Magnetically Separable Ag / CuFe 2 O 4 / Reduced Graphene Oxide Ternary Nanocomposite with High Performance for the Removal of Nitrophenols and Dye Pollutants from Aqueous Media

In this work, a novel ternary magnetic nanocomposite namely Ag/CuFe2O4/rGO was produced by a facile solvothermal route. The reduction of graphene oxide (GO) to reduced form (rGO) and the in-situ deposition of CuFe2O4 and Ag nanoparticles on rGO occurred simultaneously in a one-pot reaction. The structure, composition and morphology of the as-prepared nanocomposite were characterized by FT-IR, XRD, VSM, FESEM, EDX, TEM, and BET analyses. The results indicated that the Ag and CuFe2O4 nanoparticles were successfully loaded on the surface of rGO. The catalytic performance of the Ag/CuFe2O4/rGO nanocomposite was evaluated for the reduction of nitroarenes to corresponding amines in the presence of sodium borohydride (NaBH4) as a reducing agent. The nanocomposite exhibited the high performance in the reduction of the reduction of nitroarenes with 100% conversion within 10–30 min. The catalytic activity of the Ag/CuFe2O4/rGO was enhanced compared with Ag/CuFe2O4, Ag/rGO and CuFe2O4/rGO samples due to easier electron transfer process. Furthermore, the adsorption data revealed that cationic methylene blue (MB) dye could be removed almost completely on the Ag/CuFe2O4/rGO composite within 2 min and the composite could also selectively adsorb MB from the mixed solution with anionic methyl orange (MO). Due to the existence of magnetic CuFe2O4 nanoparticles, the Ag/CuFe2O4/rGO nanocomposite can be magnetically separated and reused without any change in structure and performance.


Introduction
][6][7] A large amount of industrial wastewaters also contain organic dye pollutants.][10][11][12][13][14][15] Hence, there is a need to find a new desirable material, which not only has the capacity of reducing the nitroarenes but also can achieve adsorption and selective separation of organic pollutants.][24][25] Graphene, single-layer carbon atoms densely packed into a two-dimensional (2D) lattice, has high surface area, excellent electrical conductivity and high mechanical Nirumand et al.: Magnetically Separable Ag/CuFe 2 O 4 /Reduced ... strength. 26It can be used as a promising catalyst support on which the metal nanocatalysts can be stabilized and result in the further enhanced catalytic performance. 27,28ne of the applications of graphene-based materials is also the removal of organic dye pollutants from water. 29,30any polymeric and inorganic adsorbents such as carbon-based materials, 31 porous metal oxides, 32 clays, 33 chitosan, 34 zeolites, 35 and metal-organic frameworks (MOFs) 36 have developed for adsorption removal of pollutants from aqueous solutions.Among them, carbonous materials such as graphene have shown excellent performance in adsorption of organic contaminants from water.However, such adsorbents are usually separated by tedious filtrating from water after the adsorption/catalytic process.To overcome this weakness, coupling with magnetic materials is highly desirable.Spinel ferrites with general formula MFe 2 O 4 (M = Mn, Fe, Co, Ni, Cu, etc.) have been of great interest in many magnetic applications. 37onsidering the above, in this work, we have synthesized the novel magnetic Ag/CuFe 2 O 4 /rGO ternary nanocomposite via solvothermal method which fully characterized by various spectroscopic techniques.During the formation of the composite, the reduction of graphene oxide (GO) to rGO and the in-situ deposition of CuFe 2 O 4 and Ag nanoparticles on rGO occurred simultaneously in a one-pot reaction.Due to high catalytic activity of Ag , the nanocomposite displayed excellent performance for catalytic reduction of nitroarenes by using NaBH 4 in aqueous solutions.Also, owing to the existence of graphene with high surface area and adsorption capacity, the Ag/ CuFe 2 O 4 /rGO nanocomposite exhibited high performance on the removal of dye pollutants from wastewater.Moreover, the magnetic properties of CuFe 2 O 4 make the separation of the nanocomposite easier, avoiding environmental contamination from processing.

