Iron and 1,3,5-Benzenetricarboxylic Metal–Organic Coordination

Article pubs.acs.org/JPCC

Iron and 1,3,5-Benzenetricarboxylic Metal−Organic Coordination Polymers Prepared by Solvothermal Method and Their Application in Efficient As(V) Removal from Aqueous Solutions Bang-Jing Zhu,†,‡ Xin-Yao Yu,† Yong Jia,† Fu-Min Peng,§ Bai Sun,† Mei-Yun Zhang,† Tao Luo,*,† Jin-Huai Liu,*,† and Xing-Jiu Huang*,† †

Research Center for Biomimetic Functional Materials and Sensing Devices, Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei, Anhui 230031, P. R. China ‡ Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China § College of Chemistry & Chemical Engineering, Anhui University, Hefei, Anhui 230039, P. R. China S Supporting Information *

ABSTRACT: Iron and 1,3,5-benzenetricarboxylic (Fe−BTC) metal−organic coordination polymers are synthesized via a simple solvothermal method. The as-synthesized Fe−BTC polymers exhibit gel behavior, which is stable in common organic solvents or in water. The Fe−BTC polymer as an adsorbent for arsenic removal from water is tested. The kinetics and thermodynamics of arsenic adsorption by the Fe− BTC polymer in aqueous solution are investigated comprehensively. The effect of pH on the adsorption is also investigated. Kinetic studies show that the kinetic data are well described by the pseudo-second-order kinetic model. The thermodynamic analysis indicates that the adsorption is spontaneous. The adsorption isotherms can be well described with the Langmuir equation. The Fe−BTC polymers show relatively high arsenic adsorption capacity, more than 6 times that of iron oxide nanoparticles with a size of 50 nm and 36 times that of commercial iron oxide powders. Hence, the as-synthesized Fe−BTC polymers possess relatively high stability and better adsorption characteristic than nanomaterials simultaneously. It also can be considered as a new method to conquer the dilemma between the excellent properties from nanoscale effect and the aggregation of small size particles in the adsorption application of nanoparticle materials.

1. INTRODUCTION Metal−organic coordination polymers, or metal−organic frameworks (MOFs), in which metal ions act as coordination centers and link together with a variety of organic bridging ligands, have developed extremely rapidly in recent years.1 Nowadays several hundred different MOFs have been identified. The structure, properties, and possible applications of MOFs have been studied.2,3 Due to the especially high surface areas, the high porosity, and the absence of hidden volumes of MOFs, the volume specific applications, e.g., gas purification,4 gas separation,3,5−8 gas storage,9−12 and heterogeneous catalysis,13−15 have been investigated comprehensively. However, the study of MOFs as adsorbents applied to remove contamination from aqueous solutions is rare. There are only a few reports16−18 on the application of MOFs to remove organic contamination from aqueous solution, including benzothiophene, methyl orange, and methylene blue. To the best of our knowledge, there have been no reports on the application of MOFs to remove arsenic pollutants from aqueous solutions. Arsenic is one of the most toxic and carcinogenic chemical elements and is regarded as the first priority issue of the toxic substances by the World Health Organization. Arsenic in © 2012 American Chemical Society

natural waters is a worldwide problem. Its practical and effective removal from groundwater remains an important and intractable challenge in water treatment.19−21 Recently, iron oxide as a promising adsorbent for removal of arsenic from aqueous solution has been investigated comprehensively22−26 because the interaction between arsenic and iron oxide is very strong and irreversible, even on nanoparticles.26,27 Especially, iron oxide nanoparticles have attracted more attention due to their large surface-to-volume ratio resulting from their small sizes. According to the literature,28 it is widely accepted that the smaller nanoparticles have higher adsorption capacities owing to larger surface-tovolume ratio. So, to obtain extremely high adsorption capacities, the nanoparticles are expected to be as small as possible. While various nanoparticle adsorbents were made, these efforts have met with the harsh challenge of the aggregation of nanoparticles, which is a general problem in nanotechnological processes because it reduces the advantages Received: December 28, 2011 Revised: March 20, 2012 Published: March 23, 2012 8601

