Iron Oxide/Chitosan Magnetic Nanocomposite Immobilized Manganese Peroxidase for Decolorization of Textile Wastewater

: Because of its effectiveness in organic pollutant degradation, manganese peroxidase (MnP) enzyme has attracted significant attention in recent years regarding its use for wastewater treatment. Herein, MnP was extracted from Anthracophyllum discolor fungi and immobilized on the surface of magnetic nanocomposite Fe 3 O 4 /chitosan. The prepared nanocomposite offered a high surface area for MnP immobilization. The influence of several environmental factors like temperature, pH, as well as storage duration on the activity of the extracted enzyme has been studied. Fourier transmission infrared spectroscopy (FT-IR), scanning electron microscope (SEM), X-ray diffraction (XRD), and transmission electron microscope (TEM) techniques were used for the characterization of the prepared MnP/Fe 3 O 4 /chitosan nanocomposite. The efficiencies of the prepared MnP/Fe 3 O 4 /chitosan nanocomposite for the elimination of reactive orange 16 (RO 16) and methylene blue (MB) industrial dyes were determined. According to the results, the immobilization of MnP on Fe 3 O 4 /chitosan nanocomposite increases its capacity to decolorize MB and RO 16. This nanocomposite allowed the removal of 96% ± 2% and 98% ± 2% of MB and RO 16, respectively. The reusability of the synthesized nanocomposite was studied for five successive cycles showing the ability to retain its efficiency even after five cycles. Thus, the prepared MnP/Fe 3 O 4 /chitosan nanocomposite has potential to be a promising material for textile wastewater bioremediation.


Introduction
Urbanization and contamination of surface drinking water resources have led to a crucial problem concerning the provision of safe water supplies in our contemporary world [1]. Uncontrolled discharges of municipal and industrial wastes in the environment can lead to the introduction of high concentrations of potentially toxic pollutants into water resources. Among the most harmful types of wastewater released are textile effluents that contain many dangerous artificial dyes like reactive orange 16 (RO 16) and methylene blue (MB). Due to its low prices, cationic dye MB is easily acquired hydrochloride were supplied via Merck (Hohenbrunn, Germany). Reactive orange 16 (RO 16), an anionic single azo dye, and Tween 80 were supplied from ACROS, Organics (Morris Plains, NJ, USA). All the used materials were of analytical grade and experimentally consumed as supplied without additional purification.

MnP Extraction from Anthracophyllum discolor
MnP enzyme was extracted from the white root fungus A. discolor strain obtained from the microbiology laboratory (Biology Department, Mansoura University). The fungus was allowed to grow at 5 °C inside tubes in the presence of media containing 10 g/L glucose (Merck, Hohenbrunn, Germany), 15 g/L agar (Merck, Hohenbrunn, Germany) and 30 g/L malt extract. The strain culture was then transferred to Petri dishes (Shanghai Joylab Medical Instruments Co., Ltd, Shanghai, China) and incubated for six days at 30 °C. Subsequently, a Kirk media containing 0.06% (v/v) surfactant tween 80, 255 μM of manganese sulfate (Al-Nasr Co., Helwan, Egypt) as MnP enzyme production inducer, and 33 g of wheat grains was prepared, and 95 mL of the Kirk media was inoculated by the mycelium of A. discolor and incubated at 30 °C for 11 days. The partially purified MnP enzyme was obtained from the media after growth. The growth media was ultra-purified using three successive filters; 1.2 μm Whatman glass filter (GE Healthcare Bio-Sciences, Pittsburgh, USA), an Amicon membrane, and 0.2 μm mixed ester-cellulose filter (Merck, Hohenbrunn, Germany). The ultrapurified enzyme was tested for MnP activity via MBTH-DMAB (3-methyl-2-benzothiazolinone hydrazine and 3-(dimethylamino)benzoic acid) (Sigma Aldrich, Steinheim, Germany) method [38], in which 10 μL hydrogen peroxide (10 mM) (Al-Nasr Co., Helwan, Egypt) was added to a mixture containing MnSO4 (20 mM, 30 μL) (Al-Nasr Co., Helwan, Egypt), ultra-purified filtrate (100 μL) (extracted from the white root fungus A. discolor ), MBTH (1.5 mM, 100 μL) (Sigma Aldrich, Steinheim, Germany), and DMAB (6.6 mM, 300 μL, buffer of succinate-lactate (100 mM, pH = 4.5, 1460 μL). A purple color was produced and measured by absorbance. Thus, the enzymatic activity was determined by reading absorbance at λ = 590 nm and was calculated with the extinction coefficient (ε = 53,000 M −1 cm −1 ) according to the literature. The amount of enzyme needed for one μM of the reaction product in one minute is equal to one unit [38].

