Bakelite-type anionic microporous organic polymers with high capacity for selective adsorption of cationic dyes from water

Chemical Engineering Journal 366 (2019) 404–414

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Bakelite-type anionic microporous organic polymers with high capacity for selective adsorption of cationic dyes from water

T

Binbin Wanga, Qian Zhanga, Gang Xionga, , Fu Dinga, Yongke Hea, Baoyi Rena, Lixin Youa, ⁎ ⁎ Xiaolei Fanb, Christopher Hardacreb, , Yaguang Suna, ⁎

a b

The Key Laboratory of Inorganic Molecule-Based Chemistry of Liaoning Province, Shenyang University of Chemical Technology, Shenyang 110142, China School of Chemical Engineering and Analytical Science, The University of Manchester, Manchester M13 9PL, United Kingdom

HIGHLIGHTS

GRAPHICAL ABSTRACT

Bakelite-type anionic micro• Two porous organic polymers have been constructed and characterized.

exhibited solid microspheres • They with remarkable high BET specific surface area.

and MOP-2 displayed out• MOP-1 standing capacities toward cationic dyes MB and MG.

and MOP-2 exhibited charge • MOP-1 and size-selectivity. endothermic adsorption processes • The are spontaneous and endothermic.

ARTICLE INFO

ABSTRACT

Keywords: Microporous organic polymers Selective adsorption Dye adsorption Hydroxyl functionalization

A new family of bakelite-type anionic microporous organic polymers (MOPs) is described in this work, which was assembled by the condensation reaction between anionic unit tri(protocatechuic aldehyde)-silicate and phloroglucinol or resorcinol by reference to the synthetic method of bakelite. The new materials exhibit uniform and solid microspheres, remarkably high BET specific surface areas of 1846.5 and 2206.7 m2/g, and a narrow pore size distribution around 1 nm region. The adsorption capacities of MOP-1 and MOP-2 toward cationic Methylene blue (MB) and Malachite green (MG) run up to 712.2, 593.6, 233.8 and 324.7 mg/g at 25 °C, respectively, but little adsorption was found for the anionic dye Methyl Orange (MO) and large size cationic dye basic blue 7 (BB7), exhibiting charge and size- selectivity. The adsorption thermodynamics and kinetics were investigated and a pseudo-second-order kinetic model and Langmuir isotherm model was found to be suitable for the adsorption process of MB and MG on MOP-1 and MOP-2. The negative values of the Gibbs free energy and the positive values of enthalpy changes indicated that adsorptions were spontaneous and endothermic. Moreover, the absorbents MOP-1 and MOP-2 can be re-used at least four times in the adsorption of MB and MG with less than 5% loss of adsorption capacity. Overall, MOP-1 and MOP-2 were found to be inexpensive, feasible and efficient materials for removal dyes, presenting future prospects in research and applications in the purification of water contaminated by organic dyes.

⁎ Corresponding authors at: The Key Laboratory of Inorganic Molecule-Based Chemistry of Liaoning Province, Shenyang University of Chemical Technology, Shenyang 110142, China (Y. Sun). E-mail addresses: [email protected] (G. Xiong), [email protected] (C. Hardacre), [email protected] (Y. Sun).

https://doi.org/10.1016/j.cej.2019.02.089 Received 12 October 2018; Received in revised form 24 January 2019; Accepted 13 February 2019 Available online 13 February 2019 1385-8947/ © 2019 Published by Elsevier B.V.

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1. Introduction

MO and large dyes such as BB7. The adsorption thermodynamics, kinetics, ionic strength, dye release behavior and recycle were investigated.

Textile wastewater contains high concentration of various organic dyes threatening to seriously to harm human health and the natural environment. Thus, some methods such as adsorption [1–3], photodegradation [3,4], biological treatment [5] and chemical oxidation [6] have been explored for the removal of organic dyes from wastewater. However, the above technologies, except for adsorption, focus on decomposing dye molecules rather than recycling them [7–8]. In fact, these dye molecules in wastewater also have considerable commercial value. Adsorption technology can not only remove dye molecules from wastewater to purify water, but also recycle them. However, high selectivity and large adsorption capacity absorbents are needed to capture the desired dyes in adsorption technology because wastewater generally contains a variety of dye components and inorganic ions [9–11]. While traditional materials such as activated carbon, zeolites and ionexchange resins always expose the disadvantages of low adsorption capacities/selectivity and poor chemical modifiability [12–14]. Therefore, it is vital to explore high selectivity and effective adsorbents to remove dyes from the wastewater and then recycle them. Up to now, a large number of materials with selective adsorption of dye molecules have been designed [15–17]. Their adsorption capacity and selectivity mainly depend on the electrostatic interaction between adsorbent and dye molecules, ion exchange mechanism and pore size selectivity [15,18–19]. In fact, depending on only one of the above mechanisms cannot achieve high selectivity and efficient adsorption and separation of dyes [20,21]. For example, the selective adsorption of dyes by electrostatic interaction is poor because most dyes contain polar groups that interact strongly with adsorbents [22–24]. Therefore, designing and synthesizing charged porous materials with narrow pore size distribution and rich, strong electrostatic force sites will remarkably contribute to the adsorption and separation of dye molecules. Based on the above considerations, we draw lessons from the preparation method of bakelite and two bakelite-type anionic MOPs (MOP1 and MOP-2) have been constructed via the condensation reaction between anionic unit tri(protocatechuic aldehyde)-silicate and phloroglucinol or resorcinol (Scheme 1). The MOP-1 and MOP-2 exhibit high thermal stabilities, solid microspheres with diameters of 2–5 μm, high BET specific surface areas of 1846.5 and 2206.7 m2/g, and a narrow pore size distribution around 1 nm region. The combination of their anionic skeleton, microporous properties and internal structures with abundant –OH groups make these materials suitable for cationic dyes capture. The adsorption capacities of MOP-1 and MOP-2 toward MB and MG can run up to 712.2, 593.6, 233.8 and 324.7 mg/g, respectively, but they do not significantly adsorb such anionic dyes such as

