Preparation of a magnetically separable CoFe₂O₄ supported Ag nanocatalyst and its catalytic reaction towards the decolorization of a variety of dyes

Abstract

This study deals with the exploration of CoFe₂O₄ supported Ag nanoparticles as a catalyst for the decolorization of various dyes (such as 4-nitrophenol, Congo red, rhodamine B) and dye mixtures by employing a reduction reaction with excess NaBH₄ in an aqueous medium. Nanocatalysts with 10 wt% Ag loading (10Ag@CoFe₂O₄) exhibited very high catalytic activity and dye solutions were found to be decolorized within 4 to 6 minutes. A simple method for the preparation of a catalyst (10Ag@CoFe₂O₄) was reported, which exhibited very high catalytic efficiency towards the decolorization of dyes with different chemical structures as well as demonstrating its magnetic properties for easy separation from reaction mixtures and its reusability.

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1. Introduction

Water pollution due to the discharge of colored effluents from various industries (such as plastics, paper and pulp manufacturing, dying of cloth, leather treatment, printing, etc.) is one of the major environmental concerns. Strong color, due to dyes and pigments, presents serious aesthetic and ecological problems to the receiving aquatic ecosystem, such as inhibition of benthic photosynthesis.¹ Moreover, some of these dyes are toxic and carcinogenic in nature.² A variety of waste water treatment methods, such as chemical, physico-chemical, biological and a combination of these methods, are generally available to treat dye-containing effluents.^1,3,4 However, most of these methods suffer from some disadvantages. Existing physico-chemical methods, such as advanced oxidation processes involving the use of Fenton’s reagent (H₂O₂/Fe²⁺), H₂O₂, ozonization, etc. are not cost effective for large scale use. Usage of polyelectrolyte, ferrous salts, lime and alum in coagulation/flocculation based methods causes the generation of a huge amount of sludge which presents a handling and disposal problem. Adsorption of dyes using activated carbon or other adsorbents is not only quite ineffective but is also uneconomical. Electrochemical based oxidation of dyes is a slow process. Photochemical oxidation of dyes under UV radiation in the presence of oxidizing agents (e.g. H₂O₂) and catalysts (TiO₂, doped TiO₂) has generated lots of interest in this perspective^5–8 but it is also a slow process and usage of UV radiation limits its large scale application. For the last few years, use of biological processes to degrade dyes has attracted the attention of researchers.^9,10 However, some limitations are also associated with these processes. The use of pure cultures (algal, fungal and bacterial) is not very effective for waste water containing various types of dyes, because most of the isolated cultures are dye specific. Hence, their application on a large scale is impractical.¹¹ Moreover, due to high stability against light, temperature, water, chemicals, etc. and their hydrophilic nature, most synthetic dyes and pigments escape conventional waste water treatment processes and persist in the environment.

Over the last few years, use of metal nanoparticle (e.g. Pd, Pt, Au, Ag, Ru, Cu, etc.) based catalysts to reduce/degrade colored dyes to their colorless form has generated immense interest due to their high catalytic activity and efficiency.^3,12–30 However, separation of nanosized metal catalysts from the reaction mixture by simple filtration or centrifugation is very difficult. This factor limits the usage of nanoparticles as catalysts. To overcome this limitation, recently, the use of magnetic nanoparticles has emerged as an attractive alternative to conventional materials for catalyst supports.^{12,14,15,31–35} This is because of the fact that their magnetic nature offers easy separation of the nanocatalyst from the reaction mixture, by applying an external magnet, which eliminates the necessity of catalyst filtration.³⁶ Although some reports are available on the use of ferrite nanoparticles as a catalyst support for the reduction of dyes, particularly 4-nitrophenol, the synthetic methods are complicated.³⁷

In this paper we report a facile chemical methodology for the preparation of a CoFe₂O₄ supported Ag nanocatalyst and demonstrated its high catalytic activity towards the decolorization of a variety of dyes (such as 4-nitrophenol (4-NP), Congo red (CR), rhodamine B (RhB)) and a mixture of these dyes. We have chosen these dyes for our study because they possess different chemical structures (Fig. S1 (ESI†)) and represent commercial dyes which are used in various industries.

