rare-earth-metal-oxide aerogels as a highly efficient adsorbent for Rhodamine-B

Journal Pre-proofs Full Length Article Facile Synthesis of Reduced-graphene-oxide/rare-earth-metal-oxide Aerogels as a Highly Efficient Adsorbent for Rhodamine-B Yong Zhang, Keding Li, Jun Liao PII: DOI: Reference:

S0169-4332(19)33193-9 https://doi.org/10.1016/j.apsusc.2019.144377 APSUSC 144377

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Applied Surface Science

Received Date: Revised Date: Accepted Date:

14 May 2019 26 August 2019 11 October 2019

Please cite this article as: Y. Zhang, K. Li, J. Liao, Facile Synthesis of Reduced-graphene-oxide/rare-earth-metaloxide Aerogels as a Highly Efficient Adsorbent for Rhodamine-B, Applied Surface Science (2019), doi: https:// doi.org/10.1016/j.apsusc.2019.144377

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Facile Synthesis of Reduced-graphene-oxide/rare-earth-metal-oxide Aerogels as a Highly Efficient Adsorbent for Rhodamine-B Yong Zhanga,b*, Keding Lib, Jun Liaoa a

State Key Laboratory of Environmental-friendly Energy Materials & School of National Defence

Science and Technology & School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, P. R. China b

Sichuan Co-Innovation Center for New Energetic Materials, Southwest University of Science and Technology, Mianyang 621010, P. R. China *

Corresponding author. E-mail: [email protected].

Abstract:

The

multifunctional

reduced-graphene-oxide/rare-earth-metal-oxide

(RGO/REMO) aerogels with three-dimensional (3D) interconnected networks were fabricated through a facile one-step hydrothermal method and freeze-drying process. It was proved that REMO nanoparticles could be easily deposited on graphene sheets in gentle conditions. To further investigate the effect of REMO on the adsorption of dyes, RGO/Nd2O3, RGO/Pr2O3 and RGO/Ce2O3 aerogels were used to adsorb the Rhodamine-B (RhB). The RGO/REMO aerogels all exhibited high adsorption -1

capacity for RhB and the maximal adsorption capacity was

. In

particular, RGO/Ce2O3 aerogel had a significant adsorption efficiency for RhB, and the adsorption capacity of RGO/Ce2O3 aerogel for RhB reached 130.1

-1

only in

one hour. Furthermore, as for RGO/Ce2O3 aerogel, the removal-rate of RhB was still more than 92% after nine repeated regeneration. The excellent adsorption capacity and high repeated utilization rate of RGO/REMO aerogels indicated that the composite adsorbent possessed potential practical application in the treatment of wastewater containing RhB. Keyword: Rare earth metal oxide, Graphene, Aerogels, Adsorption, Rhodamine-B 1. Introduction

Aerogels are a class of porous materials with 3D network structure consisting of solvent and polymer, which are expanded throughout their volume by gas [1]. As is known to all, they possess many extraordinary properties such as extremely low density, high specific surface area, high porosity, low thermal conductivity and so on[2-7]. Due to their unique spatial network structures and these features, aerogels exhibit wide potential applications in many fields such as adsorption materials[8], catalyst supports[9], thermal super-insulators[10], electrical materials and insulating materials[11]. Recently, graphene-based aerogel, a 3D carbon-based material, has been widely studied for its excellent performances and promising applications in various fields. Graphene is a two-dimensional (2D) sheet sp 2-hybridized carbon with unique advantages such as high specific surface area, outstanding electrical and optical properties, and favorable mechanical properties[12,13]. The graphene-based aerogels prepared on the graphene substrate feature the advantages of graphene and aerogel. Chen[14] used a green one-step hydrothermal method to prepare a 3D super-hydrophilic phosphoethanolamine functionalized graphene foam for water purification. Yang et al.[15] prepared a 3D RGO/zeolitic imidazolate skeleton-67 aerogel with a high adsorption capacity for dyes by in situ self-assembled zeolitic imidazolate skeleton-67 polyhedron on the 3D RGO network. Zhou et al.[16] scattered zinc oxide (ZnO) in the solution of polyurethane (PU) precursor and impregnated it into air-dried graphene aerogel (GA) to obtain the composite phase change materials (PCMs), which had excellent mechanical stability, remarkable thermal stability and high photoelectric conversion efficiency. Unfortunately, due to the hydrophobic interactions between graphene nanosheets and the influence of Van der Waals force, the aggregation between graphene nanosheets would happen which leading to the loss of surface area and other unique properties[17,18]. It was reported that constructing 2D graphene sheet structure into 3D continuous network of carbon could solve its lamellar aggregation[19,20]. Compared with 2D graphene materials, 3D graphene-based materials prepared by reducing graphene oxide such as graphene aerogel (GA)