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associated with the large surface area of single particles.29,30 The attempts to address these problems have been made in two main ways. One way is to load functional nanoparticles on common porous adsorbents, such as activated carbon,31,32 porous alumina,33 zeolite,34 diatomite,22 et al., which can partly resolve these problems by sacrifice of adsorption capacities. The other way is to design and synthesize micronano hierarchically structured adsorbents, which makes a compromise between high adsorption capacity and nanoparticle stability. Compared to nanoparticles, metal−organic coordination polymers have at least two advantages in adsorption applications: one is that the coordintion polymers are tailored nanoporous host materials on the basis of the self-assembly of metal ions linked together by specific polyatomic organic bridging ligands, which make the most striking difference from common materials and nanomaterials being probably the total lack of nonaccessible bulk volume in coordination polymer structures (demonstrated in Scheme 1). The other is that the

2. EXPERIMENTAL SECTION 2.1. Chemicals. 1,3,5-Benzenetricarboxylic acid (99%) was purchased from Aladdin Chemistry Co. Ltd. (item no. 13382). FeCl3·6H2O (A.R.) and Fe2O3 powders (99%) were purchased from Sinopharm chemical reagent company (Shanghai, China). All the chemicals were used in our experiments without further treatment. 2.2. Preparation of Adsorbents. Both the Fe−BTC polymer and Fe2O3 nanoparticles were synthesized by solvothermal method under autogenous pressure. Fe−BTC. The reactants FeCl3·6H2O and 1,3,5-benzenetricarboxylic acid (H3BTC) with a 1:1 molar ratio were dissolved in 5 mL of N,N-dimethylformamide (DMF). The mixture was heated at 150 °C for 24 h in a Teflon-lined stainless-steel autoclave with a total volume of 23 mL. After the workup, homogeneous red Fe−BTC polymer was synthesized. The assynthesized polymer was washed by DMF and ethanol several times and centrifuged and then dried at 60 °C in vacuum for 6 h. The dried polymer powder was obtained. Fe2O3 Nanoparticles. According to the literature,35 6 mmol of Fe(NO3)3·9H2O was dissolved in 78 mL of DMF. The solution was sealed in a Teflon-lined stainless-steel autoclave with a total volume of 120 mL, heated at a rate of 5 °C/min to 180 °C, and maintained at this temperature for 30 h. The red Fe2O3 nanoparticles were obtained. 2.3. Adsorption Experiments. In adsorption experiments, Fe−BTC polymer, Fe2O3 nanoparticles, and bulk Fe2O3 powders were compared. Na3AsO4 (A.R.) was used as the source of As(V). The pH values of the solutions were adjusted using HCl or NaOH. Solutions containing different concentrations of As(V) were prepared and adjusted to pH 4 for the As(V) solution. The adsorption experiments were conducted in polyethylene centrifuge tubes by using the batch technique. After a specified time, the solid and liquid were separated by centrifugation, and inductively coupled plasma-optical emission spectroscopy (IRIS Intrepid II, Thermo Electron Corporation) was used to measure the arsenic concentration in the remaining solutions. The adsorption capacity of the adsorbents for As(V) was calculated according to the following equation

Scheme 1. Schematic Illustration of the As-Proposed New Strategy for Efficient Adsorbent

qe = (C0 − Ce)

V m

where C0 and Ce represent the initial and equilibrium metal ion concentrations (mg/L), respectively. V is the volume of As(V) solution (mL), and m is the amount of adsorbent (mg).

coordination polymers possess high thermal and mechanical stability, which avoid the aggregation problem that nanoparticle materials suffer. These advantages make the coordination polymers become a more promising adsorbent candidate in applications to remove heavy metal contamination from water compared to nanoparticle adsorbents. Herein, we report the synthesis and characterization of Fe and 1,3,5-benzenetricarboxylic metal−organic coordination polymers (Fe−BTC) and the excellent adsorption properties in removal of arsenic from aqueous solutions. The assynthesized Fe−BTC polymers exhibited gel behavior and good stability. In As(V) adsorption measurements, the assynthesized Fe−BTC polymers showed considerably high arsenic adsorption capacity, more than 6 times that of Fe2O3 nanoparticles with a size of 50 nm and 36 times that of commercial Fe2O3 powders.

