Preparation of magnetic composite based on zinc oxide nanoparticles and chitosan as a photocatalyst for removal of reactive blue 198

Chitosan (90% de-acetylation) was prepared in our lab. Reactive blue 198 (RB 198) and real textile dyeing wastewater were donated by Thanh Cong textile company. All chemicals were analytical grade and used without further purification and all solutions were prepared with deionized water.

Fe₃O₄ nanoparticles were successfully fabricated by a co-precipitate method of Fe²⁺ and Fe³⁺ in ammonia solution. In this paper, synthesized Fe₃O₄ was synthesized by the method presented in previous research with some modification [23]. In brief, 0.01 mol of FeCl₃.6H₂O and 0.005 mol of FeCl_2.4.H₂O were dissolved into 50 mL of deoxygenated water. Fe₃O₄ precipitation was obtained by adding ammonia solution 25% at 80 °C for hours under a stirrer. The pH of solution was maintained at 10. Later, the precipitate was achieved by a filtration paper and washed with distilled water and ethanol for several times. Lastly, the obtained precipitate was dried at 90 °C in an oven.

There were many methods to synthesize zinc oxide nanoparticles. In this study, zinc oxide nanoparticles were simply fabricated by a sol-gel method with some adjustment [24]. Briefly, 30 mL of triethanolamine (TEA) was added into a beaker containing 20 mL of distilled water. A stirrer was used to mix solution homogeneously. During stirring, 2 mL ethanol was pipetted into the homogeneous TEA solution. Then, 0.5 M zinc acetate solution (2.7 g of Zn(CH₃COO)₂ into 50 mL H₂O) was transferred into TEA solution. The mixture was continuously stirred for 20 min at temperature of 120 °C. Afterwards, 6 mL of NH₄OH (25%) was slowly dropped into the solution. An aluminum foil was used to cover the container of the reaction, for light prevention. Immediately, there was a formation of milky precipitate of zinc oxide nanoparticles in the solution. The solution was kept to stand for 30 min to obtain the nanoparticles on the bottom of beaker. The precipitate was achieved by filtration and washed for several times by distilled water and ethanol. Finally, the resulting precipitate was dried at 90° C for 4 h.

In this study the magnetic composite based on zinc oxide nanoparticles and chitosan was synthesized by adding ZnO and Fe₃O₄ into chitosan matrix using glutaraldehyde as a cross-linker [23]. Firstly, 1% chitosan solution was prepared by dissolving 2.5 g chitosan in acetic acid 2.5%. Then, 1 g of ZnO and 1.25 g of Fe₃O₄ nanoparticles were added in the 1% chitosan solution and stirred continuously by a mechanical stirrer at 60 °C for 3 h to disperse steadily the particles into chitosan solution. Then, 0.5 mL glutaraldehyde solution was slowly pipetted into the mixture to form gel. After completing reaction, the pH of the mixture was modified to 10 using 2 M NaOH solution. The mixture was stirred and heated on a water bath for 3 h at 60 °C. The magnetic composite was formed and obtained by using a filtration paper. Then, the resulting composite was washed with acetone and distilled water and dried at 90 °C.

The x-ray diffraction was carried out in X'Pert Pro MRD (PANanalytical, Netherlands) at room temperature with Cu-Kα radiation (λ = 0.154 nm) and an incident angle (2θ), ramping from 2 to 80° at a rate of 50 s/step. Magnetic measurement of the product was conducted on a vibrating sample magnetometer (VSM-5 Model, MicroSense Co., Ltd). Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) were used to characterize the morphology and structure of prepared material. Thermogravimetric analysis (TGA) was made by using a Mettler-Toledo thermo-gravimetric analyzer.

In order to show potential environmental applications of ZnO/Fe₃O₄/CS composite for photocatalytic degradation of RB198, all experiments were carried out under UV light irradiation. 0.25 g of ZnO/Fe₃O₄/CS composite was added into a beaker containing 50 mL of RB198 solution with various initial concentrations (10, 20, 30, 40, 50 and 60 mg L⁻¹). Then, the solution was regularly mixed by a stirrer bar under UV light irradiation. After a certain time, the suspension of material was separated by a permanent magnet. The solution was obtained to analyze optical density by using UV–vis spectrophotometer at a wavelength of 625 nm. The degradation efficiency of RB198 was calculated as follows

$$\begin{eqnarray*}&&R=\frac{{{C}_{0}}-C}{{{C}_{0}}}\times 100,\end{eqnarray*}$$

where C₀ and C are concentrations of the dye solution before and after photo irradiation.

