- Email: [email protected]
Novel N-riched crystalline covalent organic framework as a highly porous adsorbent for effective cadmium removal
Journal of Environmental Chemical Engineering 7 (2019) 102907
Contents lists available at ScienceDirect
Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Novel N-riched crystalline covalent organic framework as a highly porous adsorbent for effective cadmium removal
T
Mohammad Dinari , Mohammad Hatami ⁎
Department of Chemistry, Isfahan University of Technology, Isfahan, 8415683111, Iran
ARTICLE INFO
ABSTRACT
Keywords: Covalent organic framework (COFs) Porous N-riched polymers Heavy metals Adsorption isotherms Kinetic models
One of the emerging issues in environmental remediation is the introduction of efficient adsorbents with high abundance active sites, to achieve quick uptake and high capacity for the contaminants such as cadmium. In this work, we explain how N-riched crystalline covalent organic framework (COF) which was prepared through the condensation of 2,4,6- tris(hydrazino)-1,3,5-triazine and 2,4,6-tris(p-formylphenoxy)-1,3,5-triazine serves as an ideal adsorbent for removing cadmium from aqueous solutions. More significantly, results demonstrated the adsorption isotherm is in consistent with the Langmuir model with high correlation coefficients of 0.9999 and the adsorption kinetic followed by the pseudo-second order model. Also, the result of the Langmuir constant (KL) for N-riched crystalline COF was found about of 0.39 which is related to the impressive performance in the adsorption of cadmium. Moreover, COF was characterized by XRD, TEM, BET, FE-SEM, TGA, FT-IR, CHN, and Raman spectroscopy.
1. Introduction Covalent organic frameworks (COFs) as an emerging class of porous crystalline organic polymers are constructed through strong covalent bonds. These materials represent the superior potential in various applications such as energy storage and adsorption due to the well-defined crystalline structure with the high surface area and low density [1]. In addition, the modular inherent of COFs lead the accretion of various rigid π-conjugated molecular building blocks to the growth of periodic arrays of pillar-shaped which results in the high thermal stability and permanent porosity [2]. In comparison whit metal-organic frameworks which are unstable in moisture and aqueous conditions, COFs show considerable chemical stability in the aqueous and organic environments [3–5]. In this respect, COFs can be successfully used in various applications in aqueous media such as extraction and adsorption. As a result, several linkage of B–O, C–N, and B–N have been applied for the design of frameworks [6–10], which render a significant efficiency in the fields of gas storage [11], catalysis [12], separation [11], sensing [13], drug delivery [14], and optoelectronics [15]. However, there are not enough reports concerning the application of COFs for heavy metal removal. Consequently, nowadays the development of COFs for removal of heavy metals as a new emerging application is one of the important progress [16]. In this regard, Huang and coworkers [16] reported a COF with high stability under acidic and alkaline conditions
⁎
which represented a high capacity for removal of mercury. In another study, Sun and coworkers prepared the post-synthetically modified COFs by thiol and thioether functional group which exhibited a considerable performance in mercury removal from aqueous solution [17]. The solubility of cadmium and its components is more than other heavy metals and it can be easily transferred to the bio-systems. On the other hand, Cd(II) with high toxicity causes serious diseases such as namely nausea, hypertension, stone formation in the kidney, and skeletal deformation patterns [18]. Therefore over the past decade, many efforts have been devoted to the study of cadmium removal and various methods such as ultrafiltration, nanofiltration, chemical precipitation, ion exchange, reverse osmosis, and biological treatment have been introduced [19,20]. But, these systems suffer from disadvantages such as complex processes, high consumption of energy, and high operating cost [21]. Herein, we rationally designed and synthesized an imine-linked porous COF with abundant nitrogen which supplied the high efficiency in cadmium removal, and it was briefly named as N-riched COF. Then, we applied a flexible binding block and triazine derivate monomer which results in the network resonance after polymerization. As a result, lone pair electrons are accessible and can interact with the metal ions easily.
Corresponding author. E-mail address: [email protected] (M. Dinari).
https://doi.org/10.1016/j.jece.2019.102907 Received 1 November 2018; Received in revised form 5 January 2019; Accepted 12 January 2019 Available online 14 January 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Environmental Chemical Engineering 7 (2019) 102907
M. Dinari, M. Hatami
2. Experimental
2.5. Preparation of porous N-riched COF
2.1. Materials and methods
A mixture of TFPTA (28 mg, 0.064 mmol), THAT (10 mg, 0.064 mmol), 1.0 mL of mesitylene, 1.0 mL of 1,4-dioxane and 100 μL aqueous 6 M acetic acid in a pyrex tube (10 ml) were degassed under ultrasonic conditions. Afterward, the pyrex tube was vacuumed and sealed, and then heated at 120 °C for 72 h. Then, the tube was opened and the yellow precipitate was filtered and washed with chloroform (3☓5 mL), acetone (3☓5 mL) and, THF (3☓5 mL). In the end, the yellow powder was dried in the oven at 70 °C. This product was briefly named as N-riche COF with the yield of about 80%. Anal. Calcd. For (C93H87N36O9)n: C, 60.58; H, 4.24; N, 27.35; Found: C, 58.87; H, 3.27; N, 26.18.
1,3,5-Trimethylbenzene (mesitylene), acetone, chloroform, 1,4-dioxane, tetrahydrofuran, 4-hydroxybenzaldehyde, hydrazine hydrate (80%), sodium carbonate, acetic acid, and cyanuric chloride were purchased from Merck Chemical Co. 2.2. Characterization techniques Field emission scanning electron microscopy (FE-SEM) of the sample was recorded using a HITACHI, S-4160 scanning electron microscope. Philips CM 120 microscope was used for Transmission electron microscopy (TEM) analysis. Fourier transform infrared (FT-IR) spectra of the samples were recorded on a JASCO FT-IR 680 plus spectrometer using the KBr pellet and the spectra were obtained in the range of 400-4000 cm−1 with a resolution of 4 cm−1 and an average of 64 scans. Surface area and pore size distribution were measured on a Micromeritics TriStar II Series, GA 30093 (USA instrument) system at 77 K using N2 as the adsorbate. Before the measurements, the samples were degassed at 120 °C. 1H-NMR (500 MHz) and 13C-NMR (125 MHz) were performed in CDCl3 on a Bruker Avance spectrometer. Thermogravimetry analysis (TGA) of the sample was carried out by heating at a rate of 10 °C min-1 from ambient temperature to 800 °C under N2 atmosphere using the STA503 TA instrument. X-ray diffraction (XRD) patterns were recorded using CuKα radiation on a Bruker (Germany) in the range of 2-80°. Flame atomic absorption spectrophotometer (FAAS; PerkinElmer 2380-Waltham) with a Cd (II) hollow cathode lamp was used todetermine the Cd(II) ions.
2.6. Batch adsorption tests The stock solution of 100 ppm Cd2+ was prepared from cadmium nitrate tetrahydrate in distilled water. Then 5 mg of adsorbent was added in 50 ml polyethylene bottle containing 25 mL of 10 ppm of cadmium and the pH of the solutions was adjusted in the range of 2–11 using a universal buffer. Afterward, the solutions were shaken at room temperature (180 min, 180 rpm). Then, the absorbent was separated by filter paper and the metal remained ion concentration was determined by using FAAS techniques. In order to study the adsorption isotherm, 5 mg of adsorbent was added into the solutions with different concentrations (5 to 150 mg.L−1) at the optimized pH, and the solution continuously was shaken for 180 min. The removal efficiency of Cd2+ was determined through Eq. (1): Removal efficiency = [(Ci-Ce)/Ci]×100
(1)
−1
In this equation, Ci and Ce (mg L ) are the initial and final equilibrium concentrations of metal ions, respectively. The number of metal ions which are retained in the adsorbent phase was calculated through Eq. (2):
2.3. Synthesis of 2,4,6-tris(hydrazino)-1,3,5-triazine (THTA) 2,4,6-Trihydrazino-1,3,5-triazine (THTA) was synthesized according to the previous literatures: [22,23]. Hydrazine (1 mL, 18 mmol) was dissolved in 1,4-dioxane (5 mL) in a round bottom flask. Then, a solution containing cyanuric chloride (0.552 g, 2.99 mmol) in 1,4-dioxane (5 mL) was added dropwise to the previous solution with a continuous stirring bar over 1 h. Then, it has been left for further 2 h stirring at room temperature. Then, it was allowed to reach the reflux conditions for 8 h. The resulting product was separated by filtration and washed with distilled water and 1,4-dioxane, respectively and dried at 70 °C under vacuum for 12 h. This product was briefly named THAT (yield 80% and m.p > 260 °C). Anal. Calcd for C3H9N9 (171.16): C, 21.06; H, 5.3; N, 73.64; Found: C, 20.75; H, 5.02; N, 72.94.
qe = (Ci – Ce)V/m
(2)
where V(L) and m (g) are the volumes of solution and the mass of adsorbent, respectively [30]. Kinetic studies were performed by the following procedure: Five mg of the adsorbent was added to 25 mL solution with 80 ppm cadmium ion concentration at optimized pH = 7. The solution was shaken at different time intervals up to 60 min. Finally, the adsorbent was separated by filter paper, and the filtrate was recorded using FAAS techniques. 3. Result and discussion
2.4. Synthesis of tri-(4-formacylphenoxy)-1,3,5-triazine (TFPTA)
3.1. Synthesis of monomers and porous N-riched COF
4-Hydroxybenzaldehyde (1.6 g, 13.1 mmol) and 2,4,6-trichloro1,3,5-triazine (0.6 g, 3.25 mmol) were added to the suspension of sodium carbonate (2.0 g, 18.8 mmol) in 15 mL of 1,4-dioxane in a round bottom flask. The mixture was allowed to reach the reflux conditions. After 20 h, the mixture of the reaction was cooled down and the sediments were removed by filtration and washed with 10% Na2CO3 aqueous solution and distilled water, respectively. Finally, the resulting precipitate was recrystallized with ethyl acetate, and then dried at 70 °C under vacuum for 12 h. This product was named briefly as TFPTA and characterized by FT-IR, 1H-NMR, 13C-NMR, and elemental analysis. Anal. Calcd for C24H15N3O6 (441.40): C, 65.31; H, 3.43; N, 9.52; Found: C, 65.04; H, 3.75; N, 9.44.
