Fabrication of a Magnetic Cellulose Nanocrystal ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10447-10458

Fabrication of a Magnetic Cellulose Nanocrystal/Metal−Organic Framework Composite for Removal of Pb(II) from Water Nan Wang,† Xiao-Kun Ouyang,*,† Li-Ye Yang,† and Ahmed Mohamed Omer†,‡ †

School of Food and Pharmacy, Zhejiang Ocean University, Haida Nan Load 1st, Lincheng Street, Zhoushan 316022, P. R. China Polymer Materials Research Department, Advanced Technology and New Materials Research Institute, SRTA-City, New Borg El-Arab City, P.O. Box: 21934, Alexandria, Egypt

‡

ABSTRACT: A novel composite material (MCNC@Zn-BTC) is synthesized from magnetic cellulose nanocrystals (MCNC) and a metal−organic framework (MOF), based on Zn(II) and benzene-1,3,5-tricarboxylic acid (Zn-BTC), using a simple mechanical agitation method at room temperature. The developed MCNC@Zn-BTC was characterized by SEM, TEM, FTIR, XRD, XPS, and VSM instruments. The impact of MCNC@Zn-BTC dose, initial Pb(II) concentration, contact time, pH, and adsorption temperature have been explored. Results demonstrated that only 30 min was needed to achieve the adsorption equilibrium; the maximum adsorption capacity was 558.66 mg/g at 298.2 K. Besides, data obtained were fitted to a Langmuir isotherm model (R2 = 0.9890), and the adsorption kinetics followed a pseudo-second-order model. In addition, the removal (%) of Pb(II) remained greater than 80% after five recycled times. Therefore, the novel MCNC@Zn-BTC adsorbent displays great potential to improve the quality of environmental water contaminated with heavy metals. KEYWORDS: Pb(II), Metal−organic framework, Cellulose nanocrystals, Fe3O4, Adsorption

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INTRODUCTION Pb(II) is a very stable, nondegradable, and persistent pollutant that accumulates in the environment predominantly as a result of discharges from nonferrous metal smelting, printing, dyeing, and other industrial processes.1,2 Exposure to lead(II) ions can cause damage to the nervous, digestive, renal, cardiovascular, and endocrine systems.3,4 The increasingly serious problem of lead pollution, especially in aqueous environments, necessitates the development of effective and highly efficient adsorbents that would facilitate lead removal. Methods for lead ion remediation include ion exchange,5 precipitation,6 adsorption,7 and biosorption.8 Among these, adsorption is considered to be the most efficient, economical, and simplest method for removal of Pb(II) from water. Recently, a number of new adsorbents have been assessed for their potential to remove lead, including nanostructured adsorbents9,10 and microorganism-based adsorbents.11 Metal−organic frameworks (MOFs)12 are porous crystalline materials that are usually constructed from inorganic metal ions (or metal clusters) and organic ligands that interact to form an infinite network structure. MOFs have received significant attention as a result of their high specific surface area13 and chemical and thermal stabilities.14 Changes in the lengths and dimensions of ligands can be exploited for the fabrication of MOFs with tailored pore-structure and pore-size characteristics. Recent modifications of MOFs have expanded their applicationsfor example, MOFs have been modified successfully by ligand exchange or by grafting of active functional group handles.15,16 MOFs have also been used as coatings and composites for various materials.17 Their many excellent © 2017 American Chemical Society

characteristics have led to MOFs being used widely in gas storage applications,18 drug delivery,19 catalysis,20 and adsorption.21 MOFs have shown favorable adsorption of species including heavy metals and drugs.22,23 Filtration and centrifugation are the currently most widely used methods for the recovery of MOFs. Their relatively slow speed, high cost, and inconvenient procedures, however, limit the large-scale application of MOFs. Therefore, the preparation of an easily separable MOF material will be essential for its widespread application in the future. The previously reported MOF ZnBTC, prepared from zinc acetate dihydrate and 1,3,5benzenetricarboxylic acid (BTC), is rich in carboxyl groups, which are effective for Pb(II) adsorption. Magnetic nanoparticles (nano-Fe3O4) can be dispersed easily and uniformly in a liquid phase, whether aqueous or organic, with simple oscillation or agitation. Because of their high saturation magnetization, they can be separated from the liquid phase by direct application of an external magnetic field, thus avoiding other cumbersome operating steps such as filtration and centrifugation. Conveniently, these nanoparticles can also be reused. Although the synthesis of nano-Fe3O4 is relatively simple, magnetic dipole interactions can result in significant particle aggregation, which limits their application.24 To address this deficiency, nano-Fe3O4 has been modified with carbon nanotubes,25 graphene,26 chitosan,27 and cellulose nanocrystals.28 These magnetic composite materials are usually prepared by Received: July 21, 2017 Revised: September 22, 2017 Published: October 5, 2017 10447