2.6.1. Effect of illumination time and initial concentration of RB198

2.6.2. Effect of the amount of composite loading

2.6.3. Effect of initial pH solution

The remainder of the composite after separation from all experiments was gathered in a glass bottle. Then, the composite was washed with distilled water for several times and dried at 90 °C for recycling experiments. 0.25 g of recycled composite was added into 50 mL of RB198 solution with the same optimized parameters such as pH, dosage of composite, initial concentration and illumination. After exposing to UV light, the resulting solutions were separated by a permanent magnet. Finally, the optical density of solutions was determined. The remaining composite was obtained, washed and dried to repeat to recycling experiments. The repetition of recycling was carried out until the photo-degradation efficiency of composite was about 70%.

After investigating the photocatalytic ability of composite with RB198, a performance with real textile dyeing wastewater was also carried out. Real wastewater was used from industrial textile dyeing factory with initial pH of 12. The pH of wastewater was adjusted to the optimized value from following experiments. Then, 0.25 g of composite was transferred to 50 mL of RB198 under UV light irradiation and a magnetic stirrer at 300 rpm. After certain time, the resulting solution was separated and used to measure chemical oxygen demand (COD) in order to determine the total of remaining organic compounds in wastewater.

The scanning electron microscopy (SEM) micrograph of ZnO/Fe₃O₄/CS was shown in figure 1. As seen, the result of micrograph showed that the surface of the composite was rough with tiny porous structure with opening holes on the surface. Figure 1 also revealed that porous structure of the composite had interconnection between opening holes, which help increase surface area of the catalyst contacting with dye molecules (RB198). In addition, there was one solid phase in the micrograph of the synthesized composite meaning that ZnO and Fe₃O₄ nanoparticles were relatively well-dispersed into chitosan.

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The FTIR graphs of ZnO/Fe₃O₄/CS and pure chitosan were presented in figure 2. The graphs showed several typical peaks revealing the presence of chitosan as one of the components in structure of ZnO/Fe₃O₄/CS when compared with pure chitosan [25]. The specific adsorption band at 3423.7 cm⁻¹ was attributed to the stretching vibration of O-H and N-H bonds. The characteristic peak appeared at 1345 cm⁻¹ corresponding to the stretching of C-N of -NHCO, while the peaks at 1055.69 cm⁻¹ and 2923 cm⁻¹ were ascribed to C-OH bond and C-H of alkyl group, respectively. Especially, there was a new typical peak at 1560.87 cm⁻¹ confirming the formation of imine crosslinking as a result of Schiff base reaction between –NH₂ of chitosan and −C=O of glutaraldehyde. In addition, the existence of ZnO and Fe₃O₄ nanoparticles in the composite was demonstrated by the appearance of some new characteristic peaks. The specific adsorption bands at 1413.75 and 645.78 cm⁻¹ were assigned to Zn-O bond. The peaks at 566.77 cm⁻¹ and 419.36 cm⁻¹ were characterized for Fe-O. As result, the FTIR graph proved the contribution of chitosan, ZnO and Fe₃O₄ nanoparticles, which was correlating to the results of XRD patterns.

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The XRD patterns of ZnO/Fe₃O₄/CS, Fe₃O₄, ZnO and chitosan were described in figure 3. As seen, the XRD patterns showed the straight base line and unsharp peaks which indicated that the ZnO/Fe₃O₄/CS composite was not a well crystalized material compared with separated Fe₃O₄ and ZnO nanoparticles. This may be due to the existence of chitosan on surface of ZnO and Fe₃O₄. The typical peaks at 10.12°, 19.7°, 21.5° and 42.2° were assigned to chitosan coating on nanoparticles [26], while the characteristic peaks at 31.7°, 34.5°, 36.2°, 47.5° and 56.7° were ascribed for ZnO in the composite [27]. Moreover, figure 3 also presented that diffractogram of the composite still had peaks revealing the presence of Fe₃O₄ in structure of the material with specific peaks at 35.5° and 63° [15]. The results of XRD patterns along with FTIR graphs strongly confirmed that the nanocomposite was successfully synthesized with constituents of ZnO, Fe₃O₄ and chitosan.