Preparation of Ag/CuFe 2 O 4 /rGO Nanocomposite
Graphene oxide (GO) nanosheets were prepared from natural graphite powder by a modified Hummer's method. 38The graphite oxide layers were separated from each other through ultrasonication to acquire graphene oxide sheets.For synthesis of the Ag/CuFe 2 O 4 /rGO nano-composite, the as-prepared GO (0.1 g), Fe(NO 3 ) 3 9H 2 O (2 mmol, 0.40 g) and Cu(NO 3 ) 2 3H 2 O (1 mmol, 0.12 g) were dispersed in 30 mL of ethylene glycol (EG) to form a clear solution, followed by the addition of sodium acetate (45 mmol, 3.10 g ) and AgNO 3 (1.2mmol, 0.20 g).The mixture was ultrasonicated for 120 min.Subsequently, the obtained suspension was transferred into a Teflon-lined stainless-steel autoclave.The autoclave was sealed and put into the oven, which was heated and maintained at 200 °C for 9 h, then taken out and allowed to cool naturally at room temperature.Finally, the black product was magnetically collected and washed thoroughly with water and ethanol, followed by drying at 50 °C for a few hours.It was denoted as Ag/CuFe 2 O 4 /rGO nanocomposite with 23 wt% Ag as confirmed by ICP-AES analysis.For comparison, Ag/ CuFe 2 O 4, Ag/rGO and CuFe 2 O 4 /rGO samples were synthesized under the same conditions in the absence of rGO, CuFe 2 O 4 and Ag, respectively.Also, the samples of rGO and Ag nanoparticles were prepared via the reduction of GO (0.1 g) and AgNO 3 (0.2 g), respectively, in 30 mL of ethylene glycol (EG) as solvent and reducing agent under hydrothermal conditions at 200 °C for 9 h.

3. Characterization Techniques
X-ray diffraction (XRD) measurements were carried out on a Rikagu D-max X-Ray diffractometer using Cu Kα radiation (λ = 1.540Å).Fourier transform infrared (FT-IR) spectra were recorded using Shimadzu FT-IR 160 spectrometer over the wavenumber range from 4000 to 400 cm -1 .Raman spectra were obtained using a Raman microscope with laser wavelength of 785 nm.The morphology and size of the nanocomposite particles were characterized by Mira3 Tescan, scanning electron microscope (FESEM) outfitted with energy field dispersive X-ray analyzer (EDX) for the elemental analysis of the sample.TEM images were obtained on the electron microscope (Philips CM120) at the accelerating voltage of 100 kV.The magnetic properties were measured using a vibrating sample magnetometer (Model: MDKFD, Magnetic Daneshpajoh Kashan Co., Iran) with a maximum magnetic field of 10 kOe.The Brunauer-Emmett-Teller (BET) surface area was measured by N 2 adsorption measurements at 77 K using Belsorp mini apparatus.UV-visible spectra were obtained from a Cary 100 Varian spectrophotometer in a wavelength range of 200-800 nm.The concentration of metals in the filtrate was determined by inductively coupled plasma atomic emission spectroscopy (Perkin Elmer ICP-AES).

4. Catalytic Reduction Tests
In order to study the catalytic performance of as-synthesized Ag/CuFe 2 O 4 /rGO nanocomposite in reductive processes, the reduction of some nitroaromatic compounds to corresponding amino derivatives by excess NaBH 4 in aqueous solution was investigated.For this work, freshly prepared aqueous solution of NaBH 4 (0.5 mL, 20 mM) was mixed with as-prepared aqueous solution (3 mL, 0.2 mM) of the nitroaromatic compounds, in the quartz cell (1.0 cm path length and 4 mL volume).Then, 3 mg of the Ag/CuFe 2 O 4 /rGO nanocatalyst was added into the solution and the progress of reduction was monitored by UV-Vis spectrophotometer in the range of 200-800 nm, with cycling over definite time intervals at room temperature.