3. RESULTS AND DISCUSSION 3.1. SEM and TEM Observation. The as-synthesized polymer showed gel behavior, which was red and homogeneous (Figure 1a). The polymer is stable in common solvent (alcohols, DMF, acetone, CH2Cl2) or in water. The asprepared polymer had been settled in atmosphere for 1 month without obvious changes. The SEM image (Figure 1b) shows that the dried polymer powders were particles with size on a micrometer scale. From the TEM observation (Figure 1c), it can be found clearly that the dried polymer powders were porous structure. 3.2. XPS Analysis. X-ray photoelectron spectroscopy (XPS) as a versatile analysis technique was used to analyze the composition and chemical state of the as-synthesized Fe− BTC polymer. Figure 2a shows the survey XPS data, which 8602

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3.3. TGA Analysis. Thermal gravimetric analysis (TGA) performed on as-synthesized Fe−BTC polymer revealed that the compound has high thermal stability. In Figure 3, the TGA

Figure 3. Thermal stability and the thermal gravimetric analysis (TGA) data of the as-synthesized Fe−BTC polymer. Figure 1. (a) Optical photos of the as-synthesized Fe−BTC coordination polymer gel; (b) and (C) SEM and TEM images of the dried Fe−BTC coordination polymer gel powder, respectively.

trace shows a gradual weight loss step of about 5% (48−310 °C), corresponding to escape of N,N-dimethylformamide (DMF) molecules in the pores of Fe−BTC polymer. Following it, there is a sharp weight loss at the temperature of 326−550 °C probably due to the decomposition of the coordinated 1,3,5benzenetricarboxylic molecules before the collapse of the polymer structure. 3.4. Adsorption Kinetics. The adsorption process of the as-prepared polymer was investigated compared with Fe2O3 nanoparticles and commercial Fe2O3 powders. For iron oxides, extrapolations suggest that the critical size for separation is ∼50 nm for the case of a single (nonaggregated) particle.40 For comparison purposes, we synthesized nearly monodispersed Fe2O3 nanoparticles with a size of ca. 50 nm (Figure S1a, Supporting Information). The commercial Fe2O3 powders with a size of ca. 2 μm (Figure S1b, Supporting Information) purchased from Sinopharm Chemical Reagent Co., Ltd. were used as the bulk counterpart in this study. Adsorption kinetics demonstrating the solute uptake rate is one of the most important characters which represents the adsorption efficiency of adsorbents and therefore determines their potential applications. In order to further understand the dynamics adsorption of Fe−BTC polymer, the adsorption kinetics was investigated. The effect of contact time on the removal of As(V) by Fe−BTC, Fe2O3 nanoparticles, and bulk counterparts is depicted in Figure 4a. For Fe−BTC polymer and bulk Fe2O3 powders, a rather fast adsorption of the As(V) occurred and then reached the equilibrium value within 10 min, while for the Fe2O3 nanoparticles, the equilibrium was reached within 60 min. To analyze the adsorption rate of As(V) on the three adsorbents, the pseudo-second-order rate equation was evaluated based on the experimental data

Figure 2. XPS spectra of the as-synthesized Fe−BTC polymer: (a) survey spectrum; (b) high resolution of Fe spectrum; (c) high resolution of C spectrum; (d) high resolution of O spectrum.

