Surfactant-Assisted Synthesis of Ca₃(BTC)₂ Metal-Organic Framework for the Removal of Heavy Metal

1. Introduction

Metal-organic frameworks (MOFs) are a very important new class of organic-inorganic hybrid compounds with porous and modular structures. MOFs consist of two parts: metal ion nodes or inorganic clusters, which are the secondary building units, as well as organically connected molecules or spacers. The various combinations of nodes and spacers, the flexibility with which their size, function, and geometry can be varied, all of these contribute to the preparation of large number of MOFs. Due to their extensive applications MOFs have attracted great interests in drug delivery, catalysis, optics, sensing, diagnosis, and storage[1]. In addition, the size of MOFs can be controlled at the nanoscale to generate nanoscale metal-organic frameworks (NMOFs). At the nanoscale level, Since the surface area to volume ratio of NMOF is larger, the proper connection of NMOF depends on its inner and outer surfaces. Hence, NMOF properties and behavior are considerably size and shape-dependent[2]. Depending on the synthetic process, NMOFs can exist in different forms to produce unique physicochemical properties. Small change in morphology can lead to changes in their properties.

Water contamination with heavy metals is one of the most serious environmental problems and causes global public health concerns. In the past few decades, great efforts have been attempted to minimize the impact of heavy metals in wastewater[3]. Currently, A number of techniques have been used to remove heavy metals from wastewater, such as adsorption, chemical precipitation, ion exchange, membrane filtration, electrolytic reduction, coagulation and so on[4,5,6,7,8,9]. Among them, ion-exchange is considered an efficient and facile method for wastewater treatment due to its many advantages, such as high processing capacity, high removal efficiency and fast kinetics. In addition, ion-exchange programs could not only remove dangerous metal ions, but also provide beneficial metal ions and even form some isomorphic frameworks[10,11]. In the materials used in the ion-exchange processes, synthetic resin[12,13], zeolites materials[14,15]and porous silicon[16,17]have been widely used to remove heavy metal from aqueous solutions. Due to the relatively low substitution efficiency of zeolite materials for heavy metals, porous materials with large surface area and specificity have been used as metal concentrator to detect potential metal contaminated areas, making sample transportation and processing easier. However, compared to porous materials such as zeolites, active carbons and silicon mesoporous, MOFs have been investigated for the removal of heavy metal ions due to their more extremely adjustable pore structures and much higher porosities[18,19,20]. The characteristics of well defined pore shape and pore size mean that these frameworks have significant applications in the process of metal ion-exchange and adsorption[21,22].

It is an effective method to prepare nano or microcrystals for using surfactants as soft templates[23]. The crystals are generated within or around the limitary volume or space of the template. The growth and nucleation of nanoparticles or microcrystals can be limited by the spatially and dimensionally, just like in a reaction cage, resulting in the formation of nanoparticles with complementary morphology to the template. In addition, various morphologies of MOFs can be obtained by changing the shape and dosage of the template agent.

In our previous work, Hg²⁺ adsorption from water over thiol-functionalized Cu-BTC by a facile coordination based postsynthetic strategy have been reported [24]. The thiol-functionalized samples exhibited excellent adsorption capacityfor Hg²⁺ removal (714.29 mg g-1), while the exposed [Cu₃(BTC)₂]n showed no adsorption of Hg²⁺ from the aqueous solution. To date, functionalization or grafting of MOFs based materials with different organic functional groups usually requires multiple steps to achieve selective removal of heavy metal ions from water. Compared to the large number of MOFs constructed from transition metal ions reported in the literature, the number of examples based on alkaline earth metals is relatively limited[25]. In addition, most MOFs as coordination polymers using transition metals, the secondary contamination during ion exchange may be induced, which improved the cost of sewage treatment.

Herein, not only a surfactant-assisted reaction of micro-sheets or micro-rods of MOFs Ca₃(BTC)₂ (BTC=1,3,5-benzenetricarboxylate) has been reported, but also the removal kinetics of heavy metals of Pb(II) was examined. The calcium-based MOF have an effective capture of Pb(II) through ion-exchange instead of adsorption. The rod-like Ca₃(BTC)₂ crystal is a two-dimensional (2D) layered calcium-BTC coordination polymer[26] which is synthesized by hydrothermal method. Significantly, the maximum removal capacities of Ca₃(BTC)₂ for Pb(II) is 555.6 mg/g. Clearly indicating that the Ca₃(BTC)₂ can be applied for selective removal of heavy metal ions from aqueous solution. The heavy metal ions exchange process of heavy metal ions was illustrated in Figure 1.