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The magnetic property of the composite was determined by a vibrating sample magnetometer. Magnetization curves of ZnO/Fe₃O₄/CS and Fe₃O₄ were shown in figure 4. Once again, the magnetization curve of ZnO/Fe₃O₄/CS demonstrated the presence of Fe₃O₄ in structure of the composite. The curves also revealed the superparamagnetism of Fe₃O₄ and composite. The saturation magnetization value of Fe₃O₄ was about 40 emμ g⁻¹. However, the saturation magnetization of ZnO/Fe₃O₄/CS decreased to about 10 emμ g⁻¹. The decrease of the saturation magnetization of ZnO/Fe₃O₄/CS may be due to the influence of chitosan after coating on surface of Fe₃O₄ and the dispersion of Fe₃O₄ into the matrix of chitosan. Moreover, the results also proved that the saturation magnetization of composite based on contents of Fe₃O₄ and chitosan.

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The results of thermogravimetric analysis of ZnO/Fe₃O₄/CS were presented in figure 5. Thermogravimetric (TG) and differential scanning calorimetry (DSC) curves are two characterized curves to determine changes in physical and chemical properties of composite when increasing temperature in air at a heating rate of 10 °C min⁻¹. As seen, weight loss of composite was about 45% when heating from 40–558 °C. The process of weight loss underwent two stages. For the first stage, weight loss of the composite was about 7% meaning the evaporation of water and volatile molecules in synthesized composite with temperature heating from 40–200 °C. Moreover, an endothermic peak in DSC curve revealed that a phase transition from liquid state to vapor state occurred at 55 °C. For the second stages with temperature ranging from 200–558 °C, the weight loss of prepared composite was 38% due to the decomposition of chitosan. There was the presence of two exothermic peaks at 310 °C and 440 °C. Additionally, there was no significant difference in weight loss of the prepared composite. This may be because the melting point of ZnO and Fe₃O₄ was higher than 558 °C. Therefore, remainder of the composite could be attributed to Fe₃O₄ and ZnO with weight of 55%.

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3.2.1. Effect of illumination time

The influence of illumination time and initial RB198 concentration on decolorization of RB198 was shown in figure 6. The decolorization of RB198 proportionally depended on the illumination time of the UV light system. The decolorization efficiency augmented responsively with the increase of exposing time of ZnO/Fe₃O₄/CS under UV light irradiation. Generally, the decolorization efficiency reached 100% after a certain illumination time with different concentrations. Specifically, the efficiency of 100% reached at illumination time of 40, 50, 80, 90, 100 and 120 min corresponding to RB198 concentration of 10, 20, 30, 40, 50 and 60 mg L⁻¹, respectively. As it can be seen, the equilibrium time was faster when the RB198 concentrations were higher. In addition, figure 7 showed the influence of illumination time on optical density of RB198 at concentration of 30 mg L⁻¹. When the time increased from 10 to 30 min, the optical density of RB198 augmented corresponsively. Once again, this result indicated the reduction of RB198 concentration leading to the decrease of its optical density.

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3.2.2. The effect of initial RB198 concentration

The effect of initial dye concentration was investigated by adjusting the initial concentration from 10 to 60 mg L⁻¹ and using 0.25 mg of catalyst and 50 ml of dye solution under UV light for 10 min. The obtained results were showed in figure 8. It can be seen that the photodegradation efficiency of RB 198 was inversely proportional to its concentration, meaning that the lower of the dye concentration, the higher photodegradation efficiency. Furthermore, the increase of initial dye concentration from 10 to 60 mg L⁻¹ also led to the reduction of photodegradation efficiency of RB198 from 82.3% to 38.9% after 10 min of UV irradiation. The mechanism of photodegradation was explained by the formation of critical species—hydroxyl radical, which was released during degradation process under UV light irradiation. Therefore, the initial concentration at higher concentration had stronger color intensity due to the representation of dye molecules in solution. This led to the obstacles between the active contact surface of ZnO/Fe₃O₄/CS and RB198 molecules causing the decrease of generation of ${\rm H}{{{\rm O}}^{\bullet }}$ due to the fewer active sites for the adsorption of ${\rm O}{{{\rm H}}^{-}}.$ Additionally, the route of photons going into the RB198 solution was shortened under constant intensity of UV light, which caused fewer photons interacting with catalyst surface. This was a result of reducing hydroxyl radicals that can attack RB198; therefore it leads to a decrease of photodegradation efficiency.

