Magnetically Recyclable Fe 3 O 4 / GO-NH 2 / H 3 PMo 12 O 40 Nanocomposite : Synthesis , Characterization , and Application in Selective Adsorption of Cationic Dyes from Water

In this study, the PMo12O40 3− polyanion was immobilized chemically on amino functionalized magnetic graphene oxide nanosheets. The as-prepared ternary magnetic nanocomposite (Fe3O4/GO-NH2/H3PMo12O40) was characterized by powder X-ray powder diffraction (XRD), fourier transformation infrared spectroscopy (FTIR), Raman spectroscopy, energy dispersive spectroscopy (EDX), field emission scanning electron microscopy (FESEM), BET surface area measurements, magnetic measurements (VSM) and atomic force microscopy (AFM). The results demonstrated the successful loading of H3PMo12O40 (~36.5 wt.%) on the surface of magnetic graphene oxide. The nanocomposite showed a higher specific surface area (77.07 m2/g) than pure H3PMo12O40 (≤10 m 2/g). The adsorption efficiency of this nanocomposite for removing methylene blue (MB), rhodamine B (RhB) and methyl orange (MO) from aqueous solutions was evaluated. The nanocomposite showed rapid and selective adsorption for cationic dyes from mixed dye solutions. The adsorption rate and capacity of Fe3O4/GO-NH2/H3PMo12O40 were enhanced as compared with GO, GO-NH2, Fe3O4/GO-NH2, and H3PMo12O40 samples due to enhanced electrostatic attraction and hydrogen-bonding interactions. The nanocomposite is magnetically separated and reused without any change in structure. Thus, it could be a promising green adsorbent for removing organic pollutants in water.


Introduction
Industrial activities release an increasing amount of contaminants, such as metal ions, organic dyes, and cleaning agents, which has raised public concern. 1,2So, wastewater treatment has attracted much attention in the past decades because of grievous effluent discharge of some organic dyes from plating, textile, and printing paper, plastic, cosmetic, pharmaceutical, and food industries that are resistant to biological degradation, making them quite difficult to remove from the wastewater. 3,46][7][8][9] Owing to their complex aromatic molecular structures, dyes are generally sta-ble to light, heat and oxidizing agents. 10Therefore, effective removal of dyes from dye-wastewater is essential.Among the various technologies such as photocatalytic degradation, 11 electrochemical degradation, 12 and adsorption, 13 adsorption is considered one of the most efficient and economical methods for water purification. 14Many polymeric and inorganic adsorbents such as carbonaceous nanomaterials, 15 porous metal oxides, 16 clays, 17 chitosan, 18 zeolites, 19 and so on 20,21 were developed for removing pollutants from aqueous solutions.However, such adsorbents are associated with certain problems that limit their practical applications, such as low adsorption capacity, slow adsorption rate, and difficult separation of the adsorbents. 22 are generally poor at selectively removing the targeted organic dye wastes.Hence, in this regard, it is extremely imperative to find a new desirable adsorption material, which not only is capable of reducing the organic dyes in dye-wastewater with high efficiency and fast adsorption rate but also can achieve selective separation and recovery of raw materials.
Polyoxometalates (POMs), as an outstanding class of anionic metal oxide clusters, have attracted great attention due to their earth-abundant source, rich topology and versatility, controllable shape and size, oxo-enriched surfaces, high electronegativity etc., 23 which have various applications in many fields, such as catalysis, 24 optics, 25 magnetism, 26 biological medicine, 27 and dye adsorption. 28The strong attraction of POMs to cationic dyes suggests that they are potential and suitable adsorbents for selectively capturing cationic dyes.However, there are still obvious disadvantages for POMs as adsorbents: (i) their relatively small surface area seriously obstructs the accessibility to the active sites and (ii) their excellent solubility in aqueous solution determines that they cannot be reused and recycled in the process of wastewater treatment.Therefore, plenty of remarkable work has been done to encapsulate POMs into porous solid matrices, such as activated carbon 29 and silica 30 for creating composite materials.Unfortunately, these methods sometimes lead to low POM loading; it is thus of vital significance to search for an applicable solid matrix to immobilize POMs, which might greatly improve their adsorption ability for target dyes.
2][33][34][35][36][37][38][39] In addition, in comparison with other carbonaceous nanomaterials, GO may be more environmental friendly and have better biocompatibility. 40However, it is difficult to separate it from aqueous solution because of its small particle size, causing serious health and environmental problems once it is discharged into the environment. 41The centrifugation method needs a very high rate and the traditional filtration method may cause blockages of filters.Compared with traditional centrifugation and filtration methods, the magnetic separation method is considered as a rapid and effective technique for separating nanomaterials from aqueous solution. 42,43][46][47][48] On the basis of the above discussion, in this work, amino functionalized magnetic graphene oxide (Fe 3 O 4 / GO-NH 2 ) was synthesized by a facile method and used as a novel support for immobilizing Keggin-type PMo 12 O 40 3− anions.This magnetically recoverable ternary nanocomposite material (Fe 3 O 4 /GO-NH 2 /H 3 PMo 12 O 40 ) was pre-pared by a simple acid-base electrostatic interaction between H 3 PMo 12 O 40 and amino groups of Fe 3 O 4 /GO-NH 2 .For one thing, PMo 12 O 40 3− anion with highly electronegative and hydrophilic properties and structural stability could be utilized as a potential adsorbent for removal of the cationic dyes in dye-wastewater.For another, magnetic GO possesses outstanding porosity and extremely large surface area, and it is insoluble in water, which is an appropriate solid matrix to anchor Keggin-type PMo 12 O 40 3− anions.The combination of polyoxoanions and Fe 3 O 4 / GO-NH 2 could improve the surface area and avoid the dissolution of POM.The hybrid nanomaterial exhibited superior adsorption rate and selective adsorption ability for the cationic dyes.Remarkably, this material exhibited a largescale adsorption capacity of 426.7 mg/g for MB.Hence, it is a promising and environmental friendly adsorbent for removing and separating organic pollutants in dye-wastewater.