2. Experiment 2.1. Materials and equipment All solvents and reagents for the syntheses were of analytical grade and were used as received from commercial sources without further purification. Elemental analyses (C, H and N) were carried out with a Perkin-Elmer 240C elemental analyzer. The IR spectra from 4000 to 400 cm−1 were measured using a Nicolet IR-408 spectrometer and KBr pellets. Thermogravimetric (TG) curves were recorded from room temperature to 800 °C with a heating rate of 10 °C/min on a Netzsch TG 209 instrument under N2 atmosphere. The N2 adsorption isotherms were measured by V-Sorb 2800TP surface area and pore size analyzer at 77 K. The solid 13C NMR spectra were collected on a JNM-ECZ600R. Scanning electron microscopy (SEM) analyses were conducted with JSM-4800F electron microscope (JEOL, Japan). UV–Vis spectra were measured using UV–Vis 2550 spectrophotometer (Shimadzu, Japan). The Zeta potential and grading analysis were performed on a Malvern Zetasizer Nano ZS90 (UK). 2.2. Preparation of MOP-1 and MOP-2 MOP-1: 1 mmol (0.1101 g) resorcinol and 1 mmol (0.6408 g) Siprecursor were dissolved in 10 mL methanol, and 0.1 g p-toluenesulfonic acid was added to the solution. pre-polymerization was carried out via refluxing in an oil bath at 80 °C for 1 h, and then the mixture was sealed to 25 mL Teflon reactor and heated for 3 days at 120 °C. After that, the mixture was naturally cooled to room temperature. The filtered solid was extracted by Soxhlet extraction with methanol as eluent for 72 h, then the eluted solid was dried in vacuum at 50 °C for 24 h. The activated product was 0.61 g with the yield of 81.86%. Anal. Calcd (%) for (C25H33N2O8Si)n: C, 58.01; H, 5.41; N, 6.42; Found: C, 57.95; H, 5.34; N, 6.33. MOP-2: 1 mmol (0.1261 g) phloroglucinol and 1 mmol (0.6408 g) Si-precursor were dissolved in 10 mL methanol to which 1 mL glacial acetic acid was added. Pre-polymerization was carried out via refluxing in an oil bath at 80 °C for 1 h, and then the mixture was sealed to 25 mL Teflon reactor and heated for 3 days at 120 °C. After that, the mixture was naturally cooled to room temperature. The filtered solid was extracted by Soxhlet extraction with methanol as eluent for 72 h, the eluted solid was dried in vacuum at 50 °C for 24 h. The activated

Scheme 1. Schematic representation of the synthetic pathway toward the microporous organic polymers MOP-1 and MOP-2. 405

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product was 0.68 g with the yield of 90.79%. Anal. Calcd (%) for (C39H41N2O9Si)n: C, 95.99; H, 5.82; N, 3.94; Found: C, 65.92; H, 5.78; N, 3.88.

adsorption procedures were conducted at 298.15 K. Finally, the amounts of MB and MG adsorbed on MOP-1 and on MOP-2 were calculated based on the UV–vis spectra.

2.3. Kinetics adsorption experiments

2.7. Competitive dye adsorption experiments

Exact amounts of adsorbent MOP-1 (20 mg each) was put into two 250 mL beakers, to which 150 mL 150 mg/L MB and 150 mL 120 mg/L MG solutions was added, respectively. The beakers were placed in a thermostatic shaker bath operating at 298.15 K. At the end of predetermined time intervals, the residual dye concentration in the supernatant was determined using a UV–Vis spectrophotometer at its maximum absorption wavelength of 664 nm for MB and 615 nm for MG. For the absorbent MOP-2, a similar procedure was carried out following the above experimental conditions except that the initial dye concentrations were changed to 50 mg/L for MB and 100 mg/L for MG. The adsorption capacity at time t (qt, mg/g) was calculated using the equation of qt = (c0 − ct)V/m, where c0 and ct are the initial and the concentration at time t (mg/L) of the dye solution, V is the volume of the solution (L) and m is the mass of the adsorbent (g).

To investigate the competitive adsorption of dyes with different changes and sizes in the same concentrations were monitored by UV–vis spectra. 10 mg MOP-1 (in a subsequent experiment MOP-2) were introduced, into a mixture of 25 mL aqueous solution of 60 mg/L cationic MB and 25 mL aqueous solution of 60 mg/L anionic MO, and adsorption process was monitored by UV–vis spectrophotometer at 298.15 K for 5 h. The competitive adsorption of MB and MO on MOP-2 was carried out as the same as that of MOP-1. In order to explore size selectivity, the adsorption capacity of MB, MG and BB7 on MOP-1 was carried out under the same experiment condition, the initial concentration and volume of dyes was fixed to 200 ppm and 50 mL, respectively, and the amount of absorbents, time and temperature was 10 mg, 6 h and 298.15 K, respectively. The adsorption capacity of MB, MG and BB7 on MOP-2 was carried out as the same as that of MOP-1 except changing the initial concentration of dyes to 100 mg/L.