possessed much higher specific surface area, which was conducive to adsorption process as active sites[21,22]. It was easier to recycle and reuse due to the macrostructure of 3D GA[23,24]. Moreover, the physicochemical properties of 3D GA could be easily adjusted by transforming the reaction conditions and the properties of the template, which increased the potential of practical application [25,26]. However, the adsorption efficiency was largely reduced because that GA enableed to float on the sewage surface due to its low density and high surface hydrophobicity[27]. Doping inorganic metal oxide has become a simple method to improve the physical and chemical properties of GA such as stability, compatibility and so on[28,29]. The inorganic metal oxide can meliorate the hygroscopicity of the aerogel and allow water to pass through the pores into the inside of the aerogel, which is advantageous for the enhancement of its adsorption capacity[30]. Rare earth metal oxide, which has been known as an industrial gold , has been widely used in petroleum, chemical industry, metallurgy, textile, ceramics, glass, permanent magnetic materials and other fields[31-34]. Among them, neodymium oxide (Nd2O3) nanoparticles, praseodymium oxide (Pr2O3) nanoparticles and cerium oxide (Ce2O3) nanoparticles have been extensively studied for their abundant reserves in the earth's crust and excellent properties[35-46]. As far as we know, the synthesis of reduced-graphene-oxide/rare-earth-metal -oxide (RGO/REMO) aerogels and using them as an adsorbent for dyes removal have hitherto been less studied. In this work, we prepared three kinds of 3D RGO/REMO aerogels via a facile hydrothermal reduction method and freeze-drying. Furthermore, the adsorption of RhB on different RGO/REMO (Nd2O3, Pr2O3, Ce2O3) aerogels were studied in detail. The results indicated REMO nanoparticles could efficiently improve the adsorption ability of RGO aerogels. 2. Experiment 2.1. Reagents and chemicals Natural graphite powder, neodymium nitrate (Nd(NO3)3), barium nitrate (Pr(NO3)3) and cerium nitrate (Ce(NO3)3) were purchased from Aladdin Industrial

Corporation (Shanghai, China). Rhodamine-B (RhB, 98.5%) and thiourea (CH4N2S, 99%) were provided by Chengdu Kelong Chemical Reagent Company (Chengdu, China). All reagents and chemicals were used without further purification. 2.2. Preparation of RGO/REMO aerogels GO was prepared by an improved Hummers method as described in the literature[47]. The prepared GO was formulated into an aqueous suspension (10 mg/mL). A typical process to prepare the RGO/Nd2O3 aerogel was as following. Firstly, 2 mL Nd(NO3)3 aqueous solution (20 mg/mL) was added into GO aqueous suspension (16 mL, 10 mg/mL) stored in a 80 mL glass bottle. Secondly, 40 mg CH4N2S was quickly added into the above mixed liquids and mixed evenly with a magnetic stirrer. And then 10 mL of this dispersion into a 20 mL screw type bottle and the bottle was heated in oil bath for 4 h at 90 . Finally, the prepared hydrogel was washed by deionized water and RGO/Nd2O3 aerogel was obtained after freeze-drying for 48h. The preparation methods of RGO/Pr2O3 aerogel and RGO/Ce2O3 aerogel were the same with RGO/Nd2O3 aerogel. The preparation flow chart was shown in Figure 1. 2.3. Characterizations Fourier transform infrared spectroscopy (FTIR, TENSOR27) was used to analyse the chemical composition of as-prepared RGO/REMO aerogels. The sample morphologies were obtained by using a field emission scanning electron microscope (SEM, Ultra-55, CarlZeiss, Germany) and transmission electron microscopy (TEM, Libra200, CarlZeiss, Germany). The crystal structure of RGO/REMO aerogels was detected by X-ray diffraction (XRD, X Pert pro, Panacco, Netherlands). Information concerning the chemical composition and chemical bonds on the surfaces of the RGO/REMO aerogels were obtained by X-ray photoelectron spectroscopy (XPS, Kratos Analysis XSAM800). Raman measurements were performed at room temperature using Raman microscopes (in Via, Renishaw, UK) under the excitation wavelength of 532 nm. The automated surface analyzer was applied on AR-JW-BK112 in order to investigate the Brunauer-Emmett-Teller (BET) surface

area and pore size distribution of the RGO/REMO aerogels. The thermal decomposition tests were performed with the thermal gravimetric analyzer (TGA, STA 449 F5, Netzsch) under the air atmosphere with 10

min-1 heating rate.