THTA was synthesized through the reaction between cyanuric chloride and an excess amount of hydrazinium hydroxide as shown in
Scheme 1. Preparation of 2,4,6- tris(hydrazino)-1,3,5-triazine (THTA).
2
Journal of Environmental Chemical Engineering 7 (2019) 102907
M. Dinari, M. Hatami
Scheme 2. Preparation of tri-(4-formacylphenoxy)-1,3,5-triazine (TFPTA).
Scheme 3. Synthesis of the porous N-riched COF via the condensation reaction of THTA and TFPTA.
Scheme 1. The structure of this monomer was confirmed by FT-IR and elemental analysis. The experimental results in elemental analyses of the synthesized monomers were closely matched to the calculated ones, indicating that the expected compounds were obtaine. TFPTA as a trialdehyde monomer was synthesized by a nucleophilic substitution reaction between cyanuric chloride and 4-hydroxybenzaldehyde in the presence of Na2CO3 to neutralize HCl as a byproduct. The purity and chemical structure of this monomer were approved by FT-IR, 1H-NMR, 13C-NMR, and elemental analysis techniques. The synthesis route is shown in Scheme 2. Novel porous N-riched COF was prepared through the condensation reaction of TFPTA and THAT as the trialdehyde and triamine monomers, respectively. The reaction was performed in the mixture of 1,4dioxane and mesitylene as the solvent and aqueous acetic acid as a catalyst for activation of the aldehyde group (Scheme. 3).
Fig. 1. FT-IR spectra of triamine monomer (A), trialdehyde monomer (B), and N-riched COF (C).
3
Journal of Environmental Chemical Engineering 7 (2019) 102907
M. Dinari, M. Hatami
Fig. 2. (A) 1H-NMR and (B)
13
C-NMR spectra of trialdehyde monomer.
Fig. 3. Raman spectra of triamine monomer (A), trialdehyde monomer (B), and N-riched COF (C).
3.2. Characterization of porous N-riched COF 3.2.1. FTIR results The FT-IR spectra of triamine, trialdehyde, and porous N-riched COF are depicted in Fig. 1. The spectrum of triamine displayed the clear bands which are located at 3312 and 3279 cm−1 corresponds to the vibration of the NH2 group (Fig. 1A). Also, the FT-IR spectrum of trialdehyde monomer showed two bands at 1701 and 2831 cm−1 as a consequence of the C]O and CHO groups, respectively (Fig. 1B). Part C of this figure represented the result of FT-IR for porous N-riched COF which revealed the vibration bands at 1619 and 1207 cm-1 that can be assigned to the imine stretch vibrations. These observations are in a good agreement with the corresponding values of reported COF materials [7,10,16]. Notably, the intensity of the aldehyde groups which are located at 1701 cm−1 dramatically decreased and the new stretching
Fig. 4. Nitrogen adsorption-desorption isotherms of N-riched COF. 4
Journal of Environmental Chemical Engineering 7 (2019) 102907
M. Dinari, M. Hatami
Fig. 5. XRD pattern of N-riched COF. Fig. 7. TGA curve of N-riched COF. -1
vibration band appeared at 1619 cm because of the formation of the imine bonds in the synthesized COF. In addition, the elimination of the stretch vibration band of the primary amine monomer and the appearance of a new peak at 3494 cm-1 which is related to the secondary amine stretch confirm the desired structure of the N-riched COF.
spectrum in which the presences of carbonyl, triazine and aromatic carbon atoms were approved (Fig. 2B). 3.2.3. Raman results Raman spectroscopy of trialdehyde, triamine monomers and, N-riched COF are shown in Fig. 3. Fig. 3A shows the result of analysis for the trialdehyde monomer. The clear band at 1704 cm−1 corresponds to the C]O stretching vibration. By comparing part C, it can be concluded that this vibration has been completely removed. In Fig. 3B, a weak band is observable at 988 cm−1 which is made great contributions to
3.2.2. NMR results The chemical structure of the trialdehyde monomer was further confirmed by NMR technique. Fig. 2A revealed the 1H-NMR spectrum of the TFPTA. The protons of aromatic rings are presented at 7.26 and 7.86 ppm and the protones of the aldehyde in this monomer are presented at 9.95 ppm. Also, these results were confirmed by 13C-NMR
Fig. 6. FE-SEM (a) and TEM (b) images of N-riched COF.
5
Journal of Environmental Chemical Engineering 7 (2019) 102907
M. Dinari, M. Hatami
Fig. 8. The effects of the pH of the solution (a) and adsorption time (b) on the Cd(II) removal: Adsorption conditions: adsorbent (0.005 g), 80 mg L−1 Cd(II), 60 min.
Fig. 9. Kinetic adsorption models: Elovich (a), intraparticle diffusion (b), pseudo-second-order (c), and pseudo-first-order (d) kinetic models. Adsorption conditions: adsorbent (0.005 g), 80 mg L−1 Cd(II) (25 mL), 298 K, pH = 7.
1522 and 1589 cm−1 which are corresponded to the D and G bands. This observation indicates the porous 2D hexagonal structure for synthesized COF [24–26].
NeN symmetric stretching vibrations groups. Furthermore, by comparing part C in Fig. 3, it can be seen that this vibration had a slight shift and is located at 1004 cm−1. In addition, it can be seen that the spectrum of N-riched COF (Fig. 3C) exhibited two new broad bands at
6
Journal of Environmental Chemical Engineering 7 (2019) 102907
M. Dinari, M. Hatami
3.2.4. Brunauer-Emmett-Teller (BET) The result of the adsorption/desorption isotherms of N-riched COF is shown in Fig. 4. Obviously, synthesized material implies to the characteristic of a combination of type I and IV according to the IUPAC classification N2 adsorption isotherm of the N-rich COF exhibits significant uptake at low pressure in the range of p/p0 = 0.05-0.014 that is attributed to the microporous structure as well as so narrow hysteresis on desorption which rises adsorption at the high-pressure value demonstrate mesoporous structure. Also, specific surface area value of Nriched COF was calculated by nitrogen adsorption isotherm using the Brunauer-Emmett-Teller (BET) equation and the results show the high surface area of 1935 m2 g−1. Moreover, the average of pore diameter and pore volume were calculated using the Barrett-Joyner-Halenda (BJH) method from the adsorption branches and results are in the order of 2.2 nm and 1.07 cm3 g−1, respectively.
Table 1 Kinetic parameters in the removal of Cd2+, 298 K, and optimized pH by using N-riched COF. Models Pseudo-first-order
ln
qt ) qe
=
k1 qe t
Pseudo-second-order t qt
=
1 K2 qe2
+
1 t qe
Elovich
qt =
Ln (hB ) B
+
Ln (t ) B
Intraparticle diffusion 1
3.2.5. XRD results Fig. 5 shows the XRD pattern of N-riched COF which is represented two explicit peaks at 3.4° and 6.0°. The main peak at 3.4° displays the highly ordered hexagonal structure of synthesized COF [27,28].
qt = kintra (t ) 2 + C
N-riched COF
K1 (min−1) R2
0.1362 0.9934
K2 (g mg−1. min−1) qexp (mgg−1) qcal(mgg−1) R2
0.0024 406.6676 396.8216 0.9999
B (gmg−1) R2
0.0323 0.9793
Kintra C R2
14.3239 302.8659 0.9606
results which are summarized in Fig. 8b, maximum adsorption of cadmium was achieved after 20 min.
3.2.6. SEM/TEM results FE-SEM and TEM images of N-riched COF are shown in Fig. 6(a,b), respectively. Based on the results of the FE-SEM images, a structure consisting of spherical particles for synthesizing COF is predicted. Also, the TEM image demonstrated the flake and layered structure including aggregation that occurs due to the hydrogen bonding between layers
3.3.2. Adsorption kinetic Several kinetic models such as Elovich, pseudo-first-order, pseudosecond-order, and Intra-particle diffusion were applied for perception the kinetic nature of the adsorption process and the chosen kinetic model was examined by their linear equations and slope and R2 values (Fig. 9). For the pseudo first order, the graph was drawn between Ln (qe−qt), and t. The linear equation is defined as Eq. (4).