2. Experimental

2.1. Chemicals used

The starting chemicals used were Fe(NO₃)₂·9H₂O, Co(NO₃)₂·6H₂O, ethylenediaminetetraacetic acid (EDTA), silver nitrate and tri-sodium citrate dihydrate (all from Merck, India), and rhodamine B, Congo red, 4-nitrophenol and NaBH₄ from Acros Organics, USA. All chemicals were 99.9% pure and used as received.

2.2. Synthesis of the CoFe₂O₄ supported Ag nanocatalyst

CoFe₂O₄ nanoparticles were synthesized using an aqueous-solution-based EDTA precursor method, which was developed by us and reported elsewhere.³⁸ CoFe₂O₄ supported Ag nanocatalysts were synthesized with five weight percentages of Ag (2.5, 5, 7.5, 10 and 12.5 wt%). In a typical synthesis of CoFe₂O₄ with 10 wt% Ag, 10 mL of aqueous solution containing 0.236 g of AgNO₃ was added to 1.35 g of synthesized CoFe₂O₄ in a round bottom flask and allowed to soak for 12 h in a dark chamber with constant stirring. Then, Ag⁺ ions were reduced by trisodium citrate using a 1 : 1 molar ratio of AgNO₃ : trisodium citrate.^15,39,40 30 mL of aqueous solution containing 0.408 g trisodium citrate was then added dropwise to this mixture and the temperature was maintained at 80 °C for 3 h. This Ag impregnated CoFe₂O₄ was then dried over a hot plate at 100 °C. The dried product was then washed with 250 mL of warm distilled water and dried overnight in an oven at 60 °C. The filtrate was also collected after washing and chemical analysis was performed for the detection of Ag⁺ ions. It was observed that no Ag⁺ ions were present in the filtrate. An estimation of the Ag content in the synthesised catalysts was performed by titration with potassium thiocyanate⁴¹ and it was found that the concentrations of Ag present in the samples almost matched the theoretical values.

2.3. Catalytic performance test and kinetic study

To evaluate the catalytic performance of CoFe₂O₄ supported Ag nanocatalysts, 1 mL of a 0.204 M freshly prepared NaBH₄ solution was added to a 4.5 mL 9.0 × 10⁻² mM aqueous solution of 4-NP (in the case of CR and RhB, the concentrations were 9.0 × 10⁻² mM and 2.1 × 10⁻² mM). In this solution, 1 mL of an aqueous mixture of the catalyst (0.1 g L⁻¹) and 1.5 mL of H₂O was added and mixed. 4 mL of this reaction mixture was immediately sampled in a quartz cuvette and the absorbance was recorded using a UV-vis spectrophotometer (V-570, Jasco, Japan). The progress of the reaction was then monitored by recording the time dependent UV-vis absorption spectra of the reaction mixture. The reaction was carried out at room temperature (30 ± 1 °C). All catalysis reactions were performed in triplicate.

It is a well established fact that the metal nanoparticle catalyzed reduction reactions of dyes in the presence of excess NaBH₄ proceed via pseudo first order kinetics.^{15,19,21,38,39,42,43} In the present case, the concentration of the dye in the reaction mixture (C_t) was determined from the absorbance of the peak at λ_max of the dye. The rate of the reaction was monitored by following the decrease of the absorbance of the λ_max peak with time. The ratio of the absorbance of dye A_t (measured at time t) to A₀ (measured at t = 0) is equal to the concentration ratio C_t/C₀ of the dye, where C₀ is the initial concentration of the dye. The apparent rate constant (k_app) was determined from the following rate eqn (1) and (2):

dC_t/dt = −k_appC_t (1)

ln(C_t/C₀) = ln(A_t/A₀) = −k_app (2)

The value of k_app was calculated from the ln(A_t/A₀) vs. time plot.

