Carboxylate Graphene Oxide Nanostructures

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Sandwiched Fe3O4/Carboxylate Graphene Oxide Nanostructures Constructed by Layer-by-Layer Assembly for Highly Efficient and Magnetically Recyclable Dye Removal Rong Guo, Tifeng Jiao, Ruifei Li, Yan Chen, Wanchun Guo, Lexin Zhang, Jingxin Zhou, Qingrui Zhang, and Qiuming Peng ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03635 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 19, 2017

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ACS Sustainable Chemistry & Engineering

Sandwiched Fe3O4/Carboxylate Graphene Oxide Nanostructures Constructed by Layer-by-Layer Assembly for Highly Efficient and Magnetically Recyclable Dye Removal

Rong Guo,†,‡ Tifeng Jiao,*†,‡ Ruifei Li,‡ Yan Chen,‡ Wanchun Guo,*‡ Lexin Zhang,‡ Jingxin Zhou,*‡ Qingrui Zhang,‡ and Qiuming Peng†

†

State Key Laboratory of Metastable Materials Science and Technology, Yanshan University,

Qinhuangdao 066004, P. R. China.

‡

Hebei Key Laboratory of Applied Chemistry, School of Environmental and Chemical Engineering,

Yanshan University, Qinhuangdao 066004, P. R. China.

Correspondence and requests for materials should be addressed to School of Environmental and Chemical Engineering, Yanshan University, 438West Hebei Street, Qinhuangdao 066004, P. R. China. E-mail:

T.

Jiao

([email protected]),

W.

Guo


([email protected]).

ACS Paragon Plus Environment

and

J.

Zhou

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Two-dimensional (2D) carbon nanomaterials generally display some limitations in adsorption applications due to easy agglomeration. To solve this problem, as-synthesized sandwiched nanocomposites made of Fe3O4 nanoparticles, poly(allylamine) hydrochloride molecules and carboxylate graphene oxide sheets, were prepared using a layer-by-layer (LbL) self-assembly method. The successfully-synthesized sandwiched structures in the present nanocomposites have outstanding organic dye adsorption performance, stability, and recycling. The agglomeration of carboxylate graphene oxide was reduced with increased specific surface area because the Fe3O4 nanoparticles play important roles in interpenetrating and supporting graphene oxide sheets layers. In comparison with other kinds of composite adsorbents, the preparation process of the present new sandwiched composite materials is facile to operate and regulate, which demonstrates potential large-scale applications in wastewater treatment and dye removal.

Keywords:

sandwiched nanostructures; graphene oxide; layer-by-layer assembly; dyes removal;

wastewater treatment


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Introduction Graphene oxide (GO) and its derivatives hold great promise for many applications, such as drug delivery,1-6 energy storage,7-10 bio-/chemical sensors,11-16 and wastewater treatment17 due to abundant surface hydroxyl, carboxyl, and epoxy moieties, and an excellent specific surface area resulting from its two-dimensional ultrathin structure. Among them, carboxylated graphene oxide (GO–COOH), one of the most valued functionalized graphene oxides,18-20 has a large interlayer space and abundant carboxyl groups as single active species with a strong specific affinity to organic dye, efficiently facilitating the removal of organic dye contaminants in wastewater.21 However, GO–COOH nanosheets tend to agglomerate and accumulate in applications in liquid solution due to their high surface energy, accompanied by their high specific surface area, which result in a reduction in the number of surface active sites available for absorption, and led to a decrease in absorption efficiency.22 Thus, how to reduce agglomeration of GO–COOH nanosheets during dye absorption to maintain their initial active sites has become an urgent issue. In order to overcome this obstacle, as well as develop and better utilize the performance of GO, researchers need to pay attention to the three-dimensional (3D) structure of GO and its derivatives. Luo et al. crumpled 2D sheets into fractal-dimensional paper-ball-like structures to maintain GO initial nanosheets active sites against their agglomeration during absorption applications.23 Fan et al. obtained loose and spongy porous structures of GO/PANI using in situ synthesis, which has a good adsorption capacity for Hg(II).24 Madadrang et al. introduced EDTA groups to a GO surface through a silanization process to obtain excellent adsorbents for the removal of toxic heavy metals.25 Liu et al. and Jayanthi et al. generated graphene oxide foam and sponge nanostructures by employing freeze-drying and centrifugal vacuum evaporation, respectively, which have a great potential in ACS Paragon Plus Environment


