenyl phosphate (DPP) (in the order from top to bottom).
Table 1 FTIR vibrational modes of DPP, TPE, DPP, POP-1 and P-POP-2 materials. Functional groups
Wavenumber (cm−1) DPP
TPE
TPB
POP-1
POP-2
P]O (s) PeO/CeO (s) PeO (as) C]C (s)
1279 1185 957 1592 1487
– – – 1592 1492 1445
1289 1214 949 1606 1508
1294 1200 944 1592 1487
CeH (s, as)
3069
3084–3014
– – – 1596 1492 1445 1415 3085–3014
CeH bending modes
1038 781 751 682
1080 1034 761 699
2931 (s), 2855(as) 1085 762 684
2927(s), 2850 (as) 1074 871 756 688
1074 1028 876 752 692
3.2. Removal of caffeine, diclofenac and carbamazepine using P-POPs The sorption capacity of P-POPs is investigated using caffeine, diclofenac, and carbamazepine as model pharmaceutical compounds that have presumably high affinity with the weakly acidic phosphate functional group on these sorbents. The chemical structure and properties of these compounds are summarized in Table 3. First, the influence of the solution pH on adsorption capacity of PPOP-1 was investigated at a natural pH range of aqueous medium (i.e., 3–10), as shown in Fig. 3a. When the pH increased from 5 to 7, Fig. 3a shows that the pH did not result in any statistically significant change (p > 0.05) during adsorption for caffeine and carbamazepine. A slight decrease in adsorption performance was observed at pH > 8, possibly due to the higher concentrations of basic hydroxyl ions in water competing with pharmaceutical compounds for sorption to the acidic sites (phosphate groups) on the adsorbent. Overall, the adsorption performance for caffeine and carbamazepine are relatively consistent over the pH range of 5–8. On the other hand, the diclofenac sorption to the sorbent sharply decreased as the pH increased from 4 to 8. The pKa of diclofenac is 4.15, and it is possible that the protonated form of diclofenac has a higher affinity to phosphate groups on P-POP-1 than the
871 cm−1. FTIR results show that the synthesized P-POPs have characteristic peak intensities of the P]O, PeO, CeH and C]C of precursor compounds (DPP, TPB, and TPE), suggesting successful polymer formation. Furthermore, the chemical states of the major elements in the porous polymer (i.e., C, P, O) were also analyzed using XPS, as shown in Fig. S1. The intense binding energies of C, P, and O at 285 eV, 134 eV, and 532 eV, respectively, are attributed to the successful formation of PPOP-1 and P-POP-2 materials. The phosphorous content was analyzed through inductively coupled plasma atomic emission spectroscopy and shown that 1.51 and 1.67 mmol/g for P-POP-1 and P-POP-2, respectively (Table 2). The solid-state 13C and 31P CP-MAS NMR spectra of P-POPs are also shown in Figs. S2 and S3, respectively. The peaks at approximately 127 and 143 ppm are due to the resonance of aromatic carbons (Fig. S2). More importantly, a major peak −10 ppm in Fig. S3 of 31P NMR shows Table 2 Surface properties of polymer materials. Materials
SBET (m2/g)
V
P-POP-1 P-POP-2
714 581
0.58 0.72
tot
(cm3/g)
Mesoporosity (%)
Pore Size Distribution (nm)
Phosphorus Content (mmol/g)
19.6 32.5
0.9, 1.4 0.9, 1.4, 6.8
1.51 1.67
Vtot = total pore volume at 0.999 relative pressure. 4
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Fig. 2. (a) X-ray diffraction pattern of P-POP-1 and P-POP-2; (b) N2 adsorption–desorption isotherms curves measured at 77 K for P-POP-1 and P-POP-2, respectively (inset figure: Pore size distribution of P-POP materials).
that P-POPs have much better adsorption performance than others. Having high qe values, even at relatively low Ce, suggest better applicability of P-POPs over other sorbent materials for the removal of trace-level pharmaceutical compounds.