2.4. Thermodynamics adsorption experiments 10 mg MOP-1 were added to 50 mL MB aqueous solutions with initial concentrations of 30 mg/L, 60 mg/L, 90 mg/L, 120 mg/L, 150 mg/ L, 180 mg/L, 210 mg/L, 240 mg/L, 270 mg/L, 300 mg/L and 360 mg/L, respectively. Moreover, 10 mg MOP-1 were added to 50 mL MG aqueous solutions with initial concentrations of 60 mg/L, 90 mg/L, 120 mg/L, 150 mg/L, 180 mg/L, 210 mg/L, 240 mg/L, 270 mg/L and 300 mg/L, respectively. The adsorption procedures were carried out in a thermostatic shaker bath at 298.15 K, 303.15 K and 308.15 K. The effect of the initial concentration and the adsorption thermodynamics of MB and MG on MOP-2 were carried out, the initial concentrations were changed to 20 mg/L, 30 mg/L, 40 mg/L, 50 mg/L, 60 mg/L, 70 mg/L, 80 mg/L, 100 mg/L, 120 mg/L, 140 mg/L, 160 mg/L and 180 mg/L, and 20 mg/L, 30 mg/L, 40 mg/L, 50 mg/L, 60 mg/L, 70 mg/ L, 80 mg/L, 100 mg/L, 120 mg/L, 140 mg/L, 160 mg/L and 180 mg/L, and the initial volume was 50 mL. The temperature of the thermostatic shaker bath kept at 298.15 K, 303.15 K and 308.15 K. The equilibrium adsorption capacity (qe, mg/g) was calculated using the equation of qe = (c0 − ce)V/m, where c0 and ce are the initial and the equilibrium concentrations (mg/L) of the dye solution, V is the volume of the solution (L) and m is the mass of the adsorbent (g).

2.8. Release experiment of MB and MG on MOP-1 and MOP-2 The adsorbents MOP-1 and MOP-2 that have been saturated with MB and MG were placed in pure methanol solution. UV–vis spectroscopy was employed to monitor the absorbance of the solution. When the absorbance of the solution did not increase, NaCl was added to the methanol solution to promote the desorption. 2.9. Regeneration of MOP-1 and MOP-2 The saturated NaCl methanol solution were added to the MB or MG -loaded MOP-1 and MOP-2 and was sonicated for 1 h, and then eluent was separated by centrifugation. This desorption process was performed at least three times until no dye can be detected. Finally, MOP-1 and MOP-2 were dried in vacuum at 60 °C for resue. Then the adsorption experiments carried out at 25 °C, and the initial concentrations of dyes were chose the maximum concentrations as in thermodynamics adsorption experiments.

2.5. Effect of the pH on the dye adsorption 10 mg MOP-1 were added to 50 mL MB aqueous solutions with initial concentrations of 280 mg/L, and pH of 3, 4, 5, 6, 7, 8 respectively. Moreover, 10 mg MOP-2 were added to 50 mL MB aqueous solutions with an initial concentration of 180 mg/L, and pH of 3, 4, 5, 6, 7, 8 respectively. The pH was controlled by adding 1% HCl and 0.01 M NaOH as needed. The adsorption procedures were carried out in a thermostatic shaker bath at 298.15 K. The equilibrium adsorption capacity (qe, mg/g) was calculated using the equation of qe = (c0 − ce)V/ m. 2.6. Effect of the ionic strength on the dye adsorption To explore the effect of ionic strength on the dye adsorption, solutions of NaCl and CaCl2 were employed with concentrations fixed at 200 mg of each salt/g adsorbent. For MOP-1, 25 mL of the above NaCl or CaCl2 solutions was added to 25 mL of aqueous solution containing either 420 mg/L MB or 210 mg/L MG, respectively. The same procedure was repeated for MOP-2 except that changing the initial concentration of MB and MG to 60 mg/L and 240 mg/L MG, respectively. In all experiments the amount of adsorbents were fixed to 10 mg and the

Fig. 1. FT-IR spectra of the Si-precursor, MOP-1 and MOP-2. 406

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3. Results and discussion

successfully obtained according to the reaction path we designed. In the solid state 13C NMR spectra of MOP-1 and MOP-2 (Fig. 2), the tertiary carbon formed in the condensation reaction between anionic unit tri(protocatechuic aldehyde)-silicate and phloroglucinol or resorcinol was established by the presence of the peak at 35 ppm [25]. No obvious resonance peaks at 190 ppm indicated that no aldehydes groups existed in MOP-1 and MOP-2. From this, the condensation reaction between anionic unit tri(protocatechuic aldehyde)-silicate and phloroglucinol or resorcinol was fully completed. For MOP-1, the peak at 57 ppm can be assigned to the methyl group of tetramethylammonium cation [26,27]. The tetramethylammonium cations come from the reaction methanol and triethylamine catalyzed by the ptoluenesulfonic acid. This is consistent with the similar preparation