2. Experimental methods

2.1. Materials and methods

Benzene-1,3,5-tricarboxylic acid (H₃BTC) was purchased from Aldrich. Calcium acetate (Ca(CH₃COO)₂), lead chloride (PbCl₂), was purchased from Sinopharm (Shanghai) Chemical Reagent Co.Ltd. The water used in this experiment was deionized water. Other chemical reagents and organic solvents were analytical grade, obtained from commercial suppliers, and could be used without further purification. PXRD patterns of the samples were collected by using an X-ray diffractometer with Cu target (36 kV, 25 mA) from 5°to 40. The morphologies and size or nanostructure of the products were characterized on a SU8010 field emission scanning electron microscopy (FE-SEM) equipped with an energy dispersive X-ray (Oxford Instruments INCA EDX) system. The concentration values of metal ions (such as Ca²⁺, Pb(II) were confirmed by the inductively coupled plasma mass spectrometer (ICP-MS, iCAPQ, Thermo Fisher, USA).

2.2. Synthesis of Ca₃(BTC)₂

Ca₃(BTC)₂ crystals were prepared by a hydrothermal method. 0.0316 g (0.2 mmol) Ca(CH₃COO)₂ was dissolved in deionized water(20 mL), and 0.042g(0.2 mmol) H₃BTC(BTC=benzene-1,3,5-tricarboxylate) and 2 mL absolute ethanol were simultaneously added into the above solution. The reaction mixture was vigorously stirred for 30 minutes to form a homogeneous solution, Then, different quantity (0.2 g，0.4 g，0.6 g，and 0.8 g) of CTAB were added and loaded into the Teflon autoclave. The autoclave was heated to 180℃ for 10 h, then slowly cooled to 80℃ for 36 h, and after that cooled to room temperature. The obtained white powder was filtered off, washed several times with deionized water and ethanol, and then dried at 70℃ under air atmosphere overnight in drying oven.

2.3. Capture and selectivity of Ca₃(BTC)₂ for Pb(II)

The metal salt aqueous solutions of PbCl₂ with different concentrations were immersed in a solution of deionized water (25 mL). To demonstrate the ability of Ca₃(BTC)₂ for removal of Pb(II), the ion exchange between Ca²⁺ of Ca₃ (BTC)₂ and Pb(II) was quantitatively analyzed by the inductively coupled plasma mass spectrometer experiment. The selectivity properties for Pb(II) was determined by adding 50 mg of Ca₃ (BTC)₂ with 0.2g CTAB as template to 25 mL of the above solution at room temperature for 24h. The products were collected by centrifugation and the concentration of the supernatant was measured by ICP-MS.

3. Results and discussion

3.1. Measurement of Ca₃(BTC)₂ samples before and after ion-exchange

Structure of as-synthesized Ca₃(BTC)₂ sample was primarily characterized by PXRD. The PXRD pattern of as-synthesized sample and the simulated pattern of Ca₃(BTC)₂ are shown in Figure 2. And the suface profile of Ca₃(BTC)₂ was measured by Scanning Electron Microscope (SEM). As can be seen from Figure 2, compared with the simulated Ca₃(BTC)₂ XRD patterns from its crystal structural data, all peaks from the resulting Ca₃(BTC)₂ crystals are preserved, suggesting that this pattern is in agreement with the simulated Ca₃(BTC)₂ XRD patterns[27].

XRD analysis of the MOFs for different quantity of CTAB was characterized (Figure 2). As revealed by XRD, all of the diffraction peaks for the synthesized Ca₃(BTC)₂ can be readily indexed to the simulated pattern of Ca₃(BTC)₂.

The morphology and size of samples prepared with different quantity of CTAB were comprehensively characterized by SEM (Figure 3). SEM images show that Ca₃(BTC)₂ is sheet-like or rod-like, with a width is of approximately 1 μm and a length of 5 μm. The material morphology depends on the quantity of the CTAB added.