3.2.7. TGA results In order to investigate the thermal stability of N-riched COF, TGA was performed with a heating rate of 10 °C/min under a nitrogen atmosphere from ambient temperature to 800 °C (Fig. 7). The results indicated high thermal stability up to 320 °C. The significant weight loss occurs (18%) in the range of 320–390 °C which is probably related to the decomposition of ether and imine linkages. As a valuable factor for rate the limiting oxygen index (LOI) values, the char yield can be useful. LOI designates the amount of oxygen that is needed for the burning of the materials. Based on the Van Krevelen equation (Eq. (3)), by increasing the LOI value, the char yield also increases [29]. LOI = 17.5 + 0.4CR (CR = Char yield)
(qe
Parameters
ln
(qe
qt ) qe
=
k1 qe t
(4)
By the Pseudo-second-order, the graph was drawn between t/qt and t. The linear equation is defined as Eq. (5).
t 1 1 = + t qt qe K2 qe2
(3)
(5)
In this equation, qe and qt are the maximum adsorptions at the equilibrium and the amount of adsorbed cadmium at the time (t), respectively. Also, K1 (min−1), and K2 (g. mg−1 min−1) are pseudo-first-order and pseudo-second-order rate constant of the adsorption, respectively. In the Elovich model, the graph was drawn between qt and Lnt. The linear equation is defined as Eq. (6).
Based on Eq. (1), the amount of LOI for N-riched COF was obtained around 36%. So, it can be introduced as a self-quenching material. 3.3. Cadmium removal 3.3.1. The effect of pH and contact time on the cadmium removal The effect of pH on the cadmium removal was investigated in the range of 2–11 using a 5 mg of absorbent in a solution with 10 ppm concentration. Fig. 8a shows the summary of results. Notably, the absorbent surface charge which is depended on the pH variations of solution dramatically affects the absorption of metal ions. In the mildly acidic conditions, cadmium removal is ignorable due to the higher concentration of H3O+ than metal cations. The reason is that active sites of adsorption are occupied through the protonation of the nitrogen groups. By increasing the pH of solution an anionic form of COF creates [31,32], so active centers of adsorbent are accessible. Finally, maximum removal of cadmium (98%) was achieved at pH = 7 as the best chosen. In order to optimize the time of the reaction, the removal of cadmium was studied at different times in the range of 5–60 minutes under optimized pH = 7 and 80 ppm concentration. Based on the
qt =
Ln (hB ) Ln (t ) + B B
(6)
In this equation, α and B are the initial adsorption rate and desorption constant, respectively. For Intra-particle diffusion model, the graph was drawn between qt and t1/2. The linear equation is defined as Eq. (7). qt = Kintra(t)1/2 + C −1
(7) -1/2
Herein, Kintra (mgg . min ) refers to the intraparticle diffusion rate constant, and C gives information regarding the thickness of the fringe layer. The correlation parameters were calculated for the mentioned models and the results are collected in Table 1. According to the experimental results such as regression correlation coefficient R2 and qe, Pseudo-second-order kinetic was correctly
7
Journal of Environmental Chemical Engineering 7 (2019) 102907
M. Dinari, M. Hatami
Fig. 10. Equilibrium isotherm (a), values of RL (b), Freundlich model (c), D − R model (d), Temkin model (e), and Langmuir model (f) for removing of Cd(II) by Nriched COF at 298 K.
matched to the adsorption of Cd(II) which implies to the lack of dependence of absorption on the adsorbent concentration, while it depends on the number of active sites of adsorbent. In another word, when active sites are occupied by the adsorbate, the adsorption process stops.
3.3.3. Adsorption isotherms Several isotherm models such as Langmuir, Freundlich, DubininRadushkevich, and Temkin were applied to reveal the interaction of adsorbate and adsorbent (Fig. 10). The linear Langmuir and Freundlich adsorption isotherm equations
8
Journal of Environmental Chemical Engineering 7 (2019) 102907
M. Dinari, M. Hatami
in Table 2, N-riched COF is a strong adsorbent. The Dubbinin-Radashkuich isotherm equation is explained as Eq. (10):
Table 2 Isotherm parameters on the removal of Cd2+ at 298 K and optimized pH by Nriched COF. Models Langmuir Ce qe
=
1 KL qmax
+
Ce qmax
Freundlich
logqe = logKf +
1 n
* logce
Dubinin − Radushkevich (D-R)
lnqe = lnqm
R*T lnKt bt
N-riched COF
qmax (mgg−1) KL (Lmg−1) R2
396.7594 0.3978 0.9999
Kf (mgg n R2
−1
)
149.4153 3.4575 0.9179
B
2
qm -BD ×10−7/mol2∙kj−2 ED/kJ∙mol−1 R2
376.1254 5.1527 1.4145 0.9787
+
R*T lnCe bt
KT/L∙g−1 bT/kJ∙mol−1 BT R2
9.5264 3.5735 69.6348 0.9637
Temkin
qe =
Parameters
Ln qe = lnqm – Bε2
Also, the Temkin adsorption isotherm equation is expressed as Eq. (11):
qe =
log q e = log Kf +
(8)
1 *log c e n
R*T R*T lnKt + lnce bt bt
(11)
In order to understanding, the nature of adsorption between adsorbent and adsorbate, ED (KJ. mol−1) parameter was investigated. This parameter is obtained from D–R isotherm using the equation of ED1/ (2B)½. For the value of ED < 8 (KJ.mol−1), the superior adsorption process is considered as physical adsorption [35], whilst the value of ED in the range of ED = 8–20 (KJ.mol−1) refers to the chemical adsorption. Also, bT is the Temkin constant which is related to the heat of adsorption that is shown in Table 2. Herein, bT was found about of 3.5735 and is lower than 20 kJ.mol−1 which is in a good consistency of superior physical adsorption [35]. Moreover, the result of Ed which was measured experimentally is 1.4145 kJ.mol-1. This is related to the major adsorption which is concluded as physical adsorption. Table 3 shows the results of some previous reports that have been performed to the removal of Cd(II) in comparison with the current study. It is obvious that the synthesized N-riched COF is more favorable due to the high porosity which provides a high surface to absorb the metal ions. This is because of providing a high adsorption capacity with a rapid removal up to 20 min. Hence, COF can be an effective candidate in the removal of heavy metals.
are expressed as Eqs. (8), and (9), respectively:
Ce 1 Ce = + qe KL qmax qmax
(10)
(9)
In these equations, Ce (mgL−1) and qe (mgg−1) are equilibrium concentration of Cd(II) and the amount of adsorbed cadmium at equilibrium, respectively. The regression coefficient (R2) and qe were obtained for all mentioned isotherms and the results were collected in Table 2. The Langmuir model is properly fitted and the parameters such as RL, ED, BT, and n were calculated in order to show the favorability of the adsorption. KL (L/mg) and qmax (mgg−1) is the Langmuir constant rate and the maximum adsorption capacity, respectively. Also, Kf and 1/n are the Freundlich constants in which refer to the rate of adsorption and experiential parameter as favorability of the adsorption process, respectively. If n is obtained n > 1, it implies the favorable adsorption, and n = 0 displays the irreversible adsorption [33]. The dimensionless parameter of RL is explained by the equation of 1 RL = 1 + K C , which the value of RL demonstrates the acceptable adL 0 sorption for 0 > RL > 1, unfavorable for RL > 1, and irreversible RL = 0 [30,34]. Based on the parameter of n and KL which are reported
4. Conclusions In summary, COFs have demonstrated an excellent ability in removing heavy metals due to the unique features such as high porosity, stability, large surface area, as well as more accessible active sites. This study focused on assessing the adsorption efficiency of a novel N-riched crystalline COF adsorbent which has been prepared through the condensation of triazine and trialdehyde in the mixture of 1,4-dioxane/ mesitylene as a solvent. The XRD pattern showed a sharp peak at 3.4° which confirm the highly ordered and hexagonal structure in the synthesized COF. In addition, BET results display a high surface area of 1935 m2 g−1 with a pore size of 2.2 nm. The synthesized N-riched COF exhibited excellent performance in the cadmium removal with high adsorption capacity (396 mgg-1) at optimized conditions. Also, different kinetic models were investigated. The pseudo-second-order model was determined as the best fit model. Moreover, four equilibrium isotherms were studied and the Langmuir model was properly fitted. The N-riched COF has explained a high efficiency, cost-effective, environmental-
Table 3 Comparison of cadmium adsorption capacity of N-riched COF with different adsorbents. Adsorbents
Qmax (mg g−1)
T/K
pH
Reference
Poly(AN-co-AA) Functionalized Fe3O4 magnetic nanoparticles Chitosan templated synthesis of mesoporous silica Chelating Poly acrylonitrile Magnetic LDH/guargum bionanocomposites N-riched COF
20 55 4 156 258 396
298 298 298 298 298 298
9 10 8 7 10 7
[31] [32] [36] [37] [38] This work
9
Journal of Environmental Chemical Engineering 7 (2019) 102907
M. Dinari, M. Hatami
friendliness properties as well as demonstrating more advantages such as easy manufacture and absence of by-products. So, it can be as a promising adsorbent in the environmental pollution cleanup.