3.1. Characterization of MOP-1 and MOP-2 Compared with the FT-IR spectra of Si-precursor, MOP-1 and MOP-2 (Fig. 1), the characteristic peak of 1672 cm−1 originated from stretching vibrations of aldehydes C]O can be easily observed in Siprecursor sample, but this characteristic peak disappears completely in the FT-IR spectra of MOP-1 and MOP-2, indicating that all the aldehydes were involved in the condensation reaction. The characteristic peak of about 3270 cm−1 in the FT-IR spectra of MOP-1 and MOP-2 can be assigned to stretching vibrations of phenolic hydroxyl –OH from phloroglucinol or resorcinol. Therefore, MOP-1 and MOP-2 were

Fig. 2. Solid state

13

C NMR spectra of MOP-1 and MOP-2. 407

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Fig. 3. SEM images of MOP-1(a) and MOP-2(b).

reaction of tetramethylammonium cations reported by the Hunger group, in which acidic zeolites H-Y was used as catalyst [27]. For MOP2, the resonance peaks at 10 and 43 ppm originated from the terminal methyl carbons and methylene carbons of protonated triethylamine cations which come from raw material Si-precursor. The MOP-1 and MOP-2 materials are stable on heating until 290 °C under N2 atmosphere, and they retain more than 50% of their mass at 800 °C as revealed by thermogravimetric analysis (Fig. S2). The morphology of the MOP-1 and MOP-2 materials was investigated by SEM measurements. The image revealed that morphology of powder of MOP-1 and MOP-2 were regular spherical particles with diameter 2–5 μm (Fig. 3). These spheres exhibited a smooth surface and were solid indicated by the interior of the broken balls present. PXRD revealed the amorphous nature of MOP-1 and MOP-2 by the presence of the broad band extending from about 10° to 35° (Fig. S3). N2 adsorption-desorption isotherm curves of MOP-1 and MOP-2 are given in Fig. 4. N2 isotherms exhibited type I for MOP-1 and type-IV for MOP-2 with a steep uptake at low relative pressure ((P/P0 < 0.08), revealing the microporous property of MOP-1 and MOP-2. The BET specific surface area is estimated at 1846.5 and 2206.7 m2/g for MOP-1 and MOP-2, respectively, calculated based on Multi-point BET mode (Fig. S1). Both of BET specific surface area are far higher than those of reported bakelite-type microporous organic polymers [27,28,29,30]. The total pore volumes are calculated to be 1.041 cm3/g for MOP-1 and 1.21 cm3/g for MOP-2 according to single point adsorption at P/

P0 = 0.99. The pore size distribution curves of MOP-1 and MOP-2 based on the non-local density function theory (NLDFT) method display the main peaks centered at 1.13 and 1.01 nm (Fig. 4 (Insert)), suggesting the microporous features. However, for MOP-2, there a wide and weak peak around 2.5 nm, indicating that a small number of mesoporous materials existed. These predominant pore structures with intrinsic negative charges and abundant hydroxyl groups involved in MOP-1 and MOP-2 lead them to be promising candidates in the adsorption of cationic dye molecules. 3.2. Adsorption performance of MOP-1 and MOP-2 The –OH groups, anionic skeletons and high specific surface area in MOP-1 and MOP-2 may be favorable for the adsorption of cationic species such as dyes through the combined effect of electrostatic interaction and ion-exchange. Thus MB and MG were chosen as representative cationic dyes to investigate the adsorption capacity. Initially, the influence of pH on the surface charge and the MB adsorption capacity of MOP-1 and MOP-2 was studied in aqueous solutions over the pH range 3–8, at 25 °C. The pH was controlled by adding 1% HCl and 0.01 M NaOH as needed. Both of MOP-1 and MOP-2 exhibited large negative zeta potentials (Fig. S4(a)) when dispersed in neutral aqueous solution, proving that they are intrinsically anionic polymers as expected. As pH value of the solution increased from 3 to 8, the negative charge on the surface of MOP-1 and MOP-2 increased. However, the adsorption of MB on MOP did not change significantly under aqueous conditions as a function of pH (Fig. S4(b)), therefore pH has little effect on MOP adsorption MB. Thus all further adsorption

Fig. 4. The N2 isotherm curves of MOP-1 and MOP-2. Insert: Pore size distribution analysis according to NLDFT for MOP-1 and MOP-2.

Fig. 5. The effect of contact time on MB and MG adsorption capacity at 25 °C. 408

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Table 1 The fitting results of adsorption kinetics for MB and MG on MOP-1 and MOP-2. Adsorbent

Dyes

pseudo-first-order kinetics −1

k1 (min

)

pseudo-second-order kinetics

qe (mg/g)

R

2

k2 (g/mg/min)

qe (mg/g)

