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Novel Metal−Organic Framework (MOF) Based Composite Material for the Sequestration of U(VI) and Th(IV) Metal Ions from Aqueous Environment Ayoub Abdullah Alqadami, Mu. Naushad,* Zeid Abdullah Alothman, and Ayman A. Ghfar Department of Chemistry, College of Science, Building 5, King Saud University, Riyadh 11451, Kingdom of Saudi Arabia S Supporting Information *

ABSTRACT: The combination of magnetic nanoparticles and metal−organic frameworks (MOFs) has demonstrated their prospective for pollutant sequestration. In this work, a magnetic metal−organic framework nanocomposite (Fe3O4@AMCA-MIL53(Al) was prepared and used for the removal of U(VI) and Th(IV) metal ions from aqueous environment. Fe3O4@AMCAMIL53(Al) nanocomposite was characterized by TGA, FTIR, SEM-EDX, XRD, HRTEM, BET, VSM (vibrating sample magnetometry), and XPS analyses. A batch technique was applied for the removal of the aforesaid metal ions using Fe3O4@AMCA-MIL53(Al) at different operating parameters. The isotherm and kinetic data were accurately described by the Langmuir and pseudo-second-order models. The adsorption capacity was calculated to be 227.3 and 285.7 mg/g for U(VI) and Th(IV), respectively, by fitting the equilibrium data to the Langmuir model. The kinetic studies demonstrated that the equilibrium time was 90 min for each metal ion. Various thermodynamic parameters were evaluated which indicated the endothermic and spontaneous nature of adsorption. The collected outcomes showed that Fe3O4@AMCA-MIL53(Al) was a good material for the exclusion of these metal ions from aqueous medium. The adsorbed metals were easily recovered by desorption in 0.01 M HCl. The excellent adsorption capacity and the response to the magnetic field made this novel material an auspicious candidate for environmental remediation technologies. KEYWORDS: metal−organic framework, nanocomposite, water pollutant, metals, adsorption, leaching

1. INTRODUCTION The expulsion of radioactive waste into the environment is a major issue.1 Thorium and uranium are highly toxic radioactive elements. It is known that Th(IV) and U(VI) are commonly found together in some naturally occurring minerals such as thorite and monazite.2 Some quantity of liquid radioactive waste which has U(VI) and Th(IV) is also generated because of the mining which is also dangerous to the environment. The toxic nature of U(VI) and Th(IV), even at low levels, has been a severe health hazard for many years.3 These metals can create various types of diseases such as lung and liver cancers.4 Therefore, the removal of these metals from the aqueous environment is an important issue for environmental remediation. In the past decades, several methods comprising membrane separation, 5 chemical precipitation,6 ion exchange,7−9 electroflotation,10 coagulation,11,12 and solvent extraction13 have been used for the removal of toxic metals. Most of these methods suffer from economic, technical, and health hazards related to low selectivity, low efficiency, high energy consumption, and large quantity of toxic materials used. Nevertheless, adsorption is one of the most effective methods owing to its high selectivity, ease of operation, and environmental compatibility.14−16 The selection of appropriate adsorbents for the removal of noxious metal ions from the © XXXX American Chemical Society

aqueous environment is very important. Mainly, some properties like high adsorption capacity and high regeneration ability are considered for the selection of any suitable adsorbent.17 Several adsorbents have been already described for the removal these metal ions and radioactive elements including polymer/ metal oxides,18 nano-oxides,19 zeolites,20 activated carbon,21,22 lignin,16 clays,23 polymers,24 and biopolymeric materials.25 Nevertheless, there is still an increasing demand for the synthesis of cheap, reliable, and efficient materials. The MOFs have shown a significant role in the development of new water-applicable adsorbents. The structure of metal− organic frameworks is made up of metal-oxide clusters linked by organic linkers via strong covalent bonds.26 These materials have high pore volume, highly ordered pore structure, high density of active sites, and high specific surface area which provide many advantages to their applications in watertreatment technologies. The MOFs constitute an important family receiving eminence for their several prospective applications in various industrially important areas.27 The MOFs have already been used for catalysis,28,29 gas storage and Received: July 22, 2017 Accepted: September 25, 2017

A

DOI: 10.1021/acsami.7b10768 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) FTIR spectrum of Fe3O4, NH2-MIL53(Al), AMCA-MIL-53(Al), and Fe3O4@AMCA-MIL53(Al) nanocomposite. (b) XRD of Fe3O4, NH2-MIL53(Al), and Fe3O4@AMCA-MIL53(Al) nanocomposite.