DOI: 10.1021/acssuschemeng.7b02472 ACS Sustainable Chem. Eng. 2017, 5, 10447−10458

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ACS Sustainable Chemistry & Engineering

Zn-BTC was obtained by vacuum freeze-drying. The route for the synthesis of MCNC@Zn-BTC was shown in Scheme 1.

chemical coprecipitation and in situ polymerization methods.29,30 Cellulose nanocrystals (CNC) are environmentally friendly materials that can be extracted from wood and annual plants and their residues. They retain the basic structure of cellulose and possess characteristics such as large specific surface area, high crystallinity, and high specific strength.31 CNC have received significant attention in various applications, where they have been utilized as drug carriers, scaffolds, separators, flocculants, and adsorbents. Our laboratory has focused on the modification of CNC for the adsorption of heavy metals such as Cr(VI) and Pb(II).32−34 Previously, we have shown that magnetic carboxylated cellulose nanocrystals (MCNC) display good capacity for removal of Pb(II) from water. Thus, CNC are an ideal substrate for modification with magnetic nanoparticles for efficient Pb(II) removal. In this work, the adsorption properties of magnetic cellulose nanocrystals (MCNC) toward Pb(II) are evaluated. However, their low adsorption capacity for Pb(II) (e.g., 92.24 mg g−1 at the concentration of 0.2 g L−1) and long adsorption time (4 h) require improvement before the sorbent can be applied more widely. Given the excellent adsorption performance of MOFs, we sought to develop a simple method for combining magnetic nanoparticles, CNC, and MOFs to produce a more efficient adsorbent for heavy metals. Herein, we synthesized Zn-BTC on the surface of MCNC and used this new adsorbent to remove Pb(II) from water. The findings show that the adsorption capacity was significantly improved relative to that of MCNC in isolation.

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Scheme 1. Route of Synthesis MCNC@Zn-BTC

Characterization. Scanning electron microscopy (SEM, S4800, Hitachi, Japan) was applied to examine the surface morphology of the materials, along with transmission electron microscopy (TEM, JEM2100, Lorentz, Japan). A Tensor II Fourier-transform infrared (FTIR) spectrometer (Bruker, Germany) was used to obtained FTIR spectra. A vibrating sample magnetometer (SQUID-VSM, Quantum Design, USA) was used to evaluate the magnetic properties of the prepared composite. X-ray photoelectron spectroscopy (XPS, AXIS ULTRA DLD, Shimadzu, Japan) was used to analyze the surface elements of the materials. X-ray diffraction (XRD) was analyzed with a D8 Advance Xray diffractometer from Bruker, and N2 adsorption−desorption isotherm data were observed by using the Brunauer−Emmett−Teller method. Adsorption Experiments. Pb(NO3)2 (0.8 g) was dissolved in deionized water (500 mL) to afford a Pb(II) concentration of 1000 mg/g. This solution was diluted to the desired concentration for subsequent experiments. Different doses of MCNC@Zn-BTC (5−20 mg) were added to the Pb(II) solutions (20 mL, 200 mg/L) in 100 mL conical flasks. The pH of Pb(II) solution was adjusted where required from 2 to 6 using HNO3 (0.1 mol/L) and NaOH (0.1 mol/L). The flasks containing the adsorbents and Pb(II) solutions were shaken at 150 rpm, and the temperature was maintained at 298.2 K during adsorption. Once the adsorption was completed, a magnetic field was applied to separate MCNC@Zn-BTC from solution. The Pb(II) concentration after adsorption was measured by atomic absorption spectrophotometry (AA-7000, Shimadzu). The equilibrium adsorption capacity (qe, mg/g) of MCNC@Zn-BTC was calculated using eq 1