organic pollutant dye removal.26,27 More researchers have prepared different 3D structures of GO hydrogels or aerogels with the aid of welan gum,28 various diamines,29 and gelatin,30 all of which have the advantage of being able to adsorb organic dyes. However, these three-dimensional structures may collapse as π–π stacking is a weak interaction and gelatin tends to easily disintegrate, which results in poor stability of said prepared nanocomposites. Thus, we propose a novel preparation of 3D structures of GO–COOH with the aid of Fe3O4 nanoparticles, based on the fact that Fe3O4/GO nanocomposites are synthesized easily and have efficient adsorption performances.31 In previous work, Fe3O4 particles were prepared using a hydrothermal method containing some hydroxyl groups. The interactions between Fe3O4 particles and GO do not seem strong enough for reuse in a regeneration cycle for dye removal. Here, we further improved the nanostructure of the present nanocomposites using carboxylated GO and Fe3O4 with carboxyl chemical groups. In addition, a large number of carboxyl groups in Fe3O4–COOH nanoparticles can help Fe3O4 nanoparticles to be combined with poly(allylamine) hydrochloride (PAH); their combination seemed to be more stable. Thus, Fe3O4–COOH nanoparticles, wrapped in PAH molecules, as a matrix-supporting material, can be easily and stably inserted into the pleated sheets of GO–COOH via electrostatic attraction. Therefore, sandwiched structures of GO–COOH/Fe3O4 nanocomposites were obtained using a layer-by-layer (LbL) assembly process and an inexpensive, facile, and versatile method.32 The connection in 3D structures is firm due to the solid structure of Fe3O4 nanoparticles, which effectively avoid the collapse of the structures. The obtained sandwiched nanostructure can provide obstacles for overlapping the accumulation of GO–COOH sheets and also improve specific surface area by exposing adsorption active site as much as possible. Compared with reported works, this ACS Paragon Plus Environment

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sandwiched material seems to not only improve the adsorption capacity of dyes, but also greatly enhance the stability of the composites. In addition, the nanocomposites have excellent magnetism for magnetic recyclability and dispersibility, solving the problems of recovery and reuse due to the Fe3O4 nanoparticles.33 As expected, these sandwiched-structure nanocomposites have excellent stability and display outstanding adsorption performance and removal rates for used organic dyes, demonstrating important and wide applications in the fields of environmental protection and chemical engineering. In addition, the present prepared nanocomposites are convenient for recovery and reuse, providing new ideas for reducing the agglomeration of GO in the study of two-dimensional carbon materials.

Experimental Section Materials Ferric chloride hexahydrate (FeCl3·6H2O, 98%) and ethylene glycol were purchased from Tianjin Kaitong Chemicals (Tianjin, China). Poly(allylamine) hydrochloride (PAH, average Mw ~17,500) and graphite powder (8000 mesh, 99.95%) were purchased from Aladdin Chemicals. Rhodamine B and methylene blue were obtained from Beijing Chemicals (analytical reagent grade, Beijing Chemicals, China). Ethylene glycol (average Mw = 9450) was purchased from Alfa Aesar Chemicals (Shanghai, China). All other solvents and reagents used in this work were purchased from Sinopharm Chemical Reagent Co. Ltd (analytical reagent grade, Shanghai, China). Ultra-pure water was obtained using a Millpore Milli-Q water purification system with a resistivity of 18.2 MΩ cm-1. All chemicals were used as received, without further purification.



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Synthesis of Fe3O4@(PAH/GO–COOH)n Nanocomposites Graphene oxide and the carboxyl-functionalized graphene oxide (GO–COOH) were fabricated based on the literature, and the GO–COOH sheets were freeze-dried and stored for later use.34,35 The Fe3O4–COOH nanoparticles (100 mg) were synthesized according to previous reports.36 In addition, they were soaked in an aqueous PAH solution (0.5 mg/ml) for 30 minutes with stirring. Then, the nanoparticles were collected using an external magnetic field and were washed with deionized water and ethanol three times. Then, the abovementioned samples were immersed in an aqueous GO–COOH dispersion (0.5 mg/ml using ultrasonic grinding) for 30 minutes and were stirred at the same time. The as-synthesized products, named Fe3O4@(PAH/GO–COOH)1, were collected and rinsed, followed by drying and storage. Then, the Fe3O4@(PAH/GO–COOH)1 nanocomposites were again soaked in aqueous PAH solution and aqueous GO–COOH dispersion for 30 minutes, respectively. Thus, the nanocomposites, denoted as Fe3O4@(PAH/GO–COOH)2, were obtained. Repeating the previous steps, each obtained as-synthesized product, was continuously immersed in aqueous PAH solution and aqueous GO–COOH dispersion, in turn, several times, and was denoted as Fe3O4@(PAH/GO–COOH)n (n = 1, 2, 3, 4, 5, and 6) nanocomposites, and was then subsequently dried and stored for further use.

Batch Adsorption Tests for Dyes Removal Adsorption performance was designed by taking advantage of the adsorption of two organic dyes, RhodamineB (RhB) and methylene blue (MB). UV/Vis absorption spectra were recorded for the process at wavelengths of 632 nm (MB) and 554 nm (RhB) using a UV-vis spectrometer (752-type,

Sunny

freshly-prepared

Hengping

scientific

instrument

Co.,

nanocomposite adsorbents, 41.26 ACS Paragon Plus Environment

Ltd.,

Shanghai,

mg

and

China).

