Open AccessArticle

Sustainable Removal of Phenol Dye-Containing Wastewater by Composite Incorporating ZnFe₂O₄/Nanocellulose Photocatalysts

Zan Li

^1,*

Kun Gao

¹,

Wenrui Jiang

^2,*,

Jiao Xu

³ and

Pavel Lushchyk

School of Food Engineering, Harbin University, Harbin 150086, China

School of Mechatronics Engineering, Harbin Institute of Technology, Harbin 150001, China

HE Harbin Power Plant Valve Company Limited, Harbin 150001, China

Authors to whom correspondence should be addressed.

Sustainability 2024, 16(24), 11023; https://doi.org/10.3390/su162411023

Submission received: 29 October 2024 / Revised: 6 December 2024 / Accepted: 12 December 2024 / Published: 16 December 2024

(This article belongs to the Special Issue Advanced Materials and Processes for Wastewater Treatment)

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Figure 1
XRD patterns of NC, ZFO, and hybrid nanocomposites. "> Figure 2
FT-IR spectra of NC, ZFO, and ZFO/NC nanocomposites. "> Figure 3
(a) UV–visible spectra and (b) Tauc plots for the band gap of NC, ZFO, and ZFO/NC nanocomposites. "> Figure 4
XPS survey spectra of ZFO and ZFO/NC: (a) Fe 2p, (b) Zn 2p, (c) O 1s, (d) C 1s. "> Figure 5
Characterization of synthesized NC and nanocomposites. (a,b) denote TEM images for NC and ZFO/NC, respectively. (c,d) denote SEM images for NC and ZFO/NC, respectively. "> Figure 6
Elemental compositions of (a) ZFO and NC and (b) hybrid nanocomposites; (c) EDS mapping results for ZFO. "> Figure 7
(a) Degradation curves of phenol by NC, ZFO, and xZFO/NC (x = 0.1, 0.3, 0.5, and 0.7) in the absence of a magnetic field; (b) degradation curves of phenol by NC, ZFO, and xZFO/NC under magnetic field conditions; (c) comparison of the degradation efficiency of xZFO/NC in the absence of a magnetic field and in the presence of a magnetic field; (d) the percentage increase in the photodegradation rate of xZFO/NC after the addition of a magnetic field. "> Figure 8
(a) UV–visible absorption spectra of phenol and phenol under UV–vis; (b) absorption spectra of the degradation of phenol by 0.5ZFO/NC under the condition of a magnetic field as a function of time. "> Figure 9
Cyclic experiments on photocatalytic degradation of phenol by 0.5ZFO/NC. "> Figure 10
0.5ZFO/NC in the absence of a magnetic field and in the presence of a magnetic field (MF = magnetic field; NMF = no magnetic field): (a) photocurrent response density; (b) electrochemical impedance spectroscopy (CPE = Constant Phase Angle Element). "> Figure 11
Schematic of the mechanism of visible-light photocatalytic phenol degradation by NC, ZFO, and xZFO/NC. ">

Versions Notes

Abstract

The escalating issue of phenol-containing wastewater necessitates the development of efficient and sustainable treatment methods. In this context, we present a novel composite photocatalyst comprising ZnFe₂O₄ (ZFO) nanoparticles supported on nanocellulose (NC), aimed at addressing this environmental challenge. The synthesis involved a facile hydrothermal method followed by the impregnation of ZFO nanoparticles onto the NC matrix. The morphology and structure of ZFO, NC, and ZFO/NC were investigated by TEM, SEM-EDX, UV–vis, FT-IR, XRD, and XPS analyses. ZFO, as a weakly magnetic semiconductor catalytic material, was utilized in photocatalytic experiments under magnetic field conditions. By controlling the electron spin states through the magnetic field, electron–hole recombination was suppressed, resulting in improved photocatalytic performance. The results demonstrated that 43% and 76% degradation was achieved after 120 min of irradiation due to ZFO and 0.5ZFO/NC treatment. Furthermore, the composite 0.5ZFO/NC demonstrated the highest photocatalytic efficiency, showing promising recyclability by maintaining its activity after three cycles of use. This study underscores the potential of the ZFO/NC composite for sustainable wastewater treatment, offering a promising avenue for environmental remediation.