R2

MOP-1

MB MG

0.01443 0.01220

569.4 340.8

0.9546 0.9830

3.839E−05 6.307E−05

699.3 543.5

0.9996 0.9998

MOP-2

MB MG

0.0098 0.01474

204.4 232.2

0.9646 0.9812

6.117E−05 1.187E−04

209.4 354.6

0.9953 0.9977

experiments were carried out in near neutral aqueous dye solutions (pH ≈ 6) without any pH adjustment. Fig. 5 shows the adsorption capacity of MB and MG onto MOP-1 and MOP-2 as a function of adsorption time. The adsorbed amount of the MB and MG increased with the increasing time as expected. After 40 min, the adsorption capacity rapidly increased. At the end of the adsorption process, the adsorption gradually reaches the equilibrium capacity of 637.6 mg/g for MB onto MOP-1, 506.8 mg/g for MG onto MOP-1, 200.9 mg/g for MB onto MOP-1, 334.1 mg/g for MG onto MOP2 within 420 min, respectively. To investigate the uptake rate and adsorption efficiency of adsorbent, the time- dependent experimental data were analyzed by different two kinetic models of pseudo-first-order kinetic model and pseudo-second-order kinetic model which can be expressed as ln(qe − qt) = lnqe − k1t and t/qt = 1/k2(qe)2 + (1/qe)t [31] respectively, where qt and qe are the adsorbed amounts of dye at time t and at equilibrium (mg/g), t is the time (min.); k1 and k2 is the pseudo-first-order rate constant (min−1) and the pseudo-second-order rate constant (g/mg/min), respectively. The kinetic parameters were obtained by fitting the plots of qt vs. t (Fig. S5) and listed in Table 1. A comparison with the pseudo-first-order kinetics model showed that the pseudo-second-order kinetics model is more suitable for fitting the adsorption kinetics owing to the values of the correlation coefficients R2 are closer to 1 obtained from the pseudo-second-order model. Moreover, equilibrium adsorption capacities calculated based on the pseudosecond-order model are also consistent with the experimental values, as compared with the pseudo-first-order model. Thus, the adsorption behavior of MG and MB adsorption on POP-1 and POP-2 can be described very well by the pseudo-second-order model. Considering the characteristics of the anion backbone of the adsorbent, it may be concluded that the dyes MB and MG are adsorbed mainly via chemisorption via ion exchange [32]. In order to investigate the effect of initial dye concentration and temperature on the adsorption process, the adsorption capacity at the various initial concentrations ranging from 20 mg/g to 360 mg/g and at different temperatures (25 °C, 30 °C and 35 °C) was systematically measured. The obtained qe vs. c0 and qe vs. ce curves are presented in Fig. S6 and Fig. 6, respectively. The adsorption capacity was found to increase with the increasing initial dye concentration and gradually reached adsorption saturation at the designated temperature. In the lower concentration region, the adsorption capacity increased rapidly, while a very slow increase can be found at the higher concentration. Finally, the saturation adsorption capacity was 719.4 mg/g for MB on MOP-1, 519.7 mg/g for MG on MOP-1, 234.2 mg/g for MB on MOP-2 and 347.2 mg/g for MG on MOP-2, respectively at 25 °C. Moreover, the saturation adsorption capacities of MB and MG on MOP-1 and MOP-1 increased significantly with an increase of temperature from 25 °C to 35 °C. In particular, the adsorption capacity of MB on MOP-2 increased by more than two times. The removal ratio of MB and MG on MOP-1 and MOP-2 were calculated based on isotherm adsorption curves (Fig. S7). At 25 °C, 20 mg absorbent MOP-1 was able to remove all the dye from 150 mL MB solution at a concentration of 100 mg/L and 150 mL MG solution with the concentration of 75 mg/L, respectively. The overall removal were over 99%. Similarly, 10 mg adsorbent MOP-2 can easily remove all dyes from the 80 mL 40 mg/L MB solution. As the

temperature increases, the adsorbents MOP-1 and MOP-2 were found to be able to remove all dye molecules from higher concentration solutions, for example, MOP-1 and MOP-2 can completely remove MB dye molecules from 80 mL 100 mg/L MB solution at 35 °C. Therefore, MOP1 and MOP-2 are very effective adsorbents for MB, even at higher dye concentrations. The reason why dye can be adsorbed so thoroughly is mainly due to anionic backbones and a large number of surface hydroxyl groups found in MOP-1 and MOP-2. The Langmuir and Freundlich isotherm models were applied to analyse the relationship between the adsorption capacity at equilibrium (qe, mg/g) of MOP-1 and MOP-2, and the equilibrium concentrations of the MB and MG dyes (ce, mg/L). Equations of the Langmuir (Eq. (1)) and Freundlich (Eq. (2)) isotherm models [31,33,34] are presented below:

ce c 1 = e + qe qmax kLqmax ln qe = ln kf +

1 ln ce n

(1) (2)