separation,30,31 separation of chemicals,32 metal ion removal,33,34 magnetism,35 and drug delivery.36 Several scientists have reformed the MOFs by amino groups,37 aziridine,38 iron oxide nanoparticles,39 and thiol groups40 for the removal of various types of inorganic and organic compounds. Herein, we have developed magnetite-based metal−organic framework adsorbent (Fe3O4@AMCA-MIL53(Al) for the removal of U(VI) and Th(IV) metal ions from aqueous environment. The obtained material was properly characterized by FTIR, XRD, SEM-EDX, TEM, TGA, BET, VSM (vibrating sample magnetometry), and XPS techniques. Various parameters like pH, contact time, initial metal ion concentration, and temperature were optimized. Several other important adsorption properties such as adsorption kinetics, isotherms, thermodynamics, and the adsorption mechanism were also investigated.

NH2(Al) (0.418 g; 2 mmol) was dispersed in the solution, and the reaction mixture was heated for 24 h under reflux at 80 °C. The resultant material was filtered and washed with 20 mL of DMF, 20 mL of acetonitrile, and 20 mL of H2O, separately. Then, the sample was dried at 130 °C in air atmosphere for 3 days. 2.4. Synthesis of Fe3O4@AMCA-MIL53(Al). Here, 4.3 mmol of FeCl2·4H2O and 8.7 mmol of FeCl3·6H2O were added to an aqueous solution containing different masses (0.00, 0.4, or 00.1 g) of AMCAMIL53(Al). The suspension mixture was stirred and degassed using nitrogen for 3 h, followed by the addition of 20 mL of NH3 solution to get a black suspension. The resultant material was filtered off, and repetitively washed with deionized water until the pH became neutral. 2.5. Adsorption Studies. The adsorption of metal ions using Fe3O4@AMCA-MIL53(Al) nanocomposite was examined by batch methods. Typically 0.02 g of the Fe3O4@AMCA-MIL53(Al) was shaken with 25 mL of 20 mg/L Th(IV) and U(VI) metal ion solutions separately at 100 rpm for 24 h. After equilibration was achieved, the samples were withdrawn, separated, and analyzed by ICP-MS. The adsorption capacity of the Fe3O4@AMCA-MIL53(Al) nanocomposite (mg/g) was calculated as

2. EXPERIMENTAL SECTION 2.1. Chemicals and Reagents. The chemicals and reagents which were used in the present study are given in the Supporting Information (Text S1). 2.2. Synthesis of NH2-MIL53(Al). NH2-MIL53(Al) was synthesized by the similar method reported in the literature with some modification using a solvothermal route with AlCl3·6H2O and 2aminoterephthalic acid (NH2−BDC) as the metal source and the organic linker in the molar ratio 1:1.41 Here, NH2−BDC (0.905 g, 5 mmol) and AlCl3·6H2O (1.207 g, 5 mmol) were dissolved separately in a solution with 1.5 mL of deionized water and 13.5 mL of dimethylformamide (DMF), respectively. The volume of the solvents was taken as 30 mL. These two distinct solutions were mixed into a 100 mL Teflon-lined steel autoclave and kept in an oven at 150 °C for 24 h under stationary conditions. The yellow solid product was cooled and isolated by centrifugation at 5000 rpm. The samples were activated by boiling with DMF (130 °C) for 5 h in the reflux setup to eliminate the residual water molecules.42 Afterward, the MOF was refluxed overnight with methanol (65 °C) to exchange the high-boiling DMF with the more volatile methanol. After that, the material was dried in an oven at 100 °C. 2.3. Synthesis of AMCA-MIL53(Al). AMCA-MIL53(Al) was also synthesized according to a previous method with some changes.43 Here, NH2-MIL-53(Al) was activated overnight at 150 °C under vacuum to eliminate the adhered DMF in the pores. After cooling at room temperature, 8.61 mmol (1.809 g) of citric acid was dissolved in 40 mL of acetonitrile. Then, 8.71 mmol (1.79 g) of dicyclohexylcarbodiamide (DCC) was added in the solution. After that, MIL-53-

qe = C0 − Ce

V m

(1)

The adsorbed percentage of these metals was calculated as

% adsorption =

C0 − Ce × 100 C0

(2)