2.4. Adsorption experiments RhB, a cationic dye, was used to analyze the adsorption property of the as-prepared RGO/REMO aerogels. For this study, 5 mg adsorbent was added to 20 mL aqueous solutions of ionic dye ( RhB, 100 mg L-1). Meanwhile, three kinds of aerogels with different components were studied as the adsorbents. The concentration of RhB was calculated by using the Beer-Lambert Law based on the adsorption peak at 553 nm via the UV-vis spectrophotometer at a predetermined time interval. The RhB removal rate (Rt, %) and the amount of RhB adsorbed onto the RGO/REMO aerogels (qt) were calculated basing on the following equations (1) and (2), respectively. Rt (%) = C0 Ct C0 100% qt

C0 Ct V m

(1) (2)

Where Ct (mg L-1) was the concentration of RhB at t time and C0 (mg L-1) was the initial concentration of RhB, V (L) represented the volume of the solution and m (mg) showed the weight of the RGO/REMO aerogels used as adsorbent. 3. Results and discussion 3.1. Characterizations of RGO/REMO aerogels The functional groups and REMO bonds in RGO/REMO aerogels were investigated with FTIR analysis. The FTIR spectra of GO, RGO/Nd2O3, RGO/Pr2O3 and RGO/Ce2O3 aerogels were displayed in Figure 2. As for GO, the absorption characteristic peaks at 1063.2 cm-1 and 1630.5 cm-1 were ascribed to the C-O-H and C=O extension vibrations, respectively[48]. The peaks at 1479.4 cm-1 and 2957.9 cm-1 corresponded to C=C stretching mode of aromatic ring and C-H stretching, respectively. The low intensity peaks with a spectral range between 1400 and 1000 cm-1 were due to vibrational modes associated with alkoxy and cyclooxy groups [49].

The peak of the absorption band caused by the tensile vibration of O-H was about 3423.3 cm-1, which might be due to the hygroscopicity of the aerogel [50]. The absorption peaks at about 790.8 cm-1 related with the vibrations of Nd-O, Pr-O and Ce-O bonds[51,52]. Based on the above results, it was not difficult to conclude that the RGO/REMO aerogels were formed by REMO nanoparticles and graphene sheets. As shown in Figure 3, the morphologies and interior microstructures of the prepared RGO/REMO aerogels were characterized by SEM and TEM. The SEM images (Figure 3 a1, b1 and c1) showed that the RGO/REMO aerogels possessed porous three-dimensional carbon network structures. However, the REMO nanoparticles were almost invisible from the SEM images, which could be due to their features of nanoscale structure. In order to further research the combination modes of RGO sheets and REMO nanoparticles, the TEM measurements were carried out. The TEM images (Figure 3a2, a3, b2, b3, c2 and c3) showed that the REMO nanoparticles were homogeneously dispersed and immobilized in the 3D architecture of porous RGO/REMO aerogels as indicated by the dark spots. The REMO nanoparticles can be clearly seen in all images, indicating the formation of the RGO/REMO aerogels. No severe agglomeration phenomena occurred. Besides, graphene sheets with folds also can be seen from the images. As shown in Figure 3a2, b2 and c2, the images were almost transparent and showed unique graphene wrinkles, which might be produced during the hydrothermal reduction process[53]. For example, the dashed line is a visual guide of the edge of graphene sheets and the lower left area of the dashed line is the graphene sheets as shown in Figure 3a2, which indicated that the Nd2O3 nanoparticles were well located on the surface of graphene sheets. It was not found the agglomeration of Nd2O3 nanoparticles or significant unmodified sites on RGO sheets[54]. Figure 3a3, b3 and c3 showed that the mean diameter of the REMO nanoparticles was all about 5-20 nm. In one word, the above results confirmed that the REMO nanoparticles were homogeneously distributed on the graphene sheets to form RGO/REMO aerogels. The crystal structure of RGO/REMO aerogels was detected by XRD. As shown