[19] T. Inaba, E. Kobayashi, Y. Suwazono, M. Uetani, M. Oishi, H. Nakagawa, K. Nogawa, Estimation of cumulative cadmium intake causing Itai–itai disease, Toxicol. Lett. 159 (2005) 192–201. [20] D. Sud, G. Mahajan, M. Kaur, Agricultural waste material as a potential adsorbent for sequestering heavy metal ions from aqueous solutions–a review, Bioresour. Technol. 99 (2008) 6017–6027. [21] E. Da’na, Adsorption of heavy metals on functionalized-mesoporous silica: a review, Microporous Mesoporous Mater. 247 (2017) 145–157. [22] B. Kebede, N. Retta, V. Raju, Y. Chebude, Synthesis and Characterization of 2, 4, 6tris (hydrazino)-s-triazine and its Metal Complexes, Trans. Met. Chem. 31 (2006) 19–26. [23] M.M. Naseer, D.-X. Wang, L. Zhao, Z.-T. Huang, M.-X. Wang, Synthesis and functionalization of heteroatom-bridged bicyclocalixaromatics, large molecular triangular prisms with electroN-riched and-deficient aromatic interiors, J. Org. Chem. 76 (2011) 1804–1813. [24] M.A. Aly, Infrared and Raman spectra of some symmetric azines, Spectrochim. Acta A 55 (1999) 1711–1714. [25] O. Buyukcakir, S.H. Je, S.N. Talapaneni, D. Kim, A. Coskun, Charged covalent triazine frameworks for CO2 capture and conversion, ACS Appl. Mater. Interfaces 9 (2017) 7209–7216. [26] T. Ben, K. Shi, Y. Cui, C. Pei, Y. Zuo, H. Guo, D. Zhang, J. Xu, F. Deng, Z. Tian, Targeted synthesis of an electroactive organic framework, J. Mater. Chem. 21 (2011) 18208–18214. [27] L. Xu, S.-Y. Ding, J. Liu, J. Sun, W. Wang, Q.-Y. Zheng, Highly crystalline covalent organic frameworks from flexible building blocks, Chem. Commun. 52 (2016) 4706–4709. [28] W. Liu, Q. Su, P. Ju, B. Guo, H. Zhou, G. Li, Q. Wu, A hydrazone‐based covalent organic framework as an efficient and reusable photocatalyst for the cross‐dehydrogenative coupling reaction of N‐aryltetrahydroisoquinolines, ChemSusChem 10 (2017) 664–669. [29] D.W. van Krevelen, Properties of Polymers: Their Estimation and Correlation with Chemical Structure, Elsevier Scientific Pub., Amsterdam, 1976. [30] M.H. Dindar, M.R. Yaftian, S. Rostamnia, Potential of functionalized SBA-15 mesoporous materials for decontamination of water solutions from Cr (VI), As (V) and Hg (II) ions, J. Environ. Chem. Eng. 3 (2015) 986–995. [31] N.A.M. Zahri, S.N.A.M. Jamil, L.C. Abdullah, S.J. Huey, T.C.S. Yaw, M.N. Mobarekeh, N.S.M. Rapeia, Equilibrium and kinetic behavior on cadmium and lead removal by using synthetic polymer, J. Water Process. Eng. 17 (2017) 277–289. [32] V. Devi, M. Selvaraj, P. Selvam, A.A. Kumar, S. Sankar, K. Dinakaran, Preparation and characterization of CNSR functionalized Fe3O4 magnetic nanoparticles: an efficient adsorbent for the removal of cadmium ion from water, J. Environ. Chem. Eng. 5 (2017) 4539–4546. [33] T.A. Arica, E. Ayas, M.Y. Arica, Magnetic MCM-41 silica particles grafted with poly (glycidylmethacrylate) brush: modification and application for removal of direct dyes, Microporous Mesoporous Mater. 243 (2017) 164–175. [34] D.F. Apopei, M.V. Dinu, A.W. Trochimczuk, E.S. Dragan, Sorption isotherms of heavy metal ions onto semi-interpenetrating polymer network cryogels based on polyacrylamide and anionically modified potato starch, Ind. Eng. Chem. Res. 51 (2012) 10462–10471. [35] M. Dinari, G. Mohammadnezhad, R. Soltani, Fabrication of poly (methyl methacrylate)/silica KIT-6 nanocomposites via in situ polymerization approach and their application for removal of Cu 2+ from aqueous solution, RSC Adv. 6 (2016) 11419–11429. [36] D. Tiwari, S.M. Lee, Chitosan templated synthesis of mesoporous silica and its application in the treatment of aqueous solutions contaminated with cadmium (II) and lead (II), Chem. Eng. J. 328 (2017) 434–444. [37] P. Bhunia, S. Chatterjee, P. Rudra, S. De, Chelating polyacrylonitrile beads for removal of lead and cadmium from wastewater, Sep. Purif. Technol. 193 (2018) 202–213. [38] M. Dinari, R. Tabatabaeian, Ultra-fast and highly efficient removal of cadmium ions by magnetic layered double hydroxide/guargum bionanocomposites, Carbohydr. Polym. 192 (2018) 317–326.
Acknowledgment We are grateful for the financial support from the Research Council of Isfahan University of Technology (IUT), Iran. References [1] S.-Y. Ding, W. Wang, Covalent organic frameworks (COFs): from design to applications, Chem. Soc. Rev. 42 (2013) 548–568. [2] B.J. Smith, W.R. Dichtel, Mechanistic studies of two-dimensional covalent organic frameworks rapidly polymerized from initially homogenous conditions, J. Am. Chem. Soc. 136 (2014) 8783–8789. [3] Z. Li, X. Feng, Y. Zou, Y. Zhang, H. Xia, X. Liu, Y. Mu, A 2D azine-linked covalent organic framework for gas storage applications, Chem. Commun. 50 (2014) 13825–13828. [4] X. Feng, X. Ding, D. Jiang, Covalent organic frameworks, Chem. Soc. Rev. 41 (2012) 6010–6022. [5] S. Kandambeth, A. Mallick, B. Lukose, M.V. Mane, T. Heine, R. Banerjee, Construction of crystalline 2D covalent organic frameworks with remarkable chemical (acid/base) stability via a combined reversible and irreversible route, J. Am. Chem. Soc. 134 (2012) 19524–19527. [6] A.P. Cote, A.I. Benin, N.W. Ockwig, M. O’keeffe, A.J. Matzger, O.M. Yaghi, Porous, crystalline, covalent organic frameworks, Science 310 (2005) 1166–1170. [7] R.W. Tilford, W.R. Gemmill, H.-C. zur Loye, J.J. Lavigne, Facile synthesis of a highly crystalline, covalently linked porous boronate network, Chem. Mater. 18 (2006) 5296–5301. [8] H.M. El-Kaderi, J.R. Hunt, J.L. Mendoza-Cortés, A.P. Côté, R.E. Taylor, M. O’keeffe, O.M. Yaghi, Designed synthesis of 3D covalent organic frameworks, Science 316 (2007) 268–272. [9] K.T. Jackson, T.E. Reich, H.M. El-Kaderi, Targeted synthesis of a porous borazinelinked covalent organic framework, Chem. Commun. 48 (2012) 8823–8825. [10] F.J. Uribe-Romo, C.J. Doonan, H. Furukawa, K. Oisaki, O.M. Yaghi, Crystalline covalent organic frameworks with hydrazone linkages, J. Am. Chem. Soc. 133 (2011) 11478–11481. [11] H. Ma, H. Ren, S. Meng, Z. Yan, H. Zhao, F. Sun, G. Zhu, A 3D microporous covalent organic framework with exceedingly high C3H8/CH4 and C2 hydrocarbon/CH4 selectivity, Chem. Commun. 49 (2013) 9773–9775. [12] X. Wang, X. Han, J. Zhang, X. Wu, Y. Liu, Y. Cui, Homochiral 2D porous covalent organic frameworks for heterogeneous asymmetric catalysis, J. Am. Chem. Soc. 138 (2016) 12332–12335. [13] S.-Y. Ding, M. Dong, Y.-W. Wang, Y.-T. Chen, H.-Z. Wang, C.-Y. Su, W. Wang, Thioether-based fluorescent covalent organic framework for selective detection and facile removal of mercury (II), J. Am. Chem. Soc. 138 (2016) 3031–3037. [14] Q. Fang, J. Wang, S. Gu, R.B. Kaspar, Z. Zhuang, J. Zheng, H. Guo, S. Qiu, Y. Yan, 3D porous crystalline polyimide covalent organic frameworks for drug delivery, J. Am. Chem. Soc. 137 (2015) 8352–8355. [15] J.W. Crowe, L.A. Baldwin, P.L. McGrier, Luminescent covalent organic frameworks containing a homogeneous and heterogeneous distribution of dehydrobenzoannulene vertex units, J. Am. Chem. Soc. 138 (2016) 10120–10123. [16] N. Huang, L. Zhai, H. Xu, D. Jiang, Stable covalent organic frameworks for exceptional mercury removal from aqueous solutions, J. Am. Chem. Soc. 139 (2017) 2428–2434. [17] Q. Sun, B. Aguila, J. Perman, L.D. Earl, C.W. Abney, Y. Cheng, H. Wei, N. Nguyen, L. Wojtas, S. Ma, Postsynthetically modified covalent organic frameworks for efficient and effective mercury removal, J. Am. Chem. Soc. 139 (2017) 2786–2793. [18] B.A. Fowler, Monitoring of human populations for early markers of cadmium toxicity: a review, Toxicol. Appl. Pharmacol. 238 (2009) 294–300.