For acquisition of the optimum value, the influence of contact time (1−240 min), pH (1.5−10.5), adsorbent dosage (10−80 mg), C0 (20−400 mg/L), and temperature (25−45 °C) was also analyzed by batch method. For definition of the adsorption kinetics, a 25 mL solution of each metal with initial concentrations of 20 mg/L at fixed pH was treated with 0.02 g of Fe3O4@AMCA-MIL53(Al) nanocomposite for the contact time of 5−240 min at 25 ± 1 °C and 100 rpm. The adsorption isotherm of Fe3O4@AMCA-MIL53(Al) nanocomposite was determined by shaking 0.02 g of AMCA-MIL53(Al) nanocomposite with 25 mL of each metal ion solution of dissimilar initial concentrations ranging from 20 to 400 mg/L, with pH 5.5 for U(VI) and 4.7 for Th(IV), for 300 min at temperatures 25−45 °C. The desorption study was done using different acid solvents (0.01 M H2SO4, HCl, and HNO3). For this study, 0.02 g of Fe3O4@AMCAMIL53(Al) nanocomposite was shaken with 25 mL of 20 mg/L Th(IV) and U(VI) metal ion solutions in a conical flask for 90 min under ambient temperature (298 K). After 90 min, Fe3O4@AMCAMIL53(Al) was separated and washed with Milli-Q water to eliminate the excess of metals. Then, metal-loaded Fe3O4@AMCA-MIL53(Al) B


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Figure 2. (a) TGA results of Fe3O4 and Fe3O4@AMCA-MIL53(Al). (b) N2 adsorption−desorption of NH2-MIL53(Al) and Fe3O4@AMCAMIL53(Al). (C) ζ potential of Fe3O4 and Fe3O4@AMCA-MIL53(Al). (d) VSM curve of Fe3O4 and Fe3O4@AMCA-MIL53(Al).

3.2. Characterization of Fe3O4@AMCA-MIL53(Al) Nanocomposite. The FTIR spectra of Fe3O4, NH2-MIL53(Al), AMCA-MIL53(Al), and Fe3O4@AMCA-MIL53(Al) nanocomposite were accomplished between 500 and 4000 cm−1 (Nicolet 6700, Thermo Scientific). In the spectrum of Fe3O4 nanoparticles (Figure 1a), the band at 576 cm−1 matched to FeO vibrations. The absorption bands in the spectrum of Fe3O4 in the range 3220−3380 cm−1 and at 1613 cm−1 were associated with hydroxyl (OH) groups of physically adsorbed water.44 For NH2-MIL53(Al), two sharp bands at 3367 and 3478 cm−1 were ascribed to the symmetric and asymmetric stretching vibrations of the NH2 group, respectively, which confirmed that the amino groups were free in NH2-MIL53(Al). The peak at 3684 cm−1 were assigned to  OH groups of octahedral AlO4(OH)2 chains.45 Also, the peak at 3066 was assigned to CCH. Other adsorption peaks around 1400−1715 cm−1 might be allocated to the stretching of the carboxylic functional group,41 whereas, the peaks around 1510 and 1600 cm−1 were allocated to the asymmetric stretching vibrations of the carbonyl groups coordinated with A13+.46 The peak at 1254 cm−1 may be attributed to CN vibrations. The cross-linking reaction between amino group and citric acid anhydride was evidenced by the FTIR data, because one new peak appeared at 1676 cm−1 which might be due to the formation of the amide group CONH in the frame-

nanocomposite was treated with 25 mL of the above-mentioned acid solvents at the aforesaid conditions. After 90 min, the solution was separated, and the remaining concentration of both metal ions in the solution phase was analyzed by ICP-MS. 2.6. Leaching Study. To assess the stability of Fe3O4@AMCAMIL53(Al) nanocomposite under acidic conditions, 20 mg of Fe3O4@ AMCA-MIL53(Al) was suspended in 25 mL of hydrochloric acid solutions of varying concentrations (0.01, 0.1, and 1 M). After constant contact with acid solution for 24, 48, or 72 h, the suspension was separated magnetically and the leached amount of iron in the supernatant was analyzed by ICP-MS. Fe3O4 nanoparticles were also taken for the experiments to make the comparison.

3. RESULTS AND DISCUSSION 3.1. Optimization of AMCA-MIL53(Al) mass. In order to get the best sample of Fe3O4@AMCA-MIL53(Al) nanocomposite for the maximum adsorption of Th(IV) and U(VI) metals, the mass of AMCA-MIL53(Al) was optimized. The adsorption of both metal ions was minimum when Fe3O4@AMCA-MIL53(Al) nanocomposite was synthesized without AMCA-MIL53(Al) (Table S1). The optimum mass of AMCA-MIL53(Al) was found to be 0.4 g for the synthesis Fe3O4@AMCA-MIL53(Al). Because at this mass, Fe3O4@ AMCA-MIL53(Al) showed the highest BET surface area and adsorption capacity. So, this sample was selected for the further studies in this work. C


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Figure 3. (a) SEM image of Fe3O4 nanoparticles. (b) SEM image of Fe3O4@AMCA-MIL53(Al) nanocomposite. TEM and HRTEM images of (c, e, g) Fe3O4 and (d, f, h) Fe3O4@AMCA-MIL53(Al) nanocomposite.