in Figure 4, the characteristic peak of graphene oxide was located at about 10.8° and the XRD diffraction peaks of RGO/Nd2O3, RGO/Pr2O3 and RGO/Ce2O3 aerogels were located at 17.2°, 16.9° and 16.8°, respectively. However, the XRD diffraction peak at 10.8° for GO in the RGO/REMO aerogels disappeared, which indicated that GO was reduced to RGO by thiourea during the hydrothermal reaction[55]. The right shift of the XRD diffraction peak of the hybrid aerogels indicated a decrease of the graphene sheet spacing, which was caused by the reduction of GO and the loading of REMO nanoparticles[56]. Figure S1 showed the XRD spectra of Nd2O3, Pr2O3 and Ce2O3 aerogels. Compared with the XRD spectra of RGO/REMO aerogels, it could be seen that the characteristic diffraction peaks of the REMO in RGO/REMO aerogels completely disappeared, which further indicated that the RGO/REMO aerogels were successfully prepared. The XRD structural parameters of RGO/REMO aerogels were showed in Table S1. By fitting the XRD data with Bragg s law and Scherrer s equation, the number of carbon layers in RGO/Nd2O3, RGO/Pr2O3 and RGO/Ce2O3 were calculated. The results showed that RGO/REMO aerogels had a multilayer structure and the number of carbon layers was about three. The measured XPS data of RGO/Nd2O3, RGO/Pr2O3 and RGO/Ce2O3 aerogels were shown in Figure 5. Correspondingly, the XPS spectra of pure Nd2O3, Pr2O3 and Ce2O3 aerogels were shown in Figure S2. From the Figure S2, it could be seen that there were only the characteristic peaks of Nd, Pr, Ce and O were displayed centering on 980.5 eV, 934.0 eV 886.2 and 532.1 eV, respectively. However, the XPS spectra of RGO/REMO aerogels (Figure 5a) indicated that the RGO/REMO aerogels consisted not only of Nd, Pr, Ce and O elements, but also C element, which was from GO. The peak at 283.5 eV belonged to C 1s of RGO/REMO aerogels and the characteristic peaks of Nd 3d, Pr 3d and Ce 3d were displayed centering on 980.7 eV, 934.0 eV and 886.1 eV, respectively. Figure 5(b) exhibited the BE values at 978.3 eV and 983.1 eV detected by the aerogel, which corresponded to the Nd 2O3 orbital[57]. As shown in Figure 5(c), the peaks at 933.8 eV and 934.2 eV coincided with the peak positions of Pr2O3[58]. The peaks at 887.0 eV and 885.1 eV of Ce2O3 could be obtained from

Figure 5(d)[59]. For the GO, it was well known that the three peaks situated at 284.2, 285.8 and 288.3 eV were belong to the C-C, C-O and C=O bonds, respectively[60]. As shown in Figure 5(e-g), it was remarkable that the strength of the C-O peak from the prepared aerogel was significantly lowered, which corroborated that the graphene oxide was reduced to RGO[61]. In conclusion, the results of XPS further proved that Nd2O3 nanoparticles, Pr2O3 nanoparticles and Ce2O3 nanoparticles were attached to the RGO sheets. Figure 6 showed the Raman spectra for GO, RGO/Nd2O3, RGO/Pr2O3 and RGO/Ce2O3 aerogels. There were two characteristic peaks located at about 1353.7 cm-1 and 1594.8 cm-1 corresponding to D band and G band of carbonaceous materials, respectively[62,63]. It was well known that the intensity of the D-band (A1g mode respiratory vibration) to the G-band (E2g vibration mode) (ID/IG) was often used to describe the level of chaos of the carbon material[64]. The D band was caused by the faultiness and deformation of carbon layer, and the G band was bound up with the crystal form and graphitic carbon layers structure, corresponding to the tangential vibration of the carbon atoms[65]. As seen in Figure 6, the ratio ID/IG for GO, RGO/Nd2O3, RGO/Pr2O3 and RGO/Ce2O3 aerogels were 0.727, 0.901, 0.925, 0.881, respectively. These increase in the intensity ratios were due to the decrease in the size of sp2 domains upon reduction and interaction between REMO and graphene sheets. These results again confirmed that REMO nanoparticles were present on the RGO sheets. It was generally known that the specific surface area (S BET) and pore size distribution of aerogels were highly significant to determine their adsorption properties. Therefore, in order to investigate the possibility of application of GO and RGO/REMO aerogels to adsorption, the specific surface area and pore size distribution of GO and RGO/REMO aerogels were studied by nitrogen adsorption-desorption method. As displayed in Figure 7(a), the obtained results showed that the specific surface area of 57.3 m2 g-1, 385.2 m2 g-1, 322.1 m2 g-1 and 366.2 m2 g-1 for GO, RGO/Nd2O3, RGO/Pr2O3 and RGO/Ce2O3 aerogels, respectively.