1. Materials and Characterization Techniques
Graphite powder (C, 99.95%), 3-aminopropyltriethoxysilane (APTES, 99%), phosphomolybdic acid (H 3 P-Mo 12 O 40 , 98%), toluene, sulfuric acid (H 2 SO 4 , 98%), and potassium permanganate (KMnO 4 , 98%) were purchased from Merck Chemical Co.All other chemicals were commercially purchased and used without further purification.The infrared spectra were recorded at room temperature using a Shimadzu FT-IR 160 spectrophotometer in the 4000-400 cm -1 region with KBr pellets.Powder XRD patterns were recorded on a Rigaku D-max C III X-ray diffractometer using Ni-filtered Cu Kα radiation (λ = 1.54184Å).The morphology of samples was studied using a MIRA3 TESCAN scanning electron microscope equipped with an energy dispersive X-ray analyzer (EDX) for the elemental analysis.AFM images were recorded by multi-mode atomic force microscopy (ARA-AFM, model Full Plus, ARA Research Co., Iran).Magnetic measurements were carried out at room temperature using a vibrating sample magnetometer (VSM, Magnetic Daneshpajoh Kashan Co., Iran) with a maximum magnetic field of 10 kOe.Optical adsorption spectra were obtained using a Cary 100 Varian UV-Vis spectrophotometer in a wavelength range of 200-800 nm.The Brunauer-Emmett-Teller (BET) surface area was measured by N 2 adsorption measurements at 77 K using a Nova 2000 instrument.The concentration of Mo in the composite was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES, model OEC-730).A controllable Serial-Ultrasonics apparatus (James 6MD, England) operating at an ultrasonic frequency of 100 kHz with a nominal output power of 50 W was used to disperse samples.
Furthermore, some of them are only effective for wastewater including low concentrations of dyes and they Magnetically Recyclable Fe 3 O 4 /GO-NH 2 /H 3 PMo 12 O 40 ...