A Facile Approach for the Synthesis of Zinc Ferrite/Alkali Cellulose as an Effective Magnetic Photocatalyst for the Degradation of Methylene Blue in Aqueous Solution

The spinel zinc ferrite/alkali cellulose composite has been successfully fabricated as a magnetic photocatalyst and assessed for its photocatalytic activity toward the degradation of methylene blue (MB) in an aqueous solution. The Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), thermogravimetric analysis (TGA), BET, and zeta potential were used to evaluate the magnetic photocatalyst composite and investigate its adsorption mechanism. Furthermore, the adsorption behavior of the composite was studied under various conditions. The ZnFe 2 O 4 /alkali cellulose composite effectively degraded (100%) MB after 180 min at a pH of 6.5 compared to cellulose, alkali cellulose and ZnFe 2 O 4 . The regeneration of the loaded composite was studied using the alcohol/water solution and reused upon a certain variation in the eciency after the fourth cycle. The adsorption process was found to be consistent with the pseudo-second-order kinetic model.


Introduction
High e ciency photocatalysts have been extensively studied due to their potential applications in the decomposition of toxic compounds as well as the rapid progress of modern industry (Sulaiman et al., 2016;Abboud et al., 2020). Besides, heavy metals and dyes are the most toxic e uents present in industrial wastewater, even at low concentrations. Numerous dyes are produced from different chemical industries, such as dye manufacturing, textile, paper and pulp, plastic, pharmaceutical, tanneries, cosmetics, and food processing (Bhattacharyya et al., 2015). Various treatment procedures have been described in the last three decades to remove dyes from wastewater, such as adsorption, membrane ltration, coagulation-occulation, chemical oxidation, sedimentation, and photodegradation (Abboud et al., 2020). Among these procedures, photocatalysis is considered to be the most promising and frequently used technique due to its simplicity, short reaction time, high oxidation power, reduced energy consumption, ability to remove a wide variety of dyes, and eco-friendly nature. The mechanism of photocatalysis involves UV-vis irradiation of a semiconductor compound such as TiO 2 , ZnO, Fe 2 O 3 , CdS, and ZnS as a catalyst then, electron excitation from the valence band to the conduction band (Cao et al., 2018;Wang et al., 2018). However, there was remarkable progress in the development of photocatalysis in the ultraviolet (UV) region compared to those in the visible light region. The development of visible light photocatalysts is one of the most signi cant photocatalytic research in recent years. Titania (TiO 2 ) shows high photoactivity under UV light illumination, while zinc ferrite (ZnFe 2 O 4 ) is sensitive to apparent light. ZnFe 2 O 4 cannot be utilized directly for the photocatalytic degradation of organic pollutants due to its low valence band potential and poor photoelectric conversion between zinc and iron (Valenzuela et al., 2002). TiO 2 /ZnFe 2 O 4 photocatalyst was synthesized via liquid catalytic phase conversion and sol-gel method (Jang et al., 2009;Xu et al., 2009) (Sa zade et al., 2018). Furthermore, Yttrium (Y) doped copper and zinc ferrite composites are fabricated by the solution combustion method. The resultant yttrium/zinc ferrite composite showed the highest photocatalytic activity of 94% in 180 min for the degradation of methylene blue dye as well as Mg/Mn substituted Ni/zinc ferrite composites (Ajithkumar et al., 2020). Due to the effective separation and delayed recombination of e-h pairs in ferrites, the charge carriers facilitate the oxidation of the methylene blue in solution (George et al., 2020). Consequently, the photocatalytic process requires the absorption of visible light from solar or arti cial sources (Salvetatu et al., 1999). The use of multi-walled carbon nanotubes (MWCNTs) as a reaction component in the synthesis of photocatalysts improves the photocatalytic activity of the ferrites to e ciently absorb visible light (Oh et al., 2008). ZnFe 2 O 4 /MWCNT composite was synthesized using the arc discharge method. Its photocatalytic activity toward the degradation of methylene blue showed that 84% degradation was found to result in the presence of zinc ferrite photocatalyst, while 99% degradation was observed in the composite after irradiation for 5 h under visible light (Singhal et al., 2013). Therefore, MWCNTs play an important role as an organic component in improving the structural properties of zinc ferrite. In our previous work, alkali cellulose (C 6 H 10 O 5 *NaOH) obtained via the treatment of cellulose with aqueous sodium hydroxide displayed a wide range of applications in food and pharmaceutical processes (Keshk et al., 2015(Keshk et al., , 2018Keshk & Hamdy 2019;Elkott A. et al., 2019;Marzouki et al., 2019;2021) due to its properties, such as electrical properties, colorlessness, non-toxicity, high adsorption capacity, optimal mechanical strength, relatively optimal adsorption, cost e ciency and availability. In this work, alkali cellulose was used as an effective biotemplate reactor for the dispersion of ZnFe 2 O 4 nanostructure through the porous structure of alkali cellulose for the rst time. Thus, facile fabrication of ZnFe 2 O 4 /alkali cellulose composite without any spacers was reported. Moreover, the photocatalytic activity of ZnFe 2 O 4 /(C 6 H 10 O 5 *NaOH) n was investigated using methylene blue (MB) as a toxic organic pollutant.