deprotonated form. The above results suggest that high adsorption performance was observed at pH 5 for all three pharmaceutical compounds, therefore, all other adsorption experiments from here on were carried out at pH 5. The adsorption isotherms of P-POP-1 and P-POP-2 were obtained as shown in Fig. 3. Isotherm data were fit with Langmuir and Freundlich models. Fit parameters are summarized in Table 4; the correlation coefficients (R2) indicated that the Langmuir adsorption isotherm fit the data better than the Freundlich model. Based on the Langmuir model, the maximum adsorption capacities of the three pharmaceutical compounds were obtained for the P-POPs. P-POP-2 showed a higher adsorption capacity for all the three compounds (caffeine 301 mg/g, diclofenac 217 mg/g, and carbamazepine 248 mg/g) than P-POP-1 (caffeine 245 mg/g, diclofenac 166 mg/g, and carbamazepine 224 mg/ g). This is likely due to the high pore volume and slightly excess mesoporosity, making the phosphate functional group on P-POP-2 more available as adsorption sites. In addition, the P-POP-2 possess slightly more phosphate content (1.67 mmol/g) than P-POP-1 (1.51 mmol/g) as shown in Table 2. When the adsorption capacities of P-POPs are compared to other sorbent materials such as activated carbon, mesoporous silica, and clay minerals (Table 5), P-POP-2 showed the highest maximum adsorption capacity (qm) for caffeine and diclofenac. Additionally, the equilibrium adsorption capacity (qe) when Ce = 2 mg/L was also compared for a more consistent assessment; results showed
3.3. Adsorption kinetics Adsorption kinetics for caffeine, diclofenac, and carbamazepine were monitored for both P-POP-1 and P-POP-2 as shown in Fig. S5 and Fig. 4. Within 20 min of contact time, > 70% of pharmaceutical compounds were adsorbed to P-POP-1 (Fig. S5), whereas the P-POP-2 captured > 75% during the same time interval. Similarly, For P-POP-2, the equilibrium concentration has reached at the contact time of 50 min for all compounds, whereas > 120 min was necessary for caffeine to reach the equilibrium concentration for P-POP-1. P-POP-2 showed fastest equilibrium time (teq) among other materials we have compared (Table 5). The faster adsorption rate of P-POP-2 is probably due to its high surface property, including higher and well-defined mesoporosity compared to P-POP-1. The observed kinetic data were fitted to the pseudo-first-order and pseudo-second-order kinetic models for P-POP-1 (Fig. S6 & S7) and P-POP-2 (Fig. 4), and the corresponding fitted parameters are listed in Table 5. The correlation coefficients for the pseudo-second-order model were fairly higher than those for the pseudo-first-order model (Fig. 4). The rate constant (k2) of P-POP-2 for
Table 3 The chemical structure and properties of selected pharmaceutical contaminants. Compound
Use
Caffeine
Structure
Formulae
pKa [22,41]
log Kow [3,42,43]
Mol.wt
Molecular Size (nm) [44]
stimulant
C8H10N4O2
14
−0.23
194.19
0.98 × 0.87 × 0.56
Diclofenac
Arthritis
C14H10Cl2NNaO2
4.15
0.7
318.13
0.97 × 0.96
Carbamaze-pine
Anti-epileptic
C15H12N2O
13.9
2.45
236.09
1.2 × 0.92 × 0.58
5
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Fig. 3. (a) Effect of pH on sorption of caffeine, diclofenac, and carbamazepine on P-POP-1. Conditions: Initial sorbate concentration of 50 mg/L for caffeine and 15 mg/L for diclofenac and carbamazepine, Adsorbent dose of 200 mg/L for caffeine and 100 mg/L for diclofenac and carbamazepine, and reaction time of 24 h were used. (b–d) Langmuir and Freundlich adsorption isotherm of (b) caffeine, (c) diclofenac, and (d) carbamazepine for P-POP-1 and P-POP-2. Conditions: pH 5, initial sorbate concentration of 0–53.5 mg/L for caffeine, 0–13.5 ppm for diclofenac, and 0–20 mg/L for carbamazepine, adsorbent dose of 150 mg/L for caffeine, 50 mg/L for diclofenac and carbamazepine, and reaction time of 24 h were used.