Here, the terms are as follows: θ is the Bragg’s peak; K is a constant that depends on crystallite morphology, usually assumed as unity; β is the half width of diffraction peak; and λ is the X-ray wavelength (0.154 nm). The (311) peak of the highest intensity was picked out to assess the particle diameter of the nanoparticles. The crystallite size of magnetite and nanocomposite from this equation was found to be about 11.8 and 15.04 nm, respectively. A comparatively higher particle diameter of Fe3O4@AMCA-MIL53(Al) confirmed the formation of the AMCA-MIL53(Al) coating over the Fe3O4 surface. The TGA was used to measure the loss in weight for Fe3O4 and Fe3O4@AMCA-MIL53(Al) nanocomposite with respect to temperature. The thermal degradation of Fe3O4 in Figure 2a could be allocated into two phases. In the first phase, 4.2% weight was lost up to 198 °C which might be due to the removal of adsorbed water molecules. In the second phase, the weight loss was 4.7% up to 390 °C, which was due to the transformation of some hydroxide to oxide as well as elimination of crystalline water.49 Meanwhile, the thermal degradation of Fe3O4@AMCA-MIL53(Al) nanocomposite took place in three stages: the first and second stage showed a gradual weight loss of 5% between 25 and 200 °C, conforming to the dehydration process and the removal of H2O molecules from the sample surface. In the second (between 150 and 450 °C) and third (between 450 and 700

works.42 In the spectrum for Fe3O4@AMCA-MIL53(Al) nanocomposite, a peak at 576 cm−1 confirmed the existence of Fe3O4. In the nanocomposite, the absorption bands at 1650 cm−1 were attributed to formation of the amide group CO NH in the frameworks. The peaks at 2931 and 2842 cm−1 were for CH stretching vibrations. The XRD data of Fe3O4, NH2-MIL53(Al), and Fe3O4@ AMCA-MIL53(Al) nanocomposite were acquired using an Xray diffractometer (PANalytical Empyrean). As shown in Figure 1b, the peaks at 2θ of 18.31°, 30.32°, 35.70°, 43.35°, 53.93°, 57.42°, 62.82°, and 74.90° matched to (111), (220), (311), (400), (422), (511), (440), and (622) planes of the fcc lattice of Fe3O4, respectively (JCPDS 19-0629).47 Diffraction peaks at around 2θ values of 8.98°, 12.58°, 15.32°, 25.33°, 26.93°, and 32.89° were readily recognized from the XRD. The observed diffraction peaks agree with the NH2-MIL-53(Al).48 Nine characteristic peaks for Fe3O4 and four characteristic peaks for AMCA-MIL53(Al) were preserved in the graph of the Fe3O4@ AMCA-MIL53(Al) nanocomposite, revealing that this hybrid material was composed of Fe3O4 and AMCA-MIL53(Al). The crystal size of Fe3O4 and Fe3O4@AMCA-MIL53(Al) nanocomposite was evaluated from the XRD pattern by using Debye−Scherrer’s equation: Ds = K ×

λ (β cos θ )

(3) D


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Figure 4. (a) X-ray photoelectron spectroscopic images of Fe3O4 and Fe3O4@AMCA-MIL53(Al). High-resolution XPS spectrum for (b) Fe 2p, (c) Al 2p, (d) C 1s, (e) O 1s, and (f) N 1s.