Materials And Methods
The cellulose powder used in the study was supplied by Sigma-Aldrich. All the reagents were purchased from Sigma-Aldrich and used as received.
Preparation of alkali cellulose (C 6 H 10 O 5 *NaOH) n Cellulose powder (1.0 g) was dopped in NaOH solution (2.0 M) at room temperature for 3 h. The alkali sample was collected by ltration without further washing and labeled as "alkali-cellulose" C 6 H 10 O 5 * NaOH (Marzouki et al., 2019).
Alkali cellulose (1 g) was added to the above solution and stirred vigorously at 60 °C. Subsequently, 6 g of NaOH was added to the mixture gradually within an hour to obtain a yellow wish brown precipitate under a pH of 12. The obtained yellow wish brown powder was placed in an oven under 110 °C for 6 h. The resultant composite was washed several times with distilled water and ethanol and dried at 60 °C for 48 h.

Characterization of the crystal structure and morphology
The Fourier-transform infrared (FT-IR) spectra of zinc ferrite and alkali cellulose were measured at room temperature using the Bruker FT-IR IFS 66 spectrophotometer over the range of 500-4000 cm −1 .
The X-Ray Diffraction (XRD) analysis was performed at room temperature using the Shimadzu L ab XRD-6000 with CuKα radiation of 1.54 Å and a secondary monochromator. The XRD tube was operated at a voltage of 30 kV and a current of 30 mA in the 2θ range from 10° to 60°.
The morphology of the sample was evaluated using the scanning electron microscopy FEG QUANTA 250 operated at 30 kV and equipped with an energy-dispersive X-ray instrument (EDX) for elemental analysis. The surface morphology and particle distribution of the samples were assigned using the FEI Tecnai 20 instrument operated at 200 kV.
The size distribution was determined by studying the TEM images recorded using the Tecnai 20-Philips instrument (G20, 200 kV). The powder samples were ultrasonically dispersed in ethanol.
The surface area and porosity analysis were conducted using the Novo 2000e Quanta chrome. The sample was degassed at 95 °C overnight prior to the N 2 adsorption experiment. The speci c surface area was determined by the Brunauer-Emmett-Teller (BET) model, while the pore volumes and pore radii were determined by the Barrett-Joyner-Halenda (BJH) model.
The thermogravimetric analysis (TGA) was evaluated using the Shimadzu TGA60 under N 2 atmosphere with a heating rate of 10 °C min -1 .
The size and zeta potential of different particles dispersed in water were determined by the Zetasizer Nano ZS (Malvern Paralytical).