EXPERIMENTAL SECTION

Materials. FeCl2·4H2O, FeCl3·6H2O, zinc acetate dihydrate, and benzene-1,3,5-tricarboxylic acid (C9H6O6, BTC) were purchased from Aladdin Chemical Reagent Co., Ltd. (Shanghai, China). CNC were provided by Chemkey Advanced Materials Technology Co., Ltd. (Shanghai, China). Ethanol, N,N-dimethylformamide (DMF), ammonia solution, methanol, and triethylamine (Et3N) were purchased at the purest grade available. Synthesis Procedures. Synthesis of MCNC. Based on our previous method,35 different weights of CNC (0.125−1.0 g) were dispersed in deionized water (80 mL), and in each case, the resulting solution was purged with N2 gas at 75 °C with constant mechanical stirring. After 30 min, FeCl3·6H2O (2.4 g) and FeCl2·4H2O (0.9 g) were added to the various solutions of CNC, and NH3·H2O was used to adjust the pH to 9−10. The mixtures were stirred for 1 h under nitrogen atmosphere. The precipitate was separated using a magnetic field, then washed with deionized water and ethyl alcohol for three times alternately. Finally, the sample of target MCNC was dried by vacuum freeze-drying. Fe3O4 (1.0 g) was obtained using the above-described process in the absence of CNC. Synthesis of Zn-BTC. Zn-BTC, known for its excellent adsorption properties,36 was synthesized according to the reported method with a slight modification.37 BTC (0.53 g, 2.5 mmol) and Et3N (2.7 mL) were dissolved in DMF (25 mL). Separately, zinc acetate dihydrate (0.72 g, 3.3 mmol) was also dissolved in DMF at the same volume. The two solutions were mixed and stirred for 2.5 h. The product was recovered by centrifugation and washed twice with DMF and twice more with methanol. Finally, Zn-BTC was dried at 70 °C in an oven to yield the target dry product (0.9 g). Synthesis MCNC@Zn-BTC. BTC (0.53 g, 2.5 mmol) and Et3N (2.7 mL) were dissolved in DMF (25 mL). Separately, zinc acetate dihydrate (0.72 g, 3.3 mmol) was also dissolved in DMF (25 mL) before MCNC (1 g) were added. After mechanical agitation for 30 min, these two solutions were combined, and the mixed solution was stirred for 2.5 h at 298.2 K. Thereafter, MCNC@Zn-BTC was separated magnetically and washed alternately twice with DMF and methanol before the MCNC@

qe =

(C0 − Ce)V m

(1)

where m (g) is the dose of MCNC@Zn-BTC, V (L) is the volume of the Pb(II) solution, C0 (mg/L) is the initial concentration of Pb(II), and Ce (mg/L) is the Pb(II) concentration in solution when the adsorption achieved equilibrium. The adsorption capacity (qt, mg/g) of MCNC@Zn-BTC for Pb(II) at a certain time was calculated using eq 2 qt =

(C0 − Ct )V m

(2)

where Ct (mg/L) is the concentration of Pb(II) at a certain time. The removal ratio (R) of MCNC@Zn-BTC for Pb(II) was calculated using eq 3 10448


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ACS Sustainable Chemistry & Engineering R=

C0 − Ce × 100% C0

TEM Analysis. The TEM images of CNC, MCMC, and MCNC@Zn-BTC are shown together in Figure 2. The CNC surface (Figure 2a) contains a large number of hydroxyl groups, which can interact together through hydrogen bonding and thus result in the aggregation of cellulose rods into spherical shapes.38 Comparison of Figure 2a with b, which represents the state after modification of CNC with Fe3O4, revealed that the agglomerated CNC have been dispersed. From Figure 2c and d, the average length of Zn-BTC is around 800 nm, and it is apparent that MCNC were attached to the surface of prismatic Zn-BTC, thus demonstrating that the fabrication of the target MCNC@ZnBTC was successful. FTIR Analysis. The structures of the prepared materials were characterized by FTIR. The bands at 3415 cm−1 in Figure 3a-2 correspond to the O−H stretching vibration modes. The band at 2902 cm−1 can attributed to the C−H stretching vibration modes of CNC, and those bands (1160 and 1063 cm−1) are assigned to the C−C bond vibrations.33 The peak at 581 cm−1 (Figure 3a-3) in the spectrum of MCNC corresponds to the Fe−O stretching vibration and is characteristic for Fe3O4.39 The peaks observed in the spectrum of Zn-BTC in the ranges of 1618−1539 and 1437− 1370 cm−1 (Figure 3b-4) are the asymmetric and symmetric carbonyl stretching vibrations of BTC,40 respectively. The characteristic peaks of BTC at 1618−1370 cm−1 also appeared in Figure 3b-5, indicating that Zn-BTC was successfully attached to the MCNC surface. The O−H stretching vibration caused a broad band at 3415 cm−1, indicating that the hydroxyl groups existed in this material. Figure 3b-6 shows that the peaks at 3415, 1618, and 1370 cm−1 have decreased in intensity after the loading of MCNC@Zn-BTC with Pb(II). This outcome is attributed to complexation of the carboxyl and hydroxyl groups with lead ions. The surfaces of Fe3O4 and CNC are rich in carboxyl groups, and Zn-BTC also contains a large number of these groups. The oxygen atoms present in carboxylic and hydroxyl groups possess free pairs of electrons, which allow them to interact with the empty orbital of Pb(II) and form complexes mediated by coordinative bonds. XRD Analysis. The XRD patterns for Zn-BTC and MCNC@ Zn-BTC are shown in Figure 4. Figure 4-2 shows that Fe3O4 displays six peaks at 2θ = 30.2°, 35.5°, 43.1°, 53.6°, 57.1°, and 62.6°, which are characteristic of Fe3O4.41 Additionally, a characteristic peak for CNC appeared at 15.7°, which corresponds to the (101) reflection and represents the typical diffraction pattern of type I cellulose.42 The peak characteristic for Zn-BTC at 10.9°,43 observed in Figure 4-1, appeared also in the pattern observed for MCNC@Zn-BTC after the Zn-BTC was coated on the MCNC surface (Figure 3-2). These results confirm that both the Zn-BTC crystal structure and the magnetic material are well preserved in the composite product. Vibrating Sample Magnetometry Analysis. The magnetic properties of MCNC and MCNC@Zn-BTC were analyzed by vibrating sample magnetometry (VSM). Figure 5 shows the magnetic hysteresis loops obtained for these two materials. The saturation magnetization of MCNC reached 61.69 emu/g. This value decreased to 41.42 emu/g after coating with Zn-BTC. These results indicate that the adsorbent is suitable for fast separation under the application of a magnetic field. As shown in Figure 4, the adsorbent was separated within 30 s by an external magnetic field after a vigorous shake. XPS Analysis. The survey XPS spectrum (Figure 6a) shows that the adsorbent contains elements Zn, C, O, and Fe. Additionally, an apparent Pb(II) peak appeared in the postadsorption wide-scan spectrum, thus indicating that the