adsorption selectivity for pharmaceutical compounds was assessed in the presence of 5–40 mg/L humic acid. P-POP-2 selectively removed > 95% of caffeine and carbamazepine, and > 88% of diclofenac from the solution. It was observed that the humic acid had a negligible effect on the adsorption performance of P-POP-2 for pharmaceutical compounds with basic groups. Similarly, the effect of other cations commonly presents in the aqueous solutions such as K+, Na+, Ca2+, Cu2+, and Mg2+ ions were tested. The influence of each cation was monitored at 20 mg/L concentration. Fig. 5b shows that K+ and Na+ had a small effect on the adsorption of pharmaceutical compounds, lowering its adsorption capacity by 5–8% on only the diclofenac adsorption, but the other ions had no significant effect on the adsorption of pharmaceutical compounds by P-POP-2.
Table 4 Adsorption isotherm data values of caffeine, diclofenac, and carbamazepine over P-POPs. Pharmaceutical contaminants
Caffeine Diclofenac Carbamazepine
Sorbents
P-POP-1 P-POP-2 P-POP-1 P-POP-2 P-POP-1 P-POP-2
Langmuir adsorption isotherm
Freundlich adsorption isotherm
qm (mg/ g)
KL (L/ mg)
R2
KF (mg/ g)
n
R2
245 301 166 217 224 248
1.43 1.09 4.38 6.08 7.69 6.72
0.989 0.984 0.975 0.974 0.951 0.942
118 139 58.80 71.50 92.84 87.16
3.65 3.22 4.59 4.34 4.73 5.03
0.730 0.769 0.812 0.814 0.732 0.738
3.5. Adsorption mechanism pharmaceutical compounds were considerably higher than that of PPOP-1 (Table 6), in line with the enhanced adsorption caused by larger mesoporosity of the P-POP-2 polymer matrix.
Possible adsorption mechanisms between adsorbent and aromatic adsorbate include van der Waals interactions, π-π dispersion interactions, hydrogen bonding, hydrophobic-hydrophobic mechanisms, and electrostatic interactions. Caffeine and carbamazepine exist as natural species at the pH range used in our study, suggesting electrostatic interaction is unlikely to be the dominant adsorption mechanism between P-POPs and these compounds. The pHpzc of P-POP-1 and P-POP-2 were measured to be 3.62 and 3.64, respectively (Fig. S8); both diclofenac (pKa = 4.25) and P-POP-1 have an overall negative charge at pH 5, so
3.4. Effect of competing water constituents on sorption of pharmaceutical compounds The influence of competing water constituents on adsorption performance of P-POPs are also investigated as shown in Fig. 5. First, the 6
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Table 5 Comparison of adsorption performance of P-POPs with sorbent materials reported in other literature (Bold indicates the best value in each column). Adsorbent
Caffeine
Diclofenac
Carbamazepine
qm (mg/g)
qe at Ce = 2 mg/L (mg/ g)a
teq (min)b
qm (mg/g)
qe at Ce = 2 mg/L (mg/ g)
teq (min)
qm (mg/g)
qe at Ce = 2 mg/L (mg/ g)
teq (min)
Ref
Carbon-PC Carbon xerogel Saponite SBA-15 Commercial PAC BA-smectite H3PO4 chemically activated carbon from peach stones Magnetic activated carbon P-POP-1
260 182 80.5 2.35 12.6 135 –
97 81 9 0.5 12 77.6 –
110 N/A 120 N/A N/A 1440 –
201 80 – – 25.35 – –
N/Ac 27 – – 25.11 – –
145 N/A – – N/A – –
335 – – – – – 241.6
N/Ac – – – – – 43
160 – – – – – 240
[44] [45] [46] [47] [48] [49] [37]
– 245
– 212
– 120
– 166
– 152
– 120
182.9 222
180 213
60 120
P-POP-2
301
266
50
217
189
50
246
225
50
[50] This work This work
a b c
Values of equilibrium adsorption capacity (qe) when equilibrium aqueous concentration (Ce) is 2 mg/L was taken from the data in each reference. Equilibrium time was based on adsorption kinetics data (teq ≥ t95%); listed as NA if adsorption kinetic data were not available. Adsorption isotherm in this study [44] did not have adsorption isotherm data below Ce = 20 mg/L.