indicate that the as-synthesized polymer contained the three elements Fe, O, and C. Carbon is ubiquitous and is present on all surfaces for XPS analysis, in which the carbon C 1s peak at 285 eV is often used as a reference for charge correction. The XPS spectrum of C 1s for the polymer is shown in Figure 2b, which can be deconvoluted into three peaks centered at 284.8, 288.7, and 285 eV. The peaks at 284.8 and 288.7 eV correspond to phenyl36 and carboxyl37 signals, respectively. The peak at about 285 eV is assigned to carbon on the surface of the sample. The Fe 2p spectrum (Figure 2c) can be deconvoluted into three peaks centered at 725.6, 711.8, and 716.7 eV corresponding to the peak of Fe 2p1/2, the peak of Fe 2p3/2, and the satellite peak of Fe3/2 for the Fe−BTC polymer, respectively.38 The O 1s peak at 531.7 eV (Figure 2d) corresponds to the Fe−O−C species.39

t 1 1 = + t 2 qt qe k 2qe

where qe and qt are the amount of As(V) adsorbed at equilibrium and at time t, respectively. k2 stands for the pseudosecond-order rate constant of adsorption (g/(mg·min)).41 Linear plot feature of t/qt vs t (shown in Figure 4b) was achieved, and the k2 values calculated from the slopes and 8603

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Figure 4. (a) Effect of contact time on absorption rate and (b) pseudo-second-order kinetic plots for As(V) adsorption by Fe−BTC polymer gel, Fe2O3 nanoparticles, and bulky Fe2O3 powders (pH = 4, m/V = 5.0 g/L, T = 298 K).

3.6. Adsorption Isotherms. Figure 6a shows the adsorption isotherms of As(V) on the as-obtained Fe−BTC

intercepts are summarized in Table S2 (Supporting Information). The correlation coefficients of the pseudo-second-order rate model for the linear plots are very close to 1, which suggests that kinetic adsorption of the three adsorbents can be described by the pseudo-second-order rate equation. 3.5. Effect of pH. The pH is another important factor that controls arsenic speciation. H2AsO4− dominates at low pH, which is less than about pH 6.9. At high pH, HAsO42− is dominant. H3AsO4 and AsO43− may be present in strong acid (less than pH 2.3) or base (higher than pH 11) conditions, respectively.42 To determine the optimum pH for adsorption of As(V) on Fe−BTC polymer, the uptake of As(V) as a function of pH was studied. Removal of As(V) in the range of pH 2−12 is shown in Figure 5. The removal efficiency of As(V) by Fe−

Figure 6. (a) Adsorption isotherms and (b) linearized Langmuir isotherms for As(V) adsorption by Fe−BTC gel, Fe2O3 nanoparticles, and bulk Fe2O3 powders (pH = 4, m/V = 5.0 g/L, T = 298 K).

polymer, Fe2O3 nanoparticles with the size of 50 nm, and bulk Fe2O3 powders with the size of ca. 2 μm at room temperature (298 K). Two empirical equations, Langmuir and Freundlich isotherms models, were used to analyze the experimental data. The Langmuir isotherm assumes a surface with homogeneous binding sites, equivalent adsorption energies, and no interaction between adsorbed species.43,44 Therefore, the adsorption saturates and no further adsorption occurs, while the Freundlich isotherm is based on an exponential distribution of adsorption sites and energies. It is derived from multilayer adsorption model and adsorption onto heterogeneous surfaces. The mathematical expressions of the Langmuir isotherm and the Freundlich isotherms model are Figure 5. Effect of pH on As(V) adsorption on Fe−BTC (initial As(V) concentration: 5 mg/L, m/V = 5.0 g/L, T = 298 K).

Ce C 1 = + e qe qmKL qm

BTC was above 96% in the range of pH 2−10 with the maximum removal efficiency of 98.2% at pH 4. While pH increased to 12, the removal efficiency of As(V) decreased to 35.8%. Through cautious observation, it was found that the asprepared Fe−BTC polymer was unstable in strong base condition and dissolved gradually. So, the removal efficiency of As(V) decreased drastically at pH 12. As(V) was preferably adsorbed by Fe−BTC polymer in a wide pH range. Although optimum As(V) removal existed in acidic conditions, high removal performance was still found near the neutral pH. The character of As(V) removal responding to pH variation is a key point in application. In general, the pH value of natural water is in the range of 6−8.5. So, the pH preadjustment is not needed for the contaminated water when Fe−BTC polymer is applied in the removal of As(V).