(3)

Regeneration Tests. HCl (0.1 mmol/L) was used as the eluent in regeneration tests. Spent, Pb(II)-loaded MCNC@Zn-BTC (20 mg) was added to the eluent (20 mL) and shaken for 4 h. Subsequently, MCNC@Zn-BTC was separated by application of a magnetic field. The separated MCNC@Zn-BTC was washed four times with deionized water. The above operation was repeated four times, after which no Pb(II) could be detected in the HCl solution (0.1 mmol/L). After desorbing Pb(II), the MCNC@Zn-BTC was freeze-dried under vacuum before its reuse in five successive adsorption experiments. The adsorption capacity was evaluated after each cycle.

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RESULTS AND DISCUSSION Synthesis of MCNC@Zn-BTC. During the synthesis of MCNC, the quantity of CNC (0.125−1.0 g) was varied in order to study the effect of the dose of CNC on the adsorption capacity. When this ratio of Fe3O4 and CNC is determined, in the process of coating MCNC with Zn-BTC, different mass ratios of MCNC and Zn-BTC were utilized in order to determine their optimal proportions. The concentration of Pb(II) (0.3 g/L) was applied. The volume was always 20 mL, and the amount of adsorbent was maintained at 20 mg. The binding rate (BR) was used to study the ratio of the quantities of CNC and Zn-BTC that bind the adsorbent after addition m BR = × 100% (4) M where M (g) is the actual quantity of CNC and Zn-BTC added, and m (g) is the amount of binding CNC and Zn-BTC. The results of adsorption capacity tests performed for the various materials are shown in Table 1, demonstrating that the optimum ratio of Fe3O4:CNC was 4:1 and that of MCNC:Zn-BTC was 10:9. Table 1. Effect of Mass Ratio of Fe3O4, CNC and Zn-BTC on Adsorption Capacity (at Pb(II) concentration of 300 mg/L at 298.2 K) Fe3O4:CNC (m/m)

BR (%)

qe (mg/g)

MCNC:Zn-BTC (m/m)

BR (%)

qe (mg/g)

8:1 4:1 2:1 1:1

99.83 99.16 98.09 97.33

93.06 120.74 122.09 123.38

20:9 10:9 5:9 3:9

68.32 66.67 33.35 19.83

213.62 266.90 267.17 265.33

SEM Analysis. The CNC surface was coated with spherical particles (Figure 1a) that were partially aggregated. In addition, few triangular or quadrangular structures were observed on the CNC surface. The SEM image of Zn-BTC (Figure 1b) reveals perfect prismatic pillar-like crystals. After treatment with the iron reagents, irregularly shaped particles and roughened surfaces were observed on the surface of MCNC (Figure 1c). This observation is attributed to the combination of Fe3O4 aggregates with the surface of the CNC layer. Finally, after coating with ZnBTC, pillar-like crystals were observed on the surface of MCNC (Figure 1d), thus indicating that the coating with Zn-BTC was successful. The energy-dispersive X-ray spectra (EDX) in Figure 1f shows the distribution of Fe, Zn, O, and C throughout the MCNC@Zn-BTC composite. As shown in Figure 1f, the Fe/O ratio (1.6) in MCNC@Zn-BTC is lower than that in Fe3O4 (2.6), which is attributed to the increase in oxygen content derived from CNC and Zn-BTC, suggesting the formation of MCNC@Zn-BTC. 10449


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Figure 1. SEM images of (a) CNC, (b) Zn-BTC, (c) MCNC, (d) MCNC@Zn-BTC, and SEM images and EDX (e and f) of MCNC@Zn-BTC.