5. Dye Adsorption Tests
The adsorption experiments were performed as follows: 50 mg of the Ag/CuFe 2 O 4 /rGO nanomaterial was added into 50 mL of dye solution (25 mg/L) under stirring.At predetermined time intervals, the concentrations of dye solutions, from which the adsorbent was removed by magnetic separation, were determined by measuring the absorbance of dye solution (MB,MO) using a Cary 100 Varian UV-Vis spectrophotometer.A similar experiment was also done with 50 mL of MB solutions with different concentrations (25, 50, 75 and 100 mg/L).Moreover, the UV-Vis spectroscopy was performed to determine the selective adsorption ability of Ag/CuFe 2 O 4 /rGO nanocomposite at given time intervals.The amount of dye removal efficiency was calculated by the following equation: Removal efficiency (%) = (C 0 -C t )/C 0 × 100, where C 0 and C t are the dye concentrations (mg/L) at the initial and t time, respectively.In all aqueous solutions containing dye and Ag/CuFe 2 O 4 /rGO nanocomposite, the residual concentrations of Ag and Cu after removing the adsorbent were detected to be less than 0.1 wt% by ICP-AES analysis.

1. Characterization of the Ag/CuFe 2 O 4 /rGO Nanocomposite
The XRD analysis was used to investigate the composition and structure of the products.Figure 1 shows the XRD patterns of GO, rGO, CuFe 2 O 4 , Ag and Ag/CuFe 2 O 4 / rGO samples.Figure 1(a) and (b) shows the XRD patterns of pure GO and GO after it was reduced in EG solvent under hydrothermal conditions.Figure 1(a) shows that GO exhibited a sharp and high-intensity diffraction peak at about 2θ = 11.5°.This result shows that a highly organized layer structure with an interlayer distance (d spacing) of 0.80 nm along the (002) orientation was produced.The (002) peak shifted to 2θ = 21.5 with d-spacing of 0.35 nm after reduction; thus, the d-spacing decreased from 0.80 nm to 0.35 nm.This decrease in d-spacing was caused by the elimination of oxygen-containing functional groups and H 2 O molecules from the graphite interlayers during the reduction process.As seen in Figure 1(c), the diffrac-tion peaks indexed at 2θ = 30.2°,35.6°, 43.2°, 53.6°, 57.1°, 62.7° and 74.2° are attributed to (220), (311), (400), ( 107), (511), ( 440) and (533) planes of cubic spinel structure of CuFe 2 O 4 (JCPDS-34-0425). 39The XRD pattern of Ag nanoparticles in Figure 1(d) shows peaks at 2θ values of 38.3°, 43.5°, 64.6° and 77.5° which could be assigned to the (111), ( 200), ( 220) and (311) planes of metallic silver (JCPDS-04-0783). 40From the XRD pattern of the Ag/ CuFe 2 O 4 /rGO nanocomposite (Figure 1(e)), it is clear that all appeared peaks can be attributed to CuFe 2 O 4 and Ag phases.No characteristic diffraction peaks for rGO appeared in the pattern indicating that rGO nanosheets are not stacked during the nanocomposite formation process.The reason can be attributed to the fact that Ag and CuFe 2 O 4 nanoparticles anchored on the surface of rGO nanosheets prevented the exfoliated rGO nanosheets from restacking. 41As can be seen in Figure 2(d), the characteristic band of rGO at 1615 cm −1 related to the graphitic C=C bond and the vibrational bands of CuFe 2 O 4 in the range of 600-400 cm −1 were all observed in the IR spectrum of the nanocomposite sample which demonstrate the coexistence of rGO and CuFe 2 O 4 in this hybrid nanomaterial.Moreover, the disappearance of the characteristic bands of GO confirms the formation of rGO in the nanocomposite and the restoration of a graphitic structure in graphene. 