2.4. Characterization of the synthesized catalysts

Room temperature powder X-ray diffraction (XRD) patterns of the synthesized catalysts were recorded using a powder X-ray diffractometer (Mini Flex II, Rigaku, Japan) with CuK_α radiation at a scanning speed of 2° min⁻¹. High Resolution Transmission Electron Microscope (HRTEM) images of the samples were obtained using a JEOL JEM 1400, Japan. Room temperature magnetization with respect to an external magnetic field was measured for the synthesized silver nanocomposites using a Vibrating Sample Magnetometer (VSM) (EV5, ADE Technology, USA).

3. Results and discussion

Room temperature wide angle powder XRD analysis was performed for the synthesized catalysts to identify the crystalline phases present in the samples (Fig. 1). In the XRD pattern of pure CoFe₂O₄ nanopowder (Fig. 1(a)), the presence of diffraction peaks at 2θ = 18.2, 30.1, 35.5, 37.1, 43.1, 53.4, 56.9 and 62.6° corresponding to the (111), (220), (311), (222), (400), (422), (511), and (440) planes of CoFe₂O₄ [JCPDS 22-1086] was observed. For 10Ag@CoFe₂O₄ (Fig. 1(b)), diffraction peaks at 2θ = 37.7, 43.8, and 64.2° corresponding to the (111), (200) and (220) planes of cubic Ag [JCPDS no. 4-0783] appeared along with the CoFe₂O₄ peaks. XRD patterns of the CoFe₂O₄ supported Ag nanocatalysts confirmed the presence of only Ag and CoFe₂O₄ in the catalyst. XRD analysis was also performed for the catalyst recovered after 5 cycles (Fig. 1(c)) of catalytic reaction and no significant change in the XRD pattern was observed. This fact indicates that after a catalytic reaction the structure of the catalyst remains the same.

Fig. 1 Powder XRD patterns of (a) synthesized CoFe₂O₄, (b) 10Ag@CoFe₂O₄, and (c) recovered 10Ag@CoFe₂O₄ after 5 catalytic cycles.

HRTEM micrographs of the samples (Fig. 2) revealed that Ag nanoparticles (average particle size ∼ 10 nm) are deposited on the surface of CoFe₂O₄ nanoparticles. The average particle size of the catalysts was within the range of ∼30–50 nm. The TEM micrograph of the recovered catalyst (Fig. 2(C)) also showed that the microstructure of the recovered catalyst is similar to that of the as-synthesized catalyst.

Fig. 2 HRTEM micrographs of (a) CoFe₂O₄, (b) 10Ag@CoFe₂O₄, and (c) recovered 10Ag@CoFe₂O₄ after 5 catalytic cycles.

The variation of the magnetic properties (saturation magnetization (M_s) and coercivity (H_c)) with the composition of the synthesized catalysts were investigated using a VSM at room temperature with an applied field of 15 000 Oe and the results are shown in Fig. 3. The values of M_s and H_c of CoFe₂O₄ and 10Ag@CoFe₂O₄ were found to be 67.5 emu g⁻¹, 1645.2 Oe and 59.2 emu g⁻¹, 1127.2 Oe respectively. This decrease in the values was quite obvious because the catalyst is composed of magnetic CoFe₂O₄ and non-magnetic Ag nanoparticles. As the coercivity values of the catalysts were found to be fairly high, it is expected that they can act as magnetically separable catalysts.

Fig. 3 Room temperature magnetic hysteresis loops of CoFe₂O₄ and 10Ag@CoFe₂O₄.

To understand the effect of the Ag nanoparticle loading percentage in the catalysts on the reduction of 4-NP, reactions were carried out in the presence of catalysts with various percentages of Ag nanoparticle loading (e.g. 2.5, 5, 7.5, 10 and 12.5 wt%) keeping other parameters constant. It was observed that the time required for 100% completion of the reaction decreased with increasing Ag nanoparticle loading and that the catalyst containing 10 wt% Ag nanoparticles required 4 min for completion of the reaction. A further increase in the Ag nanoparticle loading did not have much of an effect on the 100% reaction time (Fig. S2 (ESI†)). Similar behavior was observed for other dyes. Therefore, for further investigations 10 wt% Ag nanoparticle loaded CoFe₂O₄ (10Ag@CoFe₂O₄) was used as the catalyst.