Preparation of Fe3O4/Chitosan Magnetic Nanocomposite
Co-precipitation technique was used for Fe3O4 nanoparticle preparation. FeSO4 and FeCl3 solutions were mixed (1:2 molar ratios) and supplemented by aqueous ammonia with continuous stirring under nitrogen (Smgases, Beni Suef, Egypt) atmosphere for 30 min. The resulted nanoparticles were recuperated using a permanent magnet (Risheng Magnets International Co.,Ltd, Ningbo, China) and washed several times with distilled water. The Fe3O4/chitosan nanocomposite was prepared via reversed-phase suspension method in which 200 mg of washed iron oxide nanoparticles were mixed with 55 mL of mineral oil (Sigma-Aldrich, Missouri, USA) containing 0.6 mL of tween 80 (Acros Organics, Morris Plains, NJ, USA). After that, 1% (w/v) of chitosan (Winlab Company, Leicester, England) was added to the mixture. The synthesized Fe3O4/chitosan nanocomposite was sonicated and stirred for 35 min. The mixture was supplemented with 3.5 mL of 25% (w/v) glutaraldehyde solution (Sigma-Aldrich, St. Louis, MO, USA) and stirred for 5 h. Finally, the synthesized Fe3O4/chitosan nanocomposite was isolated using a magnet, washed many times using acetone, and dried at 45 °C in a vacuum. The produced Fe3O4 nanoparticles and Fe3O4/chitosan nanocomposite were characterized using different techniques including Fourier transform infrared (FT-IR) spectroscopy (Bruke EQUINOX 55, Karlsruhe, Germany), transmission electron microscope (TEM, Technai G 2 F30, Hillsboro, USA), scanning electron microscope (SEM, LEO 1450 VP, England), as well as X-ray diffraction (XRD, Rigaku MiniFlex 600 X-ray, USA).

Immobilization of MnP on the Fe3O4/Chitosan Nanocomposite
2 mL of phosphate buffer (Al-Nasr Co., Helwan, Egypt) (0.04 M, pH = 6) was used to disperse 50 mg of nanocomposite in an ultrasonic bath at 4 °C to immobilize MnP on the Fe3O4 nanocomposite.
Then, 0.6 mL of 2.5% N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (Merck, Hohenbrunn, Germany) in 2 mL phosphate buffer was added to the previous solution and ultrasonicated for a half-hour. The obtained mixture was stirred for 7 h at 1000 rpm and 4 °C. After that, the nanocomposite was collected with a permanent magnet (Risheng Magnets International Co.,Ltd, Ningbo, China) and washed several times in the phosphate buffer. For each 10 mg of the nanocomposite, 250 μL of the filtrate was added and stirred at 200 rpm and 30 °C for 30 min. Subsequently, the nanocomposite was collected by the magnet and washed by buffer solution (pH = 4.5) of sodium acetate (Al-Nasr Co., Helwan, Egypt) (50 mM) until the activity of the enzyme disappeared from the resulted solution. Finally, the obtained MnP/Fe3O4/chitosan nanocomposite was dried at 35 °C. The effect of various operating conditions including pH (3.5-9.5), temperature (30-70 °C) for 1 h, and storage duration at 20 °C (for 14 days) on immobilized MnP as well as free MnP were studied. Each experiment was performed in triplicate.