Evaluation of photocatalytic degradation
The photocatalytic degradation of ZnFe 2 O 4 /alkali cellulose composite was performed by the degradation of MB as a model of the organic pollutants in an aqueous solution. Typically, 50 mg of the composite was dipped in 10 mg L -1 of MB (200 mL). The solution mixture was subjected to sunlight for 3 h and gathered at regular time intervals during irradiation to evaluate the photocatalytic degradation. The concentration of MB was evaluated by recording its absorbance at a wavelength of 664 nm using a UVvis spectrophotometer (Analytik Jena, Germany) in order to determine the photodegradation e ciency.
The UV-vis spectra were recorded at room temperature over a wavelength of 200 nm to 800 nm. The photodegradation percentage of MB in the aqueous solution was determined as follows: where C 0 is the initial concentration, and C t is the concentration at a certain time interval.
2.5. Kinetics of the photocatalytic degradation of MB by the ZnFe 2 O 4 /alkali cellulose The degradation reactions were found to be zero-order, pseudo-rst order, and pseudo-second-order equations as listed below: where C o and C t are the initial and residual concentrations of MB after a speci c time of the reaction. k t is the rate constant of the super cial reaction obtained from the plots of the equations.
Reusability and chemical stability of the composite Reusability is a key factor for the practical application of the composite. Therefore, the reusability of the ZnFe 2 O 4 /alkali cellulose photocatalyst for the photocatalytic degradation of MB was evaluated in four runs. After each run, the ZnFe 2 O 4 /alkali cellulose was separated by an external magnetic eld, washed with alcohol/water, dried at 100 °C, and reused thereafter. Finally, the chemical stability of the ZnFe 2 O 4 /alkali cellulose was investigated after the fourth recovery run via XRD analysis.

Results And Discussion
Under concentrated alkaline solutions, physicochemical and structural modi cations of cellulose were found to take place. Native cellulose (Cellulose I) was treated with concentrated sodium hydroxide to form alkali cellulose. As a result of the penetration of the alkali onto the lattice, internal hydrogen bonds were broken, and the number of available hydroxyl groups (-OH) was increased by around 25% (Marzouki et al., 2019). Thus, zinc ferrite interacted effectively with alkali cellulose rather than pristine cellulose.
Characterization and analysis of the composite The FT-IR spectrum of alkali cellulose (Fig.1) revealed the breaking of intermolecular hydrogen bonds characteristic of the-OH group and displayed a broad band at the region of 3320 cm -1 corresponding to the OH stretching vibration that was attributed to the intramolecular hydrogen bonds (Marzouki et al., 2019). Furthermore, the C-OH…NaOH feature, i.e., the activated hydroxyl group in C6H10O5* NaOH, appeared as a weak band in the region about 1023 cm -1 (Marzouki et al., 2019). On the other hand, a decrease in the band intensity of the methylene group at 2877 cm -1 was observed in the spectrum of ZnFe 2 O 4 /alkali cellulose composite that con rmed the presence of ZnFe 2 O 4 in the network of alkali cellulose. The OH stretching vibration was associated with the water molecules coordinated to the ferrite structure and its deformation, which were observed at 3428 cm -1 and 1693 cm -1 , respectively.
The presence of zinc ferrite in the composite was con rmed by the strong stretching vibration bands of Zn-O-Fe at 1453 cm -1 (Patil et al., 2013). Furthermore, the weak bands appearing at 425 cm −1 and 570 cm −1 represented tetrahedral and octahedral modes of ZnFe 2 O 4 , which were assigned to the Fe-O and Zn-O vibrations, respectively (Shahnavaz et al., 2014). The existence of these characteristic peaks con rmed the strong interaction between spinel ZnFe 2 O 4 and alkali cellulose matrix forming the composite.