The UV-vis spectra of an aqueous solution of 4-NP showed the maximum absorption peak (λ_max) at 317 nm. After addition of NaBH₄ this peak was red-shifted from 317 to 400 nm, due to the formation of dark yellow colored 4-nitrophenolate ions.^15,23 During the reaction the successive decrease of this peak (λ_max = 400 nm) with time indicated the progress of the reduction of 4-NP. Simultaneously, the gradual development of a new peak at 300 nm indicated the formation of 4-aminophenol (4-AP). Time dependent absorption spectra of the reaction mixture showed the disappearance of the peak at 400 nm with time (Fig. 4(A)).

Fig. 4 Time dependent UV-vis spectral changes of the reaction mixtures of (A) 4-NP, (B) CR, (C) RhB, and (D) a mixture of 4-NP, CR, RhB, and NaBH₄ catalyzed by 10Ag@CoFe₂O₄, and (E) a pseudo first kinetic plot of 4-NP, CR, and RhB reduction catalyzed by 10Ag@CoFe₂O₄.

The catalytic efficiency of the 10Ag@CoFe₂O₄ catalyst was also tested towards the decolorization of Congo red and rhodamine B in the presence of excess NaBH₄ (Fig. 4(B) and (C)). It was observed that decolorization of the dyes occurred within 4–5 min, depending upon the dye.

Owing to the presence of excess NaBH₄, the rate of the reactions was assumed to be independent of NaBH₄ concentration and a pseudo first order kinetic equation was applied. Fig. 4(E) shows the ln(A_t/A₀) vs. time plot for the 10Ag@CoFe₂O₄ catalyzed reduction reactions of various dyes. The rate constants (k_app) of the reactions and the times required to complete the decolorization of the dyes are listed in Table 1.

Table 1 Completion times and rate constants of 10Ag@CoFe₂O₄ catalyzed reduction reactions

Dye	Completion time (min)	Rate constant (kapp) (min^−1)	Correlation coefficient
4-NP	4 (±0.25)	1.18	0.97
CR	5 (±0.2)	0.64	0.98
RhB	4 (±0.3)	1.42	0.97

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10Ag@CoFe₂O₄ also exhibited high catalytic efficiency towards the degradation of a mixture of dyes. The dye mixture (4-NP, CR and RhB) was decolorized (Fig. 4(D)) within 10 min when treated with 10Ag@CoFe₂O₄ ([catalyst] = 1.25 × 10⁻² g L⁻¹, [4-NP] = 5.1 × 10⁻⁵ M, [CR] = 5.1 × 10⁻⁵ M and [RhB] = 1.1 × 10⁻⁵ M).

The catalytic process of the reduction reaction using Ag nanoparticles can be explained by the electrochemical mechanism.^15,29,30 This catalytic reduction occurred via the relay of electrons from the BH₄⁻ donor to the acceptor dye molecule. In the first step of the process, BH₄⁻ and the dye diffuse from aqueous solution to the surface of the Ag nanoparticle. Hydrogen atoms, which were generated from BH₄⁻, after electron transfer (ET) to the Ag reacted with the dye molecules, leading to the production of the colorless form of the dye.^2,13–20,42 Deng et al.⁴³ have proposed that metal nanoparticles play the critical role of storing the electrons after the electron transfer from the hydride.

Ag nanoparticles serve as active catalytic sites for the reduction of various dyes and CoFe₂O₄ nanoparticles act as the magnetic support of the catalyst. Fig. 5 illustrates that CoFe₂O₄ supported Ag nanocatalysts can be easily separated from the reaction mixture after completion of the reaction by using an external magnet. After separation, the catalyst was washed with distilled water and reused several times for performing the reaction. The synthesized catalysts were reused for 5 consecutive reaction cycles and a considerable change in the activity of the catalysts was not observed (Fig. S3 (ESI†)). The reusability of these catalysts thus confirmed their stability. The Ag content of the recycled catalyst was also estimated using a titrimetric method⁴¹ and it was observed that the Ag content remains the same after the catalytic reaction. XRD (Fig. 1(c)) and TEM (Fig. 2(c)) also indicated that the structure of the catalyst does not change significantly after 5 catalytic reaction cycles.

