1 Introduction

In the last few decades, public health and environmental safety have been paid much attention [1,2,3]. Chemical contamination of surface water represents a great risk on the aquatic environment. A major reason of this contamination is the disposal of nitroaromatic compounds (NACs), as well as dyes [1, 2]. NACs have been vastly used in many industrial and agricultural fields, such as polymers, pharmaceuticals, intermediates synthesis of dye, and pesticides [4,5,6]. p-nitrophenol (4-NP) and o-nitroaniline (2-NA) are durable aromatic contaminants discharged from various industries such as dyes, explosives, pesticides, plasticizers and herbicides [7, 8]. Nitrophenol and nitroaniline and their derivatives are highly toxic even at extremely low levels [9], particularly for green plants and for aquatic organisms in surface waters [10]. Moreover, even traces of 2-NA in aquatic environment is harmful to human health and it is a potentially carcinogenic and mutagenic [11, 12]. Therefore, the United States Environmental Protection Agency has reported both nitrophenols and NACs as dangerous and high priority contaminants. Subsequently, a great challenge for chemists is to improve the methods of NACs removal [13].

Another major cause for environmental concern is the presence of dyes in water [14]. Mostly, dyes are highly stable and synthesized to resist the degradation, which makes their remediation a great challenge for chemists. Azo dyes represents about a half of synthetic textile dyes production, and about 15% of these dyes disposes in wastewaters during the dyeing processes [15]. Azo dyes are stable under aerobic treatments. For this reason, catalytic and photocatalytic degradation treatment of azo dye wastewaters received much attention recently [16, 17].

Recently, magnetic nanoparticles have been gained much interest, because of their wide range applications including magnetic ferrofluids [18], catalysis [19], energy storage [20], information storage spintronics [21] and bioseparations [22]. Furthermore, due to their biocompatibility and non-toxicity to humans [23, 24], they are widely used in biomedicine applications such as making the quality of magnetic resonance imaging (MRI) more efficient, drug delivery and also in manipulating cell membranes biomedicine [25,26,27]. Among most common forms of iron oxides, maghemite (γ-Fe2O3) is greatly applicable in medical and industrial applications [28]. It has been reported that doping of nanocrystalline γ-Fe2O3 with suitable doping metals greatly improves their properties by narrowing the energy-band gap and inhibiting electron–hole recombination [29]. Furthermore, it also can improve its activity as well as selectivity [30, 31]. Doping γ-Fe2O3 with a metal could be resulted in a greater catalytic activity compared with un-doped γ-Fe2O3 [32].

Taking into account all these considerations, the present work aimed to develop a facile, easily separable and highly efficient nanocomposite for the catalytic reduction of NACs as well as azo dyes. Thus, in this work, maghemite was used as a magnetic support to stabilize non-toxic Ag nanoparticles to obtain Ag–γ-Fe2O3 nanocomposite with exceptional activity, superparamagnetic behavior and excellent reusability in the catalytic reactions.

2 Materials and Methods

2.1 Materials

Ferrous sulphate heptahydrate (FeSO4·7H2O) was purchased from Merck. Analytical grade ferric chloride hexahydrate (FeCl3·6H2O), silver nitrate (AgNO3), sodium borohydride (NaBH4), and hydrochloric acid (HCl) were purchased from Sigma-Aldrich. All chemicals and solvents used in the synthesis processes were used as received without any further purification.

2.2 Synthesis of Maghemite Nanoparticles (γ-Fe2O3)

γ-Fe2O3 nanocomposite, was prepared according Chakrabarti et al. method [8] with some modifications. In this method salts of both ferrous and ferric ions were used. FeSO4·7H2O was the precursor of Fe2+ ions, while FeCl3·6H2O was the precursor of Fe3+ ions. FeSO4·7H2O (0.70 g) and FeCl3·6H2O (1.35 g) were dissolved in 50 ml distilled water under inert atmosphere and stirred for 30 min, and then NH4OH solution (25%) was added to the aqueous solution of Fe2+ and Fe3 ions dropwise under vigorous stirring, until pH reaches about ∼ 9.5. Instant black precipitate of Fe3O4 was obtained. This precipitate was acidified to pH ~ 2 to 3 by adding hydrochloric acid (6 M). After acidification process, the black precipitate was turned into a chocolate brown precipitate, indicating the formation of γ-Fe2O3 nanoparticles [8]. The resulted magnetic nanocomposite was stirred for another 1 h, collected by an external magnet, washed several times with distilled water and ethanol and finally dried in oven at 200 °C for 3 h.