The

6.12

mg

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Fe3O4@(PAH/GO–COOH)1 were added to 100-mL dye solutions that contained RhB (5 mg/L) and MB (10 mg/L) solution, while 13.25 mg and 7.55 mg of Fe3O4@(PAH/GO–COOH)2 adsorbents were also added to the same organic dye solutions at room temperature. The absorbance of dyes in solution were measured at different time intervals, after which the concentrations, kinetic data, and thermodynamic data were calculated using the calibration curves.

For

the

recovery

and

reuse

of

the

composites,

the

prepared

Fe3O4–COOH@(PAH/GO–COOH)1 and Fe3O4–COOH@(PAH/GO–COOH)2 nanocomposites were extracted from the dye solution using external magnetic attraction, and were washed, alternatively, using ethylene glycol and ethanol, several times. Then, the nanocomposites were washed using DI water and were dried for further use. In investigating the recovery and stability, the same Fe3O4@(PAH/GO–COOH)2 adsorbents were repeatedly used to remove the initial, fresh MB dye solution over ten consecutive cycles.

Characterization The zeta potentials were determined using a Nanozetasizer machine (ZEN 3690, Malvern Instruments, UK), which was used to measure surface charge and particle size. The specific surface areas and pore diameter distributions were determined using BET measurements (NOVA 4200-P, US). The morphologies were characterized using a field-emission scanning electron microscope (SEM) (S-4800II, Hitachi, Japan) with a 5–15 kV accelerating voltage. In addition, X-ray spectroscopy (EDXS) was carried out at an accelerating voltage of 200 kV, using an Oxford Link-ISIS

X-ray

EDXS

microanalysis

system.

In

addition,

Fe3O4@(PAH/GO–COOH)2

nanocomposites were dispersed in Spurr’s epoxy resin and solidified at 70 °C for 8 h, followed by ACS Paragon Plus Environment


being sliced into 60–100 nm sections using a LKBV microtome. The slices were attached to Cu grids for transmission electron microscopy (TEM) (HT7700, Hitachi High-Technologies Corporation, Japan). A NETZSCH STA 409 PC Luxxsi multaneous thermal analyzer (Netzsch Instruments Manufacturing Co, Ltd, Germany) was used for themogravimetry-differential scanning calorimetry (TG-DSC) characterizations, which were analyzed under air atmospheres. X-ray diffraction (XRD) analysis was performed on a X-ray diffractometer equipped with a Cu Kɑ X-ray radiation source and a Bragg diffraction setup (SMART LAB, Rigaku, Japan). Magnetic properties were measured using an MPMS-XL SQUID at 300 K. FT-IR spectra were obtained using Fourier infrared spectroscopy (Thermo Nicolet Corporation) using the KBr tablet method. Raman spectroscopy was measured using a Horiba Jobin Yvon Xplora PLUS confocal Raman microscope equipped with a motorized sample stage. The wavelength of the excitation laser was 532 nm and the laser power was maintained below 1 mW, at room temperature. X-ray photoelectron spectroscopy (XPS) was measured using a Thermo Scientific ESCALab 250 Xi, using 200 W monochromated Al Kα radiation. The 500-µm X-ray spot was used for XPS analysis and the base pressure in the analysis chamber was about 3 × 10-10 mbar.

Results and Discussion Preparation and Characterization of Nanocomposites

Figure 1 illustrates the scheme for the preparation of Fe3O4@(PAH/GO–COOH)n nanocomposites. The negatively-charged Fe3O4 nanoparticles were covered by a layer of electropositive PAH molecules; thus, the obtained Fe3O4@PAH sample shows active surface sites with positive charges. The GO–COOH sheet with a negative can provide a large number of binding sites ACS surface Paragon charge Plus Environment

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for the anchoring of Fe3O4@PAH nanoparticles. Thus, Fe3O4@(PAH/GO–COOH)1 composite via LbL assembly were obtained. However, it was reasonably speculated that some Fe3O4@PAH nanoparticles can form clusters, and are not fully anchored to the surface of the GO–COOH sheets. Therefore, after dipping in a GO–COOH solution in the second LbL cycle, some Fe3O4@PAH nanoparticles were also introduced to the GO–COOH sheets. While repeating the steps for PAH molecules and GO–COOH sheets, Fe3O4@(PAH/GO–COOH)2 was obtained via LbL self-assembly. The ζ-potential image for monitoring the LbL assembly process is shown in Figure 2a. The changes in ζ-potential present the changes in surface charge, with alternating modifications to PAH and GO–COOH in the preparation of the nanocomposites. The numbers in the x-axis represent the layer number of the modification. The point in the x-axis with a value of 0 demonstrates unmodified Fe3O4–COOH nanoparticles with a negative charge of –5 mV. The surface charge of point 1 in the x-axis, which referred to the Fe3O4@PAH nanoparticles, showed a value of –7.5 mV. After loading the GO–COOH sheets, the formed Fe3O4@(PAH/GO–COOH)1 nanocomposites at point 2 referred to a charge value of –11.5 mV. As expected, with different self-assembly steps of immersion in aqueous PAH or GO–COOH solutions, the surface ζ-potential values of the nanocomposites changed notably. Precisely, because each as-synthesized product still had extra charges on the surface, LbL self-assembly could be conducted several times; thus, it was beneficial in forming sandwich-like multilayer structures. Thus, the Fe3O4@(PAH/GO–COOH)1 nanocomposites carried out LbL self-assembly again, and the surface ζ-potential of each step was measured. The charge was –7.8

