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Biopolymer-Coated Magnetite Nanoparticles and Metal-Organic Framework Ternary Composites for Cooperative Pb(II) Adsorption Sada Venkateswarlu, Atanu Panda, Euisoo Kim, and Minyoung Yoon ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00957 • Publication Date (Web): 01 Aug 2018 Downloaded from http://pubs.acs.org on August 2, 2018

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ACS Applied Nano Materials

Biopolymer-Coated Magnetite Nanoparticles and MetalOrganic Framework Ternary Composites for Cooperative Pb(II) Adsorption

SadaVenkateswarlu, Atanu Panda, Euisoo Kim, Minyoung Yoon* Department of Nanochemistry, Gachon University, Sungnam 13120, Republic of Korea

KEYWORDS: PFe3O4, NH2-MIL-125, Nanoparticle, Composition, Pb(II) removal

*Corresponding author. Tel.: +82-31-750-8721. E-mail address:[email protected] 1

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ABSTRACT The imperfections instability, recyclability, and separation factors of MOFs limit their practical applications in the field of catalysis and water purification. Designing MOFs with benign, flexibleand separation is still a critical challenge. Up to know most of the MOFs are coated

with

conventional

synthetic

polymers,

which

are

undegradable

and

carcinogenic.However, no studies have reported the stepwise growth of biocompatible polymer capped Fe3O4 (PFe3O4) NPs onto the NH2-MIL-125 (Ti) surface (ternary composite). In this study, a simple stepwise embedding of PFe3O4NPs on NH2-MIL-125 (Ti) was successfully employed and used for efficient aquatic scavenging, which can allow synergetic cooperative adsorption with functionalities both on biopolymer and MOF surface. The obtained transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images illustrate that the PFe3O4NPs were uniformly embedded onto the surface of the MOF. The composite was employed for the quick and significant removal of Pb(II) from aqueous solution. The effect of various parameters like pH, contact time, initial metal ion concentration, interfering ions, and temperature on the adsorption capacity of the nanoporous composite were examined. The Langmuir model presented the best fitting with a maximum adsorption capacity of 561.7 mg g−1 at pH 5 and 298 K. Moreover, increasing the PFe3O4 precursor on nanoporous NH2-MIL-125 (Ti) decreased the recovery time (21 s), and enhanced the adsorption process, as the same MOF can be recycled six times without obvious loss of the adsorption capacity of Pb(II) in water. Therefore, we can finally conclude that due to the coating of Fe3O4 with biopolymer, the composite showed not only highly efficiency in metal ion adsorption, but also high stability for recycle the material, which is significant for its future practical use in treating industrial waste discharge.

2


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INTRODUCTION Releasing toxic and non-biodegradable heavy metals into the hydrosphere from various industries like mining, metal finishing, oil refining, battery, solar cell manufacturing, fertilizers, painting pigment, dyes, and ceramics has deleterious effects for human health and the environment.1,2 Among the heavy metals, Pb(II) ions cause serious neurological diseases, anemia, nausea, cancer, and renal disease.3-5 Thousands of areas affected by lead poisoning in the USA have been in the spotlight recently,6 since the maximum permissible limit of Pb(II) in drinking water is 15 and 10 µgL−1as endorsed by the United States Environmental Protection Agency (USEPA), and World Health Organization (WHO), respectively.7,8 It is increasingly important to clean wastewater from industries before dumping it into rivers and lakes. Various methods for removing Pb(II) ions from waste aqueous samples have been explored, such as coagulation, co-precipitation, solvent extraction, electrolysis, membrane filtration, ion exchange, reverse osmosis, and adsorption.9-12 These procedures are expensive and may generate a secondary sludge that eventually should be disposed of.13 However, adsorption is considered the most promising technique due to several advantages of costeffectiveness, high efficiency, environment-friendliness, and easy handling.14 Adsorbents such as zeolite, biomass, polymers, aerogels, inorganic materials including nanomaterials like ZnS, metal oxide nanoparticles, and nanotubes have been used for Pb(II) removal.15-18 However, these adsorbents exhibited either relatively low efficiencies, lack of selectivity and inadequate for recyclization. Over the last few years, a lot of attention has been focused on research to develop flexible, sustainable adsorbents for water purification. In this concern, Metal-organic frameworks (MOFs) are highly porous, crystalline, solid materials.19 Their high surface area, large pore volume, and flexible properties, generated increasing attention, and led to ample research in areas such as gathering water from air, gas 3



storage and separation, heterogeneous catalysis, optical sensing, drug delivery, proton conductivity, supercapacitors, photodynamic therapy, and pollutants removal.20-26 Moreover, the applications of MOFs as adsorbents for water purification have attracted increasing attention27,28 due to their highly tunable pore sizes, diverse structures, and open coordinate sites of MOFs allow their applications in water purification as an outstanding adsorption performance and fast adsorption kinetics. Nevertheless, their separation and recycling are difficult and time-consuming and limit their applications for water treatment. Therefore, novel adsorbent materials with high selective adsorption, fast kinetic, and easy recyclability for the removal of heavy metal ions are of great importance. The study of composite adsorbents based on nanoparticles grafted mesoporous materials, has been of significant importance which may combine their plausible advantages.29 Currently, the combination of magnetic MOF composite materials exhibits attractive properties due to the MOFs capability to selectively highly adsorb and the magnetic particles allowing localization and fast recovery.30,32 Among magnetic materials, Fe3O4 nanoparticles (NPs) have tremendous advantages: they can be easily separated under external magnetic fields based on their high ferromagnetic properties, they are easy to functionalize, and are eco-friendly materials.33 Moreover, numerous successful examples of heavy metal ion removal systems using Fe3O4 NPs have been reported.35 In addition, Fe3O4-based MOFs composites having advantages like thermal and chemical stability, high catalytic activity, eco-friendliness, biocompatibility, and excellent reproducibility can present outstanding properties, and can be used for the removal of organic pollutants, heavy metal ions, selective capture of phosphopeptides, and enzymatic digestion.35 According to the literature, the surface chemistry and functionalization of MOFs have been well established.36Given its water stability, flexibility and durable applications,37,38 we have chosen NH2-MIL-125 (Ti) as a robust adsorbent material in our study. As per the 4