where kL (L/mg) is the Langmuir constant and qmax (mg/g) is the maximum monolayer adsorption capacity. kf (mg/g) and 1/n (n, in L/ mg) represents the Freundlich constants corresponding to adsorption capacity and adsorption intensity, respectively. The qe vs. ce curves (Fig. 6) of MB and MG on MOP-1 and MOP-2 were fit by Langmuir (Fig. S8) and Freundlich (Fig. S9) isotherm models. The fitting results are given in Table 2 and Table S1. The values of the correlation coefficient R2 obtained from fitting with the Freundlich model (Table S1.) greatly differ from unity while the R2 values originated from fitting with the Langmuir model and are much closer to 1, suggesting that the Langmuir model gives better fits to the experimental data. These results further indicate that mono-layer adsorptions occurred in the adsorption process. The adsorption capacity qmax and the Langmuir adsorption constant (kL) of MB and MG on MOP-1 and MOP-2, estimated from the Langmuir model (Table 2), revealing that at 25 °C the maximum adsorption capacities of MOP-1 for MB and MG, and MOP-2 for MB and MG is 712.2, 612, 233.8 and 324.7 mg/g, respectively. These theoretical values are very close to experimental ones (qexp). Additionally, the Langmuir adsorption constant, KL values increased with an increase of temperature from 25 °C to 35 °C, indicating that the higher temperature was more favorable for the MB and MG adsorption to MOP-1 and MOP2. In the matter of adsorption capacity, the adsorption capacity of adsorbent MOP-1 toward MG reached the top-four compared with the best reported in the literature (Table 3). Therefore, it is necessary to understand the thermodynamic parameters for the adsorption of MB and MG on MOP-1 and MOP-2, such as the enthalpy change (ΔH0), the Gibbs free energy change (ΔG0) and the entropy change (ΔS0). The following thermodynamic relations were employed [38].

K0 =

qe

G0 = 409

(3)

ce

RT ln K 0

(4)

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Fig. 6. Equilibrium adsorption isotherms of MB on MOP-1(a), MG on MOP-1(b), MB on MOP-2(c) and MG on MOP-2(d) at temperatures of 25 °C, 30 °C and 35 °C.

ln K 0 =

Table 2 The fitting results by Langmuir mode for MB and MG on MOP-1 and MOP-2. Adsorbents

Dyes

T (K)

qmax (mg/g)

qexp (mg/g)

kL (L/mg)

R2

MOP-1

MB

298.15 303.15 308.15 298.15 303.15 308.15

719.4 775.2 840.3 625.0 775.2 917.4

712.2 770.9 836.8 593.6 758.6 910.6

0.7765 0.3874 0.3684 0.6022 0.8543 0.6488

0.9998 0.9995 0.9982 0.9986 0.9991 0.9991

298.15 303.15 308.15 298.15 303.15 308.15

245.1 354.6 483.1 347.2 408.2 476.2

233.8 335.3 450.4 324.7 389.1 459.2

1.046 1.516 1.418 0.1255 1.176 0.3365

0.9985 0.9988 0.9990 0.9983 0.9963 0.9987

MG

MOP-2

MB MG

S0 R

H0 RT

(5)

In these equations, K0 is the distribution coefficient for the adsorption, R is the gas constant (8.314 J/mol.K) and T (K) is the thermodynamic temperature of the dye solution. Calculation results were listed in Table 4 and Fig. S11 and presented that all Gibbs free energy values were negative, ranging from −2.611 to −6.948 kJ/mol, evidencing that adsorption procedure of MB and MG on MOP-1 and MOP-2 are spontaneous. The ΔH value for the adsorption process of MG on MOP-1, and MB and MG on MOP-2 is 89.99, 83.73 and 41.93 kJ/mol, respectively, suggesting these adsorption procedures were dominated by chemisorption originated from ion exchange and the very strong and massive hydrogen-bonding interactions between the abundant –OH groups on internal surface of the microspheres and the dye molecules [39]. While, the ΔH value of MB on MOP-1 is 20.09 kJ/mol, which

Table 3 Adsorption capacity of MOP-1 for MG in comparison with other materials. Adsorbents

Adsorbed amount(mg/g)

Ref.

ZIF-67 Granular composite hydrogel (AA-IA-APT hydrogel) magnetic β – cyclodextrin – graphene oxide nanocomposites (Fe3O4/β-CD/GO) MOP-1 MOP-1 Si-MOP-2

2430 (20 °C) 2433 990.10 (45 °C) 910 (35 °C) 758 (30 °C) 757 (35 °C)