The recombination of REMO nanoparticles with RGO flakes could be responsible for the increased specific surface area of the RGO/REMO aerogels, which meant that REMO nanoparticles had the effect of increasing the specific surface area of RGO aerogels. As shown in Figure S3-S4, the SBET and pore size distribution of REMO aerogels were also studied by nitrogen adsorption-desorption method. The SBET of pure Nd2O3, Pr2O3 and Ce2O3 aerogels were 224.2 m2 m2

-1

-1

, 232.7 m2

-1

and 259.3

, respectively. The SBET of RGO/REMO aerogels were higher than that of the

REMO aerogels, which was probably due to the synergistic effect between RGO and REMO. In addition, as shown in Figure 7(b) and S4, it could be found that the pore sizes of the RGO/REMO and REMO aerogels were all evenly distributed between 2 nm and 10 nm, which indicated that the two kinds of aerogels possessed typical mesoporous structure. However, the average pore size of RGO/REMO aerogels was higher than that of the corresponding REMO aerogels. The large S BET and average pore size might be advantageous to the improvement of the adsorption properties. Figure 8 described the GO and RGO/REMO aerogels for the TGA curves. It could be seen that the GO had the mass loss at about 260

was approximately 50.1%

due to disintegration of the oxygen-containing functional groups. However, the mass loss of the RGO/Nd2O3, RGO/Pr2O3 and RGO/Ce2O3 aerogels obtained from Figure 8 at around 260

were only 20.1%, 18.5% and 20.3%, respectively. The results showed

that the thermal stability of aerogels doped with REMO was much better than that of GO aerogel and different REMO had the same effects on the thermal stability of the RGO aerogel. It could be observed from Figure S5 that the pure REMO aerogels maintained a stable status above 1000 , indicating that the REMO aerogels presented high thermal stability. Based on the results, it was further concluded that the enhancement of the thermal stability of the RGO/REMO aerogels was due to the addition of REMO nanoparticles. 3.2. Adsorption property of RGO/REMO aerogels towards RhB Due to the high specific surface area and mesoporous structure, RGO/REMO aerogels were valuable in adsorption applications. In order to investigate the effect of

REMO on the adsorption of dyes, RGO/Nd2O3, RGO/Pr2O3, and RGO/Ce2O3 aerogels were used to adsorb RhB. There was a positive correlation between absorbance and dye concentration on the basis of the Beer-Lambert Law. Based on this, UV-vis spectrometer was used to determine the concentration of RhB at different times (Figure 9-11 (a)). Obviously, as shown in Figure 9-11 (b), it was well seen that the adsorption capacity of RhB increased as time went on. It could be calculated that the adsorption capacities of RGO/Nd2O3, RGO/Pr2O3 and RGO/Ce2O3 aerogels at equilibrium were 243.4 mg g-1, 226.0 mg g-1 and 235.7 mg g-1, respectively. In addition, the adsorption performance of the RGO/REMO aerogels were compared with other adsorbents shown in the recent literature and summarized in Table 1. It was found that the RGO/REMO aerogels showed remarkable equilibrium adsorption quantity and high removal rate for RhB compared to the other adsorbents, indicating their practicability in the treatment of wastewater containing RhB. Beyond that, the three RGO/REMO aerogels all had a high adsorption rate in the initial stage of adsorption process and then gradually decreased until achieving adsorption equilibrium. There are two main reasons for this. On one hand, the adsorption sites were occupied with the time went on, which caused the decrease of adsorption rate. On the other hand, due to the steric hindrance effect between the aqueous phase and the RhB attached to the RGO/REMO aerogels, unoccupied surface adsorption sites were hard to be used effectively[66]. It could be obviously seen that the adsorption capacities of RGO/Nd2O3, RGO/Pr2O3 and RGO/Ce2O3 aerogels in the first hour of adsorption process

-1

-1

-1

, respectively.