Particle size distribution. Figure 7b shows the size distribution of MCNC@Zn-BTC which measured by zatameter, and the average particle size is 979 nm, close to the result of TEM. Comparison of Adsorption Capacity. As shown in Figure 8a, the adsorption times required for Fe3O4, MCNC, M@ZnBTC, and MCNC@Zn-BTC to reach equilibrium were 120, 180, 30, and 30 min, respectively. It was indicated that the addition of Zn-BTC was beneficial to the increase in the adsorption rate. The qe reached 177.27 ± 0.74 mg/g after Fe3O4 was modified with Zn-BTC, but the removal ratio was only 88.67%. After the MCNC were coated with Zn-BTC, the qe of MCNC@Zn-BTC increased to 198.18 ± 0.54 mg/g and the removal ratio exceeded 99.10%; i.e., Pb(II) was almost completely removed by this absorbent. Thus, the novel combination of Fe3O4, MCNC, and Zn-BTC to form MCNC@Zn-BTC shows significant promise as an ideal adsorbent for Pb(II) removal from water. In order to establish the conditions that would allow optimum absorption of Pb(II), the effects of changing the adsorbent dose, Pb(II) initial concentration, adsorption time, pH, and temperature were evaluated.

adsorbent has taken up Pb(II) efficiently. The binding energies at 724.2 and 710.6 eV correspond to Fe 2p1/2 and Fe 2p3/2.44 Although a satellite peak typical of Fe2O3 is usually observed near 714 eV,45 this peak was not observed in Figure 5c as a result of its overlap with a peak arising from Fe2+. Thus, the XPS spectrum supports the formation of Fe3O4 in the adsorbent.46 The appearance of the Pb 4f 7/2 peak at 138.8 eV after adsorption (Figure 6b) confirms that MCNC@Zn-BTC can adsorb Pb(II) successfully.47 BET Analysis. As shown in Figure 7a, the N2 adsorption− desorption isotherms of MCNC@Zn-BTC show the first inflection point at P/P0 < 0.1, indicating the completion of the adsorption of monomolecular layers. As the pressure increases, the adsorption of a second layer occurs. In the saturated vapor pressure, the number of adsorption layers is infinite. These types of adsorption isotherms are type II isotherms. The surface area and pore volume of MCNC@Zn-BTC is 65.10 m2/g according to the multipoint BET (Brunauer−Emmett−Teller) method. The pore diameter and volume calculated by the Barrett− Joyner−Halenda (BJH) method are 3.82 nm and 0.49 cm2/g, respectively. 10450


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Figure 2. TEM images of (a) CNC, (b) Zn-BTC, (c) MCNC, and (d) MCNC@Zn-BTC.

Figure 3. FTIR spectra of (a) Fe3O4 (1), CNC (2), and MCNC (3) and (b) Zn-BTC (4), MCNC@Zn-BTC (5), and Pb(II)-loaded MCNC@Zn-BTC (6).

Figure 5. VSM magnetization curves for (1) MCNC and (2) MCNC@ Zn-BTC. Inset images show that the adsorbent was separated by an external magnetic field.

Figure 4. XRD spectra of (1) Zn-BTC and (2) MCNC@Zn-BTC.

Effect of MCNC@Zn-BTC Dose. In order to examine the effect of MCNC@Zn-BTC concentration, the dose of the adsorbent was increased from 5 to 20 mg, and the volume of 10451


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Figure 6. XPS spectra of MCNC@Zn-BTC before and after adsorption: (a) wide scan, (b) Pb 4f, and (c) Fe 2p spectra.

Pb(II) solution (200 mg/L) was kept fixed at 20 mL. The adsorption capacity was calculated after the adsorption reached equilibrium, and the results are shown in Figure 9. As the

Figure 9. Influence of MCNC@Zn-BTC dose on adsorption capacity (at Pb(II) concentration of 200 mg/L at 298.2 K). Figure 7. N2 adsorption−desorption isotherms for (a) MCNC@ZnBTC. (b) Size distribution of MCNC@Zn-BTC.

adsorbent dose increased, qe decreased from 396.07 ± 2.09 to 198.19 ± 0.54 mg/g. When the adsorbent dose was lower, the

Figure 8. Comparison of the adsorption rate (a) adsorption capacities and removal rates (b) for the various adsorbents (at Pb(II) concentration of 200 mg/L at 298.2 K). 10452


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Figure 10. (a) Influence of the C0 on the adsorption capacity at 298.2 K. Fitted curves obtained for (b) the Langmuir and (c) the Freundlich isotherm models for the adsorption of Pb(II) by MCNC@Zn-BTC.