Fig. 4. (a) Adsorption kinetics of caffeine, diclofenac, and carbamazepine on P-POP-2; Conditions: Initial sorbate concentration of 50 mg/L for caffeine and 15 mg/L for diclofenac and carbamazepine, adsorption dose of 200 mg/L for caffeine and 100 mg/L for diclofenac and carbamazepine, and pH = 5 were used. (b) kinetic data fitted with the pseudo-first-order kinetic model, and (c) kinetic data fitted with the pseudo-second-order kinetic model.
interactions between aromatics in adsorbent and pharmaceutical compounds may also contribute to their sorption. However, P-POPs showed much better adsorption performance compared to commercial powdered activated carbons (Table 5, [47]), in which π-π interactions are reported to play a dominant role. Thus, it is likely that other adsorption mechanisms may be more important for P-POPs. For our adsorbent, the less hydrophobic compound (i.e., caffeine) showed higher sorption capacity than the more hydrophobic compound (i.e., diclofenac), suggesting hydrophobic-hydrophobic mechanism contributes a minor role especially for caffeine and carbamazepine. For P-POPs, we speculate that hydrogen bonding between the phosphate site on the adsorbent and adsorbate plays a dominant role in adsorption (Scheme 2). Phosphate group (PO3OH) has four oxygen atoms connected with P atom, in which one oxygen is connected with a double bond and one oxygen is connected with a hydrogen atom, thus providing multidentate sites for hydrogen bonding. Pharmaceutical compounds (e.g., caffeine, carbamazepine, diclofenac) have multiple heteroatom (nitrogen) and carbene hydrogens that could interact with the phosphate unit via hydrogen bonding. However, it is possible that hydrophobic-hydrophobic mechanism or other mechanism can play more dominant roles at different solution conditions or when other pharmaceutical compounds sorb to P-POPs.
Table 6 Adsorption kinetics of caffeine, diclofenac sodium, and carbamazepine over PPOPs. Pharmaceutical contaminants
Caffeine Diclofenac Carbamazepine
Porous organic Polymer
P-POP-1 P-POP-2 P-POP-1 P-POP-2 P-POP-1 P-POP-2
Psuedo-1st-order model
Psuedo-2nd-order model
k1 (min−1)
R2
k2×10−3 (g mg−1 min−1)
R2
0.0401 0.0660 0.0403 0.0401 0.0407 0.0621
0.925 0.905 0.714 0.707 0.833 0.787
0.824 1.280 0.701 0.942 0.712 0.949
0.999 0.998 0.997 0.996 0.998 0.998
electrostatic attraction should not occur between the two. Minimal influence of other cations (e.g., Na+, Ca2+) on adsorption performance of the pharmaceutical (discussed in Section 3.4) also confirms this. However, adsorption capacity for diclofenac has sharply decreased as pH increases, possibly due to the electrostatic repulsion between P-POP surface and diclofenac inhibiting diclofenac sorption to the phosphate site. Since P-POPs has a benzene ring backbone, π-π dispersion 7
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Fig. 5. Effect of (a) humic acid and (b) cations on the adsorption of caffeine, diclofenac, and carbamazepine over P-POP-2; Conditions: Initial sorbate concentration of 50 mg/L for caffeine and 15 mg/L for diclofenac and carbamazepine, adsorbent dose of 200 mg/L for caffeine and 100 mg/L for diclofenac and carbamazepine, reaction time of 3 h, and pH 5 were used.