42he morphology and microstructure of pure GO and Ag/CuFe 2 O 4 /rGO nanocomposite were investigated by SEM analysis.The SEM micrograph of pure GO in Figure 3(a) shows the highly porous and layered structure of GO having large stacks, possibly consisting of hundreds of GO nanosheets.It should also be noted that the surfaces of the GO sheets are quite flat and smooth.Figure 3(a) shows the SEM of GO nanosheets which stacked together due to the dispersive forces between them. 45As can be seen in Figure 3(b), the surfaces of graphene nanosheets were covered with the agglomerated Ag and CuFe 2 O 4 nanoparticles.It should also be noted that the pores between the graphene sheets were evenly filled up with the Ag and CuFe 2 O 4 nanoparticles.In addition, the image of the nanocomposite clearly indicated that the surface properties of the modified GO product were strongly affected.Compared to pure GO, the surfaces of rGO nanosheets in the nanocomposite are rough, and the edges are highly crumpled.
Further investigation was carried out by energy dispersive X-ray spectroscopy (EDX) to characterize the composition of as-prepared Ag/CuFe 2 O 4 /rGO nanocomposite.The existence of Ag, Cu, Fe, C and O elements in the nanocomposite could be proved by the EDX elemental   Raman analysis is a powerful technique to monitor significant structural changes of GO nanostructures  According to the Raman spectra of GO and rGO, the two characteristic bands at about 1318 and 1589 cm −1 can be attributed to the disordered structure (D band, sp 3 carbon atoms of disorders and defects) and graphitic structure (G band, sp 2 carbon atoms in graphitic sheets) of GO nanosheets, respectively. 46As can be seen in Figure 6(c), the D and G bands still exist in the Raman spectrum of Ag/ CuFe 2 O 4 /rGO composite, besides a broad band around 600 cm -1 related to the A 1 g stretching vibrartion of Fe-O bond in the FeO 6 groups of CuFe 2 O 4 nanoparticles. 47hen, this finding demonstrates the coexistence of CuFe 2 O 4 and rGO in the nanocomposite.Compared to pure GO and rGO samples, the position of D and G bands slightly shifted likely due to the strong interaction of the Ag and CuFe 2 O 4 nanoparticles with the rGO nanosheets.Furthermore, the D band to G band intensity ratios for rGO (I D /I G = 1.48) and Ag/CuFe 2 O 4 /rGO (I D /I G = 1.41) are larger compared with that of pure GO (I D /I G = 1.09), which can be attributed to the increase in the degree of disorder and defects, and also the decrease in the average size of sp 2 domains. 48The increase in the I D /I G ratio also confirms that the GO has been deoxygenated and reduced to rGO.
The magnetic properties of CuFe 2 O 4 and Ag/ CuFe 2 O 4 /rGO samples were examined using a vibrating sample magnetometer (VSM) at ambient temperature.From Figure 7(a) and (b), the saturation magnetizations (Ms) of the CuFe 2 O 4 and Ag/CuFe 2 O 4 /rGO were found to be 68 emu/g and 48.2 emu g −1 , respectively.The slope defined as S-like shape of the magnetic hysteresis loops as well as the fact that both materials exhibit zero coercivity (Hc) and very low remanence (Mr) at ambient tempera-  (silver and graphene) in the nanocomposite.From the inset Figure 7, it is evident that homogeneous black Ag/ CuFe 2 O 4 /rGO suspension can be easily separated from solution under an external magnet influence.Figure 8 shows the nitrogen adsorption-desorption isotherms and corresponding pore size distributions curves (the inset) of rGO and Ag/CuFe 2 O 4 /rGO samples.As seen in Figure 8(a) and (b), the isotherms of GO and Ag/CuFe 2 O 4 /rGO nanocomposite can be classified to type-IV with H 4 -type hysteresis loop for rGO and H 3 -type hysteresis loop for the nanocomposite at high relative pressures according to the IUPAC classification, which indicate the presence of mesopores. 50The BET surface area (S BET ) of the Ag/CuFe 2 O 4 /rGO nanocomposite is 90.20 m 2 /g with the total pore volume of 0.261 cm 3 /g, which are greater than the values of rGO (S BET = 79.15m 2 /g and total pore volume = 0.123 cm 3 /g).It can be concluded that immobilizing Ag and CuFe 2 O 4 nanoparticles on the suface of graphene nanosheets increases the surface area and porosity which are favorable factors for improving the catalytic and adsorption performances.