Keywords:

ZnFe₂O₄; nanocellulose; photocatalyst; phenol

1. Introduction

Phenol-containing wastewater poses a significant environmental challenge due to its harmful effects on ecosystems and human health. The standard set by the U.S. EPA regarding phenols indicates that at a concentration of 2.56 mg/L in phenol solutions, chronic toxicity to freshwater aquatic organisms may occur, while 3.5 mg/L is the threshold concentration at which these compounds may pose harm to humans. The discharge of untreated wastewater containing phenol can lead to water pollution, disrupting aquatic life and endangering public well-being [1]. In response to this pressing issue, researchers have been exploring innovative and sustainable approaches for the efficient removal of phenol from wastewater.

The textile, paper, and plastic industries are major contributors to the environmental discharge of phenolic and other synthetic dyes. These dyes, once released into waterways, can lead to severe ecological damage and pose health risks to both aquatic life and human populations. In response to this challenge, numerous treatment techniques have been developed over the years, including activated carbon adsorption and coagulation–flocculation processes. However, these conventional methods have demonstrated limitations in terms of their efficiency, cost-effectiveness, and ability to handle a wide range of dye types. In light of these shortcomings, there is a pressing need for innovative and sustainable solutions that can effectively mitigate the environmental impact of dye discharge while being economically viable and environmentally friendly. Conventional wastewater treatment methods often struggle to efficiently remove phenol, leading to the need for advanced treatment technologies. Among these, photocatalysis has emerged as a promising approach due to its ability to harness light energy to degrade organic pollutants [2].

In recent years, composite photocatalysts have gained attention for their potential to enhance the photocatalytic performance and stability of semiconductor materials. ZnFe₂O₄ (ZFO), a class of magnetic semiconductor materials, has found extensive use in the conversion of solar energy [3], photocatalytic processes [4,5], and the photochemical production of hydrogen from water [6]. Its distinctive attributes, such as its ability to absorb visible light, high photochemical stability, and affordability, contribute to these applications. Furthermore, research indicates that magnetic ZFO particles exhibit intrinsic peroxidase-like activity, enabling them to react with hydrogen peroxide (H₂O₂) to generate highly reactive hydroxyl radicals (·OH), which are effective in degrading organic contaminants [7,8]. Despite these advantages, ZFO is not suitable for use as an efficient photocatalyst on its own due to its high rates of charge recombination following photoinduction [9]. Therefore, it is crucial to enhance the efficiency of charge separation induced by light in ZnFe₂O₄ to improve its photocatalytic efficacy.

Additionally, nanocellulose (NC), derived from renewable biomass sources, has emerged as an environmentally friendly support matrix for photocatalytic nanoparticles, offering advantages such as a high surface area, biodegradability, and a low cost [10,11,12,13].

This study aims to investigate the effectiveness of a composite photocatalyst incorporating ZFO nanoparticles supported on nanocellulose for the sustainable removal of phenol-containing wastewater. With the integration of ZFO nanoparticles onto an NC matrix, we anticipate synergistic effects that will enhance the photocatalytic activity and stability of the composite material. Furthermore, the utilization of ZFO/NC composite photocatalysts aligns with the principles of green chemistry, offering a promising solution for the remediation of phenol pollutants in wastewater.

In this paper, we present the synthesis and characterization of the ZFO/NC composite photocatalyst, followed by an evaluation of its photocatalytic performance for phenol degradation. The findings of this study contribute to the advancement of sustainable wastewater treatment technologies, with potential applications in mitigating environmental pollution and safeguarding water quality.

2. Materials and Methods

2.1. Materials

A nanofibrillated nanocellulose (diameter: 4–10 nm, length: 200 nm) suspension in water (6% w/v) was obtained from Macklin (Shanghai, China). Zinc chloride (ZnCl₂), iron(III) chloride (FeCl₃), and sodium hydroxide (NaOH) were procured from Macklin. All the chemicals utilized were of high analytical quality; no additional purification procedures were conducted.