Fig. 5 (A) Rhodamine B solution, (B) rhodamine B and catalyst mixture before the reaction, and (C) the colorless reaction mixture after the reaction and magnetic separation of the catalyst.

4. Conclusions

We have successfully synthesized CoFe₂O₄ supported Ag nanocatalysts using a simple chemical method in an aqueous medium. An EDTA precursor method was employed to prepare CoFe₂O₄ nanopowders, which acted as magnetic supports for the catalysts. The EDTA precursor method is a novel but cost effective synthetic route to prepare CoFe₂O₄ nanoparticles and the advantages offered by the EDTA precursor method over traditional solid-state, sol–gel, co-precipitation or high-energy ball milling methods has been discussed elsewhere.³⁸ Ag nanoparticles were deposited on the surface of CoFe₂O₄ by employing the reduction of AgNO₃ with trisodium citrate in an aqueous medium. The exquisiteness of the overall catalyst preparation methodology lies in its simplicity.

The synthesized CoFe₂O₄ supported Ag nanocatalysts not only exhibited catalytic activity towards the decolorization of various dyes as well as dye mixtures but also exhibited easy magnetic separation with a good external magnet. This character makes a CoFe₂O₄ supported Ag nanocatalyst attractive because it overcomes the intrinsic separation problem of nanostructured catalysts.

In summary, simple preparation methodology, high catalytic efficiency towards the decolorization of dyes, very good reusability and magnetic separation ability makes the CoFe₂O₄ supported Ag nanocatalyst a promising candidate in the field of catalysis particularly for waste water treatment.

In our previous studies we have observed that Ag nanoparticle loaded materials (e.g. folic-acid-functionalized AgCl/TiO₂ (ref. 44) and Ag nanoparticle loaded mesoporous silica (SBA-15))⁴⁵ exhibit high antibacterial activity against bacterial strains (Gram-positive; M. luteus, S. aureus and Gram-negative; E. coli, P. aeruginosa) and a fungal strain (C. albicans). The mechanism of the antimicrobial action of the Ag nanoparticles involves the binding of Ag⁺ ions to the thiol (sulfhydryl) groups of proteins and enzymes, which causes inactivation and inhibition in cell processes. Hence, we expect that the synthesized CoFe₂O₄ supported Ag nanocatalyst will also exhibit antibacterial activity and we are now studying its antibacterial activity. Moreover, investigations on the catalytic efficiency of this synthesized catalyst in the presence of different coexisting ions (e.g. Na⁺, K⁺, Cl⁻, SO₄²⁻, CO₃²⁻, etc.) by performing the catalysis reactions in simulated water media as well as in real dye-containing industrial effluents are in progress. Our future applications of this novel catalyst will also concentrate on the degradation of common herbicide and pesticide pollutants such as trifluralin, pendimethalin, etc. We will communicate the results in the near future.

Acknowledgements

Dr N. N. Ghosh gratefully acknowledges financial support from the Council of Scientific and Industrial Research India (Project no. 02(0147)/13/EMR-II). S. Hazra thanks CSIR, India, for a Senior Research Fellowship. The authors thank Prof. Paul A. Millner and Mr Martin Fuller, University of Leeds, UK for the TEM analysis.

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1 Chemical structures of (A) 4-nitrophenol, (B) Congo red, (C) rhodamine B; Fig. S2 effect of Ag nanoparticle loading percentage on the time required for the completion of the reduction of 4-nitrophenol; Fig. S3 conversion percentage of 4-NP, CR and RhB in successive uses of 10Ag@CoFe₂O₄ catalyst. See DOI: 10.1039/c5ra00298b

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