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literature, up to now, most of the MOFs are coated with conventional synthetic polymers.39-40 In general, synthetic polymers are undegradable, expensive, carcinogenic can cause cancer, skin diseases, liver damage, chronic bronchitis, birth defects and contaminate the aquatic system.41 Recently, interest towards the use of natural polymers has become a great concern because of their eco-friendly and cost-effective benefits. Therefore, to enhance cost effective, stability, flexibility and sustainability of the MOF composite it is an important approach for developing bio agro polymer capped Fe3O4 NPs. We found no reports on the composition of biocompatible polymer capped Fe3O4 NPs embedded into NH2-MIL-125 (Ti) composite as a metal removal adsorbent. We demonstrate, for the first time, a simple and facile method to synthesize a biocompatible polymer capped Fe3O4NPs embedded NH2-MIL-125 (Ti) composite. At first, the polymer capped Fe3O4 (PFe3O4) NPs were prepared using waste potato peel extract as a capping as well as a reducing agent. Potato peel waste is a classic example of material capable to generate biocompatible polymers. Industrial processing generates nearly 140,000 tons of peel waste worldwide annually.42 Potato peel waste is a rich source of starch, polysaccharides, lignin, etc., and we found several applications for these peels mentioned in the literature.43,44 Aside from using a huge underutilized biopolymer resource, this method has the advantage of being environmentally friendly.45 During the second step, the PFe3O4@NH2-MIL-125 (Ti) composite was prepared using the hydrothermal method. The surface interactions between the MOF, PFe3O4NPs, and Pb(II) were investigated using Fourier-transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). It was expected that the composite material can show interesting adsorption behavior toward heavy metal ions because of its unique functionalities for both biopolymer on PFe3O4 and pore surface of MOF, 5



which can make cooperative metal ion adsorption. The PFe3O4@NH2-MIL-125composite presented a high selective removal performance towards Pb(II) ions in aqueous solution (561.7 mg g−1). The kinetic experiments confirmed fast accessibility of the Pb(II) ions to the adsorbent surface, reaching equilibrium within a few minutes. Several parameters were systematically evaluated, including solution pH, dosage, zeta potential, initial Pb(II) concentration, the effect of interfering ions, contact time, and the leaching tendency. Thermodynamic parameters like Gibbs free energy, enthalpy, and the entropy changes were also studied for the adsorption of Pb(II) onto PFe3O4@NH2-MIL-125 (Ti) MOF composite. Compared to other adsorbents this PFe3O4NPs embedded Ti based framework has excellent water stability and adsorption capacity even after six cycles. Moreover, its use also avoided the potential for secondary pollution. Considering the above advantages this material is one of the best adsorbents for water purification as well as other industrial applications.

EXPERIMENTAL SECTION Synthesis of PFe3O4 NPs and PFe3O4@NH2-MIL-125 (Ti) composite. For the reaction,1350mg, 5.00 mmol of FeCl3·6H2O and 2100mg, 25.0 mmol of sodium acetate were added to 40.0 mL mixture of extract (See experimental section in Supporting Information) and propylene glycol (3:1, v/v) and then the mixture was sonicated for 15 min followed by stirring vigorously for 30 min at 70 °C. Subsequently, the mixture was poured in to a Teflonlined stainless-steel autoclave and applied 175 °C temperature for 6 h. The obtained product was isolated by applying an external magnetic field and washed five times with ethanol and water. The collected particles were dried overnight in a vacuum oven at 70 °C and stored in a stoppered vial until further use. In the above reaction, propylene glycol can act as an additive to the extract for the growth of the benign polymer. Moreover, the prepared PFe3O4 NPs were 6


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split into various amounts like 25, 75 and 150 mg for the synthesis composite materials with different Fe3O4 NP loading. To prepare bioinspired magnetic MOF, 0.177 mL (0.600 mmol) of Ti(IV) isopropoxide and 2-aminoterephthalic acid (217 mg, 1.20 mmol) were added to a 10.0 mL mixture of DMF and CH3OH (1:1, v/v) in a glove box and stirred for 5 min subsequently by the addition of prepared PFe3O4NPs (75 mg, 0.300 mmol) into the above solution. Later, the substrate mixture was poured into a 20 mL Teflon-lined autoclave and treated at 160°C for 12 h in an oven. After the solvothermal treatment, the fabricated mixture was cooled to 25 °C, and a dark brownish pale-yellow composite was recovered by applying an external magnetic field. The composite was washed three times with DMF to remove the unreacted organic ligand followed by an additional wash with CH3OH to exchange DMF. Finally, the synthesized PFe3O4@NH2-MIL-125 was dried under dynamic vacuum at 90 °C for 12 h and was kept in a glass vial until further use. The schematic route of the synthesis process illustrated in Scheme 1. For further comparison studies, bare Fe3O4 nanoparticles without extract/polymer capped and bare Fe3O4@NH2-MIL-125 composite were also prepared to gauge the stability and flexibility.

Batch adsorption experiments. The batch experiments were conducted to estimate the adsorption behavior of the bare PFe3O4 and PFe3O4@NH2-MIL-125 composite towards Pb(II) ions in a pH of 2−10 at 298 K. The effect of additional parameters like the solution pH, contact time, adsorbent dose, initial Pb(II) concentration, and interfering ions on the equilibrium adsorption capacity was also studied. A stock solution of Pb(II) (1000 mg L−1) was developed by dissolving the certain amount of lead nitrate in ultrapure Millipore water. The solution concentrations were maintained at 10–100 mg L−1 by diluting the stock solution. The Pb(II) solution pH was adjusted using 0.1 M HNO3/NaOH. Subsequently, 10 mg of the 7



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PFe3O4@NH2-MIL-125 composite was added to a 100-mL solution containing Pb(II) at a pH of 5. Afterward, the mixture was immersed into on sonication bath for 5 min at 25 °C and subsequently poured to Erlenmeyer flask (125 mL), which was shaken on a thermostatic incubator at the desired temperature (298 K) with a speed of 200 rpm. The magnetite MOF adsorbent was detached from the solution using an external magnet. An inductively coupled plasma-atomic emission spectroscopy (ICP-AES; Thermoscientific iCAP7400-DUO) apparatus was used to determine the concentration of Pb(II) ions. The adsorption kinetics was investigated using an initial Pb(II) concentration of 10 mg L−1 at pH 5 with an adsorbent dose of 10 mg. The mixture was endorsed to react with the adsorbent for a specific time (between 5 and 240 min). The adsorption of the materials at various temperatures (298, 308, 318, and 328 K) was measured using a temperature controlled shaking incubator to understanding the adsorption thermodynamics. The adsorption procedure was carried out in triplicate. The adsorbed amount and the adsorption percentage of Pb(II) ions (adsorbed amount (mg) per g of adsorbent) on PFe3O4@NH2-MIL-125 composite at equilibrium was determined using the following equations:

qe =

(C i − C e )V M

Adsorption (%) =

(1)

(C i − C e ) × 100 Ci

(2)

where qe(mg g−1), Ci and Ce (mg L−1) are the equilibrium adsorption capacity of Pb(II), initial and equilibrium concentration of Pb(II) respectively, V (L),M (mg) are the volume of the solution, adsorbent dosage.