2.3 Synthesis of Silver-Doped Maghemite Nanocomposite (Ag–γ-Fe2O3)

Previously prepared γ-Fe2O3 (0.50 g) was dispersed in 50 ml deionized water and sonicated for 30 min, then AgNO3 aqueous solution (10 ml, 0.042 g) was added dropwise under stirring, then freshly prepared NaBH4 aqueous solution (10 ml, 0.019 g) was added dropwise to the suspension (in order to reduce Ag+ ions), and the mixture was sonicated for 1 h. The formed silver doped maghemite (Ag–γ-Fe2O3) nanocomposite was collected by an external magnet, washed with deionized water and ether, then dried under vacuum at 50 °C for 6 h.

2.4 Catalytic Activity Experiments

2.4.1 Reduction of NACs Experiment

In catalytic reduction of 4-NP experiment, freshly prepared aqueous NaBH4 solution (0.5 ml, 0.2 M) was added to aqueous 4-NP solution (2 ml, 0.12 mM). Upon the addition of NaBH4 the yellow solution of 4-NP turned into deep yellow solution indicating the formation of 4-nitrophenolate [13]. Subsequently, Ag–γ-Fe2O3 nanocomposite (100 µl, 1 mg ml− 1) was added to the deep yellow mixture which turned colorless at the end of the reaction. To examine the reusability of Ag–γ-Fe2O3 nanocomposite, the used nanocomposite was separated from the reaction mixture using an external magnet after the completion of the reaction. The recovered nanocomposite was washed with ethanol and water repeatedly for the subsequent use. Reduction process of 4-NP was repeated twelve times using the recycled nanocomposite.

Similarly, in catalytic reduction of 2-NA experiment, freshly prepared aqueous NaBH4 solution (0.5 ml, 0.2 M) was added to aqueous solution of 2-NA (2 ml, 0.12 mM), and then Ag–γ-Fe2O3 nanocomposite (100 µl, 1 mg ml− 1) was added to the mixture. The yellow colored solution of 2-NA became colorless, confirming the reduction of 2-NA. For the two catalytic reduction reactions, the temperature was kept at 25 °C.

2.4.2 Catalytic Degradation of Methyl Orange

In catalytic degradation of methyl orange (MO) experiment, 2 ml of 0.5 M aqueous NaBH4 solution was added to 20 ml of MO solution (100 mg l−1), and then nanocomposite (1 mg) was added while stirring. The color of MO vanished gradually, indicating the catalytic degradation of MO. The temperature of the process was kept at 25 °C. UV–Vis absorption spectra technique was used to monitor all of the three reactions.

2.5 Characterization

Thermogravimetric analysis (TG) was carried out using a Linseis STA PT-1000. The rate of heating was 10 °C min−1. The morphology and particles size were determined using transmission electron microscope (JEOL, JEM 100 CX, Japan). Crystal phase and crystallite size are obtained by X-ray diffractometer (XRD BRUKER D8 Advance Cu target, Germany), operating with CuKα radiation (λ = 1.54 Å) generated at 40 KV and 40 mA. Scans are performed for 2θ values between 10° and 80° with a 2θ step of 0.02 for 0.4 s per point. Room temperature magnetic characteristics of Ag–γ-Fe2O3 were determined by using a vibrating sample magnetometer (VSM) in an external magnetic field of 20 KG (20 KOe). The FT-IR spectra of the magnetic nanocomposite were taken in potassium bromide disc using Perkin Elmer spectrophotometer (Model 1430) covering frequency range 400–4000 cm−1. The instrument was calibrated by a polystyrene film (1602 ± 1 cm− 1). UV–Vis absorption spectra were monitored by a PG-spectrophotometer instrument.

3 Results and Disucssion

3.1 Catalyst Characterization

The total mass loss was evaluated to be ∼ 13.5% according to the TG analysis up to 600 °C, as shown in Fig. 1.