mV

(point

3)

and

–12.5

mV

(point

4),

respectively,

which

meant

that

Fe3O4@(PAH/GO–COOH)1 nanocomposites attached to the GO–COOH sheets again, and the samples of Fe3O4@(PAH/GO–COOH)2 nanocomposites were obtained. With layer number increments, the zeta potentials of ACS the Paragon formed Plus nanocomposites Environment became more unstable after four


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layers of LbL self-assembly, which can correspond to poor dispersion in a water solution, which mainly originated from aggregation and precipitation. In addition, the average size of the Fe3O4–COOH nanoparticles in

Figure

2b is

about 300

nm,

while

the

values

of

Fe3O4@(PAH/GO–COOH)1 and Fe3O4@(PAH/GO–COOH)2 nanocomposites are about 400 nm and 550 nm, respectively. Thus, the obtained Fe3O4@(PAH/GO–COOH)1 with layer number of 2 and Fe3O4@(PAH/GO–COOH)2 nanocomposites with layer number of 4, were chosen for further characterization and shown in next sections. Figure 3 shows the micro-morphology of the three samples. According to the SEM and TEM images, the Fe3O4–COOH nanoparticles were irregular mono-dispersed particles. After completion of the first PAH and GO–COOH self-assembly cycle, wrinkled and flaky nanostructures were observed, as shown in Figure 3b. The large folded sheets and innumerable nanoparticles could be identified as GO–COOH sheets and Fe3O4 nanoparticles, respectively. As shown in Figure 3d, many Fe3O4@PAH particles were clearly loaded on the border, and both surfaces of the GO sheets, demonstrating that it was a good substrate for next assembly process. After two cycles of LbL self-assembly, the formed Fe3O4@(PAH/GO–COOH)2 nanocomposites showed an obvious sandwiched structure. As shown in Figure 3c, it is clear that many Fe3O4@PAH nanoparticles were dispersed on both sides of the GO layers, which resulted in larger spaces between the GO–COOH sheets. In addition, TEM measurements further and definitively confirmed the abovementioned view of the sandwiched structures. Fe3O4@PAH nanoparticles play important roles in providing barriers for the easy agglomeration of GO sheets, and increasing interlamellar space, as shown in Figure 3e. It can be clearly observed that many Fe3O4@PAH nanoparticles formed interlayers between the layered GO sheets. In addition, according to the TEM image of the composite slice in Figure 3f, the sheets of GO–COOHACS trimly overlapped and Fe3O4 nanoparticles were inserted into Paragon Plus Environment

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the interspace between the GO sheets, suggesting the formation of a sandwich-like nanostructure. The obtained special sandwiched structure was speculated to have specific surface areas and improved active sites for dye adsorption. Wide-angle XRD spectra were further collected to characterize the prepared nanocomposites, as shown in Figure 4. The characteristic diffraction peaks of Fe3O4–COOH nanoparticles appeared at 2θ values of 30.0°, 35.4°, 43.2°, 53.5°, and 57.0°, which were indexed to the (220), (331), (400), (422), and (511) lattice planes.37 It was apparent that Fe3O4@(PAH/GO–COOH)1 and Fe3O4@(PAH/GO–COOH)2 nanocomposites have the same characteristic peaks at the abovementioned sites, demonstrating that the crystal structure of the material Fe3O4–COOH nanoparticles were not destroyed. Similar results were obtained using Raman spectroscopy, being that the shifts at 692 cm-1, 345 cm-1, and 487 cm-1, could be considered as an effect of the Fe3O4 nanoparticles

(Figure

S2),

which

was

reported

previously.38,39

Moreover,

the

Fe3O4@(PAH/GO–COOH)1 and the Fe3O4@(PAH/GO–COOH)2 nanocomposites presented same broad peak centered at 2θ value of 24.0°, compared with the GO–COOH sheet. The same result could be seen in the FTIR spectra and Raman spectroscopy (Figures S1 and S2). All three samples had the same peak type at the same peak position, except for the characteristic absorption peaks at 2970 and 2920 cm-1, which could be attributed to the C–H stretching vibration of PAH and GO–COOH.40-43 However, Fe3O4@(PAH/GO–COOH)2 composite showed the above characteristic peaks due to more numbers of LbL assembly. In addition, the Raman shift of the G and D bands, at 1615 cm-1 and 1354 cm-1, implied that the nanocomposites only had GO–COOH sheets and Fe3O4, without any other impurities.31 In addition, from the XPS and TG curves shown in Figures S3 and S4, the two, chief, oxygen-containing groups, with characteristic double peaks at 531.7 eV and 534 eV, came from the carbonyl group and carbon oxygen bond of GO–COOH; the weight loss was ACSthe Paragon Plus Environment