The result confirmed that the absorption efficiency of RGO/Ce2O3 aerogel was much higher than that of other two aerogels in the first hour of adsorption process, indicating that the RGO/Ce2O3 aerogel had a higher adsorption efficiency for RhB in the initial stage of the adsorption process. This might be explained that the outer surface of the RGO/Ce2O3 aerogel had more pores and adsorption sites than that of the other two aerogels, which facilitated the diffusion of the RhB from the outer surface to the internal adsorption points. As shown in Figure S6, the adsorption properties of pure Nd2O3, Pr2O3 and Ce2O3 aerogels for the removal of RhB were also

tested. It was found that REMO aerogels had almost no effective adsorption capacity for RhB. To further analyze the experiments and investigate the adsorption mechanism, two kinetic models, which were the pseudo-first-order and the pseudo-second-order kinetic models, were applied. The two kinetic models (pseudo-first-order and pseudo-second-order models)[76,77] expressed as equations (3) and (4), respectively, were

Figure 10-12 (c) and (d). ln q1 qt t qt

ln q1 k1t

1 k 2 q22

(3) (4)

t q2

Where k1 (h-1) and k2 (g (mg h)-1) were the adsorption rate constant, which came from pseudo-first-order and pseudo-second-order kinetic models, respectively. The equilibrium adsorption capacity of RhB corresponding to the two adsorption models were q1 (mg g-1) and q2 (mg g-1). In addition, the adsorption capacity of RhB on the adsorbent at time t was qt (mg g-1). The parameters of two adsorption kinetic equations were shown in Table 2. From the pseudo-first-order kinetic model, the theoretical adsorption capacity of -1

RGO/Nd2O3 aerogel for than the experimental one at 243.4

-1

(R12=0.956), which was much lower

. However, for the pseudo-second-order

kinetic model, the ideal maximum adsorption capacity of RGO/Nd2O3 aerogel for RhB was approximately 244.5 mg g-1 (R22=0.975), which was very close to the experimental q2 (243.4 mg g-1). It demonstrated that the kinetics data for RGO/Nd 2O3 aerogel fitted well with the pseudo-second-order kinetic model. As for RGO/Pr2O3 aerogel

and

RGO/Ce2O3

aerogel,

the

correlated

coefficients

(R22)

of

pseudo-second-order kinetic model were both more than 0.990. Furthermore, the calculated q2 for RGO/Pr2O3 aerogel (229.9 mg g-1) and RGO/Ce2O3 aerogel (241.5mg g-1) were very close to the experimental q2 for RGO/Pr2O3 aerogel (226.0 mg g-1) and RGO/Ce2O3 aerogel (235.7 mg g-1), respectively. Comparison of the correlation coefficients of the two kinetic models, it could be obtained that the

adsorption process of RhB by RGO/REMO aerogel was more in line with the pseudo-second-order kinetic model, suggesting that the rate limiting steps involved the sharing or exchanging of electrons between RGO/REMO aerogels and RhB [78]. The hydrophobic interactio

-

RhB and the grapheme layers of RGO/REMO aerogels [79], which were the major removal process of RhB. The positive charge of RhB+ might also contribute to -

cing its electron-accepting ability[80]. At

the same time, it could be obtained from the XPS spectrum that the rare earth metal ions were in a cationic state, hence a cation-

ight occur between the

positive rare earth metal metal ions and the aromatic ring of RhB[81]. In addition, due to a small amount of unreduced oxygen-containing functional groups on the graphene sheets of the RGO/REMO aerogels, the electrostatic attraction and hydrogen bonding might exist between the RGO/REMO aerogels and RhB[82,83]. The removal mechanism of the RhB by RGO/REMO aerogels were shown in Figure 12. To further investigate the effect of REMO nanoparticles on the adsorption of dyes, the maximum adsorption capacity and color change of adsorption process for three kinds of aerogels were shown in Figure 13. From Figure 13(a), for RhB solution -1

adsorption capacity of RGO/Nd2O3

), it could be seen that the maximum -1

, which was

higher than that of the other two aerogels. There might be two reasons for the difference in adsorption capacity. On the one hand, the hybrid aerogel loaded with Nd2O3 nanoparticles had a higher specific surface area than the other two hybrid aerogels. On the other hand, the effect of Nd2O3 nanoparticles on the interaction between graphene sheets and RhB might be better than that of other two nanoparticles[84]. The color change of adsorption process of the RGO/REMO aerogels for RhB were shown in Figure 13(b) and the color change of the RhB solution was basically same for the different adsorbents. This might be explained by the fact that the color of the original solution of RhB was too deep to make the color change obviously in the early stage of adsorption.