X-ray diffraction (XRD)
The XRD diffractogram of alkali cellulose displayed the typical diffraction peaks of the crystalline structure of cellulose II at 2θ values of 11.8°, 12.3°, 20.1°, and 21.7° corresponding to the ( ), (110), (102) and (200)  The void plays a de nite role in the diametric swelling of the composite. Also, the cylindrical structure contributed to an increase in the tensile strength (high mechanical stability) of the composite. However, the presence of these elongated voids did not result in an asymmetric extension under stress within the composite. The EDX spectra exhibited different intense peaks associated with Zn, Fe, O, and C atoms, indicating that a highly pure composite was formed without residual side products.

High-resolution transmission electron microscopy (HRTEM)
To further con rm the interaction between ZnFe 2 O 4 and cellulose in the composite, the micrograph of the ZnFe 2 O 4 /alkali cellulose was investigated by TEM. The HRTEM images of the ZnFe 2 O 4 /alkali cellulose under different magni cations are presented in Fig.4. The micro images suggest that the ZnFe 2 O 4 nanoparticles were successfully supported on the alkali cellulose. The ZnFe 2 O 4 nanoparticles exhibited spherical particles ranging from 4 to 8 nm. It is worth mentioning that the nanoparticles showed some agglomeration, which is attributed to their small particle size and absence of steric hindrance (Tedjieukeng H.M. K. et al 2018). The average d-spacing of the observed lattice fringes was around 0.26 nm, which was assigned to the (311) plane of ZnFe 2 O 4 . The size of the obtained crystallite from the analysis of TEM images was in good agreement with that determined by the Debye-Scherrer approximation.
The crystallite sizes were obtained from the analysis of TEM images and were consistent with the zeta potential and investigation of particle size (Table 3).