distinctly increased due to thermal decomposition of the carbon skeletons in PAH and GO–COOH molecules. Based on this, the Fe3O4@(PAH/GO–COOH)1 and Fe3O4@(PAH/GO–COOH)2 nanocomposite samples were successfully synthesized. Utilizing N2 adsorption–desorption isotherms, the microstructures of the obtained composites was further investigated and is shown in Figure 5; the properties of the samples are summarized in Table 1. The hysteresis loops of Fe3O4-COOH@(PAH/GO–COOH)2 can be presented at a higher relative pressure (P/P0 of 0.50–0.95), which means that the nanocomposites have mesoporous structures with pore diameter ranges from 2 nm to 50 nm. The smaller aperture purports a larger specific surface area which is favorable for adsorbing dye molecules. In addition, Fe3O4–COOH@(PAH/GO–COOH)1 nanocomposites are at a higher relative pressure (P/P0 of 0.50–0.70). These two hysteresis loops can be classified into H3 or H4 hysteresis effects, indicating that the formed adsorbent composite is a plate-like particle or narrow slit pore material, and does not exhibit any adsorption limit in the relatively-high pressure region. At medium pressure of the adsorbed volume, the tardy increase and the desorption hysteresis denote developed mesoporosity in the samples, which are major prerequisites for adsorption. In addition, the relative pressure, with a sharp increment near 1.0, indicates the presence of macroporosity, resulting from the sandwiched structures of the samples. The pore size distribution curves calculated using BJH methods can be seen in Figure 5b. Fe3O4–COOH@(PAH/GO–COOH)2 has a higher BET specific surface area of 42.27 m2g-1, while Fe3O4–COOH@(PAH/GO–COOH)1 has an area of 31.65 m2g-1; therefore, the former can adsorb more nitrogen. In addition, the pore volume and average pore diameter were also larger than that of Fe3O4–COOH@(PAH/GO–COOH)1. The sandwiched structure of the nanocomposites resulted in higher specific surface areas, which increased the number of adsorbent activity points. Moreover, larger pore diameters and Environment pore volumes offer abundant channels for a dye ACS Paragon Plus

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solution; therefore, the sandwiched structure of Fe3O4–COOH@(PAH/GO–COOH)2 has outstanding adsorption performance. Magnetic measurements and research on the several hierarchical nanostructures were conducted using magnetization hysteresis loops at room temperature. Obviously, the field-dependent magnetization curves are completely reversible, as shown in Figure 6, indicating that all materials were super-paramagnetic without coercivity and remanence.36 The Fe3O4–COOH nanoparticles had a saturation magnetization value of 73.6 emu/g at 20 kOe. After loading one and two layers of GO–COOH sheets, the saturation magnetization value dropped to 57.2 emu/g and 21.5 emu/g, as, with the increase in the number of loading cycles, the content of non-magnetic materials in PAH and GO–COOH molecules in composites increased. Although there was a decrease in saturation magnetization, the strong magnetic response of Fe3O4@(PAH/GO–COOH)2 could still ensure a controllable magnetic recovery.

Adsorption Performances toward Dye Removal

GO–COOH nanocomposites have abundant functional groups and large surface area, which are the most dominant key factors in the efficient removal of dyes, as has been previously reported in the literature.5,20

Our

sandwiched-structure

samples,

Fe3O4@(PAH/GO–COOH)1

and

Fe3O4@(PAH/GO–COOH)2 nanocomposites, were investigated with respect to their capacity to remove organic dyes by spectral monitoring. As is shown in Figure 7, two kinds of nanocomposites had a continuous adsorption process; in addition, the colors of the organic dyes changed markedly (Figure 8). It is clear that the color of the MB solution turns light blue, and the RhB solution turns light pink after a period of time. The adsorption equilibrium times of MB and RhB were 20 min and ACS Paragon Plus Environment


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150 min, respectively. The obtained equilibrium times could be applied in the adsorption experiments. For both RhB and MB, the dye adsorption efficiency of Fe3O4@(PAH/GO–COOH)2 was better than that of Fe3O4@(PAH/GO–COOH)1. The adsorption kinetics were further evaluated by fitting experimental data using a pseudo-first-order model and pseudo-second-order model adsorption equation. The pseudo-first-order model is demonstrated by Equation (1):

log(qe − qt ) = log qe −

k1 t 2.303

(1)

where t is the adsorption time, qe is the adsorption capacity at equilibrium, k1 is the pseudo-first-order model rate constant, and qt is the adsorption capacity at time t. The pseudo-second-order model is demonstrated by Equation (2): t 1 1 = + t 2 Q t k 2 Qe Qe