multilayer adsorption processes.49 RL is a dimensionless separation factor (eq 7) that is used to predict the adsorption behavior of [email protected]

surface of the adsorbent was surrounded by adequate quantity of Pb(II) ion, and thus, the adsorption sites were fully occupied. As the amount of adsorbent increased, the presence of excess adsorption sites for Pb(II) resulted in a lower adsorption capacity. The removal ratio approached 100% as the amount of adsorbent was increasedwhen the adsorbent dose reached 1.0 mg/mL, the removal ratio was 99.09 ± 0.27%. Taking into consideration the removal ratio, the adsorbent dose of 1.0 mg/ mL was adopted in the subsequent adsorption experiments. Effect of Pb(II) Concentration. To investigate the influence of the initial concentration of Pb(II) on the adsorption capacity, the concentrations of the Pb(II) solution from 10 to 800 mg L−1 were used to observe the adsorption isotherm, as shown in Figure 10a. The qe of MCNC@Zn-BTC increased from 10 to 502 ± 1.38 mg g−1 with increasing C0. The amount of adsorbent exceeded that of the metal ions at lower Pb(II) concentrations, and thus, Pb(II) was removed completely at C0 values below 200 mg L−1. When the concentration of Pb(II) was higher, the adsorption sites could still easily capture the metal ions, but the rate of increase in the adsorption capacity gradually decreased. This reduction occurred as a result of the decrease in the amount of unoccupied adsorption sites as the C0 increased. The adsorption ratio decreased from nearly 100% to 62.76 ± 0.17%. Taking into consideration both the adsorption capacity and removal ratio, the 200 mg L−1 Pb(II) solution was used for all subsequent experiments. Equilibrium adsorption isotherms can help explain the observed adsorption behavior. Adsorption equilibrium data for Pb(II) removed by MCNC@Zn-BTC were evaluated by fitting to both Langmuir and Freundlich isotherm models. The Langmuir isotherm model (eq 5) can explain monolayer adsorption, the same or equivalent adsorption without lateral interactions or spatial/steric hindrance. This model can explain monolayer adsorption, i.e., the same or equivalent adsorption without lateral interactions or spatial/steric hindrance.48 By contrast, the Freundlich isotherm model (eq 6) is used to describe reversible adsorption, and it can be applied to explain

Ce C 1 = + e qe bqm qm log qe = log KF +

RL =

(5)

1 log Ce n

(6)

1 1 + bC0

(7)

In these equations, qm (mg/g) is the maximum adsorption capacity about removing Pb(II) by MCNC@Zn-BTC, n is the Freundlich constant about the adsorption intensity, and b and KF (mg/g) are Langmuir and Freundlich constant, respectively, about the adsorption capacity. Figures 10b and c show the curves obtained from the adsorption data fitted by the Langmuir model and the Freundlich model. Table 2 shows corresponding Table 2. Parameters of Langmuir and Freundlich Models for Pb(II) Removal by MCNC@Zn-BTC Langmuir isotherm T (K) 298.2

qm (mg/g) 558.66

b (L/mg) 0.05

Freundlich isotherm R

2

0.9890

KF −5

1 × 10

n

R2

0.37

0.8205

parameters. Comparison of the fitted curves and the data reveals that R2 of the Langmuir isotherm model was considerably higher (0.9890) than the R2 of the Freundlich isotherm (0.8205). So the adsorption process conforms to the Langmuir isotherm model. The qm fitted by the Langmuir isotherm was 558.66 mg g−1 at room temperature. The value of b obtained from the Langmuir model was greater than 0, so the theoretical saturated adsorption capacity exceeds the experimental value. Table 3 shows that the positive values of RL, determined for different initial concen10453


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ACS Sustainable Chemistry & Engineering Table 3. RL Values for Pb(II) Removal by MCNC@Zn-BTC Based on the Langmuir Model C0(mg/L)

RL

10 50 100 200 300 500 800

0.67 0.29 0.17 0.091 0.063 0.038 0.024

t 1 t = + 2 qt qe k 2qe

In these equations, k1 (min−1) is the rate constant for the pseudo-first-order model, and k2 (g/mg min) is the rate constant for the pseudo-second-order model. Figure 11b and c show the fitted curves; these parameters pertaining to the two formulas are given in Table 4. Table 4. Parameters of Kinetic Models Examined for Pb(II) Removal by MCNC@Zn-BTC Pseudo-first-order model