8


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RESULTS AND DISCUSSION Biopolymer coated Fe3O4-MOF composite and its binding with lead ions. The morphological information like structure and composition of the PFe3O4@NH2-MIL-125 MOF was evaluated by TEM analysis. Figure1a, b shows the plain octahedral plate-like structure of the bare NH2-MIL-125 framework with an average diameter of 400−450 nm. Although we cannot find SAED pattern of pure MOF (Figure 1a inset) because of strong electron beam may destroy structural integrity of the MOF, the PXRD data clearly indicate crystalline nature of MOF. PFe3O4 NPs with sizes of 9−13 nm are shown in SI (Figure S1a and b). The particles covered with a polymer layer are clearly marked using arrows. The SAED pattern illustrates the polycrystalline nature of the PFe3O4 NPs (Figure S1b inset). The TEM images (Figure 1c, d, and e) show that the concentration of the PFe3O4 NPs on the surface of NH2-MIL-125 MOF varies. However, the size of the PFe3O4 NPs on the composite is 16–19 nm, most probably due to the temperature related polymer cluster type formation and the magnetic dipolar interaction between the PFe3O4 nanoparticles.47 Figure 1c reveals that the MOF surface is covered partially due to the low concentration of PFe3O4 (25 mg, 0.10 mmol). While increasing the concentration of the PFe3O4 source (75 mg, 0.30 mmol) almost the entire surface of the MOF has been decorated and some of the particles entered its interstitial spaces, which is a direct confirmation of the successful magnetization of NH2MIL-125 with PFe3O4 nanoparticles (Figure 1d). In addition, the HRTEM images (Figure 1f, g, and h) clearly indicate that some of the nanoparticles have entered the interstitial cavity of the MOF (yellow dotted circles) and the lattice fringes of the magnetite material with an interplanar distance of 0.25 nm belongs to the d-spacing of the Fe3O4 (311) with a polymeric layer. The crystalline nature is further confirmed by the presence of an array of bright diffraction dotes indexed to reflections from the (220), (311), and (400) crystal planes, which 9



are assigned to the planes of magnetite in the SAED pattern (Figure 1i). Finally, the external and internal composition of the PFe3O4@NH2-MIL-125 composite was further investigated by STEM-EDX elemental mapping analysis, which reveals that Ti, Fe, C, O, and N are present in the magnetite MOF composite shown in (Figure 2b–f). Figure 2c clearly visualizes that PFe3O4NPs can be found both on the external surface and in the inner shell cavity of the MOF (indicated by white circle with arrows). In addition, no other elemental compositions were identified, attesting to the purity of the product. Moreover, the amount of the PFe3O4 was further increased up to 150 mg, and the particles have predominately occupied the surface of the MOF as shown in (Figure 1e). The above results were further confirmed by powder X-ray diffraction (PXRD) analysis. TEM images of the bare Fe3O4 without extract and 75 mg of bare Fe3O4 NPs doped NH2-MIL-125 composite are shown in (Figure S2a and b). The XRD pattern of the as-prepared NH2-MIL-125, PFe3O4, bare Fe3O4, PFe3O4@NH2-MIL125 (25, 75and 150 mg of Fe precursor) and 75 mg of bare Fe3O4 embedded NH2-MIL-125 composites are shown in Figure 3i–viii. The diffraction peaks and relative intensities of the NH2-MIL-125 in line (ii) are very similar to those of the simulated patterns in line (i) of NH2MIL-125.46 No characteristic peaks for TiO2 phases, such as rutile, and anatase were observed which indicates the purity of the framework. The XRD pattern of PFe3O4, bare Fe3O4 and PFe3O4@NH2-MIL-125 (25,75, and 150 mg of Fe precursor) composite shows the (220), (311), (400), (333), (440), and (533) planes which correspond to diffraction peaks of Fe3O4 at 2θ = 30.07°, 35.4°, 43.05°, 56.93°, and 62.52°, respectively. These peaks are well indexed to the face-centered cubic inverse spinel structure of crystalline Fe3O4 (JCPDS card no. 821533). Additionally, neither Fe2O3 nor other phases were not detected, illustrating the high purity of the sample. However, the diffraction pattern of the PFe3O4@NH2-MIL-125 10


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composite (lines v–vii) remained nearly unchanged compared to lines ii and iii, which could explain why the presence of Fe3O4 did not influence the structure of the framework. Nevertheless, due to the high Fe precursor, the relative MOF peak intensity decreased as shown in Figure 3, line vii. PXRD diffraction peaks corresponding to Fe3O4 (red triangles) and NH2-MIL-125 (green hexagonal) (Figure 3, line viii) clearly confirm the presence of the composite material. In addition, we have also calculated the crystallite size of PFe3O4 NPs using the Debye-Scherrer equation (D = 0.89λ/βcosθ), where D, λ, β and θ are the average crystallite size, wavelength of the Cu Kα irradiation, intensity at the full width at halfmaximum of the diffraction peak, and diffraction angle of the (311) PFe3O4 plane respectively. It should be noted that PFe3O4 has an average crystallite size of 11 nm, whereas the PFe3O4 crystallites in the composite have an average size of 18 nm. The results agree with the TEM analysis. The surface functional groups and the chemical reactions were analyzed by FT-IR and XPS. We used FTIR spectroscopy to analyze the chemical functional groups of the potato peel extract, PFe3O4, NH2-MIL-125 and PFe3O4@NH2-MIL-125 composites shown in Figure S3 (i-v). A broad band appeared at 3520 cm−1 indicating the OH stretching vibration polysaccharides, hemicellulose, and suberin/lignin polymers (Figure S3, line i). The peaks observed at 2931, 2869, 1731,1522, 1090, and 874 cm−1 indicate the CH2, C=O, C=C and C– O–C stretching vibrations of the above biofunctional groups. Figure S3, line ii shows a new peak at 589 cm−1 indicating the Fe–O stretching vibration of PFe3O4, while the peaks at 3485, 2926, 2843 and 1722 cm–1 indicate the involvement of the O-H, polymeric CH2 and C=O stretching vibrations of polysaccharides and carbonyl functional groups of potato peel extract (Figure S3, line ii). We found peaks at 3497, 3354, 1332 cm−1 (Figure S3, line iii) indicating the stretching modes of the –NH and C–N groups of NH2-MIL-125, and the peaks at 1697, 11