(2)

where Qe is the adsorption capacity at equilibrium, k2 is the pseudo-second-model rate constant, and Qt is the adsorption capacity at time t. The fitting results are summarized in Table 2. The kinetic adsorption data demonstrated that the pseudo-two-order model had a higher correlation coefficient (R2 > 0.99), which was more accurate than the pseudo-first-order model. In addition, the dye removal efficiency of Fe3O4@(PAH/GO–COOH)2 nanocomposites for MB reached 35.958 mg/g, while Fe3O4@(PAH/GO–COOH)1 nanocomposites only reached 20.995 mg/g. In comparison, for the RhB system, the removal efficiency reached values of 22.124 mg/g and 15.252 mg/g, respectively. Apparently, the obtained Fe3O4@(PAH/GO–COOH)2 nanocomposites had a better dye adsorption capacity due to the sandwiched structure, and had better specific surface area active sites. In addition, from the point of view of practical or industrial applications, the recovery and ACS Paragon Plus Environment

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reuse of adsorbents is necessary for economic development and environmental protection. Benefiting from the rapid and efficient magnetic separation, the hybrid materials reported here can be easily regenerated; therefore, the used Fe3O4@(PAH/GO–COOH)2 nanocomposites can be separated using an external magnetic field for further adsorption reuse. From the graphic illustration in Figure 9, Fe3O4@(PAH/GO–COOH)2 nanocomposites still showed excellent adsorption effects for MB dye after repeated regeneration (over eight times) of adsorption, and the adsorption rate still could reach 82%, which showed that Fe3O4@(PAH/GO–COOH)2 could effectively adsorb organic dyes. Moreover, the Fe3O4@(PAH/GO–COOH)2 material can be recovered and reused several times after a simple washing and drying process, without significant loss of adsorption. For further comparative studies, the dye adsorption efficacy of GO–COOH sheets was also carried out. The morphology and surface area of GO–COOH were investigated in previous reports.44-46 As shown in Figure S5, GO–COOH sheets show an adsorption capacity of about 20 mg/g for MB removal. However, these GO sheets are difficult to extract from aqueous dye solutions for subsequent regeneration and utilization. In addition, due to the easy agglomeration between GO–COOH sheets, after repeated regeneration (over six times) of adsorption, the removal capacities of MB sharply decreased to a value of 45%, as shown in Figure S6. Therefore, this also demonstrates the superiority of the presented sandwiched structure in terms of dye adsorption. Moreover, compared with the composites reported in previous works (listed in Table 3), it is clearly indicated that different kinds of composite absorbents showed various characteristics.47-57 For examples, Cheng et al. prepared magnetic GO/poly(vinyl alcohol) composite gels via a simple preparation process, including stirring and the addition of ammonia solution, and the obtained composite gels showed excellent performance in terms of stability and dye removal capacity.57 Our present work showed great advantages in adsorption capacity and stability, as well as being an eco-friendly preparation ACS Paragon Plus Environment


process. It can be expected that the present, obtained composites, with sandwiched structures due to LbL assembly, can serve important and broad applications in the fields of sustainable chemistry and environmental engineering, providing new clues for the design of novel two-dimensional carbon materials.

Conclusions In this work, we have demonstrated the facile preparation and dye adsorption capacities of new, sandwiched, composite absorbent materials (Fe3O4@(PAH/GO–COOH)n (n = 1~6)) by taking advantage of a layer-by-layer self-assembly. According to the size and zeta potential analyses, Fe3O4@(PAH/GO–COOH)1 and Fe3O4@(PAH/GO–COOH)2 nanocomposites were selected to carry out characterization and dye removal performances. According to the morphological and BET results, Fe3O4@(PAH/GO–COOH)2 demonstrated a three-dimensional, sandwiched nanostructure with carboxylated Fe3O4 nanoparticles inserted into pleated GO sheets, which showed the advantages of a larger specific surface area and more active sites. Based on the adsorption experiments of organic dyes, the Fe3O4@(PAH/GO–COOH)2 nanocomposites, with better specific surface sites, demonstrated a more outstanding adsorption performance. In addition, the nanocomposites could be easily separated from wastewater dye solution using an extra magnetic field, and demonstrated excellent reusability. The present research work proposed a novel approach to increase the specific surface area and reduce agglomeration of GO sheets using LbL self-assembly, which can provide a new exploration for dye removal applications in wastewater treatment.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXX. FT-IR, TG, Raman, XPS characterizations, adsorption and regeneration of GO-COOH sheets (PDF)

AUTHOR INFORMATION Corresponding Authors E-mail:

T.

Jiao


W.

Guo


and

J.

Zhou

([email protected]).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 21473153 and 51503178), Support Program for the Top Young Talents of Hebei Province, China Postdoctoral Science Foundation (No. 2015M580214), and Scientific and Technological Research and Development Program of Qinhuangdao City (No. 201701B004).