Fig. 1
figure 1

The TG curve of Ag–γ-Fe2O3 nanocomposite

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The TG curve for Ag–γ-Fe2O3 is shown in Fig. 1. In the TG curve, the mass loss by dehydration of physically adsorbed water molecules from the catalyst surface was observed at ∼ 100 °C. This was evaluated to be 5.5% according to the TG analysis, then the sample mass gradually decreased (3.6%) at higher temperatures up to 180 °C which is due to chemically adsorbed water [33]. Additionally, TG curve shows small losses in two consecutive steps, firstly between 220 °C and 300 °C (2.7%) is due to the complete transition of Fe3O4 to γ-Fe2O3, while the other loss between 420 and 540 °C (2%) is due to the transition of γ-Fe2O3 into α-Fe2O3 [34]. The total mass loss was evaluated to be ∼ 13.5% according to the TG analysis up to 600 °C, as shown in Fig. 1.

It may be observed from TEM Fig. 2a–c that, the as-synthesized γ-Fe2O3 nanoparticles and Ag–γ-Fe2O3 nanocomposite are spherical in shape with nano-dimensions ranging from 4 to 6 nm for pure γ-Fe2O3 and from 6 to 10 nm for Ag–γ-Fe2O3. TEM micrograph showed that doping maghemite with Ag increased the agglomeration of the particles.

Fig. 2
figure 2

TEM of pure γ-Fe2O3 nanoparticles (a), TEM of Ag–γ-Fe2O3 nanocomposite (b), HTEM of Ag–γ-Fe2O3 nanocomposite (c)

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The crystallinity of Ag–γ-Fe2O3 nanocomposite was examined by XRD study as shown in Fig. 3. The diffractogram fits very well with the cubic symmetry (space group P4132) of γ-Fe2O3 (Card No. 039-1346). In addition, the obtained lattice parameters a = b = c = 8.3565 Å and the sharp characteristics of the peaks confirm the presence of the cubic structure and good crystallinity [35]. Diffraction peaks at around 2θ = 30.4°, 35.6°, 43.3°, 53.7°, 57.2°, and 63.1° corresponding to the (220, 311, 400, 422, 511, and 440) planes respectively, have been assigned to a spinel structure with characteristics of maghemite [8], while the diffraction peaks of silver at 2θ = 38.1°, 44.6°, 64.7°, and 77.5° corresponding to the (111, 200, 220 and 311) planes identify the sample as a face-centered cubic lattice of Ag (space group Fm-3m No. 225) with cell constants of a = 4.074 Å (JCPDS Card File No. 003-0921) [12].

Fig. 3
figure 3

The X-ray diffraction pattern of: (a) pure γ-Fe2O3 nanoparticles, (b) Ag–γ-Fe2O3 nanocomposite

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The magnetic property of Ag–γ-Fe2O3 nanocomposite was characterized by VSM. Figure 4 shows a typical room temperature magnetization curve of Ag–γ-Fe2O3 nanocomposite. The Ms (saturation magnetization) of Ag–γ-Fe2O3 nanocomposite as shown from the magnetization curve is 63.76 emu g−1. This value is slightly less than the value of pure γ-Fe2O3 nanoparticles (65.24 emu g−1), due to the introduction of non-magnetic silver nanoparticles. Our results showed that the Hc of nanocomposite approaches zero confirming the supermagnetic behavior of the nanocomposite with Mr (remanent magnetization) of 1.0025 emu g−1.

Fig. 4
figure 4

The hysteresis loop of pure γ-Fe2O3 nanoparticles and Ag–γ-Fe2O3 nanocomposite at room temperature

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FT-IR spectra of γ-Fe2O3 nanoparticles and Ag–γ-Fe2O3 nanocomposite are shown in Fig. 5. It is obvious that Ag–γ-Fe2O3 has similar spectrum to that pure γ-Fe2O3. However, the two peaks at 560 and 630 cm− 1 observed in case of pure γ-Fe2O3 which related to the Fe–O group were replaced by a new peak at 578 cm− 1 upon doping γ-Fe2O3 with Ag. The adsorbed water shows a broad band between 3430 and 3440 cm− 1 assigned to O–H stretching in H-bonded water and bands located at 1633 in pure γ-Fe2O3 and at 1640 cm− 1 in Ag–γ-Fe2O3 are due to the O–H bending vibration of molecular water [11]. Moreover, no peaks were observed for silver nanocomposite. This is mainly because silver nanocomposite do not have absorption in the Infrared region [36].