trations based on the Langmuir model, are less than 1, thus showing that this adsorption is favorable.51 The value of n obtained from the Freundlich isotherm model was between 1 and 10, which further corroborates that this adsorption process is favorable in terms of Pb(II) removal. Effect of Adsorption Time. The adsorption time varied from 5 to 70 min was used to analyze the influence of adsorption time on the adsorption capacity of MCNC@Zn-BTC. The adsorbent loading was always 20 mg, and the concentration and dosage of Pb(II) were 200 mg/mL and 20 mL, respectively. The adsorption capacities, measured over the different contact times, are shown in Figure 11a. The results show that value of qt increased rapidly during the first 5 min and reached a value of 186.33 ± 0.87 mg g−1 as a result of the sufficient adsorption capacity for uptake of Pb(II). At more prolonged contact times, the adsorption sites became gradually occupied by Pb(II), thus producing a slower increase in adsorption capacity. After 30 min, the adsorption capacity remained static, with a removal ratio of >99%. The adsorption mechanism was clarified by fitting the results according to pseudo-first-order and pseudo-second-order kinetics models (eqs 6 and 7, respectively). ln(qe − qt ) = ln qe − k1t

(9)

Pseudo-second-order model

qe, exp

qe, cal

k1

R2

qe, cal

k2

R2

198.19

7.21

0.07733

0.7432

199.6

0.00217

0.9999

The R2 obtained using the pseudo-second-order kinetic model was greater than 0.999; thus, the adsorption process conforms better to this kinetic model, affording a calculated qe of 199.6 mg/ g. This is close to the experimental value of 198.19 mg/g, and thus the adsorption of Pb(II) by MCNC@Zn-BTC conforms to the pseudo-second-order kinetics model, implying that this adsorption process is closer to chemisorption.52 Effect of pH. pH has an obvious influence on the adsorption capacity of the adsorbent because it can affect the adsorbent’s surface charge and the morphology of the metal ions. For the current system, for example, changes in pH to alkaline levels can induce the precipitation of Pb(II). The pH of the Pb(II) solution at 200 mg/L was 5.45. The influence of pH on the adsorption capacity was evaluated by varying the pH from 2 to 6. MCNC@ Zn-BTC (20 mg) was used to adsorb Pb(II) at different pH values, and the adsorption values were calculated after adsorption equilibrium was reached. Figure 12 shows that the qe and removal ratio improved with increasing pH. The qe and removal ratio remained essentially unchanged for pH values in the range from 5

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Figure 11. (a) Effect of contact time on adsorption capacity (at Pb(II) concentration of 200 mg/L at 298.2 K). Fitting of adsorption of Pb(II) by MCNC@Zn-BTC to (b) a pseudo-first-order and (c) a pseudo-second-order kinetic model. 10454


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The changes in qe as a function of temperature allow us to calculate (Table 5) the Gibbs free energy (ΔG, kJ/mol), enthalpy Table 5. Parameters of Thermodynamics about Adsorption Process of MCNC@Zn-BTC for Removing Pb(II) ΔG(kJ/mol) T (K) 293.2

298.2

303.2

308.2

ΔH (kJ/mol)

ΔS (J/mol K)

−5.09

−5.29

−5.45

−5.66

5.87

−0.037

(ΔH, kJ/mol), and entropy (ΔS, kJ/mol) of the process using eq 10 q ⎛ ΔH ΔS ⎞⎟ ΔG = −RT ln e = −RT ⎜ − + ⎝ RT Ce R ⎠ (10)

Figure 12. Effect of pH on adsorption capacity (at Pb(II) concentration of 200 mg/L at 298.2 K).

where T (K) is the thermodynamic temperature, and R is the universal gas constant. The values of ΔG are negative, indicating that this adsorption process is spontaneous. As for ΔH, the value is positive; in combination with the effects of temperature, this confirm that this adsorption process is endothermic. Therefore, an appropriate increase in temperature can promote the adsorption process. In addition, the ΔS suggests that this adsorption process is also entropy driven. Regeneration Analysis. Pb(II) was desorbed from the adsorbent by treatment with acid (HCl). Once isolated, MCNC@Zn-BTC was reused for adsorptive Pb(II) removal in further five cycles (Figure 14). After five successive cycles, the

to 6. However, for pH < 4, the adsorption capacity of MCNC@ Zn-BTC was significantly reduced. At low pH, the oxygen electrons can interact with hydrogen cations, which hinders their interactions with Pb(II) ions. At optimal pH (5−6), the carboxyl groups present in Zn-BTC can be used to chelate metal ions. More specifically, the negatively charged COO− ions present on the composite surface can bind positive Pb(II) ions through electrostatic interactions. This hypothesis was further corroborated by the increase in the removal ratio of MCNC observed after their modification with Zn-BTC. According to the result, the lead solution should not be adjusted (i.e., pH 5.45 is optimal). Influence of Temperature. The temperature varied from 293.2 to 308.2 K and was applied to study the effect of temperature on qe. In these experiments, the Pb(II) concentration was 300 mg/L, and the volume was fixed at 20 mL. MCNC@Zn-BTC (20 mg) was mixed with the solution and shaken at different temperatures until adsorption equilibrium was attained. The adsorption capacities and removal ratios at different temperatures are summarized in Figure 13. The qe value