Page 18 of 47

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Table 1. Physical data of the obtained LbL-assembled composites.

Samples

Specific surface area (m2g-1)

Average pore Diameter (nm)

Pore volume (cm3g-1)

Fe3O4-COOH@(PAH/GO-COOH)1

31.6489

4.82725

0.043367

Fe3O4-COOH@(PAH/GO-COOH)2

42.2699

9.79122

0.071766



Page 26 of 47

Table 2. Kinetic parameters of the obtained LbL-assembled composites for MB and RhB removal at 298 K (experimental data from Fig. 7). Pseudo-first-order model MB Fe3O4-COOH@(PAH/GO-COOH)1 Fe3O4-COOH@(PAH/GO-COOH)2

qe (mg/g) 20.471 31.01

R2

K1 (min-1)

0.9243 0.0604 0.9890 0.00322

Pseudo-first-order model RhB Fe3O4-COOH@(PAH/GO-COOH)1 Fe3O4-COOH@(PAH/GO-COOH)2

qe (mg/g) 16.8340 22.1726

R2

K1 (min-1)

0.9616 0.03445 0.9887

0.02579


Pseudo-second-order model qe (mg/g) 20.9952 35.9583

R2 0.9920 0.9903

K2 (g/mg·min) 0.007622 0.001542

Pseudo-second-order model qe (mg/g) 15.2522 22.1239

R2 0.9962 0.9957

K2 (g/mg·min) 0.002777 0.001348

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Table 3. Comparative characteristics and adsorption capacities of relative adsorbents for MB in reported literatures. No.

Adsorbent

Adsorption

Characteristics

reference

capacity (mg/g) 1

Graphene oxide-based

334.45

Poor stability

17

hydrogels 2

GO sponge

396.83

Poor

stability,

3

pPDA-GO hydrogel

235.8

Toxic raw materials

29

4

GO/Fe3O4 nanohybrids

Poor stability

31

5

D@GO-COOH@(PEI/PAA8

43.86

Expensive raw materials

32

6

Rice hull ash

17.1

Week adsorption capacity

47

7

Oak sawdust

29.94


48

26

remarkable capacity

9

Living fungus

10

Reduced

graphene

32

oxide

1.17

Poor removal of dye

49

7.85


50

based hydrogel 11

PVA/PAA/GO-COOH@TiO2

30.45

Complexed preparation

51

12

Fe3O4@MnO2

core–shell

9.71


52

Magnetic multi-wall carbon

11.89


53

3.50


54

Excellent

55

nanocomposite 13

nanotube 14

GO-CS

nanocomposite

hydrogels 15

GO-DETA hydrogel

884.96

capacity

Unfriendly environment 16

PVA/PAA/GO-COOH@PDA

17

Magnetic GO/poly(vinyl

26.92

Complexed preparation

56

270.94

Facile

57

composite membrane alcohol) composite gels 18

Present work

fabrication,

well

capacity, good stability 35.96

Eco-friendly well stability


preparation,


Page 28 of 47

Figure captions Figure

1.

Schematic

illustration

of

the

fabrication

and

dye

adsorption

of

Fe3O4-COOH@(PAH/GO-COOH)n nanocomposites by LbL assembly.

Figure 2. (a) Zeta potential during the assembly of multilayers on Fe3O4 nanoparticle; (b) size distribution of Fe3O4@(PAH/GO-COOH)n (n=0,1,2). The numbers in the x-axis of image a represent the layer number of the modification.

Figure 3. (a) SEM and inserted TEM images of Fe3O4-COOH nanoparticles; (b-c) SEM images of Fe3O4@(PAH/GO-COOH)1 and Fe3O4@(PAH/GO-COOH)2 nanocomposites with inserted EDXS image

in

image

c;

(d-e)

TEM

images

of

Fe3O4@(PAH/GO-COOH)1

and

Fe3O4@(PAH/GO-COOH)2; (f) TEM image of Fe3O4@(PAH/GO-COOH)2 nanocomposites slice.

Figure 4. XRD patterns of all the obtained composites. Figure 5. (a) N2 adsorption–desorption isotherms and (b) pore size distribution of the obtained Fe3O4@(PAH/GO-COOH)1 and Fe3O4@(PAH/GO-COOH)2 composites.

Figure 6. Magnetization hysteresis loops of Fe3O4-COOH nanoparticles and the obtained nanocomposites.

Figure 7. Kinetic adsorption of (a) qt versus t plots and (b) t/qt versus t plots for MB; (c) qt versus t plots and (d) t/qt versus t plots for RhB.

Figure 8. Photographs of MB solutions with Fe3O4@(PAH/GO-COOH)2 nanocomposites and magnet at 1 min (a) and 50 minutes (b); RhB solutions with Fe3O4@(PAH/GO-COOH)2 nanocomposites and magnet at 1 min (c) and 100 minutes (d).