1533, 1260, 811 cm−1 indicate the aromatic C–H and acid O–C=O group chelation to the titanium oxo-cluster (–O–Ti–O–: 500 and 800 cm−1).48 However, after reacting with Fe3O4 the peaks at 3354 and 1332 cm–1 slightly shifted to 3341 and 1311 cm−1, respectively indicating bonding between the OH group of PFe3O4 and the NH2 group of the MOF, as well as some of the magnetite particles entering the MOF inner shell cavity electrostatically.49,50 The above results comprehensively indicate the formation of the PFe3O4@NH2-MIL-125 composite (Figure S3, line iv). This hybrid PFe3O4 embedded NH2-MIL-125 ternary composite is used for Pb(II) removal. X-ray photon spectroscopy (XPS) data can provide complete information about the chemical composition and electronic structure of the PFe3O4, PFe3O4@NH2-MIL-125 (with 25(low), 75 (medium) and 150 mg (high) of PFe3O4 NP) composite before and after the adsorption of the Pb(II) ions. Figure 4a, line i shows four main peaks from C 1s, N 1s, O 1s, and Ti 2p, which exist in the NH2-MIL-125 framework. Figure 4a, line ii shows the 284, 533, and 710– 725 eV peaks indicating the presence of C 1s, O 1s, and Fe 2p in biogenic PFe3O4. However, the survey spectra of the PFe3O4@NH2-MIL-125 composite (Figure 4a line iii–v) illustrate the photoelectron peaks of C 1s, N 1s, O 1s, Ti 2p, and Fe 2p indicating the presence of Fe3O4 within the MOF framework; the atomic ratios of N, O, C, Fe, and Ti are shown in (Table S1). In addition, the survey spectra of PFe3O4 and bare Fe3O4 (without extract) (Figure 4i, line i and ii) indicate that line ii has a higher intensity Fe peak than line i. This may be due to line i representing the sample with a polymer layer on its surface, which reduces the intensity of the Fe 2p peak. The high-resolution spectrum of Ti 2p (Figure 4b) exhibiting two binding energy peaks at 459.2 and 464.6 eV that can be attributed to Ti 2p3/2 and Ti 2p1/2, represents the phase purity of the framework. Meanwhile, the high-resolution Fe 2p spectra of the PFe3O4@NH2-MIL-125 composites (low, medium and high), PFe3O4 and bare Fe3O4 12


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exhibited two peaks with binding energies of 710.5 and 724.7 eV (Figure 4c, line i–v), belongs to Fe 2p3/2 and Fe 2p1/2. Moreover, the peak positions of all Fe spectra are the same. No additional satellite peak at 719 eV (maghemite phase), was observed illustrating the phase purity of Fe3O4 in the composite. These results illustrate the unchanged state of Fe during the synthesis of the magnetic MOF composite. Figure 4d, line i shows the composite deconvoluted C 1s spectrum binding energies at 284.7, 285.9, 286.8, and 288.4 eV, which were attributed to the C–C/C=C, C-N, C-O, and O-C=O groups respectively.51 The O 1s peak in the NH2-MIL-125 can be deconvoluted by the three sub peaks at 529.8, 531.5, and 534.3 eV indicating the presence of Ti–O bonds of oxygen atoms from the titanium-oxo cluster, while the O–C/C=O groups are the carboxylate oxygen in the framework (Figure 4e, line i). Moreover, Figure 4e, line ii shows a 530.1 eV peak in the O 1s spectra of the PFe3O4, corresponding to the anionic oxygen of PFe3O4. In addition, a strong intensity 533.7 eV peak of the OH functional group of PFe3O4 confirmed the existence of many hydroxyl groups on the surface of the PFe3O4 spheres, whereas, after reaction with MOF it was shifted to 533.5 eV (Figure 4f, line i). Based on the above result, PFe3O4 polymer spheres play a supporting role in the bonding of Pb(II) with ‘N’ in the MOF composite as a joint contribution,49,50 thus enhancing the removal capacity of the composite. Whereas, the 399.7 and 402.1 eV peaks are attributed to the N 1s spectrum of the –NH groups (Figure 4g, line i). The above results confirm the formation of successful formation of the ternary composite. Thermogravimetric analysis (TGA) was carried out to understand the thermal stability of the synthesized materials. (Figure S4, line i) shows that the weight loss of pure NH2-MIL-125 occurs in three stages. The initial weight loss of 1.9% from 35 to 100 ºC represented the decomposition of the guest methanol molecules. The main weight loss of 30.1% occurred between the second and third steps from 110 to 382 ºC and corresponded to the solvent 13



molecules and organic linker decomposition. An approximately 10.2% weight loss was detected when the temperature increased from 400 to 700 ºC, which indicated the degradation of the framework and lead to TiO2 as a precursor. However, for PFe3O4 (Figure S4, line v) the weight loss occurred in two steps: the first step at 100 ºC the 1.6% loss indicated the loss of water molecules, while the 3.4% weight loss between 170 and 350 ºC was attributed to the decomposition of the polymeric surfactant functional groups. Figure S4, line ii–iv shows the TGA profile of the PFe3O4@NH2-MIL-125 composites. Compared to the pure MOF (42%), the total weight of hybrid magnetite MOF composites with low, medium and high PFe3O4 NP content presented weight losses of 38.5%, 22.1%, and 12.5%, respectively. These were due to the PFe3O4 NPs present both on the external surface and inside the internal shell cavity of the MOF enhancing the stability of the composite. The N2 adsorption-desorption isotherms of the as-synthesized samples are shown in Figure 5. The specific surface area of the pure NH2-MIL-125 and PFe3O4@NH2-MIL-125 (25, 75 and 150 mg of PFe3O4 NP) composites were 1277.92, 740.26, 610.58 and 145.37 m2 g−1 respectively, as calculated using the Brunauer–Emmett–Teller (BET) technique shown in Figure 5a–d. In addition, we also compared the surface area of pure NH2-MIL-125 with the other reported values (Table S2). The isotherms exhibit a typical type IV isotherm pattern with an H4-type hysteresis loop, indicating the mesoporous feature of the nanostructure according to the IUPAC classification. The pore-size distributions estimated using the density functional theory (DFT) model for pure NH2-MIL-125 was 0.65 to 1.26 nm, for composite materials Barrett−Joyner−Halenda (BJH) model was applied and pore-size were 3.3, 5.4, and 25 nm corresponding to PFe3O4@NH2-MIL-125 (low, medium and high, respectively), thus illustrating the changing in porosity from micro- to mesoporosity. Based on the single-point adsorption, the total pore volume of the NH2-MIL-125 and 75 mg of PFe3O4 embedded NH214