In addition to the removal rate and adsorption capacity, regenerative capacity and recyclability of adsorbents were quite significance for their practical application. The regenerative capacity and recyclability of RGO/REMO aerogels were explored as shown in Figure 14. After the first adsorption, the aerogels were dislodged 98.4%, 98.1% and 97.8% of the adsorbed dyes from the RGO/Nd2O3, RGO/Pr2O3 and RGO/Ce2O3 aerogels, respectively. However, after 9 cycles, three kinds of RGO/REMO aerogels still retained 90.1%, 86.5%, and 92.6% of the initial adsorption capacity for RhB, respectively. In addition, the adsorption capacity of RGO/Ce 2O3 aerogel after regeneration was higher than that of the other two aerogels, indicating that the structure of RGO/Ce2O3 aerogel was more stable. The high repeated utilization rate of RGO/REMO aerogels indicated that the RGO/REMO aerogels possessed potential practical application in the treatment of wastewater containing RhB. 4. Conclusion In summary, we combined the REMO (Nd2O3, Pr2O3, Ce2O3) with the GA by decorating the REMO nanoparticles on the RGO sheets to enhance the adsorption property. According to such ideas, three kinds of 3D RGO/REMO aerogels were prepared by a facile hydrothermal method and freeze-drying. The adsorption properties of RGO/Nd2O3, RGO/Pr2O3 and RGO/Ce2O3 aerogels for RhB were fully investigated. Compared with other two kinds of aerogels, the RGO/Ce 2O3 aerogel exhibited both excellent dye removal efficiency and reproducibility towards RhB. The adsorption capacity of RGO/Ce2O3 aerogel for RhB reached 130.1

-1

in only one

hour. Furthermore, the adsorption-rate of RGO/Ce2O3 aerogel remained above 92% after nine regeneration. In one word, the prepared RGO/REMO aerogels will be a promising adsorbent with a significant application prospect for the treatment of wastewater containing dyes. Acknowledgment We are grateful to Institute of Chemical Materials and Research Center of Laser Fusion, China Academy of Engineering Physics. This work was financially supported

by the National Natural Science Foundation of China (Grant No. number), the Foundation of Sichuan Educational Committee (No.16ZB0151), Career Development Funding of CAEP (No.2402001),and the Research Fund for the Doctoral Program of Southwest University of Science and Technology (No.17zx7135).

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Figure 1. Schematic for preparing RGO/REMO aerogels: (a) GO dispersion solution, (b) rare earth metal nitrates solution, (c) GO/REMO hybrid solution after stirring, (d) RGO/REMO hydrogel after hydrothermal reduction, (e) RGO/REMO aerogels after freeze-drying.

GO

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Wavenumbers (cm ) Figure 2. FTIR spectra of GO, RGO/Nd2O3 aerogel, RGO/Pr2O3 aerogel and RGO/Ce2O3 aerogel.

Figure 3. SEM (a1, b1, c1) and TEM (a2, a3, b2, b3, c2, c3) spectra of RGO/Nd 2O3 aerogel (a1, a2, a3), RGO/Pr2O3 aerogel (b1, b2, b3) and RGO/Ce2O3 aerogel (c1, c2, c3).

RGO/Ce2O3

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GO

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Figure 4. XRD spectra of GO, RGO/Nd2O3 aerogel, RGO/Pr2O3 aerogel and RGO/Ce2O3 aerogel.

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Figure 5. (a) XPS spectra of RGO/Nd2O3 aerogel, RGO/Pr2O3 aerogel and RGO/Ce2O3 aerogel, (b) core-level Nd3d of RGO/Nd2O3 aerogel, (c) core-level Pr3d

of RGO/Pr2O3 aerogel, (d) core-level Ce3d of RGO/Ce2O3 aerogel, (e) C1s XPS spectra of RGO/Nd2O3 aerogel, (f) C1s XPS spectra of RGO/Pr2O3 aerogel, (g) C1s XPS spectra of RGO/Ce2O3 aerogel.

1353.7 1594.8

ID/IG 0.727

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0.901

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0.925

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0.881

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Wavenumbers (cm ) Figure 6. Raman spectra of GO, RGO/Nd 2O3 aerogel, RGO/Pr2O3 aerogel and RGO/Ce2O3 aerogel.

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Figure 7. The nitrogen adsorption/desorption isotherms (a) and BJH analysis of pore size distributions (b) of GO, RGO/Nd2O3 aerogel, RGO/Pr2O3 aerogel and RGO/Ce2O3 aerogel.

100

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)

Figure 8. TGA curves of GO, RGO/Nd2O3 aerogel, RGO/Pr2O3 aerogel and RGO/Ce2O3 aerogel.