10
Contents lists available at ScienceDirect
Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Novel N-riched crystalline covalent organic framework as a highly porous adsorbent for effective cadmium removal
T
Mohammad Dinari , Mohammad Hatami ⁎
Department of Chemistry, Isfahan University of Technology, Isfahan, 8415683111, Iran
ARTICLE INFO
ABSTRACT
Keywords: Covalent organic framework (COFs) Porous N-riched polymers Heavy metals Adsorption isotherms Kinetic models
One of the emerging issues in environmental remediation is the introduction of efficient adsorbents with high abundance active sites, to achieve quick uptake and high capacity for the contaminants such as cadmium. In this work, we explain how N-riched crystalline covalent organic framework (COF) which was prepared through the condensation of 2,4,6- tris(hydrazino)-1,3,5-triazine and 2,4,6-tris(p-formylphenoxy)-1,3,5-triazine serves as an ideal adsorbent for removing cadmium from aqueous solutions. More significantly, results demonstrated the adsorption isotherm is in consistent with the Langmuir model with high correlation coefficients of 0.9999 and the adsorption kinetic followed by the pseudo-second order model. Also, the result of the Langmuir constant (KL) for N-riched crystalline COF was found about of 0.39 which is related to the impressive performance in the adsorption of cadmium. Moreover, COF was characterized by XRD, TEM, BET, FE-SEM, TGA, FT-IR, CHN, and Raman spectroscopy.
1. Introduction Covalent organic frameworks (COFs) as an emerging class of porous crystalline organic polymers are constructed through strong covalent bonds. These materials represent the superior potential in various applications such as energy storage and adsorption due to the well-defined crystalline structure with the high surface area and low density [1]. In addition, the modular inherent of COFs lead the accretion of various rigid π-conjugated molecular building blocks to the growth of periodic arrays of pillar-shaped which results in the high thermal stability and permanent porosity [2]. In comparison whit metal-organic frameworks which are unstable in moisture and aqueous conditions, COFs show considerable chemical stability in the aqueous and organic environments [3–5]. In this respect, COFs can be successfully used in various applications in aqueous media such as extraction and adsorption. As a result, several linkage of B–O, C–N, and B–N have been applied for the design of frameworks [6–10], which render a significant efficiency in the fields of gas storage [11], catalysis [12], separation [11], sensing [13], drug delivery [14], and optoelectronics [15]. However, there are not enough reports concerning the application of COFs for heavy metal removal. Consequently, nowadays the development of COFs for removal of heavy metals as a new emerging application is one of the important progress [16]. In this regard, Huang and coworkers [16] reported a COF with high stability under acidic and alkaline conditions
⁎
which represented a high capacity for removal of mercury. In another study, Sun and coworkers prepared the post-synthetically modified COFs by thiol and thioether functional group which exhibited a considerable performance in mercury removal from aqueous solution [17]. The solubility of cadmium and its components is more than other heavy metals and it can be easily transferred to the bio-systems. On the other hand, Cd(II) with high toxicity causes serious diseases such as namely nausea, hypertension, stone formation in the kidney, and skeletal deformation patterns [18]. Therefore over the past decade, many efforts have been devoted to the study of cadmium removal and various methods such as ultrafiltration, nanofiltration, chemical precipitation, ion exchange, reverse osmosis, and biological treatment have been introduced [19,20]. But, these systems suffer from disadvantages such as complex processes, high consumption of energy, and high operating cost [21]. Herein, we rationally designed and synthesized an imine-linked porous COF with abundant nitrogen which supplied the high efficiency in cadmium removal, and it was briefly named as N-riched COF. Then, we applied a flexible binding block and triazine derivate monomer which results in the network resonance after polymerization. As a result, lone pair electrons are accessible and can interact with the metal ions easily.
Corresponding author. E-mail address: [email protected] (M. Dinari).
https://doi.org/10.1016/j.jece.2019.102907 Received 1 November 2018; Received in revised form 5 January 2019; Accepted 12 January 2019 Available online 14 January 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Environmental Chemical Engineering 7 (2019) 102907
M. Dinari, M. Hatami
2. Experimental
2.5. Preparation of porous N-riched COF
2.1. Materials and methods
A mixture of TFPTA (28 mg, 0.064 mmol), THAT (10 mg, 0.064 mmol), 1.0 mL of mesitylene, 1.0 mL of 1,4-dioxane and 100 μL aqueous 6 M acetic acid in a pyrex tube (10 ml) were degassed under ultrasonic conditions. Afterward, the pyrex tube was vacuumed and sealed, and then heated at 120 °C for 72 h. Then, the tube was opened and the yellow precipitate was filtered and washed with chloroform (3☓5 mL), acetone (3☓5 mL) and, THF (3☓5 mL). In the end, the yellow powder was dried in the oven at 70 °C. This product was briefly named as N-riche COF with the yield of about 80%. Anal. Calcd. For (C93H87N36O9)n: C, 60.58; H, 4.24; N, 27.35; Found: C, 58.87; H, 3.27; N, 26.18.
1,3,5-Trimethylbenzene (mesitylene), acetone, chloroform, 1,4-dioxane, tetrahydrofuran, 4-hydroxybenzaldehyde, hydrazine hydrate (80%), sodium carbonate, acetic acid, and cyanuric chloride were purchased from Merck Chemical Co. 2.2. Characterization techniques Field emission scanning electron microscopy (FE-SEM) of the sample was recorded using a HITACHI, S-4160 scanning electron microscope. Philips CM 120 microscope was used for Transmission electron microscopy (TEM) analysis. Fourier transform infrared (FT-IR) spectra of the samples were recorded on a JASCO FT-IR 680 plus spectrometer using the KBr pellet and the spectra were obtained in the range of 400-4000 cm−1 with a resolution of 4 cm−1 and an average of 64 scans. Surface area and pore size distribution were measured on a Micromeritics TriStar II Series, GA 30093 (USA instrument) system at 77 K using N2 as the adsorbate. Before the measurements, the samples were degassed at 120 °C. 1H-NMR (500 MHz) and 13C-NMR (125 MHz) were performed in CDCl3 on a Bruker Avance spectrometer. Thermogravimetry analysis (TGA) of the sample was carried out by heating at a rate of 10 °C min-1 from ambient temperature to 800 °C under N2 atmosphere using the STA503 TA instrument. X-ray diffraction (XRD) patterns were recorded using CuKα radiation on a Bruker (Germany) in the range of 2-80°. Flame atomic absorption spectrophotometer (FAAS; PerkinElmer 2380-Waltham) with a Cd (II) hollow cathode lamp was used todetermine the Cd(II) ions.
2.6. Batch adsorption tests The stock solution of 100 ppm Cd2+ was prepared from cadmium nitrate tetrahydrate in distilled water. Then 5 mg of adsorbent was added in 50 ml polyethylene bottle containing 25 mL of 10 ppm of cadmium and the pH of the solutions was adjusted in the range of 2–11 using a universal buffer. Afterward, the solutions were shaken at room temperature (180 min, 180 rpm). Then, the absorbent was separated by filter paper and the metal remained ion concentration was determined by using FAAS techniques. In order to study the adsorption isotherm, 5 mg of adsorbent was added into the solutions with different concentrations (5 to 150 mg.L−1) at the optimized pH, and the solution continuously was shaken for 180 min. The removal efficiency of Cd2+ was determined through Eq. (1): Removal efficiency = [(Ci-Ce)/Ci]×100
(1)
−1
In this equation, Ci and Ce (mg L ) are the initial and final equilibrium concentrations of metal ions, respectively. The number of metal ions which are retained in the adsorbent phase was calculated through Eq. (2):
2.3. Synthesis of 2,4,6-tris(hydrazino)-1,3,5-triazine (THTA) 2,4,6-Trihydrazino-1,3,5-triazine (THTA) was synthesized according to the previous literatures: [22,23]. Hydrazine (1 mL, 18 mmol) was dissolved in 1,4-dioxane (5 mL) in a round bottom flask. Then, a solution containing cyanuric chloride (0.552 g, 2.99 mmol) in 1,4-dioxane (5 mL) was added dropwise to the previous solution with a continuous stirring bar over 1 h. Then, it has been left for further 2 h stirring at room temperature. Then, it was allowed to reach the reflux conditions for 8 h. The resulting product was separated by filtration and washed with distilled water and 1,4-dioxane, respectively and dried at 70 °C under vacuum for 12 h. This product was briefly named THAT (yield 80% and m.p > 260 °C). Anal. Calcd for C3H9N9 (171.16): C, 21.06; H, 5.3; N, 73.64; Found: C, 20.75; H, 5.02; N, 72.94.
qe = (Ci – Ce)V/m
(2)
where V(L) and m (g) are the volumes of solution and the mass of adsorbent, respectively [30]. Kinetic studies were performed by the following procedure: Five mg of the adsorbent was added to 25 mL solution with 80 ppm cadmium ion concentration at optimized pH = 7. The solution was shaken at different time intervals up to 60 min. Finally, the adsorbent was separated by filter paper, and the filtrate was recorded using FAAS techniques. 3. Result and discussion
2.4. Synthesis of tri-(4-formacylphenoxy)-1,3,5-triazine (TFPTA)
3.1. Synthesis of monomers and porous N-riched COF
4-Hydroxybenzaldehyde (1.6 g, 13.1 mmol) and 2,4,6-trichloro1,3,5-triazine (0.6 g, 3.25 mmol) were added to the suspension of sodium carbonate (2.0 g, 18.8 mmol) in 15 mL of 1,4-dioxane in a round bottom flask. The mixture was allowed to reach the reflux conditions. After 20 h, the mixture of the reaction was cooled down and the sediments were removed by filtration and washed with 10% Na2CO3 aqueous solution and distilled water, respectively. Finally, the resulting precipitate was recrystallized with ethyl acetate, and then dried at 70 °C under vacuum for 12 h. This product was named briefly as TFPTA and characterized by FT-IR, 1H-NMR, 13C-NMR, and elemental analysis. Anal. Calcd for C24H15N3O6 (441.40): C, 65.31; H, 3.43; N, 9.52; Found: C, 65.04; H, 3.75; N, 9.44.