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MIL-125 composite were calculated to be 0.5493 cm3 g−1 at P/P0 = 0.9929 and 0.4013 cm3 g−1 at P/P0 = 0.9929. Moreover, the size distribution of the corresponding materials is shown in the insets of Figure 5a–d. Even though the dissemination of PFe3O4 on the surface of the MOF resulted in a decrease in the pore volume and surface area of the PFe3O4@NH2-MIL125 composites compared to the pure MOF, the composite still has a high mesoporous surface area, which is excellent for the adsorption process. In addition, the specific surface area of bare PFe3O4 is 45.80 m2 g−1and the pore-size 27 nm as shown in the inset of Figure S5. The tiny PFe3O4 nanoparticles are well covered by a polymer layer that may have intrinsic porosity due to the polymer coated on the surface of the nanoparticles. The polymer layer shows the pore size of 27 nm, which can lead mesoporous character. Both of the polymer layer and the MOF can act as a non-magnetic layer resulting in the reduction of the original magnetic saturation of MNPs. Vibrating sample magnetometer (VSM) measurements were performed at room temperature in the −10 000 to 10 000 G field range to gauge the magnetization of the as-synthesized magnetic materials composite. Figure 6A, line i–iv shows the hysteresis loops of PFe3O4 and the PFe3O4@NH2-MIL-125 composite (150, 75, and 25 mg of PFe3O4 NP), with saturation magnetization (Ms) values of 69.5, 62.7, 51.6, and 29.4 emu g−1, respectively, with nonzero remnant magnetization (Mr), and coercive force (Hc) with a nonlinear hysteresis loop illustrating the ferromagnetic character. The enlarged hysteresis loops at field strengths between −700 and 700 G was shown in Figure 6A, inset confirmed the ferromagnetic character of PFe3O4 and the PFe3O4@NH2-MIL-125 composites. Compared to bare Fe3O4 (75.6 emu g−1, Figure S6), the saturation magnetization (Ms) of PFe3O4, PFe3O4@NH2-MIL125 composite was decreased due to the biopolymer act as a protecting layer on the surface of magnetic nanoparticles. The biopolymer and MOF in the ternary composite contributes as a 15



non-magnetic material with respect to magnetic nanoparticles. Partial oxidation of Fe2+ in the Fe3O4 to Fe3+ can also reduce the original magnetic saturation of Fe3O4.52 However, saturation magnetization of the composite is still strong enough for the separation of pollutant particles and gases using an external magnetic field. We conducted a physical experiment to test the separation capacity of the synthesized adsorbent. Figure 6B, a–f shows that the 75 mg PFe3O4 embedded NH2-MIL-125 composite was separated using an external magnetic field within 21 s. This is one of the fast-magnetic separations compared to other reported materials see (Table S3). In addition, the first time we have performed the test, the Tyndall effect (Figure 6B, g–i): as the separation time increased, the particles were gradually separated, and the laser beam intensity was decreased. After 21 s, we achieved complete separation and the beam disappeared due to the absence of the Tyndall effect, which confirmed the accurate separation of the adsorbent by the external magnetic field. PFe3O4@NH2-MIL-125 composite have high surface area, good dispersibility and short time response emergent for water purification technology.

Effect of pH and initial concentration of Pb(II) on adsorption. The adsorption capacity of heavy metal ions mainly depends on the solution pH as it can significantly affect the activity of the metal binding sites. Thus, pH effect on the adsorption of Pb(II) ions was investigated as shown in Figure 7a. A series of experiments was conducted at pH ranging between 2 and 10 at 298 K with a 10 mg L−1 initial concentration of Pb(II). The efficiency of Pb(II) ion removal for PFe3O4, pure NH2-MIL-125 and the PFe3O4@NH2-MIL-125 composite was low at pH < 3. As the pH value decreased, the surface functional groups are protonated (H+) and compete with the Pb(II) adsorption. While increasing the pH, the removal efficiency reached its highest at pH 5 (99.1%), since to the surface functional groups are deprotonated and the 16


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molecules exist in nearly neutral form. However, the removal efficiency gradually decreased at pH > 7 due to the increase in anionic character of the functional groups. Moreover, pHdependent surface protonation/deprotonation is gauged by the ζ potential method. The isoelectric points of PFe3O4 and magnetic MOF adsorbent were located between pH 5–6 and 6–7, respectively (Figure 7b, line i–ii). The slight change in isoelectric point is due to the presence of the amino group of the MOF. The net surface charge is positive at pH < 5 which is unfavorable for the Pb(II) adsorption due to Columbic repulsion force between the Pb(II) ion and adsorbent. However, at pH > 7 Pb(II) exists as Pb(OH)+ at pH 8, and the precipitated forms: Pb(OH)2, and Pb(OH)3 exist at pH>8.52 Consequently, pH 5 was chosen as the optimal pH for the succeeding experiments. It can be observed that the rate of adsorption capacities of PFe3O4 (line i), pure NH2-MIL-125 (line ii), and PFe3O4@NH2-MIL-125 composites (150, 25, and 75 mg of PFe3O4 NP (lines iii, iv, and v)) were 60.3%, 83.9%, 87.1%, 90.6%, and 99.1%, respectively. For PFe3O4 and MOF, the functional groups –OH and –NH2 participate individually to remove Pb(II) ions. By contrast, for the PFe3O4 embedded NH2-MIL-125 composite both groups can act as chelating agents, due to the multifunctional active sites on the surface of the composite. According to Person and Brown, who researched the surface, aromatic –NH- and [M-O]-H+ exhibit strong metal-ligand interactions with the Pb(II) ion,53 which not only enhance the adsorption efficiency, but also magnify the separation process. In addition, we tested the removal efficiency of the bare Fe3O4 and Fe3O4@NH2-MIL-125 composite as shown in (Figure S7, line i and ii), and the removal efficiency was determined to be 42% and 85.3%, respectively. Therefore, 75 mg of the PFe3O4 embeddedNH2-MIL-125 composite was used as an optimum adsorbent throughout the experiments. Furthermore, to gauge the leaching behavior of the synthesized materials, we compared the without polymer capped bare Fe3O4 and Fe3O4@NH2-MIL-125 composite. Figure S8 illustrates the leaching 17