ln qe =

1 ln Ce + ln KF n

where qm and KL in the Langmuir equation represent the maximum adsorption capacity of adsorbents (mg/g) and the energy of adsorption, respectively. KF and n are Freundlich constants related to adsorption capacity and adsorption intensity, respectively. The parameters of the Langmuir and Freundlich models were calculated (Table S1, Supporting Information), and the curve fitting results of the Langmuir model are shown in Figure 6b. From the correlation coefficients, it can be seen that the adsorption data of Fe−BTC, Fe2O3 nanoparticles, and Fe2O3 bulk powders fit the Langmuir isotherm mode better than the Freundlich isotherm model. From Table S1 (Supporting Information), the maximum adsorption capacities qm of Fe− BTC polymer, Fe2O3 nanoparticles, and bulk counterparts are 8604

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As(V) adsorption on Fe2O3 nanoparticles and bulk counterparts are about −27 kJ/mol, which indicate that arsenic adsorption is spontaneous and the interaction between As(V) and iron oxides is very strong, consistent with the literature.26,27 The ΔG0 values of Fe−BTC polymer are very close to the values of Fe2O3 bulk powders and nanoparticles, which demonstrate that the similar arsenic adsorption occurred in the Fe−BTC polymer, proving that the effective adsorption sites in Fe−BTC polymer, Fe2O3 nanoparticles, and bulk counterparts are the iron oxide clusters. Also, the ΔG0 values of Fe−BTC polymer are less than the values of nanoparticles, indicating that the effective sites in Fe−BTC polymer are more active than the ones on Fe2O3 nanoparticles. The result of thermodynamic study proves that the as-proposed design for efficient adsorbents is reasonable and effective. 3.8. Proposed Mechanism of As(V) Adsorption on Fe− BTC. To further understand the mechanism of As(V) adsorption on Fe−BTC polymer, XPS and IR spectra were used to characterize the samples. In comparative study between the sample of Fe2O3 nanoparticles after adsorption of As(V) and the sample of Fe−BTC polymer after adsorption of As(V) by XPS (Figure 8a), there was an interesting phenomenon that the As sign could not be found in the XPS spectrum of Fe− BTC polymer but could in that of Fe2O3 nanoparticles, even though the result of the adsorption isotherm shows that Fe− BTC polymer adsorbed more arsenic than Fe2O3 nanoparticles. However, the IR spectra (Figure 8b) of the sample of Fe−BTC polymer before and after adsorption of As(V) show clearly that there is a new peak at 824 cm−1 in the spectrum of the sample after the adsorption compared to the one before adsorption. According to the literature,24 the 824 cm−1 band can be assigned to the Fe−O−As groups, which confirms that Fe− BTC polymer did adsorb arsenic ions. A reasonable explanation of the interesting phenomenon proposed is that the arsenic ions adsorbed onto the interior of the Fe−BTC polymer, not on the outer surface, so the adsorbed As cannot be detected by XPS, which is the solid surface detective technique. The TEM phase mapping was used to prove the above proposal. The TEM images of the sample of Fe−BTC polymer after adsorption of As(V) with corresponding phase mappings of Fe Kα1 and As Kα1 are shown in Figure 8c−e, which reveal information about the As element (green color) in the Fe− BTC polymer structure. The result of TEM phase mapping characterization demonstrates that the arsenic ions really adsorbed onto the interior of the Fe−BTC polymer, which is in good agreement with the design (shown in Scheme 1).