2. Catalytic Reduction of Nitroarenes over the Ag/CuFe 2 O 4 /rGO Nanocomposite
The catalytic activity of the Ag/CuFe 2 O 4 /rGO nanocomposite was evaluated for the reduction of 2-nitroaniline (2-NA), 4-nitroaniline (4-NA), 2-nitrophenol (2-NP) and 4-nitrophenol (4-NP) in the presence of sodium borohydride (NaBH 4 ).In the reduction process, the initial concentrations of NaBH 4 and nitroaromatics were 20 mM and 0.2 mM, respectively.Figure 9(a)-(d) shows the typical UV-visible spectra and concentration changes of nitroaromatic compounds in the presence of Ag/CuFe 2 O 4 /rGO nanocomposite and NaBH 4 .2][53][54][55][56] The intensity of this peak decreases when the reduction proceeds in the presence of Ag/CuFe 2 O 4 /rGO nanocatalyst.The characteristic absorption peaks of 2-NA, 4-NA, 2-NP and 4-NP were disappeared in 25, 30, 10 and 15 min, respectively.Since the reductive reactions were carried out under the same experimental conditions, the different rates can be related to the structures of organic compounds.The pseudo first-order kinetics can be applied to evaluate the rate constants in the reduction process, because the concentration of NaBH 4 is higher compared to that of nitroarenes and it can be considered as a constant during the reaction time.The concentration of mentioned compounds at time t is defined as C t and the initial concentration of them at t = 0 is regarded as C 0 .The C t /C 0 is measured from the relative intensity of absorbance (A t / A 0 ).As shown in the insets of Figure 9   ture allowed ranking them as superparamagnetic. 49The lower range of magnetization (emu g −1 ) for the Ag/ CuFe 2 O 4 /rGO can be related to the decrease of CuFe  The study of the reaction mechanism helps to design a catalyst with better output.Figure S2  -to 4-NP, leading to the production of 4-aminophenol (4-AP). 21After the reaction is completed, the Ag/CuFe 2 O 4 /rGO nanocatalyst can be separated easily from the solution by a magnet and applied for next reaction.

Adsorption Performance of the Ag/ CuFe 2 O 4 /rGO Nanocomposite
There are some methods on the effective elimination of hazardous substances from aqueous solutions, such as photo and sonocatalytic degradation, oxidation and adsorption that among them, adsorption is a good choice due to its higher efficiency and simpler operation. 73In this context, considering the structure of the packed sp 2 -bonded carbon atoms of rGO and magnetic property of CuFe 2 O 4 , it is expected that the Ag/CuFe 2 O 4 /rGO can be used as a novel and effective adsorbent for the removal of organic aromatic pollutants from wastewaters.The adsorption capacity was evaluated using two dyes: positively charged methylene blue (MB) and negatively charged methyl orange (MO).As indicated in Figure 10(a), the characteristic UV-vis absorption band of MB at 664 nm, decreased and disappeared within only 2 min.The extraordinary removal of MB could be largely ascribed to the π-π stacking between dyes and rGO nanosheets. 40The electrostatic interactions between the positively charged dye and negatively charged residual oxygen-containing functional groups of rGO, were also effective in the adsorption process.

1. Effect of the Initial Concentration of MB
As shown in Figure 11(a), the effect of the initial concentration of MB solution on adsorption efficiency was studied.Accordingly to the obtained results, the adsorption percentage of the nanocomposite towards MB can reach up to about 100% at lower initial concentrations (15 and 25 and 75 mg/L).However, for MB with concentration of 100 mg/L, adsorption percentage decreased to 70% within 30 minutes and the Ag/CuFe 2 O 4 /rGO nanocomposite could not adsorb more dye molecules due to saturation of adsorption capacity.The high adsorption performance of graphene sheets mainly is due to the large surface area and graphitized basal plane structure of it, which could provide excellent conditions for adsorption of dyes. 31However, in the presence of Ag/CuFe 2 O 4 /rGO nanocomposite, the removal efficiency of MB is increased to 100% indicating the as-prepared nanocomposite is more suitable and superior.The Ag and CuFe 2 O 4 nanoparticles on the surface of graphene prohibit the restacking of its layers and thus increase the surface area and adsorption capacity of the composite.This result is consistent with BET analyses.