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behavior of the adsorbents at different pH. The leaching experiment result clearly indicates that the PFe3O4 embedded NH2-MIL-125 composite was propitious for the enhanced removal of Pb(II) ions. Figure S9 shows the adsorption of Pb(II) onto the PFe3O4@NH2-MIL-125 composite using initial Pb(II) concentrations between 10 to 100 mg L−1 at pH 5.0 and 298 K. The adsorbent removed virtually all the Pb(II) ions at low concentrations. Despite high concentrations, metal ions quickly occupied the active sites of the adsorbent, and the number of available adsorption sites was limited. Moreover, the adsorption decreased as the repulsive forces between the adsorbed Pb(II), and Pb(II) ions in the solution was increased. It can be concluded that the initial concentrations of Pb(II) plays a significant role in the adsorption process. Given real water analysis, high Pb(II) removal at low concentrations is considered of great importance. Therefore, a 10 mgL−1concentration of Pb(II) was used as an optimum concentration throughout the experiments. In addition, effect of adsorbent dosage, kinetic and thermodynamic studies are also presented in supporting information (Figure S10-S13 and Table S4, S5). Lead ion adsorption isotherms. The relationship between the equilibrium adsorption capacity and equilibrium concentration at a certain temperature was studied using adsorption isotherms, Langmuir’s and Freundlich’s being the most commonly used ones. Langmuir’s model is based on the assumption that maximum adsorption corresponds to the formation of an adsorbate monolayer on the adsorbent surface. The energy of adsorption is constant, and no adsorbate transmigration occurs on the surface. The mathematical form of Langmuir equation is:

(3) where qe is the equilibrium adsorption capacity of the metal on the adsorbent concentration 18


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(mg g−1), Ce is the metal ion concentration at equilibrium in the solution (mg L−1), qm is the maximum capacity of PFe3O4@NH2-MIL-125 (mg g−1), and b (L mg−1) is the equilibrium constant with respect to the sorption energy. Figure S14a illustrates a linear relationship between Ce/qe and Ce at pH5.0 at various temperatures. The maximum adsorption capacity (qm) values are 561.7, 476.9, 416.6 and 322.5 mg g−1 at 298, 308, 318, and 328 K, respectively. These values decreased as temperature increased, revealing the exothermic nature of the Pb(II) adsorption process. The decrease in Langmuir constant (b) values from 561.7 to 322.5 mg g−1 with the rise in temperature between 298 to 328 K indicated the poor affinity of Pb(II) for the as prepared PFe3O4@NH2-MIL-125 composite at a higher temperature. The values of the regression coefficient: 0.999–0.996 were close to 1 indicating the isotherm is well fitted. Moreover, the Pb(II) adsorption capacity of PFe3O4 and pure NH2MIL-125 was also calculated. A linear relationship between Ce/qe and Ce at pH 5.0 at 298 K is illustrated in Figure S15a and b. The adsorption capacities for PFe3O4 and pure NH2-MIL125are 51.15 (R2=0.9945) and 232.55 (R2=0.9965) mg g-1, respectively. For PFe3O4 and NH2MIL-125, the absence of cooperative adsorption of -OH in PFe3O4 and -NH2 in MOF decreases the adsorption capacity of Pb(II) ions compared to the composite material. In addition, an essential characteristic of the Langmuir isotherm can be described by the dimensionless equilibrium factor (RL), which can be calculated using the following equation:

(4) where Co (mg g−1) is the initial metal concentration and b (L mg−1) is the Langmuir constant. For favorable sorption, it is suggested that the RL value ranges between 0 and 1. In this study, the calculated RL value for the adsorption of Pb(II) onto the adsorbent was 0.3703 at 298 K, which lies between 0 and 1. This indicates that the adsorption of Pb(II) is favorable for the 19



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PFe3O4@NH2-MIL-125 composite. The sorption energy is not homogeneous for all sorption sites, since this involves equilibrium on a heterogeneous surface, as explained by the Freundlich model. The linear form of the Freundlich adsorption isotherm model is expressed as follows: (5) where Kf (mg g−1) and n are the Freundlich isotherm constants relating to the adsorption amount and the intensity of adsorbents; Kf and n were estimated by the slope and intercept of lnCe vs lnqe graph at various temperatures as shown in (Figure S14b). The calculated values for the parameters of both Langmuir’s and Freundlich’s models were summarized in Table S6, where all n values are greater than 1, indicating that the adsorption is favorable at the studied conditions. PFe3O4 and pure NH2-MIL-125 was also studied using the Freundlich’s isotherm model (Figure S16 a, and b), where the correlation coefficient R2=0.9572 and 0.9783. Considering the correlation coefficient (R2), it can be clearly seen that the adsorption process can be well fitted using the Langmuir adsorption model, due to lower correlation coefficient (R2) of Freundlich’s isotherm model. This result indicates that the sorption and binding energy is uniform on the whole surface of theFe3O4@NH2-MIL-125 composite. The calculated qm is 561.7 mg g−1, which is a superior value comparable to that of most reported recent adsorbents as seen in Table 1.55-64 Our material shows high efficiency in Pb(II) removal resulting in as low as 10 ppm level with no other leaching element from the composite. Moreover, the adsorbent may also useful for very low concentration to achieve recommended WHO level.

Lead metal ion adsorption mechanism analysis. The state of the arts of this work is 20


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providing unique platform for highly efficient lead metal ion adsorption with high recyclability. For the explanation of the high efficiency of the composite in lead adsorption, adsorption mechanism should be analyzed. Spectroscopic analysis results including FT-IR and XPS can give crucial idea to understand the Pb(II) adsorption mechanism on PFe3O4@NH2-MIL-125 composites. One of the major drawbacks of bare particles is huge agglomeration and poor stability in acidic medium, which is confirmed by leaching experiments (Figure S8 and Scheme 2a). To give better stability Fe3O4 nanoparticles, the bare particles were capped with a biopolymeric layer, which can act as glue to make better contact with MOF. In addition, the biopolymer layer can help to enhance lead adsorption for aminoMOF with cooperative lead adsorption. The FT-IR peaks at 3485 and 3341 cm−1 were shifted to 3450 and 3323 cm−1, respectively after Pb(II) adsorption illustrating the binding of Pb(II) ions by the –NH2 on MOF and -OH on biopolymer (Figure S3, line v). Moreover, the C–N stretching of the aromatic amine at 1311 cm−1 was also shifted to 1271 cm–1, which confirms the formation of the –C-N-Pb(II)-O-Fe bond.50 XPS data further supports the result of FT-IR data. A new XPS peak at approximately 140 eV attributed to the presence of Pb(II) on the surface of the PFe3O4@NH2-MIL-125 composite (Figure 4a, line v). After the Pb(II) adsorption, the deconvoluted C 1s, O 1s, N 1s spectra (Figure 4d (ii), f (ii), and g (ii)) presented new low intensity peaks at 285.2, 533.1, 399.1, and 401.6 eV, respectively indicating that Pb(II) was coordinated via O and N atoms in the PFe3O4@NH2-MIL-125 composite. This result implies that the Pb(II) removal mechanism predominantly contributed the substitution of −OH on biopolymer and amine of NH2-MIL-125. The quantitative XPS analysis data illustrates that the atomic ratio of N and Pb (Table S7) was 1.3:1 which demonstrates the 1:1 binding between the PFe3O4@NH2-MIL-125 MOF and Pb(II) ion. Finally, the high-resolution XPS spectra of the Pb(II)-adsorbed PFe3O4@NH2-MIL-125 21