°C) stages, 8.5% and 5% weight loss might be the releasing of DMF and NH2−BDC from the framework observed from the adsorbents, respectively.50 The N2 adsorption−desorption measurements (Quadrasorb evo, Quantachrome) were applied to find the surface area, pore volume, and pore size of NH2-MIL53(Al) and Fe3O4@AMCAMIL53(Al) (Figure 2b). The adsorption isotherms were of type IV for NH2-MIL53(Al) as well as Fe3O4@AMCA-MIL53(Al) which favored the nonporous structure. The BJH pore size distribution of NH2-MIL53(Al) and Fe3O4@AMCA-MIL53(Al) was found using the adsorption−desorption isotherm, and the pore size was found to be 20.7 and 1.85 nm. For NH2MIL53(Al) and AMCA-MIL53(Al), the BET surface area was 479.7 and 197.8 m2/g, and the total pore volume was found to be 0.809 and 0.0277 cm3/g, respectively. The surface charge of Fe3O4 and Fe3O4@AMCA-MIL53(Al) nanocomposite was measured as the ζ potential in the pH range 2.3−9.11. The isoelectric point for Fe3O4 and Fe3O4@ AMCA-MIL53(Al) nanocomposite was 7.1 and 5.6, respectively (Figure 2c). Therefore, these nanoparticles had net negative surface charge at pH > pHpzc and positive surface charge at pH < pHpzc. A higher electric charge on the surface of the Fe3O4@AMCA-MIL53(Al) nanocomposite prevented the agglomeration of magnetic nanoparticles in solution because of the high repulsive forces among particles, and a high ζ potential (negative or positive) suggested stable systems.50 The ζ potentials for Fe3O4 and Fe3O4@AMCA-MIL53(Al) were 30.55 and 35.39 mV, respectively, at low pH, while, at high pH, they were −23 and −35.39 mV for Fe3O4 and Fe3O4@ AMCA-MIL53(Al), respectively. After the coating of Fe3O4 nanoparticles, the high negative value of the ζ potential showed that the Fe3O4@AMCA-MIL53(Al) suspension had colloidal stability.

The magnetic properties of Fe3O4 and Fe3O4@AMCAMIL53(Al) nanocomposite were characterized by vibrating sample magnetometry (VSM). The VSM graphs of these samples are shown in Figure 2d. As is obvious from Figure 2d, the saturation magnetization values of Fe3O4 and Fe3O4@ AMCA-MIL53(Al) nanocomposite were 78.4 and 49.7 electromagnetic units per gram (emu/g), respectively. The low saturation magnetization value of Fe3O4@AMCA-MIL53(Al) in comparison to that of the Fe3O4 nanoparticles might be due to the presence of a low content of Fe3O4 in the Fe3O4@ AMCA-MIL53(Al) nanocomposites. Although the saturation magnetization value of Fe3O4@AMCA-MIL53(Al) was low, it was enough to be simply separated from a solution by external magnetic fields. The SEM (JSM-6380 LA, Tokyo, Japan) images of Fe3O4 and Fe3O4@AMCA-MIL53(Al) nanocomposite are shown in Figure 3a,b. The morphology of Fe3O4@AMCA-MIL53(Al) nanocomposite was completely different from the morphology of Fe3O4. The TEM and HRTEM (HRTEM JEOL 2100) of Fe3O4 and Fe3O4@AMCA-MIL53(Al) are given in Figure 3c,e,g and Figure 3d,f,h, respectively. TEM images showed that Fe3O4 nanoparticles were aggregated because of the participation of the OH groups and magnetic interaction. The average particle size of Fe3O4 was 6 nm (Figure 3c). On the other hand, the particle size of Fe3O4@AMCA-MIL53(Al) nanocomposite was 8.5 nm due to the coating of AMCAMIL53(Al) on Fe3O4 (Figure 3d). The lattice fringes (Figure 3g,h) with an interfering distance of 0.296 and 0.252 nm corresponded to (220) and (311) planes of Fe3O4 crystal, respectively. Similarly, the interfering distance of 0.251 and 0.211 nm corresponded to (311) and (400) planes of Fe3O4@ CA-NH2-MIL53(Al), respectively.49 Figure S1 shows the particle size distribution for Fe3O4 and Fe3O4@AMCAE


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Figure 5. Removal of U(VI) and Th(IV) metal ions using Fe3O4@AMCA-MIL53(Al) nanocomposite at different values of (a) time, (b) pH, and (c, d) Fe3O4@AMCA-MIL53(Al) dose.