12.287, 6.365, and 1.098 mg/g, respectively. To evaluate the extent of active sites on the three adsorbents, qm should be converted into the same standard value via the equation m qm′ = qm · adsorbent m iron where qm and qm′ represent the maximum adsorption capacity related to the mass of adsorbent and the mass of iron that the adsorbent contains, respectively. madsorbent and miron are the mass of adsorbent and the mass of iron in the adsorbent. By the conversion, qm′ values of Fe−BTC polymer, Fe2O3 nanoparticles, and bulk Fe2O3 powders are 57.705, 9.093, and 1.569 mg/g, which indicate that the effective adsorption sites in the Fe−BTC polymers are nearly 6.5 times that of iron oxide nanoparticles with a diameter of 50 nm and 36.8 times that of commercial iron oxide powders with sizes of ca. 2 μm. 3.7. Adsorption Thermodynamics. The thermodynamic parameters of adsorbents are actually helpful in the practical application of the process. Hence, the adsorption thermodynamics were studied. Thermodynamic parameters were calculated from the thermodynamic equilibrium constant K0, which is defined as follows K0 =

as v C = s s ae ve Ce

where as is the activity of adsorbed As(V), ae is the activity of the As(V) in solution at equilibrium, vs is the activity coefficient of the adsorbed As(V), ve is the activity coefficient of the As(V) in solution, Cs is the amount of As(V) adsorbed per mass of adsorbents (mmol/g), and Ce is the concentration of As(V) in solution at equilibrium (mmol/mL).41,45 As the As(V) concentration in the solution decreases and approaches zero, K0 can be calculated by plotting ln(Cs/Ce) vs Ce (Figure 7) and extrapolating Cs to zero. The values of K0 can

Figure 7. Plots of ln Cs/Ce vs Cs for As(V) adsorption by Fe−BTC polymer gel, Fe2O3 nanoparticles, and bulky Fe2O3 powders (pH = 4, m/V = 5.0 g/L, T = 298 K).

4. CONCLUSIONS The Fe−BTC coordination polymers were synthesized using a simple solvothermal method. The self-assembly of iron ions, which acted as coordination centers, linked together with 1,3,5benzenetricarboxylic bridging ligands, had formed tailored porous polymer with high chemical and structural stability. The Fe−BTC coordination polymer shows excellent adsorption capacity toward As(V), which is 6.5 times more than that of 50 nm Fe2O3 nanoparticles. The Fe−BTC polymer also shows better dynamic adsorption ability than the as-prepared iron oxide nanoparticles with the size of 50 nm. The Fe−BTC polymer is a promising adsorbent in the application of arsenic removal from aqueous solutions.

be obtained from the intercept of the plot. The Gibbs energy (ΔG0) of adsorption is calculated from the equation ΔG 0 = −RT ln K 0

where R is the ideal gas constant (8.3145 J/(mol·K)) and T (K) is the absolute temperature. The calculated values of thermodynamic parameters are listed in Table S2 (Supporting Information). The negative Gibbs’ free energy values (ΔG0) confirm that the adsorption is spontaneous under the ambient conditions. The ΔG0 values of 8605

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Figure 8. (a) XPS spectra of Fe2O3 nanoparticles and Fe−BTC polymer gel after adsorption of As(V); (b) IR spectra of Fe−BTC polymer gel before and after adsorption of As(V); (c) TEM image of Fe−BTC polymer gel after adsorption of As(V); (d) and (e) corrresponding Fe Kα1 and As Kα1 phase maps, respectively.

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ASSOCIATED CONTENT

S Supporting Information *

TEM images of Fe2O3; parameters of Langmuir and Freundlich models for adsorption of As(V); kinetics and thermodynamics parameters of adsorbed As(V). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.L.); [email protected] (X.J.H.); [email protected] (J.H.L.) Fax: 86 551 5592420. Tel.: 86 551 5595607. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Basic Research Program of China (No. 2011CB933700) for its support. We also acknowledge the National Natural Science Foundation of China (Grant Nos. 60801021, 21103198, and 20907035) and the One Hundred Person Project of the Chinese Academy of Sciences and the China Postdoctoral Science Foundation (No. 20110490386). B.S. thanks the Open Fund Program from the Key Laboratory of Cigarette Smoke of State Tobacco Monopoly Administration, Technical Center of Shanghai Tobacco Corporation (K2010-106), for funding.

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