showed two XPS peaks at 138.7 and 143.5 eV, illustrating the Pb(II) binding energies of the 4f7/2 and 4f5/2 orbitals. In addition, no peak was observed at 136.5 eV, thus indicating the absence of neutral lead. This dual chelating stance enhances the removal percentage of Pb(II) from aqueous solutions. Therefore, the biopolymer on Fe3O4 and amine functional group on MOF play a cooperative role for highly efficient adsorption of lead (Scheme 2b).

Metal ion adsorption selectivity. For real water treatment applications, an adsorbent should not only have high sorption capacity and fast kinetics but should also exhibit excellent selectivity toward the target metal ion in mixed metal ions solutions. Thus, it is necessary to determine the influences of the co-existing heavy metals on the removal of Pb(II). Adsorbent ion selectivity experiments were carried out with diverse coexisting ions like Ag+, Be2+, Cd2+, Zn2+, Ni2+, Mn2+, Mg2+, As3+, and Cr3+, with 10 times higher concentration (100ppm) compared to the target ion Pb(II) (10 ppm). Figure S17 shows the PFe3O4@NH2-MIL-125 (Ti) composite exhibits grater removal efficiency towards Pb(II) ions. This was attributed to the fact that hard acids prefer to associate with hard bases (Pearson classification),65 while borderline acids prefer to bind to borderline hetero atoms containing the Ph–NH– and–O– groups. Coexisting ions like Be2+, Mg2+, Cr3+, and As3+ acted as hard acids, while Ag+ and Cd2+ are soft acids and had no negative impact on the removal efficiency of Pb(II). However, Zn2+ and Ni2+ acted as borderline acids similar to Pb(II). Nevertheless, according to the hydration energy values, the metal with the lowest hydration energy will be effectively bound by the heteroatoms.66 Thus, Pb(II) (−1485 kJmol−1) has the lowest hydration energy compared to Zn2+ (−2047 kJ/mol) and Ni2+ (−2096 kJ/mol) and can be efficiently and rapidly captured by the magnetic MOF composite even in a competitive metal environment, therefore enhancing the adsorption selectivity. 22


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Desorption and reusability studies. From an industrial and sustainable development point of view, the reusability of the adsorbent is most important. Before analyzing the reusability of the adsorbent, we carried out desorption processes in batch mode using different eluents, like 0.1 M NaOH, 0.1 M HNO3, and 0.1 M EDTA. First, the adsorbent was added to a 0.1 M HNO3 solution and the mixture was sonicated for 15 min. The same procedure was repeated for the 0.1 M NaOH and 0.1 M EDTA solutions. It was found that the desorption percentage using them were 42.8, 85, and 97.9%, respectively. Due to the high chelating stability constant of EDTA with Pb(II) (logK = 18.04), EDTA showed excellent regeneration properties without leaching of adsorbent.67 We also examined the stability of the composite during the regeneration cycles (Figure S18 and S19). Six cycles of adsorption-desorption study were conducted to examine the capability of the PFe3O4@NH2-MIL-125 composite (Figure S20). In addition, we also tried to remove Pb(II) using hot water condition. However, the composite adsorbent cannot be regenerated using only hot water. In addition, we have used different pH values for regeneration. At pH < 4, iron leaching was observed, whereas MOF structure was destroyed slightly at pH > 11. For our composite material, 0.1 M EDTA solution treatment may be the best condition for regeneration and a similar regeneration process was also recently reported by others. 67 For simple removal of Pb(II) and regeneration of adsorbent, this process will be efficient because the adsorbent can be simply separated from the solution by filtration. The adsorption capacity of the adsorbent decreased only by 17% during the cycles, which indicates that thePFe3O4@NH2-MIL-125 composite is a suitable adsorbent for water purification not only for metal ions but also for organic molecules pollution and as a catalyst.

23



CONCLUSION MOFs have fascinating advantages over other adsorbents like their tunable pore sizes, structures, and open coordinate sites, which allow to capture diverse hazardous cationic and other organic species. We constructed a biopolymer capped Fe3O4 NPs embedded MOFs ternary composite. The PFe3O4 nanoparticles were synthesized using eco-friendly waste potato peel extract, and propylene glycol used as an additive. Due to high boiling point and hydrophilic nature allow not only modify the physical properties of biopolymers but can also improve strength and flexibility. The particles were covered with a biopolymer layer, which can prevent the chemical decomposition of the nanoparticles, enhanced their stability and allowed them to be easily dispersed on the surface of the MOF. The nanoparticles growth on the surface of the MOF was well controlled by hydrothermal method by varying the concentration of PFe3O4. The obtained novel magnetically driven multifunctional MOF composite possessed superior ferromagnetic properties with high efficiency, excellent separation, and quick recyclable properties. The multifunctional groups on the surface of the composite enhanced the removal capacity of the Pb(II) ions and the proposed mechanism was corroborated through XPS and FTIR characterization. The removal capacity of the PFe3O4@NH2-MIL-125 MOF composite: 561.7 mg g−1was one of the best values compared to the existing literature. It should be noted that the cooperative adsorption between biopolymer and MOF, the composite shows very large adsorption capacity with fast equilibration. In addition, the Pb(II) ions could be effectively desorbed by a 0.1 M EDTA solution, thus the composite could be reused for successive cycles. Due to the enhanced stability by biopolymer coating, the composite can be reusable more than 5 times without using its adsorption efficiency. The high saturation magnetic value 51.6 emu g−1 lead to one of the fastest separations (21 s) in aqueous solution. The novel multifunctional composite 24


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material may also useful for gas sorption-desorption, catalytic applications, drug delivery and energy storage devices. Additional research regarding such applications is now in progress.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website and include chemicals, extraction procedure, TEM analysis, FT-IR analysis, TGA analysis, BET analysis, ζ-potential measurement, adsorption parameters, tables and figures summarizing Pb(II) adsorption kinetics, isotherms, and thermodynamics studies, leaching experiments, selective metal ion adsorption, and regeneration/recycling experiments (PDF).