MIL53(Al) nanocomposite. It was observed that the average diameters for Fe3O4 and Fe3O4@AMCA-MIL53(Al) were 9.7 and 10.4 nm, respectively. The XPS was used to analyze the structural composition of Fe3O4 and Fe3O4@AMCA-MIL53(Al) nanocomposite. Figure 4 shows the XPS spectra and high-resolution XPS spectra of Fe3O4 and Fe3O4@AMCA-MIL53(Al) nanocomposite. The spectrum of Fe3O4 in Figure 4a shows two peaks at 530.2 and 711.2 eV matched to O 1s and Fe 2p, respectively. On the other hand, the spectrum of Fe3O4@AMCA-MIL53(Al) nanocomposite showed five peaks at 715, 530.4, 399.8, 284.6, and 74.8 eV which were attributed to Fe 2p, O 1s, N 1s, C 1s, and Al 2p, respectively (Figure 4a). The binding energy of the metal Al 2p was 74.8 eV, which was due to the formation of AlO4(OH2) in the NH2-MIL53(Al) framework (Figure 4c). Figure 4b shows the high-resolution XPS spectrum of Fe 2p that exhibits two prominent peaks attributed at 724.23 and 711.19 eV, conforming to the Fe 2p1/2 and Fe 2p3/2 spin−orbit intense peaks of pure magnetite (Fe3O4). The Fe 2p peak in pure Fe3O4 was higher than that of Fe3O4@AMCA-MIL53(Al), which showed the interaction between Fe atoms and the AMCA-MIL53(Al). In the high-resolution XPS spectrum for C 1s, there were three peaks (Figure 4d) at 284.52, 285.8, and 288.2, attributed to CC/CC, CN, and CO, respectively.45 The high-resolution XPS spectrum for O 1s showed three peaks (Figure 4e) at 529.9, 530.8, and 531.45, corresponding to the FeO, AlO and CO, respectively. In the high-resolution XPS spectrum for N 1s (Figure 4f), there

were two peaks at 399.74 and 401.2 attributed to NC and NHCO, respectively. The surface composition of naked Fe3O4, Fe3O4@AMCAMIL53(Al), and metal-adsorbed Fe3O4@AMCA-MIL53(Al)/ U(VI)/Th(IV) was performed by EDX (Figure S2). Oxygen and iron were present in all of the samples with oxygen abundance less than iron. The existence of AMCA-MIL53(Al) in the nanocomposite was confirmed by the presence of N, C, and Al. The adsorption of Th(IV) and U(VI) metals on the surface of Fe3O4@AMCA-MIL53(Al) nanocomposite was also confirmed by the EDX analysis. 3.4. Adsorption Parameters. The adsorption of U(IV) and Th(IV) metal ions onto Fe3O4@AMCA-MIL53(Al) was studied at various time intervals (1−140 min). The removal efficiency for U(VI) and Th(IV) metal ions increased with time until equilibrium is reached. For both metals, the adsorption was fast at the start, and the equilibrium was achieved in 90 min (Figure 5a). However, the Th(IV) was adsorbed somewhat faster than U(IV). At 5 min, the adsorption capacity (qe) for U(VI) was 10.3 mg/g while it was 14.8 mg/g for Th(IV) metal ion. The optimum time for adsorption was chosen as 90 min because no substantial change in the adsorption was noticed after 90 min. Because of the existence of freely available adsorption sites on the surface of Fe3O4@AMCA-MIL53(Al), the adsorption of both metal ions was fast at the start.51 The high adsorption of U(VI) and Th(IV) metal ions by Fe3O4@ AMCA-MIL53(Al) (Figure 5a) was due to the presence of various electron-rich nitrogen and oxygen groups. Consequently, there was electrostatic interaction between the F


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Figure 6. (a, b) Effect of initial ion concentration on the adsorption of U(VI) and Th(IV) metal ions onto Fe3O4@AMCA-MIL53(Al) nanocomposite at different temperatures. (c, d) Adsorption−desorption studies.

increasing concentrations of these metal ions (Figure 6a,b). This might be explained by the fact that more targets of metal ions could afford the higher driving force to enable the ion diffusion from the solution phase to Fe3O4@AMCA-MIL53(Al), and more collisions took place between Th(IV) and U(VI) metal ions and active sites of Fe3O4@AMCA-MIL53(Al). However, with respect to the temperature, the qe for both metals increased with the increase in temperature which showed the endothermic nature of adsorption. 3.5. Rate Constant Study. The adsorption kinetics for both metal ions onto Fe3O4@AMCA-MIL53(Al) was tested by the pseudo-first-order and pseudo-second-order kinetic models.55,56 The pseudo-first-order equation is as follows:

electropositive metal ions and electron-rich atoms (nitrogen and oxygen). The effect of pH was examined in the range 2−10. It can be seen from Figure 5b that the adsorption capacities were increased significantly with increasing pH up to 6 for both metal ions. At pH 6, the qe values for Th(IV) and U(VI) metal ions were 24.5 and 22.7 mg/g, respectively. However, at pH > 6.0, the adsorption was decreased for both metal ions because of the formation of metal hydroxides.52 In the acidic medium, the adsorption was low because the concentration of H+ ions was high, which is due to the fact that the protonation of active sites of Fe3O4@AMCA-MIL53(Al) was conquered over the adsorption process.53 The effect of Fe3O4@AMCA-MIL53(Al) dose on the qe of U(VI) and Th(IV) metal ions is given in Figure 5c,d. Here, the percent adsorption was increased, and qe was decreased with increasing Fe3O4@AMCA-MIL53(Al) dose. The reduction of qe with increasing adsorbent dosage was ascribed to the aggregation or overlapping of adsorption sites resulting in a reduction in the total surface area of adsorbent available for metals and in an increase in diffusion path length.54 The effects of C0 (20−400 mg/L) of both metals were also analyzed at varying temperatures (298−328 K). It was noted that the qe of Th(IV) and U(VI) metals was enhanced with

log(qe − qt ) = log qe −

k1t 2.303

(4)