Scheme 2. Possible host guest hydrogen bonding interaction mechanism of a) caffeine, b) diclofenac, and c) carbamazepine with phosphate functional sites of PPOPs at pH 5.
that after 5 repeated cycles there was a slight decrease (< 10%) in the adsorption capacity of P-POP-2 (Fig. S10), suggesting that the sorbent has a good recyclability over extended periods of use. Furthermore, FTIR results (Fig. S11) showed that the P-POP polymers are highly stable even after the 24 h treatment with either the acidic (1 M HCl) or basic (1 M NaOH) aqueous solution. Similarly, the thermal stability of P-POPs was also analyzed using thermal gravimetric analysis (Fig. S12), and the results suggested that the polymers are stable up to 250 °C.
Comparing the physicochemical properties of caffeine, carbamazepine, and diclofenac, the influence of adsorbate characteristics on the adsorption capacity of P-POPs were also investigated. Some factors that often affect sorption of adsorbate to adsorbent are molecular size, hydrophobicity (i.e., log(Kow)), and pKa, as well as types and structure of substituents on aromatic compounds [51,52]. For P-POPs, no noticeable trend between adsorption capacity and log(Kow) of the three pharmaceutical compounds was observed (Fig. S9), suggesting that hydrophobicity of adsorbates have minimal influence on their sorption to PPOPs. An inverse relationship between the molecular weight of pharmaceutical compounds and their adsorption capacities to the porous polymer adsorbents were observed for both P-POP-1 and P-POP-2 (Fig. S9), in which the smaller molecule (i.e., caffeine) had higher qm than the larger molecule (i.e., diclofenac). This is likely due to smaller molecules having easier and higher accessibility to all available sites (even ones within small micropores) on the adsorbent. Further studies with a larger number of pharmaceutical compounds are needed to more carefully identify the key characteristics that influence adsorption capacities of these pharmaceutical compounds on P-POPs.
4. Conclusions Novel sorbents, phosphate-based porous organic polymers P-POP-1 and P-POP-2, were synthesized for the removal of pharmaceutical compounds in water, and their adsorption performance was evaluated using three pharmaceutical compounds (i.e., caffeine, diclofenac, and carbamazepine). Overall, P-POPs exhibited excellent adsorption capacities compared to the other sorbent materials due to their uniform polymer structure containing phosphate group. P-POP-2 exhibited excellent textural properties with more mesopores than P-POP-1, thus showing higher adsorption capacity as well as faster equilibrium time. Fits with the isotherm model and kinetic model showed that the adsorption of pharmaceutical compounds on P-POPs follow a Langmuir isotherm with pseudo-second-order kinetics. The dominant adsorption mechanism is likely to be multidentate hydrogen bonding between the adsorbate and phosphate group on P-POPs. Both P-POP-1 and P-POP-2 showed high selectivity (> 98%) for the pharmaceutical compounds in
3.6. Adsorbent regeneration To verify the recyclability and stability of P-POP-2, repeated cycles of adsorption–regeneration tests were carried out. In each cycle, the experimental conditions for the adsorption step was kept the same. After each adsorption step, P-POP-2 was regenerated by washing with acidified ethanol followed by drying in a vacuum. It was demonstrated 8
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the presence of humic acids and other common cations (i.e., Na+, Mg2+, Ca2+) in water. They also showed high chemical stability over repeated use without deterioration in adsorption capacity. The results from this study show that phosphate-based porous organic polymer material could be a promising adsorbent for removing pharmaceutical compounds in water and wastewater. Additionally, coupling porous organic polymer with the target functional group explores a new possibility for selective physicochemical interaction not only for water treatment but also for other versatile industrial applications, including heterogeneous catalysis and energy applications.
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