Effect of Adsorbent Dosage
The influence of the adsorbent dosage on MB adsorption in Figure 11(c) indicates that with the increase of the adsorbent dosage from 25 to 35 mg, the removal efficiency of MB is enhanced to 78%, and when the dosage is 50 mg, it reaches 100% within 2 min.The finding indicates that 50 mg is the suitable dosage for the adsorption of 25mg/L MB solution.

4. Effect of pH
As shown in Figure 11(d), the initial solution pH plays an important role in the adsorption of MB on the surface of Ag/CuFe 2 O 4 /rGO nanocomposite.The adsorption of MB increases gradually as pH value increases from 2 to 7 and finally maintains the high level with increasing pH.In acidic pHs, the surface of Ag/CuFe 2 O 4 /rGO nanocomposite is positively charged due to protonation reaction.Thus, the electrostatic repulsion between positively charged MB dye and Ag/CuFe 2 O 4 /rGO nanocomposite leads to the low adsorption of MB in this pH range.However, at pH ≥ 7, the surface of the adsorbent is negatively charged and more available to retain the cationic MB dye, resulting in the increase of MB adsorption.In the pH range of 7-11, the removal of MB reaches maximum and remains approximately constant.

5. Recyclability of the Ag/CuFe 2 O 4 /rGO Nanocomposite
To evaluate the stability and reusability of the Ag/ CuFe 2 O 4 /rGO nanocomposite which are important factors for practical applications, recycling experiments were performed.The composite was separated from the reaction mixture by an external magnet after the first use in the adsorption of MB and washed thoroughly with ethanol.The recovered nanocomposite was found to be reusable for at least five runs without significant loss in activity.As shown in Figure S3, the slight decrease of performance could be observed after five runs.
The amounts of Ag and Cu metals in aqueous solution after each run were analyzed by ICP-AES.It was shown that the leaching of these metals from the catalyst is negligible, confirming the stability of the composite.Furthermore, the structural stability was investigated by XRD, FT-IR, EDX and SEM analyses after five runs.As shown in Figure S4, these results did not show significant change after the fifth run in comparison with those of the fresh sample.

Conclusions
In summary, the Ag/CuFe 2 O 4 /rGO nanocomposite was prepared through a facile one-top solvothermal route.The Ag/CuFe 2 O 4 nanoparticles were loaded on the surface of GO and the nanocomposite displayed magnetic property.The catalytic performance of this composite was excellent for reduction of nitroarenes in the presence of NaBH 4 .The adsorption capacity of MB and MO dyes was also investigated and indicated that the removal efficiency of MB was 100% within only 2 min.Interestingly, due to the fast adsorption of MB molecules, the Ag/CuFe 2 O 4 /rGO nanocomposite could be used as effective and selective adsorbent for the removal of MB molecules from the mixed MB+MO dye solution.The Ag/CuFe 2 O 4 /rGO was separated from solution for reusing by simply applying an external magnetic field.Results obtained in this work open a way to the fabrication of effective nanomaterials for serving as an ideal platform to study the various heterogeneous catalytic processes and wastewater purification.

Figure 4 (
b) shows a representative SEM image of the nanocomposite with corresponding EDX elemental mappings.The distribution of corresponding elemental mappings confirmed that the elements were uniformly distributed over the nanocomposite, confirming the homogeneity of the sample.The EDX results further indicated that the Ag and CuFe 2 O 4 nanoparticles were successfully loaded on the surface of rGO.Figure5shows the TEM images of Ag/CuFe 2 O 4 and Ag/CuFe 2 O 4 /rGO samples.Figure5(a)shows the typical TEM image of the Ag/CuFe 2 O 4 , displaying that the sample consists of a large quantity of nearly uniform spheres nanoparticles which loosely aggregated.As can be seen in Figure5(b), the almost transparent graphene sheets are fully exfoliated and decorated homogeneously with Ag and CuFe 2 O 4 nanoparticles having an average diameter of 30 nm in consistent with the average particle size calculated from Debye-Scherer formula.No obvious aggregation was seen in Figure5(b).The rGO sheets could not only prevent agglomeration of the Co 3 O 4 nanoparticles and enable a good dispersion of these spherical particles, but also substantially enhance the specific surface area of the composite.