The pseudo-second-order equation is as follows: t 1 t = + qt qe k 2qe 2

(5)

The values of the correlation coefficient (R2) and different constants from these two kinetic models are given in Table 1. G


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Table 1. Kinetic Model Constants for the Adsorption of U(VI) and Th(IV) Metal Ions onto Fe3O4@AMCA-MIL53(Al) Nanocomposite pseudo-first-order

pseudo-second-order

metal ion

C0 (mg/L)

qe, exp

qe, calcd

K1 (min−1)

R2

qe, calcd (mg/g)

K2 (g mg−1 min−1)

R2

U(VI) Th(IV)

20 20

21.3875 22.01

13.83 7.12

0.039 0.024

0.983 0.947

22.32 22.27

0.0058 0.0112

0.999 0.999

Table 2. Isotherm Parameters for the Adsorption of U(VI) and Th(IV) Metal Ions onto Fe3O4@AMCA-MIL53(Al) Nanocomposite Langmuir isotherm

Freundlich isotherm

metal ion

temperature (K)

qm, exp (mg/g)

qm, calcd (mg/g)

b (L/mg)

RL

R2

Kf (mg/g)(L/mg)(1/n)

N

R2

U(VI)

298 308 318 298 308 318

198.7 212.5 225.0 208.7 235.0 261.3

192.3 217.4 227.3 208.3 210.0 285.7

0.040 0.044 0.065 0.054 0.077 0.080

0.559 0.534 0.436 0.482 0.393 0.383

0.995 0.993 0.993 1.000 0.998 0.994

14.73 17.84 23.6 19.73 19.94 30.63

1.98 2.03 2.19 1.59 1.60 1.61

0.953 0.944 0.927 0.981 0.972 0.955

Th(IV)

The results indicated that the adsorption of both metal ions was best defined by the pseudo-second-order equation because of the high coefficient values (R2 ≥ 0.999) for this model. 3.6. Adsorption Isotherms. The adsorption isotherms were studied using Langmuir and Freundlich isotherm models.57,58 The linear form of the Langmuir isotherm equation may be written as

1 1 1 = + qe qm bqmCe

Table 3. Comparison of Adsorption Capacity of Fe3O4@ AMCA-MIL53(Al) with Different Adsorbents adsorption capacity adsorbent

(6)

The important characteristic of the Langmuir isotherm can be expressed in terms of the dimensionless equilibrium parameter RL, which is given as59 RL =

1 1 + bC0

(7)

1 ln Ce n

Th(IV)

88.32 140 170 156.3

227.3

96.15 285.7

ref 60 61 62 63 64 this work

of the correlation coefficient (R2 > 0.99), the Langmuir isotherm model fitted quite well with the experimental data. 3.7. Effect of Temperature and Thermodynamic Studies. The thermodynamic parameters, that is, ΔH° (enthalpy change), ΔG° (standard free energy), and ΔS° (entropy change), for the adsorption of Th(IV) and U(VI) metals onto Fe3O4@AMCA-MIL53(Al) were calculated. The values of ΔH° and ΔS° were calculated from the slopes and intercepts of the plots of ln Kc versus 1/T by the following equation:

For favorable adsorption, the value of RL should be between 0 and 1.0. It is clear from Table 2 that the values of qm (mg/g) increased from 208.3 to 285.7 and 192.3 to 227.3 with increasing the temperature from 298 to 318 K for Th(IV) and U(VI) metals, respectively. The values of b were also enhanced with increasing temperature, which showed that the intensity of adsorption was increased at higher temperatures. The higher values of b for Th(IV) metal ions showed that the energy of Th(IV) ion adsorption was more than that of U(VI) ion adsorption. The values of RL for both metal ions were HNO3 (59.8) > H2SO4 (49.6) and HCl (84.6) > HNO3 (68.7) > H2SO4 (56.2), respectively (Figure S5). The better recovery of both metal ions in 0.01 M hydrochloric acid was due to the smaller size Cl− ions in