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Figure 9. (a) UV-vis spectra of RhB solution adsorbed by RGO/Nd 2O3 aerogel, (b) Time-dependent adsorption of RhB on the RGO/Nd2O3 aerogel, (c) Application of pseudo-first-order equation model to analyse the adsorption of RhB by RGO/Nd 2O3 aerogel and (d) Application of pseudo-second-order equation model to analyse the adsorption of RhB by RGO/Nd2O3 aerogel.

a

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Figure 10. (a) UV-vis spectra of RhB solution adsorbed by RGO/Pr 2O3 aerogel, (b) Time-dependent adsorption of RhB on the RGO/Pr2O3 aerogel, (c) Application of pseudo-first-order equation model to analyse the adsorption of RhB by RGO/Pr 2O3 aerogel and (d) Application of pseudo-second-order equation model to analyse the adsorption of RhB by RGO/Pr2O3 aerogel.

a

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200

400

600

800

1000

1200

Time(min)

0

200

400

600

800

1000

Time(min)

Figure 11. (a) UV-vis spectra of RhB solution adsorbed by RGO/Ce2O3 aerogel, (b) Time-dependent adsorption of RhB on the RGO/Ce2O3 aerogel, (c) Application of pseudo-first-order equation model to analyse the adsorption of RhB by RGO/Ce 2O3 aerogel and (d) Application of pseudo-second-order equation model to analyse the adsorption of RhB by RGO/Ce2O3 aerogel.

1200

Table 1 Comparison of qmax and removal rate of RhB by various adsorbents. Material 3,5-diacrylamidobenz oic acid based resin (APEADA) MgAl-LDH (O3D-LDH) Biobased magnetic hollow particles (BMHPs) Fe3O4 nanoparticles synthesized (Fe3O4 NPs) N,N-dimethylformami de (DMF) ZnO nanostructures/guar gum nanocomposites (ZnO NPs/GG) Zeolitic imidazolate frameworks (ZIFs) N, P-codoped reduced graphene oxide (PA-RGO) Fe3O4 nanoparticles functionalized/activate d carbon (Fe3O4/AC) RGO/Pr2O3 aerogel RGO/Ce2O3 aerogel RGO/Nd2O3 aerogel

qmax (

-1

)

Removal rate (%)

Refs.

23.28

65.20

[67]

48.29

40.00

[68]

50.40

50.40

[69]

51.81

98.00

[70]

63.85

91.20

[71]

72.00

90.00

[72]

116.20

38.70

[73]

149.00

97.50

[74]

182.48

90.00

[75]

226.00 235.70 243.40

98.00 99.20 almost 100

This work This work This work

Table 2 The parameters of the diffusion model for RhB adsorption. Pseudo-first-order equation

Pseudo-second-order equation k2 qe(cal) (g mg-1 R22 -1 (mg g ) min-1)

Samples

k1 (min-1)

qe(cal) (mg g-1)

R1 2

RGO/Nd2O3

2.35×10-3

170.7

0.956

3.55×10-5

244.5

0.975

RGO/Pr2O3 RGO/Ce2O3

2.84×10-3 2.23×10-3

131.5 133.1

0.979 0.985

6.17×10-5 4.19×10-5

229.9 241.5

0.997 0.992

Figure 12. A schematic illustrating the adsorption mechanisms of RGO/REMO aerogels for RhB.

300

a

200

100

0

RGO/Nd2O3

RGO/Pr2O3

RGO/Ce2O3

Figure 13. (a) The adsorption capacity of RGO/Nd 2O3 aerogel, RGO/Pr2O3 aerogel and RGO/Ce2O3 aerogel for RhB and (b) The solution color change in the process of RhB adsorption by using different RGO/REMO aerogels.

100

RGO/Nd2O3 RGO/Pr2O3

98

RGO/Ce2O3

96 94 92 90 88 86 0

2

4 6 Repetition adsorption times

8

10

Figure 14. The relationship between the adsorption efficiency of RGO/Nd 2O3 aerogel, RGO/Pr2O3 aerogel and RGO/Ce2O3 aerogel and the recycle times after the adsorption procedure is repeated 9 times.

Highlights 1) The reduced-graphene-oxide/rare-earth-metal-oxide (RGO/REMO) aerogels with three-dimensional interconnected networks was prepared under mild conditions. 2) The RGO/REMO aerogel presented remarkable adsorption capacity. 3) The RGO/Ce2O3 aerogel showed significant adsorption efficiency, which adsorption capacity for RhB reached 130.1

-1

in only one hour.

4) The RGO/Ce2O3 aerogel exhibited excellent recycling regeneration property, which adsorption-rate remained above 92% after nine regeneration.