Figure 9. Regeneration studies of as-prepared Fe3O4@(PAH/GO-COOH)2 nanocomposites towards MB for different consecutive cycles at room temperature.


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Figure

1.

Schematic

illustration

of

the

fabrication

and

Fe3O4-COOH@(PAH/GO-COOH)n nanocomposites by LbL assembly.


dye

adsorption

of


Figure 2. (a) Zeta potential during the assembly of multilayers on Fe3O4 nanoparticle; (b) size distribution of Fe3O4@(PAH/GO-COOH)n (n=0,1,2). The numbers in the x-axis of image a represent the layer number of the modification.


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Figure 3. (a) SEM and inserted TEM images of Fe3O4-COOH nanoparticles; (b-c) SEM images of Fe3O4@(PAH/GO-COOH)1 and Fe3O4@(PAH/GO-COOH)2 nanocomposites with inserted EDXS image

in

image

c;

(d-e)

TEM

images

of

Fe3O4@(PAH/GO-COOH)1

and

Fe3O4@(PAH/GO-COOH)2; (f) TEM image of Fe3O4@(PAH/GO-COOH)2 nanocomposites slice.



Figure 4. XRD patterns of all the obtained composites.


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Figure 5. (a) N2 adsorption–desorption isotherms and (b) pore size distribution of the obtained Fe3O4@(PAH/GO-COOH)1 and Fe3O4@(PAH/GO-COOH)2 composites.



Figure 6. Magnetization hysteresis loops of Fe3O4-COOH nanoparticles and the obtained nanocomposites.


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Figure 7. Kinetic adsorption of (a) qt versus t plots and (b) t/qt versus t plots for MB; (c) qt versus t plots and (d) t/qt versus t plots for RhB.



Figure 8. Photographs of MB solutions with Fe3O4@(PAH/GO-COOH)2 nanocomposites and magnet at 1 min (a) and 50 minutes (b); RhB solutions with Fe3O4@(PAH/GO-COOH)2 nanocomposites and magnet at 1 min (c) and 100 minutes (d).


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Figure 9. Regeneration studies of as-prepared Fe3O4@(PAH/GO-COOH)2 nanocomposites towards MB for different consecutive cycles at room temperature.



For Table of Contents Use Only

Synopsis Hierarchical sandwiched nanocomposites containing Fe3O4 nanoparticles and carboxylate graphene oxide sheets are prepared via layer-by-layer assembly, demonstrating high-efficient dyes removal and outstanding recycling for wastewater treatments.


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Figure 1. Schematic illustration of the fabrication and dye adsorption of Fe3O4-COOH@(PAH/GO-COOH)n nanocomposites by LbL assembly. 71x31mm (300 x 300 DPI)



Figure 2. (a) Zeta potential during the assembly of multilayers on Fe3O4 nanoparticle; (b) size distribution of Fe3O4@(PAH/GO-COOH)n (n=0,1,2). The numbers in the x-axis of image a represent the layer number of the modification. 65x26mm (300 x 300 DPI)


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Figure 3. (a) SEM and inserted TEM images of Fe3O4-COOH nanoparticles; (b-c) SEM images of Fe3O4@(PAH/GO-COOH)1 and Fe3O4@(PAH/GO-COOH)2 nanocomposites with inserted EDXS image in image c; (d-e) TEM images of Fe3O4@(PAH/GO-COOH)1 and Fe3O4@(PAH/GO-COOH)2; (f) TEM image of Fe3O4@(PAH/GO-COOH)2 nanocomposites slice. 82x42mm (300 x 300 DPI)



Figure 4. XRD patterns of all the obtained composites. 88x64mm (300 x 300 DPI)


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Figure 5. (a) N2 adsorption–desorption isotherms and (b) pore size distribution of the obtained Fe3O4@(PAH/GO-COOH)1 and Fe3O4@(PAH/GO-COOH)2 composites. 60x22mm (300 x 300 DPI)



Figure 6. Magnetization hysteresis loops of Fe3O4-COOH nanoparticles and the obtained nanocomposites. 59x21mm (300 x 300 DPI)


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Figure 7. Kinetic adsorption of (a) qt versus t plots and (b) t/qt versus t plots for MB; (c) qt versus t plots and (d) t/qt versus t plots for RhB. 106x81mm (300 x 300 DPI)



Figure 8. Photographs of MB solutions with Fe3O4@(PAH/GO-COOH)2 nanocomposites and magnet at 1 min (a) and 50 minutes (b); RhB solutions with Fe3O4@(PAH/GO-COOH)2 nanocomposites and magnet at 1 min (c) and 100 minutes (d). 105x78mm (300 x 300 DPI)


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Figure 9. Regeneration studies of as-prepared Fe3O4@(PAH/GO-COOH)2 nanocomposites towards MB for different consecutive cycles at room temperature. 93x72mm (300 x 300 DPI)


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