[35] [36] [37] This work This work 15

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saturation and c1/2 is the concentration at half saturation, NM is the density of receptor sites. The qe vs. ce curves (Fig. 6) of MB and MG on MOP-1 and MOP-2 were fitted by the monolayer model with single energy isotherm models (eq.6). The fitting plots and results were given in Fig. 7 and Table 5. The good correlation coefficient values R2 obtained from fitting with the monolayer model for single compound adsorption (Table 5) indicated that strong correlations between the models and the experimental adsorption isotherms. [43] The adsorption capacity qmax and the number of ions linked per receptor site of adsorbent (n) of MB and MG on MOP1 and MOP-2 have been estimated from the monolayer model for single compound adsorption (Table 5). The results revealed that at 25 °C the numbers of MB and MG linked per receptor site in MOP-1 was 0.5025 and 0.3445, respectively, and the numbers for MB and MG linked per receptor site in MOP-2 was 0.6947 and 0.7847, respectively. The adsorption capacity of MB and MG on MOP-1 is proportional to the number of dye molecules linked to each receptor site (n). The same phenomenon was observed on adsorbent MOP-2. The maximum adsorption capacities of MOP-1 for MB and MG, and MOP-2 for MB and MG is 782.0, 637.8, 234.4 and 376.6 mg/g, respectively, very being close to experimental values (qexp). In addition, the density of receptor sites (Nm) of MOP-1 and MOP-2 on MB and MG is 5.473, 5.620, 1.186 and 1.457 mmol/g, respectively. High receptor site density indicated that absorbents can offer very abundant receptor sites to capture the dye molecules. Obviously, the density of receptor sites of MOP-1 on MB and MG far higher than that of MOP-2 on MB and MG, this phenomenon may be interpreted that the mesoporous distribution from 2 nm to 3.5 nm in MOP-2 is harmful to the exposure of receptor sites, leading to relatively few sites to capture dye molecules. Correspondingly, the adsorption capacities of MB and MG absorbed by MOP-1 are far higher than those by MOP-1. To further investigate the role of ion exchange in the selective adsorption of charged dyes on our MOPs, the anionic dye methyl orange (MO) was chosen as the competitor to the cationic dye MG. From UV–vis spectra (Fig. 8) of an aqueous solution containing MO (30 mg/g) and MG (30 mg/g) as adsorbed by MOP-1 (left) and MOP-2 (right), it is obvious that the intensity of the maximum absorption peak (617 nm) of MG decreased with time to completion after 120 and 300 min, respectively. In contract, the intensity of the maximum absorption peak (463 nm) of MO remained practically unchanged, indicating that MO was not adsorbed at all, being negatively charged and in spite that MO was less bulky than MG. The results provided support to the fact that the adsorption processes were dominated by ion exchange, as also indicated by the thermodynamic analysis. This also provides evidence that the designed adsorbents are highly selective for the adsorption for cationic dyes like MG and MB. From the study of adsorption behavior of MOP-1 and MOP-2 on MB and MG, it can be seen that the main driving forces of adsorption are different, which leads to the difference of adsorption capacity. Actually, the amount of MB adsorbed by MOP-1 is composed of two parts, ion exchange and physical adsorption, which can be confirmed by its release curve in methanol (Fig. S13) and charge-selective adsorption behavior. However, the adsorption of MB by MOP-2 is only ion exchange behavior. Therefore, the adsorption amount (712.2 mg/g) of MOP-1 toward MB is much larger than that (233.8 mg/g) of MOP-2 toward MB. In addition, mesoporous distribution exists in MOP-2, which is disadvantageous to the dye adsorption of small molecules. In addition, MOP-1 exhibits a higher density of receptor sites than that of MOP-2 (Table 5), this phenomenon can be explained by the fact that the mesoporous distribution in MOP-2 is not conducive to the exposure of adsorption sites, so there are relatively few sites to capture dye molecules. Therefore, the adsorption of MB and MG by MOP-2 is less than that by MOP-1. As shown by the thermodynamic and competitive dye adsorption experiments, ion exchange is the main mechanism e for the dye adsorption process. Metal cations present in the solution interfere with the

Table 4 Thermodynamic parameters for adsorption of MB and MG on MOP-1 and MOP2. Adsorbents

Dyes

T (K)

lnK0

ΔG0 (kJ/ mol)

ΔH0 (kJ/ mol)

ΔS0 (J/ mol.K)

MOP-1

MB

298.15 303.15 308.15 298.15 303.15 308.15

2.342 2.463 2.605 1.540 2.119 2.719

−5.806 −6.208 −6.675 −3.819 −5.341 −6.966

20.09

86.81

89.99

314.6

298.15 303.15 308.15 298.15 303.15 308.15

0.6498 1.150 1.748 1.180 1.447 1.650

−1.611 −2.899 −4.479 −2.926 −3.647 −4.228

83.78

286.3

41.93

130.4

MG

MOP-2

MB MG

indicates that the physical adsorption existed in this procedure [40]. The positive values of ΔS0 indicated the increasing of disorder and randomness at the solid/solution interface during the adsorption [39,40]. From the release experiments in methanol solution of MOP-1 and MOP-2 after absorbing saturated MB and MG, different release behaviors can be observed (Figs. S13 and 14). MB on MOP-1, and MB and MG on MOP-2 did not release any dye molecules after 30 min. However, once NaCl was added to these methanol solutions, the absorbance intensities remarkably increased, suggesting that a large number of dye molecules rapidly escaped from the adsorbents. These release behaviors indicated that few dye molecules were physisorbed to the surface of the adsorbents, with the majority bound via an ion exchange process. As the adsorption in the pore is mainly associated with the driving forces of ion exchange and strong electrostatic forces such as hydrogen bondings and pi-pi stacking between dye molecules and the adsorbent skeleton, a large enthalpy change (ΔH) was observed in the adsorption procedure. While, the released behavior of MB from absorbent MOP-1 was different from those of the other systems, the MB molecules slowly released from the adsorbent MOP-1 and reached the desorption equilibrium at about 60 min. This stage of desorption can be considered as the desorption procedure of MB on the adsorbent surface, indicating that this part of MB molecule was physically adsorbed on the surface of absorbent MOP-1. When solid NaCl was added, the desorption rate of MB showed a rapid increase which is consistent with the ion exchange adsorption process with some of the MB molecules physisorbed on the adsorbent. Therefore, chemical adsorption and physical adsorption existed simultaneously in the process of MB adsorption on MOP-1, which can explain relatively small adsorption enthalpy change in this adsorption process. The Langmuir model, which is an ideal single-layer localization adsorption theory, assumes that the surface properties of the adsorbent are uniform and the adsorption sites are evenly distributed. In fact, the assumptions are too perfect to deviate from the actual conditions, usually leading to significant errors between its fitting results and actual values. Recently, the Sellaoui group have developed the monolayer model for single compound adsorption based on the statistical physics theory to improve the understanding of the adsorption process and simultaneously provided a new interpretation of the adsorption isotherm. [41] The adsorption model can be expressed by Sellaoui et al [42]:

Q=

nNM 1+

( )

C1/2 n Ce

(6)

in which Q is the adsorption capacity, n is the number of dyes linked per receptor site of the adsorbent, Q0 = nNM is the adsorbed quantity at 411

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Fig. 7. Fitting the equilibrium adsorption data with the monolayer model for single-compound adsorption at 25 °C (MB on MOP-1(a), MG on MOP-1(b), MB on MOP2(c) and MG on MOP-2(d)).

adsorption process. Sodium and calcium ions were employed to study interference effects on the adsorption amount of the dye. The concentration of each metal ion was set at 100 mg/g, which is the concentration of these metal ions in hard water. The results (Fig. S12) shown that sodium and calcium ions hardly affected the adsorbed amount of MB and MG on MOP-1, perhaps the reason is that these the size of sodium or calcium ions is close to that of tetramethylammonium cations. By contrast, sodium and calcium ions played a negative role in the adsorption of MB and MG on MOP-1, triggering a significant decrease in the amount of adsorbed dye, which may be caused by the competition between sodium or calcium cations and the cationic dyes

Table 5 The fitting results by the monolayer model with single energy for MB and MG absorbed by MOP-1 and MOP-2 at 25 °C. Adsorbents

Dyes

qmax (mmol/g)

qmax (mg/g)

qexp (mg/g)

n

NM (mmol/g)

R2

MOP-1

MB MG

2.750 1.936

782.0 637.8

712.2 593.6

0.5025 0.3445

5.473 5.620

0.9487 0.9927

MOP-2

MB MG

0.8242 1.143

234.4 376.6

233.8 324.7

0.6947 0.7847

1.186 1.457

0.9604 0.9827

Fig. 8. UV–vis spectra of a mixture of MO and MG aqueous solutions adsorbed by MOP-1 (left) and MOP-2 (right). 412

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Acknowledgements This work was supported by National Natural Science Foundation of China (21671139 and 21501122), and the Doctoral Scientific Research Foundation of Liaoning Province (201601193) and the Distinguished Professor Project of Liaoning province(No. 2013204). Program for Liaoning Innovative Research Team in University. XF thanks the partial support by The Royal Society International Exchange Award (IE161344) to enable the collaboration. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.02.089. References [1] M. Rafatullaha, O. Sulaimana, R. Hashima, A. Ahmad, Adsorption of methylene blue on low-cost adsorbents: a review, J. Hazard. Mater. 177 (2010) 70–80. [2] M.T. Yagub, T.K. Sen, S. Afroze, H.M. Ang, Dye and its removal from aqueous solution by adsorption: a review, Adv. Colloid Interfaces 209 (2014) 172–184. [3] B.D. Wu, D.X. Zhu, S.J. Zhang, W.Z. Lin, G.Z. Wu, B.C. 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Fig. 9. Recyclability of regenerated MOP-1 and MOP-2 for the adsorption of MB and MG.

for exchanging with triethylamine cations. Since the synthesized adsorbents MOP-1 and MOP-2 have narrow pore size distribution, they may be exhibited size-selective adsorption on some dye molecules. Therefore, the large-size dye molecule Basic blue 7 (BB7) (Fig. S1) was used as the probe. The adsorption results were presented in Fig. S15 and revealed BB7 cannot be absorbed by MOP-1 and MOP-2. Thus, absorbents MOP-1 and MOP-2 exhibited sizeselective adsorption on dye molecules. The reuse of adsorbent can greatly reduce the cost of the adsorption process and meet the development requirements of green chemistry. Therefore, it is necessary to explore the recycling performance of adsorbents MOP-1 and MOP-2. The used adsorbents were regenerated in saturated NaCl methanol solution with the help of ultrasound irradiation. After four adsorption-desorption cycles, the adsorption capacities of MOP-1 and MOP-2 still retained above 95% of respective initial adsorption capacity (Fig. 9). The excellent recycling performance indicates that they are efficient and economic absorbents for removing organic dyes from wastewater. The simplicity and sustainability of the regeneration procedure of MOP-1 and MOP-2 cannot only reuse adsorbents, but also recycle adsorbed dyes. 4. Conclusion In summary, this paper has presented a simple method to prepare two bakelite-type anionic MOPs microspheres with remarkable high BET specific surface areas from cheap raw materials. These microspheres exhibited high adsorption capacities on MB and MG. Furthermore, they can completely removal MB and MG at relatively high initial MB concentrations. The high adsorption capacity of the microspheres can be ascribed to ion exchanged and the strong electrostatic interaction between the abundant –OH groups on the internal/ external surface of the microspheres and the dye molecules. In addition, they have shown charged and size-selective adsorption. The regenerated samples still kept a high adsorption capacity and can be repeatedly used for at least 4 cycles without significant decrease after regenerated in the saturated NaCl methanol. Consequently, the bakelite-type anionic MOPs microporous polymer materials opens up an effective and economical way of adsorption and separation materials, which will stimulate further research on the construction of new microporous polymer materials with a large number of functions and charges, which can improve the selective adsorption and separation ability on desired organic dyes from aqueous solutions. 413

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