THTA was synthesized through the reaction between cyanuric chloride and an excess amount of hydrazinium hydroxide as shown in
Scheme 1. Preparation of 2,4,6- tris(hydrazino)-1,3,5-triazine (THTA).
2
Journal of Environmental Chemical Engineering 7 (2019) 102907
M. Dinari, M. Hatami
Scheme 2. Preparation of tri-(4-formacylphenoxy)-1,3,5-triazine (TFPTA).
Scheme 3. Synthesis of the porous N-riched COF via the condensation reaction of THTA and TFPTA.
Scheme 1. The structure of this monomer was confirmed by FT-IR and elemental analysis. The experimental results in elemental analyses of the synthesized monomers were closely matched to the calculated ones, indicating that the expected compounds were obtaine. TFPTA as a trialdehyde monomer was synthesized by a nucleophilic substitution reaction between cyanuric chloride and 4-hydroxybenzaldehyde in the presence of Na2CO3 to neutralize HCl as a byproduct. The purity and chemical structure of this monomer were approved by FT-IR, 1H-NMR, 13C-NMR, and elemental analysis techniques. The synthesis route is shown in Scheme 2. Novel porous N-riched COF was prepared through the condensation reaction of TFPTA and THAT as the trialdehyde and triamine monomers, respectively. The reaction was performed in the mixture of 1,4dioxane and mesitylene as the solvent and aqueous acetic acid as a catalyst for activation of the aldehyde group (Scheme. 3).
Fig. 1. FT-IR spectra of triamine monomer (A), trialdehyde monomer (B), and N-riched COF (C).
3
Journal of Environmental Chemical Engineering 7 (2019) 102907
M. Dinari, M. Hatami
Fig. 2. (A) 1H-NMR and (B)
13
C-NMR spectra of trialdehyde monomer.
Fig. 3. Raman spectra of triamine monomer (A), trialdehyde monomer (B), and N-riched COF (C).
3.2. Characterization of porous N-riched COF 3.2.1. FTIR results The FT-IR spectra of triamine, trialdehyde, and porous N-riched COF are depicted in Fig. 1. The spectrum of triamine displayed the clear bands which are located at 3312 and 3279 cm−1 corresponds to the vibration of the NH2 group (Fig. 1A). Also, the FT-IR spectrum of trialdehyde monomer showed two bands at 1701 and 2831 cm−1 as a consequence of the C]O and CHO groups, respectively (Fig. 1B). Part C of this figure represented the result of FT-IR for porous N-riched COF which revealed the vibration bands at 1619 and 1207 cm-1 that can be assigned to the imine stretch vibrations. These observations are in a good agreement with the corresponding values of reported COF materials [7,10,16]. Notably, the intensity of the aldehyde groups which are located at 1701 cm−1 dramatically decreased and the new stretching
Fig. 4. Nitrogen adsorption-desorption isotherms of N-riched COF. 4
Journal of Environmental Chemical Engineering 7 (2019) 102907
M. Dinari, M. Hatami
Fig. 5. XRD pattern of N-riched COF. Fig. 7. TGA curve of N-riched COF. -1
vibration band appeared at 1619 cm because of the formation of the imine bonds in the synthesized COF. In addition, the elimination of the stretch vibration band of the primary amine monomer and the appearance of a new peak at 3494 cm-1 which is related to the secondary amine stretch confirm the desired structure of the N-riched COF.
spectrum in which the presences of carbonyl, triazine and aromatic carbon atoms were approved (Fig. 2B). 3.2.3. Raman results Raman spectroscopy of trialdehyde, triamine monomers and, N-riched COF are shown in Fig. 3. Fig. 3A shows the result of analysis for the trialdehyde monomer. The clear band at 1704 cm−1 corresponds to the C]O stretching vibration. By comparing part C, it can be concluded that this vibration has been completely removed. In Fig. 3B, a weak band is observable at 988 cm−1 which is made great contributions to
3.2.2. NMR results The chemical structure of the trialdehyde monomer was further confirmed by NMR technique. Fig. 2A revealed the 1H-NMR spectrum of the TFPTA. The protons of aromatic rings are presented at 7.26 and 7.86 ppm and the protones of the aldehyde in this monomer are presented at 9.95 ppm. Also, these results were confirmed by 13C-NMR
Fig. 6. FE-SEM (a) and TEM (b) images of N-riched COF.
5
Journal of Environmental Chemical Engineering 7 (2019) 102907
M. Dinari, M. Hatami
Fig. 8. The effects of the pH of the solution (a) and adsorption time (b) on the Cd(II) removal: Adsorption conditions: adsorbent (0.005 g), 80 mg L−1 Cd(II), 60 min.
Fig. 9. Kinetic adsorption models: Elovich (a), intraparticle diffusion (b), pseudo-second-order (c), and pseudo-first-order (d) kinetic models. Adsorption conditions: adsorbent (0.005 g), 80 mg L−1 Cd(II) (25 mL), 298 K, pH = 7.
1522 and 1589 cm−1 which are corresponded to the D and G bands. This observation indicates the porous 2D hexagonal structure for synthesized COF [24–26].
NeN symmetric stretching vibrations groups. Furthermore, by comparing part C in Fig. 3, it can be seen that this vibration had a slight shift and is located at 1004 cm−1. In addition, it can be seen that the spectrum of N-riched COF (Fig. 3C) exhibited two new broad bands at
6
Journal of Environmental Chemical Engineering 7 (2019) 102907
M. Dinari, M. Hatami
3.2.4. Brunauer-Emmett-Teller (BET) The result of the adsorption/desorption isotherms of N-riched COF is shown in Fig. 4. Obviously, synthesized material implies to the characteristic of a combination of type I and IV according to the IUPAC classification N2 adsorption isotherm of the N-rich COF exhibits significant uptake at low pressure in the range of p/p0 = 0.05-0.014 that is attributed to the microporous structure as well as so narrow hysteresis on desorption which rises adsorption at the high-pressure value demonstrate mesoporous structure. Also, specific surface area value of Nriched COF was calculated by nitrogen adsorption isotherm using the Brunauer-Emmett-Teller (BET) equation and the results show the high surface area of 1935 m2 g−1. Moreover, the average of pore diameter and pore volume were calculated using the Barrett-Joyner-Halenda (BJH) method from the adsorption branches and results are in the order of 2.2 nm and 1.07 cm3 g−1, respectively.
Table 1 Kinetic parameters in the removal of Cd2+, 298 K, and optimized pH by using N-riched COF. Models Pseudo-first-order
ln
qt ) qe
=
k1 qe t
Pseudo-second-order t qt
=
1 K2 qe2
+
1 t qe
Elovich
qt =
Ln (hB ) B
+
Ln (t ) B
Intraparticle diffusion 1
3.2.5. XRD results Fig. 5 shows the XRD pattern of N-riched COF which is represented two explicit peaks at 3.4° and 6.0°. The main peak at 3.4° displays the highly ordered hexagonal structure of synthesized COF [27,28].
qt = kintra (t ) 2 + C
N-riched COF
K1 (min−1) R2
0.1362 0.9934
K2 (g mg−1. min−1) qexp (mgg−1) qcal(mgg−1) R2
0.0024 406.6676 396.8216 0.9999
B (gmg−1) R2
0.0323 0.9793
Kintra C R2
14.3239 302.8659 0.9606
results which are summarized in Fig. 8b, maximum adsorption of cadmium was achieved after 20 min.
3.2.6. SEM/TEM results FE-SEM and TEM images of N-riched COF are shown in Fig. 6(a,b), respectively. Based on the results of the FE-SEM images, a structure consisting of spherical particles for synthesizing COF is predicted. Also, the TEM image demonstrated the flake and layered structure including aggregation that occurs due to the hydrogen bonding between layers
3.3.2. Adsorption kinetic Several kinetic models such as Elovich, pseudo-first-order, pseudosecond-order, and Intra-particle diffusion were applied for perception the kinetic nature of the adsorption process and the chosen kinetic model was examined by their linear equations and slope and R2 values (Fig. 9). For the pseudo first order, the graph was drawn between Ln (qe−qt), and t. The linear equation is defined as Eq. (4).