Figure 14. Reusability of MCNC@Zn-BTC for Pb(II) adsorption (at Pb(II) concentration of 200 mg/L at 298.2 K).

adsorption capacity decreased slightly from 198.18 ± 0.54 to 163.31 ± 1.89 mg/g, but the Pb(II) removal ratio remained at 82.07 ± 0.31%. Thus, these results demonstrate that MCNC@ Zn-BTC is an efficient and cost-effective adsorbent with good potential for regeneration and reuse. Adsorption Selectivity. Pb(NO3)2, CuSO4, Cd(NO3)2· 4H2O, and ZnCl2 were dissolved in deionized water to prepare a standard solution of heavy metal ions, where each metal ion was present at a concentration of 0.05 g/L. This solution was utilized in the selectivity experiment, where the volume was maintained at 20 mL and the amount of adsorbent at 20 mg. The adsorption capacity and removal rate of each metal ion are shown in Figure 15. Although the mixed solution contained four different types of metal ions and three anions, MCNC@Zn-BTC maintained a high removal rate for Pb(II) (96.49%). In addition, MCNC@ZnBTC also showed good adsorption properties toward Cu2+ and

Figure 13. Effect of temperature on adsorption of Pb(II) by MCNC@ Zn-BTC at Pb(II) concentration of 300 mg/L.

increased from 262.78 ± 0.99 to 270.93 ± 0.89 mg/g with increasing adsorption temperature, and the removal ratio increased from 88.97 ± 0.45% to 90.11 ± 0.34%. Currently, there is no obvious explanation for the increases in these values. However, the curve shown in Figure 13 suggests that the increasing qe is correlated with the increasing temperature. Thus, the adsorption of Pb(II) by MCNC@Zn-BTC is suggested to be an endothermic process. 10455



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Research Article

AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +86-580-2554781. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are grateful for the support of the National Natural Science Foundation of China (Grant No. 21476212) and the Foundation of Science and Technology Department of Zhejiang Province (Grant No. 2017C33126).

■

Figure 15. Adsorption selectivity of MCNC@Zn-BTC toward Pb2+, Cu2+, Cd2+, and Zn2+ ions at 298.2 K.

Cd2+, thus indicating that this adsorbent can be used more broadly for heavy metal removal. The adsorption capacity of MCNC@Zn-BTC was also compared with those of several previously reported adsorbents (Table 6). The results of this comparison indicate that MCNC@ Table 6. Comparison of Adsorption of Pb(II) by MCNC@ZnBTC with Those of Other Reported Adsorbents Sorbent

qm (mg/g)

Adsorption time

Ref

LS-GO-PANIa POP-NH2b MCGOc MHC/OMCNTsd MCNC@Zn-BTC

216.4 523.6 79 116.3 558.66

24 h 300 min 40 min 120 min 30 min

7 53 54 55 this work

REFERENCES

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a

LS-GO-PANI = lignosulfonate−graphene oxide−polyaniline. bPOPNH2 = amine functionalized porous organic polymer. cMCGO = magnetic chitosan/grapheme oxide. dMHC/OMCNTs = magnetic chitosan/oxidized multiwalled carbon nanotubes.

Zn-BTC, as a novel adsorbent for Pb(II), displays the highest adsorption capacity (558.66 mg/g) and the lowest adsorption contact time (30 min). For this reason, it is hoped that the current results will facilitate wider application of MCNC@ZnBTC as a highly promising adsorbent capable of fast and effective adsorption of Pb(II).

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CONCLUSION In this study, MCNC were synthesized using a chemical coprecipitation method, and the surfaces of the MCNC were subsequently coated with Zn-BTC via an Et3N-catalyzed process under mild conditions. The resulting MCNC@Zn-BTC material was characterized by SEM, FTIR, XRD, XPS, and VSM, which confirmed its successful synthesis. This novel material was applied to remove Pb(II) from water, and the adsorption conditions were optimized through a series of comprehensive experiments. According to the adsorption data, the adsorption process follows pseudo-second-order kinetics and conformed to the Langmuir isotherm model. The qm value, determined by fitting of the adsorption results to the Langmuir isotherm model, was 558.66 mg/g. The adsorption time that reached equilibrium was 30 min, and the optimum pH was established as 5.45, i.e., the pH of the original Pb(II) solution. The simple preparation and successful regeneration of the adsorbent (over five successive cycles) suggest that MCNC@Zn-BTC can be applied to remove Pb(II) from contaminated water. 10456


Research Article

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