Fig. 5
figure 5

The FT-IR spectra of pure γ-Fe2O3 nanoparticles and Ag–γ-Fe2O3 nanocomposite

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Figure 6a, b represents the UV–Visible spectra of pure γ-Fe2O3 and Ag–γ-Fe2O3, respectively. The intensity of absorption for the samples increased with decreasing wavelength as expected for nanomaterials [37]. Tauc’s relationship was used calculate the energy gap as follows: αhν = A(hν − Eg)2, Where α is the absorption coefficient, A is a constant, h is Planck’s constant. The extrapolation of the linear region of a plot of (ahν)2 versus hν, gives the value of the band gap of the samples. The energy gap was found to be 2.14 eV for Ag–γ-Fe2O3 and 2.59 eV for pure γ-Fe2O3 which is in good agreement with a energy gap value of 2.65 eV for Fe2O3 [38].

Fig. 6
figure 6

Variation of absorption coefficient of pure γ-Fe2O3 nanoparticles (a) and Ag–γ-Fe2O3 nanocomposite (b)

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3.2 Catalytic Tests

3.2.1 Reduction of Nitroaromatic Compounds

Ag–γ-Fe2O3 nanocatalyst was tested in the reduction of NACs (4-NP and 2-NA). Both reduction reactions of 4-NP and 2-NA were followed by measuring the change in UV–Vis absorbance using spectrophotometry technique since a single product is resulted in each reaction [39, 40]. Furthermore, the reduction reaction of 4-NP was selected to investigate the reusability as well as the activity of the synthesized Ag–γ-Fe2O3 nanocomposite.

4-NP has a specific λmax at 317 nm which is shifted to 400 nm upon addition of aqueous NaBH4 solution, indicating the formation of the corresponding phenolate ions [13, 41, 42]. This λmax at 400 nm corresponding to p-nitrophenolate ions did not change over time (2 days) in absence of the nanocatalyst affirming that the reaction did not proceed by aqueous NaBH4 solution. Furthermore, when γ-Fe2O3 nanoparticles (100 mg) were added to 4-NP and NaBH4 mixed solution, it takes more than 2 h to be 5% completed (results not shown here). However, the addition of 1 mg of Ag–γ-Fe2O3 nanocatalyst to a mixture of 4-NP and freshly prepared NaBH4, led to a quick decrease in the peak intensity at 400 nm, while another peak at 300 nm (assigned to 4-aminophenol) increased (Fig. 7a) indicating the reaction progress. On the other hand, 2-NA has two distinct absorption peaks located at 410 nm and 285 nm [13] with no apparent change in the position of the two peaks upon the addition of NaBH4. Addition of pure γ-Fe2O3 (100 mg) to 2-NA and NaBH4 mixed solution showed no change in the peaks intensities. However, the addition of 1 mg of Ag–γ-Fe2O3 to a mixture of 2-NA and freshly prepared NaBH4, led to an instant decrease in the peaks intensities at 410 nm and 285 nm (Fig. 7b) confirming the reduction of 2-NA.

Fig. 7
figure 7

The UV–Vis spectra for: reduction of 4-NP to 4-AP (a), reduction of 2-NA to o-PDA (b), over Ag–γ-Fe2O3 nanocomposite

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Catalytic reduction of the 4-NP and 2-NA to the corresponding amino compound (Scheme 1) in the presence of Ag–γ-Fe2O3 were completed in 6 and 10 min, respectively. For the reduction process to be occur, NACs and borohydride ions were adsorbed on Ag–γ-Fe2O3 and an electron relaying from the borohydride ions to the NACs ions [43].

Scheme 1
scheme 1

Reduction of NACs to the corresponding amino compounds over Ag–γ-Fe2O3 nanocomposite

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Both reactions were adjusted to be first order, by taking NaBH4 in much higher concentration than those of nitro compound and can be considered as a constant during the reaction period. The first order kinetic equation can be expressed as:

$$\ln \left( {{{\text{C}}_{\text{t}}}/{{\text{C}}_0}} \right)= - {\text{kt}}$$
(1)

where; Co, Ct are the concentration of NACs at reaction time zero and t, respectively and k is the observed first-order rate constant (s− 1), t is the reaction time (s).