3.2.7. TGA results In order to investigate the thermal stability of N-riched COF, TGA was performed with a heating rate of 10 °C/min under a nitrogen atmosphere from ambient temperature to 800 °C (Fig. 7). The results indicated high thermal stability up to 320 °C. The significant weight loss occurs (18%) in the range of 320–390 °C which is probably related to the decomposition of ether and imine linkages. As a valuable factor for rate the limiting oxygen index (LOI) values, the char yield can be useful. LOI designates the amount of oxygen that is needed for the burning of the materials. Based on the Van Krevelen equation (Eq. (3)), by increasing the LOI value, the char yield also increases [29]. LOI = 17.5 + 0.4CR (CR = Char yield)
(qe
Parameters
ln
(qe
qt ) qe
=
k1 qe t
(4)
By the Pseudo-second-order, the graph was drawn between t/qt and t. The linear equation is defined as Eq. (5).
t 1 1 = + t qt qe K2 qe2
(3)
(5)
In this equation, qe and qt are the maximum adsorptions at the equilibrium and the amount of adsorbed cadmium at the time (t), respectively. Also, K1 (min−1), and K2 (g. mg−1 min−1) are pseudo-first-order and pseudo-second-order rate constant of the adsorption, respectively. In the Elovich model, the graph was drawn between qt and Lnt. The linear equation is defined as Eq. (6).
Based on Eq. (1), the amount of LOI for N-riched COF was obtained around 36%. So, it can be introduced as a self-quenching material. 3.3. Cadmium removal 3.3.1. The effect of pH and contact time on the cadmium removal The effect of pH on the cadmium removal was investigated in the range of 2–11 using a 5 mg of absorbent in a solution with 10 ppm concentration. Fig. 8a shows the summary of results. Notably, the absorbent surface charge which is depended on the pH variations of solution dramatically affects the absorption of metal ions. In the mildly acidic conditions, cadmium removal is ignorable due to the higher concentration of H3O+ than metal cations. The reason is that active sites of adsorption are occupied through the protonation of the nitrogen groups. By increasing the pH of solution an anionic form of COF creates [31,32], so active centers of adsorbent are accessible. Finally, maximum removal of cadmium (98%) was achieved at pH = 7 as the best chosen. In order to optimize the time of the reaction, the removal of cadmium was studied at different times in the range of 5–60 minutes under optimized pH = 7 and 80 ppm concentration. Based on the
qt =
Ln (hB ) Ln (t ) + B B
(6)
In this equation, α and B are the initial adsorption rate and desorption constant, respectively. For Intra-particle diffusion model, the graph was drawn between qt and t1/2. The linear equation is defined as Eq. (7). qt = Kintra(t)1/2 + C −1
(7) -1/2
Herein, Kintra (mgg . min ) refers to the intraparticle diffusion rate constant, and C gives information regarding the thickness of the fringe layer. The correlation parameters were calculated for the mentioned models and the results are collected in Table 1. According to the experimental results such as regression correlation coefficient R2 and qe, Pseudo-second-order kinetic was correctly
7
Journal of Environmental Chemical Engineering 7 (2019) 102907
M. Dinari, M. Hatami
Fig. 10. Equilibrium isotherm (a), values of RL (b), Freundlich model (c), D − R model (d), Temkin model (e), and Langmuir model (f) for removing of Cd(II) by Nriched COF at 298 K.
matched to the adsorption of Cd(II) which implies to the lack of dependence of absorption on the adsorbent concentration, while it depends on the number of active sites of adsorbent. In another word, when active sites are occupied by the adsorbate, the adsorption process stops.
3.3.3. Adsorption isotherms Several isotherm models such as Langmuir, Freundlich, DubininRadushkevich, and Temkin were applied to reveal the interaction of adsorbate and adsorbent (Fig. 10). The linear Langmuir and Freundlich adsorption isotherm equations
8
Journal of Environmental Chemical Engineering 7 (2019) 102907
M. Dinari, M. Hatami
in Table 2, N-riched COF is a strong adsorbent. The Dubbinin-Radashkuich isotherm equation is explained as Eq. (10):
Table 2 Isotherm parameters on the removal of Cd2+ at 298 K and optimized pH by Nriched COF. Models Langmuir Ce qe
=
1 KL qmax
+
Ce qmax
Freundlich
logqe = logKf +
1 n
* logce
Dubinin − Radushkevich (D-R)
lnqe = lnqm
R*T lnKt bt
N-riched COF
qmax (mgg−1) KL (Lmg−1) R2
396.7594 0.3978 0.9999
Kf (mgg n R2
−1
)
149.4153 3.4575 0.9179
B
2
qm -BD ×10−7/mol2∙kj−2 ED/kJ∙mol−1 R2
376.1254 5.1527 1.4145 0.9787
+
R*T lnCe bt
KT/L∙g−1 bT/kJ∙mol−1 BT R2
9.5264 3.5735 69.6348 0.9637
Temkin
qe =
Parameters
Ln qe = lnqm – Bε2
Also, the Temkin adsorption isotherm equation is expressed as Eq. (11):
qe =
log q e = log Kf +
(8)
1 *log c e n
R*T R*T lnKt + lnce bt bt
(11)
In order to understanding, the nature of adsorption between adsorbent and adsorbate, ED (KJ. mol−1) parameter was investigated. This parameter is obtained from D–R isotherm using the equation of ED1/ (2B)½. For the value of ED < 8 (KJ.mol−1), the superior adsorption process is considered as physical adsorption [35], whilst the value of ED in the range of ED = 8–20 (KJ.mol−1) refers to the chemical adsorption. Also, bT is the Temkin constant which is related to the heat of adsorption that is shown in Table 2. Herein, bT was found about of 3.5735 and is lower than 20 kJ.mol−1 which is in a good consistency of superior physical adsorption [35]. Moreover, the result of Ed which was measured experimentally is 1.4145 kJ.mol-1. This is related to the major adsorption which is concluded as physical adsorption. Table 3 shows the results of some previous reports that have been performed to the removal of Cd(II) in comparison with the current study. It is obvious that the synthesized N-riched COF is more favorable due to the high porosity which provides a high surface to absorb the metal ions. This is because of providing a high adsorption capacity with a rapid removal up to 20 min. Hence, COF can be an effective candidate in the removal of heavy metals.
are expressed as Eqs. (8), and (9), respectively:
Ce 1 Ce = + qe KL qmax qmax
(10)
(9)
In these equations, Ce (mgL−1) and qe (mgg−1) are equilibrium concentration of Cd(II) and the amount of adsorbed cadmium at equilibrium, respectively. The regression coefficient (R2) and qe were obtained for all mentioned isotherms and the results were collected in Table 2. The Langmuir model is properly fitted and the parameters such as RL, ED, BT, and n were calculated in order to show the favorability of the adsorption. KL (L/mg) and qmax (mgg−1) is the Langmuir constant rate and the maximum adsorption capacity, respectively. Also, Kf and 1/n are the Freundlich constants in which refer to the rate of adsorption and experiential parameter as favorability of the adsorption process, respectively. If n is obtained n > 1, it implies the favorable adsorption, and n = 0 displays the irreversible adsorption [33]. The dimensionless parameter of RL is explained by the equation of 1 RL = 1 + K C , which the value of RL demonstrates the acceptable adL 0 sorption for 0 > RL > 1, unfavorable for RL > 1, and irreversible RL = 0 [30,34]. Based on the parameter of n and KL which are reported
4. Conclusions In summary, COFs have demonstrated an excellent ability in removing heavy metals due to the unique features such as high porosity, stability, large surface area, as well as more accessible active sites. This study focused on assessing the adsorption efficiency of a novel N-riched crystalline COF adsorbent which has been prepared through the condensation of triazine and trialdehyde in the mixture of 1,4-dioxane/ mesitylene as a solvent. The XRD pattern showed a sharp peak at 3.4° which confirm the highly ordered and hexagonal structure in the synthesized COF. In addition, BET results display a high surface area of 1935 m2 g−1 with a pore size of 2.2 nm. The synthesized N-riched COF exhibited excellent performance in the cadmium removal with high adsorption capacity (396 mgg-1) at optimized conditions. Also, different kinetic models were investigated. The pseudo-second-order model was determined as the best fit model. Moreover, four equilibrium isotherms were studied and the Langmuir model was properly fitted. The N-riched COF has explained a high efficiency, cost-effective, environmental-
Table 3 Comparison of cadmium adsorption capacity of N-riched COF with different adsorbents. Adsorbents
Qmax (mg g−1)
T/K
pH
Reference
Poly(AN-co-AA) Functionalized Fe3O4 magnetic nanoparticles Chitosan templated synthesis of mesoporous silica Chelating Poly acrylonitrile Magnetic LDH/guargum bionanocomposites N-riched COF
20 55 4 156 258 396
298 298 298 298 298 298
9 10 8 7 10 7
[31] [32] [36] [37] [38] This work
9
Journal of Environmental Chemical Engineering 7 (2019) 102907
M. Dinari, M. Hatami
friendliness properties as well as demonstrating more advantages such as easy manufacture and absence of by-products. So, it can be as a promising adsorbent in the environmental pollution cleanup.