In the absence of CTAB, the synthesized material is blocky and about 5 μm in length. As the quantity of CTAB increased to 0.2g, a few of microrods with pores on the surface were formed (Figure 3b). As the quantity of CTAB increased to 0.4g, some microsheets were formed (Figure 3c). With improving the quantity of CTAB from 0.4-0.6 g, more and more regular micro-rods have emerged(Figure 3d). The continuous increase in the quantity of CTAB to 0.8g leads to the generation of regular microrods (Figure 3e). These results show that the the addition of CTAB is critical for the final morphology and size, Therefore, it is reasonable to speculate that CTAB plays an important role in controlling the size and morphology of Ca-MOFs. In addition, the morphology of Ca₃ (BTC)₂ after ion exchange with Pb(II) has also been studied. The results showed that the morphology of the materials changed greatly after ion exchange (Figure 3f)，due to ion exchange, the morphology of the product changed from rod-shaped to sheet-shaped, the exchange process is further verified.

To confirm the process of ion exchange, the metal ion exchange process of heavy metals by Energy Dispersive Spectrometer(EDX) was followed (Figure 4). The EDX images showed that Ca²⁺ in Ca-MOF was completely exchanged with Pb(II) in solution after 24 hours of reaction (Figure 4d). The main reason is that the coordination ability between Ca²⁺ and H₃BTC is lower than that of Pb(II), Pb(II) will seize some organic functional groups, leading to partial crystal dissolution, and the dissolved composite anions interact with Pb(II) to form composite precipitates. With the progress of the reaction, an increasing number of organic functional groups were complexed with Pb(II) to form three-dimensional solid materials. Due to the reaction conditions at room temperature and atmospheric pressure, the resulting heavy metal complexes were not three-dimensional ordered crystals with uniform size, but rather disordered solid materials.

3.2. Adsorption isotherms for Pb(II)

The adsorption isotherm was used to better understand the mechanism on heavy metal removal, as shown in Figure 5. The capture capacities of Pb(II) increases rapidly at lower Pb(II) concentration (less than 7.81 mg/L). When the concentration of Pb(II) is higher than this value, further increases in Pb(II) concentration will not enhance the capture capacity of Pb(II). These experimental results indicate the saturated exchange capacity of Pb(II) on Ca₃(BTC)₂ is approximately 530.8 mg/L. The maximum exchange capacity of Pb(II) was estimated by fitting the equilibrium capture data with the Langmuir adsorption model, which can be described as[28,29,30]:

Ce/qe = Ce/qm +1/qmKL

where Ce(mg/L) is the equilibrium concentration of remaining Pb(II) in the solution, qe(mg/g) is the equilibrium exchange capacity, which is the displacement Pb(II) per mass unit of Ca-MOF at equilibrium, qm is the saturation capacity of Ca-MOF and KL(L/mg) is the Langmuir constant.

The linear regression between Ce/qe and Ce was calculated, as shown in Figure 5b. The correlation coefficient is 0.9958, it is shown that the capture of Pb(II) is well followed by Langmuir adsorption model. The saturation capacity for the capture of Pb(II) is determined to be 555.6 mg/g by fitting the equilibrium capture data with the Langmuir adsorption model (see Figure 5). It is notable that the maximum capacity of the material obtained here is much greater than the maximum capacity of many other porous materials[31,32,33,34].

3.3. Adsorption kinetics

To evaluate the adsorption kinetics, the adsorption kinetics of Ca₃(BTC)₂ on Pb(II) were studied. As the concentration of Pb(II) increases, the growth rate of Ca²⁺ slows down, indicates that the exchange capacities of Pb(II) tended to achieve the maximum after 20h (1200min), and almost 86% of Pb(II) was replaced by Ca²⁺ from the framework within 20h. (see Figure 6). All the exchange was observed by ICP-MS analysis. It can be observed that the removal efficiency of Ca²⁺ is relatively high in all cases.

4. Conclusions

In this paper, a facile approach for the removal of heavy metal ions through bare Ca₃(BTC)₂. Pb(II) can be replaced by Ca²⁺ of Ca₃(BTC)₂ efficiently. Ca₃(BTC)₂ material exhibited excellent capture ability for Pb(II). During the ion-exchange process, the materials not only exhibit different topological structures, but also showed extremely high removal ability for Pb(II), reaching 555.6 mg g^-1. In addition, the Ca-MOF exhibited high adsorption affinity and the Kd value was calculated to be 1.97×10³ for Pb(II). Due to the process involves the replacement between heavy metal ions and Ca²⁺ of Ca₃(BTC)₂, the cation size and coordination abilities of metal ions play an important role in the facile ion exchange process. In summary, this work provides a green and facile method for selective removal of heavy metal ions by Ca-MOF, and even provides a synthesis method for heterostructure materials including different MOFs.