Figure 8 represents the plots of (Ct/C0) and ln (Ct/C0) for reduction of NACs against reaction time (s) in the presence of Ag–γ-Fe2O3 at 25 °C. The linear relationship of ln (Ct/Co) versus reaction time (t) indicates that both reduction reactions follow the pseudo first-order kinetics with respect to NACs concentrations. The specific rate constants were calculated to be 6.92 × 10− 3 s− 1 and 4.34 × 10− 3 s− 1 for reduction of 4-NP and 2-NA, respectively. Table 1 shows a comparison of the results of Ag–γ-Fe2O3 with the literature already reported for catalytic reduction of NACs [44, 45]. The superior catalytic activity of Ag–γ-Fe2O3 nanocomposite may be due to high dispersion of Ag on magnetic based material with enhanced activity. Besides, the magnetic behavior of Ag–γ-Fe2O3 adds another advantage for its ease separation and reuse.

Fig. 8
figure 8

Plots of: (Ct/C0) and ln(Ct/C0) versus reaction time for reduction of 4-NP and 2-NA in the presence of Ag–γ-Fe2O3 nanocomposite at 25 °C

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Table 1 Comparison of rate constant and activity parameter of Ag–γ-Fe2O3 nanocomposite for reduction of NACs with reported catalysts
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The activity parameter k′ (k/M, where M is the total mass of the whole catalyst used) was investigated for Ag–γ-Fe2O3 nanocomposite. Table 1 shows the comparative results of Ag–γ-Fe2O3 with the other catalysts reported for catalytic reduction of 4-NP and 2-NA. The activity parameters for the reduction of both 4-NP and 2-NA by Ag–γ-Fe2O3 nanocomposite were found to be 69.2, and 43.4 s− 1 gcat−1, respectively. The activity parameters values of Ag–γ-Fe2O3 nanocomposite for both reactions are significantly higher than the reported catalysts [13, 44, 46,47,48,49,50,51,52,53], reflecting the superior activity of Ag–γ-Fe2O3 nanocomposite towards the reductions of NACs.

Because of their importance reusability and activity of Ag–γ-Fe2O3 were investigated. The activity was estimated based on the decrease of the rate of each cycle compared to the rate of first cycle. At the end of the reaction of 4-NP, Ag–γ-Fe2O3 nanocomposite was recovered by using an external magnet and washed with water and ethanol several times for further reuses. Figure 9 shows that the reaction is completed each time using the recovered Ag–γ-Fe2O3 nanocomposite. Ag–γ-Fe2O3 nanocomposite was recycled for twelve cycles for the 4-NP reaction with a transformation of about 98% during 6 min. On the other hand, the activity of the Ag–γ-Fe2O3 nanocomposite slightly decreased to 96% at the twelfth cycle.

Fig. 9
figure 9

The reusability and activity of Ag–γ-Fe2O3 nanocomposite for the reduction of 4-NP of 25 °C

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However, the actual catalyst in the as-synthesized composite is the Ag nanoparticles, since γ-Fe2O3 has negligible activity toward the NACs reduction reactions. A normalized activity parameter named specific activity parameter ҟ (k/M′, where M′ is the mass of the Ag in the catalyst used) was calculated and compared to other Ag-based catalysts reported for the reduction of 4-NP as a model reaction (Table 2). Hence, if the amount of Ag loading (4.8%) is considered, the specific activity parameter ҟ is calculated for Ag–γ-Fe2O3 to be 1441.7 and 904.2 s− 1 gAg−1 for both 4-NP and 2-NA respectively, which are exceptionally high as compared to the values reported in the literature. It is obvious from Table 2 that the as-synthesized Ag–γ-Fe2O3 nanocomposite possesses a significantly much higher (2.6–1442 times) specific activity parameter than other reported Ag-based catalysts [13, 36, 54,55,56,57,58,59]. Moreover, the as-prepared Ag–γ-Fe2O3 nanocomposite can be recovered and reused easily and successfully for at least twelve times with a conversation rate higher than 98%, indicating the Ag–γ-Fe2O3 with much higher activities, as well as more reusability than other Ag-based catalysts including the magnetic based towards the reduction of 4-NP. Moreover, the as-synthesized Ag–γ-Fe2O3 nanocomposite showed much enhanced activity and reusability in the reduction of NACs compared to the reported magnetic-based silver catalysts Ag-Fe2O3 microboxes [54], Ag-Fe2O3 [55], Fe3O4@SiO2–Ag [56] and Fe3O4@SiO2–Ag [36]. These results reflect that our synthetic route of Ag–γ-Fe2O3 is greatly optimize the activity of silver nanoparticles with retention of superparamagnetic magnetic behavior of γ-Fe2O3.