Figure 4 .
Figure 4. (a) EDX spectrum and (b) a representative SEM image with corresponding EDX elemental mappings of the Ag/CuFe 2 O 4 /rGO nanocomposite.
Figure 6 presents the Raman spectra of GO, rGO and Ag/CuFe 2 O 4 / rGO nanocomposite samples.Figure 6(a) and (b) shows the Raman spectra of the pure GO and GO sample after its reduction in EG solvent under hydrothermal conditions.
(a)-(d), the linear relationship of ln[A]/[A 0 ] versus time (t) indicates that the reduction of nitrophenols and nitroanilines over Ag/ CuFe 2 O 4 /rGO nanocomposite follows the pseudo first-order kinetics.

Figure 7 .
Figure 7. Magnetic hysteresis loops of (a) pure CuFe 2 O 4 and (b) Ag/ CuFe 2 O 4 /rGO at room temperature.The inset image shows the Ag/ CuFe 2 O 4 /rGO suspension in the absence and presence of an external magnetic field.
shows the catalytic mechanism of 4-NP reduction by the Ag/CuFe 2 O 4 /rGO nanocomposite in the presence of NaBH 4 .During the hydrogenation reduction process, BH 4 -and 4-NP are first adsorbed on the surface of the catalyst, and then the Ag nanoparticles transfer electrons from BH 4

Figure 11 (
Figure 11(b) represents the adsorption capacities of Ag/CuFe 2 O 4 , rGO and Ag/CuFe 2 O 4 /rGO samples toward MB dye.As seen, the removal efficiency of MB by rGO and Ag/CuFe 2 O 4 nanoparticles are 78% and 25% respectively.The high adsorption performance of graphene sheets mainly is due to the large surface area and graphitized basal plane structure of it, which could provide excellent conditions for adsorption of dyes.31However, in the presence of Ag/CuFe 2 O 4 /rGO nanocomposite, the removal efficiency of MB is increased to 100% indicating the as-prepared nanocomposite is more suitable and superior.The Ag and CuFe 2 O 4 nanoparticles on the surface of graphene prohibit the restacking of its layers and thus increase the surface area and adsorption capacity of the composite.This result is consistent with BET analyses.

Table 1 .
Comparison of the results obtained for the reduction of 4-NP in the present work with those obtained by some reported catalysts.NP at 400 nm was observed in the absence of Ag/CuFe 2 O 4 /rGO, confirming that the reduction is mostly catalyzed by the Ag/CuFe 2 O 4 /rGO nanocomposite.In the presence of Ag/CuFe 2 O 4 /rGO nanocomposite and NaBH 4 , the nitroarenes are easily reduced in aqueous solutions.
The Ag/CuFe 2 O 4 nanaoparticles could be acted as spacers to minimize the agglomeration of the rGO nanosheets.The adsorption ability of Ag/CuFe 2 O 4 / rGO nanocomposite towards MO dye was also investigated (Figure10(b)).From the figure, the absorbance of MO at 463 nm decreased slightly after 60 min and the removal efficiency of MO was about 61%.Moreover, to confirm the ability of the Ag/CuFe 2 O 4 /rGO nanocomposite to separate MB and MO dye molecules, a mixed dye solution was selected to determine the selective adsorption capacity of the solid composite.From Figure10(c), the absorption peak of MB disappeared quickly; just leaving the absorption peak of MO, indicating that Ag/CuFe 2 O 4 /rGO composite could selectively adsorb the cationic MB dye when exposed to the mixed solutions of MB+MO.The finding indicates that the Ag/CuFe 2 O 4 /rGO nanocomposite also possesses selective adsorption ability towards the cationic dyes in wastewater.