[19] T. Inaba, E. Kobayashi, Y. Suwazono, M. Uetani, M. Oishi, H. Nakagawa, K. Nogawa, Estimation of cumulative cadmium intake causing Itai–itai disease, Toxicol. Lett. 159 (2005) 192–201. [20] D. Sud, G. Mahajan, M. Kaur, Agricultural waste material as a potential adsorbent for sequestering heavy metal ions from aqueous solutions–a review, Bioresour. Technol. 99 (2008) 6017–6027. [21] E. Da’na, Adsorption of heavy metals on functionalized-mesoporous silica: a review, Microporous Mesoporous Mater. 247 (2017) 145–157. [22] B. Kebede, N. Retta, V. Raju, Y. Chebude, Synthesis and Characterization of 2, 4, 6tris (hydrazino)-s-triazine and its Metal Complexes, Trans. Met. Chem. 31 (2006) 19–26. [23] M.M. Naseer, D.-X. Wang, L. Zhao, Z.-T. Huang, M.-X. Wang, Synthesis and functionalization of heteroatom-bridged bicyclocalixaromatics, large molecular triangular prisms with electroN-riched and-deficient aromatic interiors, J. Org. Chem. 76 (2011) 1804–1813. [24] M.A. Aly, Infrared and Raman spectra of some symmetric azines, Spectrochim. Acta A 55 (1999) 1711–1714. [25] O. Buyukcakir, S.H. Je, S.N. Talapaneni, D. Kim, A. Coskun, Charged covalent triazine frameworks for CO2 capture and conversion, ACS Appl. Mater. Interfaces 9 (2017) 7209–7216. [26] T. Ben, K. Shi, Y. Cui, C. Pei, Y. Zuo, H. Guo, D. Zhang, J. Xu, F. Deng, Z. Tian, Targeted synthesis of an electroactive organic framework, J. Mater. Chem. 21 (2011) 18208–18214. [27] L. Xu, S.-Y. Ding, J. Liu, J. Sun, W. Wang, Q.-Y. Zheng, Highly crystalline covalent organic frameworks from flexible building blocks, Chem. Commun. 52 (2016) 4706–4709. [28] W. Liu, Q. Su, P. Ju, B. Guo, H. Zhou, G. Li, Q. Wu, A hydrazone‐based covalent organic framework as an efficient and reusable photocatalyst for the cross‐dehydrogenative coupling reaction of N‐aryltetrahydroisoquinolines, ChemSusChem 10 (2017) 664–669. [29] D.W. van Krevelen, Properties of Polymers: Their Estimation and Correlation with Chemical Structure, Elsevier Scientific Pub., Amsterdam, 1976. [30] M.H. Dindar, M.R. Yaftian, S. Rostamnia, Potential of functionalized SBA-15 mesoporous materials for decontamination of water solutions from Cr (VI), As (V) and Hg (II) ions, J. Environ. Chem. Eng. 3 (2015) 986–995. [31] N.A.M. Zahri, S.N.A.M. Jamil, L.C. Abdullah, S.J. Huey, T.C.S. Yaw, M.N. Mobarekeh, N.S.M. Rapeia, Equilibrium and kinetic behavior on cadmium and lead removal by using synthetic polymer, J. Water Process. Eng. 17 (2017) 277–289. [32] V. Devi, M. Selvaraj, P. Selvam, A.A. Kumar, S. Sankar, K. Dinakaran, Preparation and characterization of CNSR functionalized Fe3O4 magnetic nanoparticles: an efficient adsorbent for the removal of cadmium ion from water, J. Environ. Chem. Eng. 5 (2017) 4539–4546. [33] T.A. Arica, E. Ayas, M.Y. Arica, Magnetic MCM-41 silica particles grafted with poly (glycidylmethacrylate) brush: modification and application for removal of direct dyes, Microporous Mesoporous Mater. 243 (2017) 164–175. [34] D.F. Apopei, M.V. Dinu, A.W. Trochimczuk, E.S. Dragan, Sorption isotherms of heavy metal ions onto semi-interpenetrating polymer network cryogels based on polyacrylamide and anionically modified potato starch, Ind. Eng. Chem. Res. 51 (2012) 10462–10471. [35] M. Dinari, G. Mohammadnezhad, R. Soltani, Fabrication of poly (methyl methacrylate)/silica KIT-6 nanocomposites via in situ polymerization approach and their application for removal of Cu 2+ from aqueous solution, RSC Adv. 6 (2016) 11419–11429. [36] D. Tiwari, S.M. Lee, Chitosan templated synthesis of mesoporous silica and its application in the treatment of aqueous solutions contaminated with cadmium (II) and lead (II), Chem. Eng. J. 328 (2017) 434–444. [37] P. Bhunia, S. Chatterjee, P. Rudra, S. De, Chelating polyacrylonitrile beads for removal of lead and cadmium from wastewater, Sep. Purif. Technol. 193 (2018) 202–213. [38] M. Dinari, R. Tabatabaeian, Ultra-fast and highly efficient removal of cadmium ions by magnetic layered double hydroxide/guargum bionanocomposites, Carbohydr. Polym. 192 (2018) 317–326.
Acknowledgment We are grateful for the financial support from the Research Council of Isfahan University of Technology (IUT), Iran. References [1] S.-Y. Ding, W. Wang, Covalent organic frameworks (COFs): from design to applications, Chem. Soc. Rev. 42 (2013) 548–568. [2] B.J. Smith, W.R. Dichtel, Mechanistic studies of two-dimensional covalent organic frameworks rapidly polymerized from initially homogenous conditions, J. Am. Chem. Soc. 136 (2014) 8783–8789. [3] Z. Li, X. Feng, Y. Zou, Y. Zhang, H. Xia, X. Liu, Y. Mu, A 2D azine-linked covalent organic framework for gas storage applications, Chem. Commun. 50 (2014) 13825–13828. [4] X. Feng, X. Ding, D. Jiang, Covalent organic frameworks, Chem. Soc. Rev. 41 (2012) 6010–6022. [5] S. Kandambeth, A. Mallick, B. Lukose, M.V. Mane, T. Heine, R. Banerjee, Construction of crystalline 2D covalent organic frameworks with remarkable chemical (acid/base) stability via a combined reversible and irreversible route, J. Am. Chem. Soc. 134 (2012) 19524–19527. [6] A.P. Cote, A.I. Benin, N.W. Ockwig, M. O’keeffe, A.J. Matzger, O.M. Yaghi, Porous, crystalline, covalent organic frameworks, Science 310 (2005) 1166–1170. [7] R.W. Tilford, W.R. Gemmill, H.-C. zur Loye, J.J. Lavigne, Facile synthesis of a highly crystalline, covalently linked porous boronate network, Chem. Mater. 18 (2006) 5296–5301. [8] H.M. El-Kaderi, J.R. Hunt, J.L. Mendoza-Cortés, A.P. Côté, R.E. Taylor, M. O’keeffe, O.M. Yaghi, Designed synthesis of 3D covalent organic frameworks, Science 316 (2007) 268–272. [9] K.T. Jackson, T.E. Reich, H.M. El-Kaderi, Targeted synthesis of a porous borazinelinked covalent organic framework, Chem. Commun. 48 (2012) 8823–8825. [10] F.J. Uribe-Romo, C.J. Doonan, H. Furukawa, K. Oisaki, O.M. Yaghi, Crystalline covalent organic frameworks with hydrazone linkages, J. Am. Chem. Soc. 133 (2011) 11478–11481. [11] H. Ma, H. Ren, S. Meng, Z. Yan, H. Zhao, F. Sun, G. Zhu, A 3D microporous covalent organic framework with exceedingly high C3H8/CH4 and C2 hydrocarbon/CH4 selectivity, Chem. Commun. 49 (2013) 9773–9775. [12] X. Wang, X. Han, J. Zhang, X. Wu, Y. Liu, Y. Cui, Homochiral 2D porous covalent organic frameworks for heterogeneous asymmetric catalysis, J. Am. Chem. Soc. 138 (2016) 12332–12335. [13] S.-Y. Ding, M. Dong, Y.-W. Wang, Y.-T. Chen, H.-Z. Wang, C.-Y. Su, W. Wang, Thioether-based fluorescent covalent organic framework for selective detection and facile removal of mercury (II), J. Am. Chem. Soc. 138 (2016) 3031–3037. [14] Q. Fang, J. Wang, S. Gu, R.B. Kaspar, Z. Zhuang, J. Zheng, H. Guo, S. Qiu, Y. Yan, 3D porous crystalline polyimide covalent organic frameworks for drug delivery, J. Am. Chem. Soc. 137 (2015) 8352–8355. [15] J.W. Crowe, L.A. Baldwin, P.L. McGrier, Luminescent covalent organic frameworks containing a homogeneous and heterogeneous distribution of dehydrobenzoannulene vertex units, J. Am. Chem. Soc. 138 (2016) 10120–10123. [16] N. Huang, L. Zhai, H. Xu, D. Jiang, Stable covalent organic frameworks for exceptional mercury removal from aqueous solutions, J. Am. Chem. Soc. 139 (2017) 2428–2434. [17] Q. Sun, B. Aguila, J. Perman, L.D. Earl, C.W. Abney, Y. Cheng, H. Wei, N. Nguyen, L. Wojtas, S. Ma, Postsynthetically modified covalent organic frameworks for efficient and effective mercury removal, J. Am. Chem. Soc. 139 (2017) 2786–2793. [18] B.A. Fowler, Monitoring of human populations for early markers of cadmium toxicity: a review, Toxicol. Appl. Pharmacol. 238 (2009) 294–300.
10