Table 2 Comparison of specific activity of Ag–γ-Fe2O3 nanocomposite for the reduction of 4-NP as a model reaction with reported Ag-based catalysts
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3.2.2 Catalytic Degradation of Methyl Orange

According to literature the degradation of MO by NaBH4 in absence of a catalyst is thermodynamically favorable, but it is kinetically difficult [60]. This can be altered using metal nanocomposite which provides an alternative pathway with lower activation energy. The magnetic Ag–γ-Fe2O3nanocomposite was used as catalysts for the catalytic degradation of MO by NaBH4. The kinetics of the catalytic degradation of MO was followed by measuring the absorbance of samples at specific λmax = 465 nm at different time intervals as shown in Fig. 10. The catalytic degradation reaction was adjusted to be first order with respect to MO, and the degradation rate can be easily calculated from the absorbance at 465 nm at different time intervals. When degradation reaction proceeded the absorption peak at 465 nm was gradually decrease with time and the solution turned colorless, confirming complete degradation of the dye [61].

Fig. 10
figure 10

The UV–Vis. spectra for: a MO and NaBH4 before adding the catalyst, b successive degradation of MO after adding Ag–γ-Fe2O3 nanocomposite at 25 °C

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Catalytic degradation of MO with NaBH4 did not proceed in absence of catalyst or even in the presence of pure γ-Fe2O3 (2 days). However, addition of 1 mg of Ag–γ-Fe2O3 to a mixture of MO (20 ml, 100 mg/l) and freshly prepared NaBH4 (2 ml, 0.5 M) led to an instant decrease in the peak intensity at 465 nm corresponding to MO as shown in Fig. 10. Results showed that Ag–γ-Fe2O3has high activity towards catalytic degradation of MO, where it takes ∼ 30 min. to be 100% decomposed.

In a similar manner to NACs reduction reactions, the pseudo first-order kinetics applied and the rate constants for the catalytic degradation of MO was calculated. Plot of ln(Ct/Co) versus reaction time (Fig. 11) confirm the pseudo-first order kinetics for catalytic degradation of MO. Figure 11 represents plots of (Ct/C0) and ln(Ct/C0) for degradation of MO against reaction time (s), respectively. Plot of ln(Ct/C0) showed that the degradation reaction is first order with specific rate constant k = 1.53 × 10− 3 s− 1. Table 3 shows a comparison of the results of Ag–γ-Fe2O3 with the literature already reported for catalytic degradation of MO. Our synthesized Ag–γ-Fe2O3 nanocomposite has efficiency ≥ 99% in less than 30 min. It is obvious that the catalytic activity and efficiency of the synthesized Ag–γ-Fe2O3 nanocomposite in the catalytic degradation of MO are significantly higher than these reported nanoparticles, with magnetic behavior advantage.

Fig. 11
figure 11

Plots of (Ct/C0) and ln(Ct/C0) versus reaction time for catalytic degradation of MO in the presence of Ag–γ-Fe2O3 nanocomposite at 25 °C

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Table 3 Comparison of catalytic activity of Ag–γ-Fe2O3 for the degradation of MO with reported catalysts
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4 Conclusion

Ag–γ-Fe2O3 nanocomposite with superparamagnetic behavior and superior activity was synthesized. The synthesized Ag–γ-Fe2O3 nanocomposite exhibited superior catalytic activity for the reduction of NACs (4-NP and 2-NA) in the presence of NaBH4. Besides, it showed a superior activity and high efficiency for the catalytic degradation of MO in the presence of NaBH4. Moreover, Ag–γ-Fe2O3 nanocomposite was recycled for twelve times in the reduction of 4-NP as a model reaction and showed promising recycling results. Ag–γ-Fe2O3 nanocomposite provides an efficient simple scheme for the synthesis of other magnetic-based transition metal nanocomposites with superior activity and easy separation from the reaction mixture. These separable magnetically-based transition metal nanocomposites are highly efficient in numerous catalytic reduction reactions.