Wastewater Treatment Experiment
In order to study the removal efficiency of MnP, Fe3O4/chitosan nanocomposite, and MnP/Fe3O4/chitosan nanocomposite toward dyes RO 16 and MB, synthetic wastewater containing 50 mg/L of each dye was prepared. A 100 mL Erlenmeyer flask (Sigma Aldrich, Steinheim, Germany) was used in the adsorption experiment, in which 50 mL of synthetic wastewater was mixed with 10 mg of Fe3O4/chitosan nanocomposite, and MnP/Fe3O4/chitosan nanocomposite. In the case of the free MnP, the same volume as the immobilized ultra-purified filtrate was used. The wastewater decolorization experiment was conducted at pH 7, temperature 27 °C, 100 rpm and, for a contact time ranged from 10 to 50 min. The dye concentrations before and after adsorption were analyzed using a UV/VIS spectrophotometer (SHIMADZU, UV-2401pc, Addison, IL, USA) at 495 nm and 660 nm for RO 16 and MB, respectively. After each experiment, the magnetic nanocomposites were collected using a permanent magnet. Each experiment was performed in triplicate.

Characterization of the Synthesized Nanocomposite
Different techniques for characterization including XRD, SEM, FT-IR, and TEM were used to identify chitosan, synthesized Fe3O4 nanoparticles, and synthesized Fe3O4/chitosan nanocomposite. SEM images of Fe3O4 and Fe3O4/chitosan nanoparticles are presented in Figure 1a,b, respectively. SEM showed a homogeneous distribution of Fe3O4 nanoparticles, which is related to the presence of chitosan offering high surface for the immobilization of the MnP enzyme. Furthermore, a spherical shape of iron oxide nanoparticles and a size less than 35 nm were determined from the SEM image. Based on the TEM images ( Figure 1c,d for Fe3O4 and Fe3O4/chitosan, respectively), the prepared nanoparticles showed uniform spherical morphology with crystalline structure.

Purified MnP Measured Activity
The activity of MnP enzyme in the A. discolor filtrate was measured, as described in the experimental section according to the literature [38]. The MnP activity was found to be 4000 U/L, which is considered as an excellent result for the ultra-purified filtrate.

The Temperature and pH Effect on the Immobilized and Free MnP Activity
The immobilized MnP on the surface of Fe3O4/chitosan magnetic nanocomposite and the free MnP activities were examined at various temperatures and pHs. The working temperature ranged from 30 °C to 70 °C, while the working pH ranged from 3.5 to 9.5 as shown in Figure 3a,b, respectively. According to Figure 3a, the highest activity for both immobilized and free MnP was observed at 50 °C, with values two times higher for the immobilized enzyme (98 ± 2%). This is related to the higher stability of the immobilized enzyme compared with the free enzyme. Generally, the interaction between the support and the enzyme makes the immobilized enzyme resistant to temperature variation. The difference in activity between the immobilized and free enzymes was reported by many researchers [41,42]. According to Figure 3b, there is no significant difference between the immobilized and free MnP activity over the pH range (4.0-8.0). However, at pH = 8.5, there is a significant difference between the activity of the immobilized and the free MnP. These results might be related to the changes of enzyme conformation associated with the microenvironment change and the formation of covalent bonds upon immobilization. Consequently, the immobilization process makes the enzyme stable and resistant to pH change as reported by many researchers [43,44].

The Effect of Storage Duration on Immobilized and Free MnP Activity
The effect of storage at 20 °C on the immobilized MnP on the surface of Fe3O4/chitosan magnetic nanocomposite and on free MnP activities was studied over two weeks (Figure 3c). According to Figure 3c, there is higher stability of the immobilized MnP compared to the free enzyme during the storage period. The free enzyme showed 45 ± 2% decrease of its activity during the first five days and 90 ± 2% decrease after ten days. However, the immobilized MnP showed a slight decrease of its activity reaching 60 ± 2% after 14 days. This behavior could be related to the presence of interactions between the enzyme and the nanocomposite support, which enhance the enzymatic stability compared to the free enzyme.