Thermogravimetric analysis (TGA)
The thermal stability of cellulose, alkali cellulose, and its composite were analyzed under air conditions at a heating rate of 5 °C/min. The TGA curve in Fig. 5 showed that the decomposition temperature of alkali cellulose was higher than that of cellulose, owing to the deformation in the crystal structure of cellulose (by breaking the hydrogen bond).
Furthermore, a small weight loss was observed at about 150 °C due to the evaporation of residual water from cellulose and alkali cellulose. A major weight loss (second weight loss) was observed at about 300°C for alkali cellulose and 350 °C for cellulose, which was attributed to the degradation of lignin. On comparing alkali cellulose and composite, the behavior was found to be different, where the rst and second weight losses for the composite were shifted to higher values, indicating a good interaction between zinc ferrite and alkali cellulose matrix that enhanced its thermal stability.
The weight loss percentage at both 300 °C and 600 °C indicated that the Zn ions in ZnFe 2 O 4 were incorporated into the sodium cellulose (alkali cellulose) matrix and formed strong linkage bonds with higher binding energy, glass transition temperature, and thermal stability during the fabrication process.
These data were consistent with the published data on ZnO/alkali cellulose composite (Keshk & Hamdy, 2018) Brunauer-Emmett-Teller (BET) analysis of the surface area The textural properties of the photocatalyst were characterized by the BET analysis. Fig. 6 shows the N 2 adsorption-desorption isotherm of the ZnFe 2 O 4 /alkali cellulose that is characterized as the type-V isotherm.
The estimated BET speci c surface area was 5.3 m 2 /g, with a total pore volume of 0.028 cm 3 /g. Using the Barrett-Joyner-Halenda (BJH) method, the pore radius values were found in the range of 0.2-3.5 nm, indicating that the composite was composed of microporous and mesoporous structures (Groen et al., 2003).
Photocatalytic activity of the magnetic photocatalyst Fig. 7 shows the UV-vis absorbance of methylene blue in the reaction system after a speci c time interval. The structure of MB contains phenothiazine and dimethylamine on each side. The zinc ferrite-based photocatalyst recombined with alkali cellulose showed relatively high degradation potential (Fig. 7). The surface charge of ZnFe 2 O 4 /alkali cellulose facilitated the interaction between the composite and MB, as indicated by the zeta potential and particle size investigation (Table 3).
The photocatalytic activity of the composite depends on its composition, speci c surface area, porosity, size, distribution of particles, and nature of the crystalline phase. The ZnFe 2 O 4 /alkali cellulose achieved complete degradation of MB (100%) within 3 h, whereas ZnFe 2 O 4 /cellulose showed low e ciency (60%).
The effect of pH of the reaction on the degradation of MB was shown in Fig. (8).
The degradation of methylene blue increased gradually as the pH increased from 4 to 6.5. Thereafter, a noticeable decrease in degradation was observed for the pH values from 6.5 to 9. Methylene blue was protonated in the acidic medium and excess H + ions bonded with the OH group, which consequently reduced the degradation of the dye. Furthermore, the dye was deprotonated in the alkaline medium, resulting in repulsion between the surface of the adsorbent and dye. Therefore, all the experiments were conducted at a pH of 6.5 as it showed the best results. The observed trend was in agreement with the previously reported results in the literature (Nasrullah A. et al 2018) (Liu Y. et al 2018). The degradation data were tted to the zero-order, pseudo-rst order, and pseudo-second-order equations. The obtained k values are listed in Table 1. Accordingly, the data for both catalysts were consistent with the pseudosecond-order kinetics. The effect of the catalyst loading in the range of 10-100 mg on the photocatalytic degradation of 200 ml of 10 mg L -1 MB was evaluated. Accordingly, the degradation of MB by the ZnFe 2 O 4 /alkali cellulose composite increased from 30% to 98% as the dosage of the catalyst increased from 10 mg to 50 mg.
However, any further increase in the catalyst loading resulted in a slight decrease in its performance. This could be attributed to the increase in the agglomeration of the catalyst induced by the increase in its loading, which consequently reduced its surface area and number of active sites involved in the photocatalytic reaction and therefore decreased the percentage of degradation ( The obtained results indicated that 50 mg was the optimal mass of the catalyst under the speci ed reaction conditions. Furthermore, the zeta potential and particle size of the alkali cellulose and its composite were determined in water after 2 h of agitation. The particle size of ZnFe 2 O 4 , cellulose, and alkali cellulose varied between 105 to 180 nm under minimum charges (  Table 2, ZnFe 2 O 4 /alkali cellulose composite was found to be the most effective catalyst for the degradation of MB compared to cellulose, alkali cellulose, or ZnFe 2 O 4 due to the maximum particle size of the composite (970 nm) and a higher positive charge (19 mV) than that observed in MB (-5.01 mV). Moreover, the reusability and chemical stability results indicated that the ZnFe 2 O 4 /alkali cellulose was easily regenerated and still showed good chemical stability after being reused for four cycles, suggesting its potential for industrial applications.

Conclusion
A new magnetic ZnFe 2 O 4 /alkali cellulose was synthesized as a photocatalyst via a facile approach. The characterization of the composite was performed using FT-IR, XRD, FESEM, EDS, TEM, BET, zeta potential, and UV techniques that con rmed the presence of spinel ferrites in the alkaline cellulose matrix. The photocatalytic experiments indicated that the composite was photo catalytically active. The degradation e ciency of this method increased with the photocatalyst dosage and irradiation time and decreased under alkaline and acidic pH values. The optimum conditions for the maximum degradation e ciency were a pH value of 7, initial MB concentration of 2 mg/mL, photocatalyst mass of 50 mg, and irradiation time of 180 min. The ZnFe 2 O 4 /alkali cellulose was easily separated using a magnet and regenerated without any signi cant loss of the photocatalytic activity after being used for four consecutive cycles.
After the fourth regeneration cycle, the prepared composite showed good chemical stability and reusability. The degradation of MB was found to be 80% in the fourth cycle. The kinetic study of the process showed that the photocatalytic degradation of MB followed pseudo-second-order kinetics (R 2 = 0.9744). Therefore, it is concluded that the developed biocomposite has a high potential for the degradation of MB from aqueous solutions and thus utilized as an appropriate photocatalyst with high e ciency, cost-effectiveness, easy operation, and simple preparation. Declarations