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (M.Y.). Author Contributions All authors contributed to this study. All authors have given approval to the final version of the paper. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Ministry of Education (NRF-2016R1D1A1B03930948, M. Y.) and by Ministry of Science and ICT (NRF-2017R1C1B5076834, S. V.). The X-ray diffraction experiment at 25



PLS-II, 2D-SMC beamline was supported in part by MEST and POSTECH.

ABBREVIATIONS BET, Brunauer−Emmett−Teller; FT-IR, Fourier transform infrared; HRTEM, high resolution transmission electron microscopy; MOF, metal organic framework; PFe3O4 NPs, polymer capped magnetite nanoparticles; SAED, selected-area electron diffraction pattern; TEM, transmission electron microscopy; TGA, thermogravimetric analysis; VSM, vibrating sample magnetometer; XPS, X-ray photoelectron spectrometry; XRD, X-ray diffraction

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Schemes

Scheme 1. Schematic representation of the PFe3O4@NH2-MIL-125 (Ti) composite synthesis.

35



Scheme 2. a) Synthesis and stability of bare Fe3O4, PFe3O4 and PFe3O4@NH2-MIL-125 composite; b) Proposed Mechanism for the chelating of Pb(III) on the surface of the PFe3O4@NH2-MIL-125 (Ti) composite.

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Figures

Figure 1. TEM images of (a, b) pure NH2-MIL-125, and (c, d and e) various concentration 25, 75 and 150 mg of PFe3O4 NPs embedded NH2-MIL-125 composites; (f, g and h) HRTEM images of PFe3O4@NH2-MIL-125 (75 mg of PFe3O4 NP) composite; and (i) SAED patterns of MOF composite.

37



Figure 2. a) TEM image of PFe3O4@NH2-MIL-125 (75 mg of PFe3O4 NP) composite; Elemental mapping of b) titanium, c) iron, d) nitrogen, e) oxygen and f) carbon.

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Figure 3. X-ray diffraction data of (i, ii) stimulated and synthesized pure NH2-MIL-125, (iii), PFe3O4, (iv) bare Fe3O4, (v–vii) PFe3O4@NH2-MIL-125 composites (25, 75 and 150mg of PFe3O4 NP, respectively), and (viii) bare 75 mg of Fe3O4 embedded NH2-MIL-125 composite having with peak positions of Fe3O4 (red triangles) and NH2-MIL-125 (green hexagons).

39



Figure 4. a) Typical X-ray photoelectron survey scan spectra of (i) pure NH2-MIL-125, (ii) – (iv) PFe3O4@NH2-MIL-125 composite (25, 75, and 150 mg of PFe3O4 NP, respectively), and (v) PFe3O4@NH2-MIL-125 (75 mg) after the adsorption of Pb(II);b) and c) high-resolution XPS of Ti 2p and Fe 2p: (i)–(iii), PFe3O4@NH2-MIL-125 (25, 75 and 150 mg, of PFe3O4 NP, respectively), (iv) PFe3O4 and (v) bare Fe3O4; d) C 1sin PFe3O4@NH2-MIL-125 (i) before and (ii) after Pb(II) adsorption; e) O 1s in (i) pure NH2-MIL-125 and (ii) pure PFe3O4; f) O 1s in PFe3O4@NH2-MIL-125 (i) before and (ii) after Pb(II) adsorption; g) N 1s in PFe3O4@NH2-MIL-125(i) before and (ii) after Pb(II) adsorption; h) Pb 4f in Pb(II)-adsorbed PFe3O4@NH2-MIL-125; and i) survey scan spectra of (i) PFe3O4 and (ii) bare Fe3O4. 40


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Figure 5. N2 adsorption−desorption isotherms (77 K) of a) pure NH2-MIL-125 and b)–d) PFe3O4@NH2-MIL-125 composites (25, 75 and 150 mg of PFe3O4 NP, respectively); inset: pore size distribution.

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Figure 6. (A) Magnetization curves of (i) PFe3O4 (black) (left inset: expanded view of line i), (ii–iii) PFe3O4@NH2-MIL-125 composites (150, 75, and 25 mg of PFe3O4 NP, respectively) (right inset: enlarged loops of lines ii, iii, and iv). (B) Magnetic separation over time (a, 0 s; b, 3 s; c, 6 s; d, 10 s; e, 16 s; f, 21 s) for a composition of 2 mg in 20 mL. The Tyndall effect with respect to time and magnetic separation (g, 10 s; h, 16 s; and i, 21 s).

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Figure 7. (a) Effect of pH on the adsorption of Pb(II) by (i) PFe3O4 (black), (ii) NH2-MIL125, (iii) PFe3O4@NH2-MIL-125 (150 (high), 25 (low), and 75 (medium) mg of PFe3O4 NP, respectively); (b) ζ- potential of (i) PFe3O4 and (ii) PFe3O4@NH2-MIL-125 composite.

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Tables

Table 1. Comparison of adsorption capacities of Pb(II) ions onto the PFe3O4@NH2-MIL-125 (Ti) composite for various adsorbents. Type of adsorbent

Titanate/Fe3O4-ICPS

Adsorption Equilibrium Removal Reusability Ref. capacity qm time (min) efficiency cycles (mg g−1) (%) 382.3 60 >90 2 55

Fe3O4-TETA-CMCS-FAAS

370.6

90

97.8

5

56

Fe3O4-SO3H MNP

108.9

120

99.0

10

57

CD-Fe3S4

256.0

1440

94.3

-

58

NH2-functionalized

166.7

120

99.9

-

59

819.7

60

98.4

5

60

Fe-modified MOF-5

344.8

120

-

-

61

Amino-functionalized

1795.3

-

61.4

6

10

POP-NH2

523.6

300

-

4

62

Zn(II)-based MOF

616.6

180

99.21

4

63

Carbomethoxy-

63.0

5

96

-

64

561.7

90

99.1

6

This

Zr-MOFs HPA-GO

Zr-MOFs-CUF

functionalized MOF PFe3O4@NH2-MIL-125

work

composite

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TOC Graphics

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Adsorption - ACS Publications - American Chemical Society

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