Textile Wastewater Decolorization Using Mmobilized and Free MnP
The removal efficiencies of MB and RO 16 dyes using the Fe3O4/chitosan nanocomposite, the free and the immobilized MnP enzyme on Fe3O4/chitosan nanocomposite are shown in Figure 4a,b. Experiments were conducted at 27 °C and pH 7, the values measured for the synthetic wastewater without any adjustment. Therefore, the operating conditions (27 °C and pH 7) that mimic the real environment are more logical for the application of the process at large scales. This could reduce the process cost at large scale. According to Figure 4a,b, free MnP allowed the removal of 40 ± 2% and 43 ± 2% of the MB and RO 16 after 10 min, respectively. After 50 min, the removal efficiency of MB and RO 16 reached 52 ± 2% and 65 ± 2%, respectively. The obtained results indicate the advantages offered by the enzyme for the removal of organic pollutants from wastewater. Generally, enzymes are characterized by a notable efficiency and substrate specificity. Ligninolytic enzymes, especially MnP, can degrade organic compounds, such as polyaromatic hydrocarbons, with high efficiency [36]. They act at different environmental conditions (pH, temperature, etc.), they are effective at low pollutant concentrations and they are not inhibited by xenobiotic and microbial inhibitors [45,46]. The results suggest that the problem associated with free enzymes can be reduced by the immobilization technology [47,48].  As illustrated in Figure 4a,b, the magnetic Fe3O4/chitosan nanocomposite showed high removal efficiency for MB and RO 16 of 77 ± 2% and 79 ± 2% after 10 min, and these values reached 88 ± 2% (for MB) and 89 ± 2% (for RO 16) after 50 min, respectively. As shown in FT-IR bands (Figure 2a), the nanocomposite presents available adsorption sites, including amino and hydroxyl groups allowing an enhanced adsorption efficiency. The higher removal of both MB and RO 16 was obtained after 10 min, due to the availability of active sites on the surfaces of nanocomposite that interact easily with a large number of dye molecules. After this period, only a few active sites remain available, explaining the slight enhancement of the adsorption at the end of the experiment (50 min). This fact has been investigated by many researchers [49,50].
The magnetic Fe3O4/chitosan nanocomposite was introduced in order to immobilize the MnP enzyme. After 50 min, the immobilized enzyme allowed a decolorization rate of 96% ± 2% and 98% ± 2% for MB and RO 16, respectively (Figure 4a,b). Compared to the free enzyme, a significant improvement of the decolorization of textile wastewater was offered by the immobilized enzyme. The immobilization process increases the surface/volume ratio that enhances the binding capacity of the enzyme and helps to overcome the diffusion limitation [47]. Additionally, the small dimension of the enzyme carrier's nanoparticles can enhance the catalytic activity of the immobilized enzyme [48].

The Reusability of MnP/Fe3O4/Chitosan Nanocomposite
The reusability of MnP/Fe3O4/chitosan nanocomposite was examined due to its significant influence on the wastewater treatment processing cost, which is in part linked to the use of the nanocomposite. The nanocomposite reusability for the removal of MB and RO 16 was studied for successive five cycles as shown in Figure 4c. After each run, the magnetic nanocomposite was collected by a permanent magnet and washed many times to fit for the next run. After five cycles, the prepared MnP/Fe3O4/chitosan nanocomposite showed remarkable stability with slight decrease toward the removal of MB and RO 16 from wastewater. The removal efficiency of MB after cycle 1, 2, 3, 4, and 5 was 96 ± 2%, 94 ± 2%, 91 ± 2%, 88 ± 2%, and 85 ± 2%, respectively. Likewise, the removal efficiency of RO 16 was 98 ± 2%, 96 ± 2%, 93 ± 2%, 89 ± 2%, and 86 ± 2% after cycle 1, 2, 3, 4, and 5, respectively. These features make the prepared MnP/Fe3O4/chitosan nanocomposite a promising means of MB and RO 16 removal from wastewater.

Conclusions
Fe3O4/chitosan nanocomposite was an excellent support for the immobilization of MnP enzyme extracted from A. discolor. The produced material MnP/Fe3O4/chitosan was tested for decolorization of synthetic textile wastewater. Interestingly, the immobilized enzyme showed higher stability than the free enzyme at different operating conditions (pH, temperature, and storage duration). The prepared nanocomposite is considered as an ideal material with excellent properties as reported in the present study. For MB and RO 16, significant removal rates were achieved using the prepared nanocomposite. Moreover, the nanocomposite reusability was studied up to five cycles with ease recovery using a permanent magnet. However, more investigation is needed to explore other types of dyes under specific operating parameters (pH, temperature, etc.). The efficiency of this process should be studied for real textile wastewater and in real-world environmental conditions. Further, experiments using other enzymes should be conducted and the prepared nanocomposite could be explored for the bioremediation of other harmful pollutants.