(8)

It was noted that the values of Kf were increased with increasing temperature, which is exhibited in the endothermic nature of adsorption (Table 2). The values of n for both metals at all temperatures were between 1 and 10, designating again that the adsorption was favorable. Because of the higher values H


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ACS Applied Materials & Interfaces Table 4. Thermodynamic Parameters for the Adsorption of U(VI) and Th(IV) Metal Ions onto Fe3O4@AMCAMIL53(Al) Nanocomposite

Table 5. Iron Content Leached from Fe3O4 and Fe3O4@ AMCA-MIL53(Al) under Acidic Conditions leached Fe content (%)

−ΔG° (kJ/mol) metal ion

C0 (mg/L)

ΔH° (kJ/mol)

U(VI)

50 100 150 50 100 150

29.35 32.87 15.48 19.44 21.31 23.65

Th(IV)

Fe3O4@ AMCA-MIL53(Al)

Fe3O4

ΔS° [J/(mol K)]

298 K

308 K

318 K

111.6 117.3 57.8 80.43 83.95 88.70

3.97 2.14 1.83 4.64 3.86 2.66

4.88 3.16 2.16 5.11 4.21 3.94

6.21 4.50 3.00 6.26 5.56 4.42

concentration of HCl solution (M)

24 h

48 h

72 h

24 h

48 h

72 h

0.01 0.1 1.0

7.2 49.84 91.25

8.4 80.16 92.78

8.6 87.13 93.33

0.15 18.06 26.67

1.22 18.54 26.90

1.37 19.86 30.16

MIL53(Al) with the same treatment, showing a significantly improved stability of Fe3O4@AMCA-MIL53(Al) nanocomposite under acidic conditions. The leaching percentage of Fe was at its minimum in 0.01 M HCl. Because of the high stability of Fe3O4@AMCA-MIL53(Al) nanocomposite in 0.01 M HCl, the saturated Fe3O4@AMCA-MIL53(Al) was regenerated using 0.01 M HCl solution.

comparison to the NO3− and SO42− ions. The results showed that the adsorption capacity of Fe3O4@AMCA-MIL53(Al) for both metals is restored satisfactorily, suggesting that Fe3O4@ AMCA-MIL53(Al) is regenerable and can be used several times (Figure 6c,d). The suggested mechanism for the adsorption− desorption behavior for U(VI) and Th(IV) metal ions is given in Figure 7. There were two types of interactions: (i) the electrostatic interactions between the electropositive metal ions and electron-rich oxygen, and (ii) the coordinate bonding between nitrogen and metal ions. The stability of Fe3O4@AMCA-MIL53(Al) nanocomposite was tested in acidic conditions by observing the leached Fe content, and the results are given in Table 5. The leached percentage of Fe in Fe3O4 was 7.2% and 91.25% when the material was suspended in 0.01 and 1 M HCl for 24 h, respectively; it was 0.15% and 26.67% for Fe3O4@AMCA-

4. CONCLUDING REMARKS AND PERSPECTIVES In the present research, a novel magnetic metal−organicframework composite composed of iron oxide magnetic nanoparticles and AMCA-MIL-53(Al) was synthesized and characterized by various techniques. This magnetic metal− organic-framework nanocomposite was used for the removal of Th(IV) and U(VI) metal ions from aqueous environment. The adsorption of both metal ions using Fe3O4@AMCA-MIL53(Al) nanocomposite was fast, and the equilibrium was recognized within 90 min for both metal ions. The leaching studies showed

Figure 7. Synthesis of Fe3O4@AMCA-MIL53(Al) nanocomposite and its adsorption−desorption behavior for metal ions. I


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a considerably improved stability of Fe3O4@AMCA-MIL53(Al) nanocomposite under acidic conditions. As a result of the thermodynamic studies, it was shown that the adsorption of U(VI) and Th(IV) onto Fe3O4@AMCA-MIL53(Al) nanocomposite was spontaneous and endothermic in nature. Hence, MOFs have shown extraordinary performances. Thus, we anticipate that extreme research efforts will be directed toward MOFs and their composites, with emphasis on their practical applications in wastewater treatment.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b10768. Chemicals and reagents, optimization of the AMCAMIL-53(Al) mass, particle size distribution, and EDX studies (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +96614676094. ORCID

Mu. Naushad: 0000-0001-6056-587X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors extend their appreciation to the Deanship of Scientific Research at King Saud University for funding this work through the Research Group No. RG-1436-034.

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REFERENCES

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