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3.2.3. Effect of catalyst loading

The effect of ZnO/Fe₃O₄/CS dosage on the photodegradation efficiency of RB198 was observed by taking different amounts of catalyst ranging from 0.1 to 0.5 mg into 50 mL of 30 mg L⁻¹ dye solution with pH of distilled water under UV light irradiation for 10 min. The obtained results are depicted in figure 9. As seen, when increasing the catalyst loading from 0.1 to 0.5 mg, the photodegradation efficiency of RB198 also raised from 42% to 100%. This may be explained due to the increase of contact area between active regions with RB198 corresponding to the raise of ZnO/Fe₃O₄/CS dosage. Particularly, the highest photodegradation efficiency was about 100% at 0.5 gram of the composite, while the lowest photodegradation efficiency was 41.6% at 0.1 g of the composite.

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Moreover, figure 10 also presented the UV–vis absorption spectra for degradation of RB 198 by using different ZnO/Fe₃O₄/CS catalyst dosage. As can be seen, the UV–vis absorption spectrum of RB198 at 0 g was highest due to no active regions to attack RB198. On the other hand, when raising the amount of ZnO/Fe₃O₄/CS catalyst dosage from 0.1 to 0.5 g, the absorption spectrum of RB198 trended to induce. This was because of the increase the active sites of hydroxyl radicals. Therefore, the lowest absorption spectrum of RB198 was at 0.5 g.

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3.2.4. Effect of pH of RB198 solution

It has been well-known that the pH value could influence the rate of photocatalytic degradation of some organic compounds. The impact of pH of RB198 was investigated in this study. The experiments were carried out in the pH range intervals of 6–8 by using 0.25 g of ZnO/Fe₃O₄/CS catalyst and 50 mL of 30 mg L⁻¹ RB198 solution under UV light irradiation for 10 min. Figure 11 demonstrated the result of photodegradation efficiency of RB198 with different pH values. The result revealed that the photodegradation efficiency decreases with the increase of pH value. Concretely, the photodegradation efficiency reached to 85% at low pH value (pH = 6). Inversely, the lowest photodegradation efficiency was 57.76% at pH of 8. This indicated that photodegradation efficiency was higher in an acidic medium than that of a basic medium. Because chitosan matrix can be swollen in acidic medium due to the hydrogen bonds between H⁺ and –OH groups in chitosan. Therefore, the swollen chitosan matrix made the porous holes larger in acidic medium, which increased the contact area with BR198 in solution. Besides, NH₂ groups of chitosan can be easily positive charged by protonation of H⁺ protons in acidic solution that help attract negative charged RB198 molecules owning to electrostatic interaction.

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3.2.5. Recycling experiments of ZnO/Fe₃O₄/CS catalyst

Favorably, the ZnO/Fe₃O₄/CS composite has magnetic activity which can easily recycle by using the magnet. Therefore, recycling experiments were repeated in several times. The results were shown in figure 12. As seen, the photocatalytic degradation of RB198 decreased from 96.17% to 76.28% after three times of usage. The results proved that ZnO/Fe₃O₄/CS catalyst can be used as a promising photo-catalytic composite for removal of dye solution with reusable property. In addition, after three times of usage, the decolorization of RB198 still reached over 70%. This indicated that ZnO/Fe₃O₄/CS was applicable for wastewater treatment. Interestingly, the chemical oxygen demand (COD) value of BR198 was 112 mg L⁻¹ before the treating with ZnO/Fe₃O₄/CS. However, it significantly decreased to 40 mg L⁻¹ after using the photocatalytic composite for treatment. Once again, the COD value investigated the power of ZnO/Fe₃O₄/CS in removal of RB198.

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3.2.6. Real wastewater performance