2.2. Methods

2.2.1. Preparation of ZFO/NC Nanocomposite

Nanocellulose was suspended in distilled water and sonicated for 15 min to break up any fiber agglomerations. The standard synthesis process for ZFO/NC, which was produced through the solvothermal method, was typically carried out as follows: We prepared a 0.1 M FeCl₃ solution by dissolving FeCl₃·6H₂O in deionized water. Similarly, we created a 4.4 M NaOH solution by dissolving NaOH in deionized water. Subsequently, we introduced 0.34 g, 1.01 g, and 1.68 g of NC into the FeCl₃ solution, which had a volume of 70 mL, and also added 0.1 M ZnCl₂ to it. We mixed the solution thoroughly under magnetic stirring (200 rpm, 30 min). We gradually added 10 mL of the prepared NaOH solution to the mixture while maintaining the same conditions. We continued stirring for another 30 min. Once the reaction was completed, we transferred the suspension to a high-pressure vessel lined with 100 mL of Teflon. We heated the vessel at 200 °C for an overnight period. Afterward, we rinsed the product with water and allowed it to dry to obtain the synthesized compound.

2.2.2. Instrumentation and Characterization

Characterization of the surface morphology and elemental composition was conducted with Scanning Electron Microscopy (SEM, model JEOL JSM-6510 LV, Tokyo, Japan) and Energy Dispersive Spectroscopy (EDS), respectively. To facilitate analysis, a thin layer of gold was sputter-coated onto the samples using a JEOL JFC 1200 FINE COATER (Tokyo, Japan). FT-IR spectroscopy was used to identify functional groups and chemical bonds within the nanocomposite. Fourier Transform Infrared Spectroscopy (FT-IR) was performed at ambient temperature with a VERTEX 70 V instrument (Bruker, Karlsruhe, Germany) in the Attenuated Total Reflectance (ATR) mode, scanning in the range of 4000 to 400 cm⁻¹. XRD was crucial for determining the crystal structure, phase purity, and crystallite size of the nanocomposites. An X-ray diffraction (XRD) analysis was conducted using an XPERT PRO-PAN Analytical system. TEM offers even higher resolution and can visualize the internal structure of nanocomposites, including the size, shape, and arrangement of the nanoparticles at the atomic level. Transmission Electron Microscopy (TEM) images were captured on a JEOL GEM-1010 microscope (Tokyo, Japan) at an accelerating voltage of 76 kV. X-ray Photoelectron Spectroscopy (XPS) was employed with an ESCALAB 250XI+ instrument from Thermo Scientific (K-ALPHA, Waltham, MA, USA), using Al Kα radiation at an energy of 1486.6 eV. The analysis parameters were set at a spot size of 500 μm, with the energy resolution calibrated against the Ag3d5/2 and C1s lines at 0.45 eV and 0.82 eV, respectively, and under a vacuum of 10–8 mbar. The full spectrum had a pass energy of 50 eV, while the narrow spectrum used a pass energy of 20 eV. The data processing was conducted using Casa XPS software (version 2.3.26) and MultiPak software (version 8.2). UV–vis spectroscopy was employed to determine the optical properties of the nanocomposites, such as the bandgap, electronic transitions, and presence of surface defects. UV–vis DRS was recorded on a PerkinElmer Lambda 750 (Hopkinton, MA, USA). The transient photocurrent (I-t) and electrochemical impedance spectroscopy (EIS) (Chenhua 660e, Shanghai, China) were used to test the carrier separation efficiency of the catalysts.

2.2.3. Photodegradation Measurements

To mimic organic wastewater, a phenol solution with a concentration of 15 mg/L was prepared. Subsequently, 125 mL of this phenol solution, 0.18 g of the catalyst ZFO/NC, and 15 µL of hydrogen peroxide were combined. By adding sodium hydroxide, the pH of the solution was adjusted to 7. A custom-made photocatalytic reactor equipped with magnets was employed to initiate the photocatalytic reaction. The reaction was allowed to proceed for 60 min. Following the reaction, a sample was taken, which was then subjected to 8 min of centrifugation at high speed (10,000 rpm). The supernatant from the centrifuge tube was aspirated, and the filtrate was filtered. The concentration of phenol in the solution was determined using a UV–vis spectrophotometer. Finally, the degradation rate of phenol was calculated based on the absorbance readings before and after the reaction. For the photodegradation experiment, a double-beaker setup was used, and a constant temperature was maintained with a circulating water bath. To achieve adsorption/desorption equilibrium in the dark, a stirring (200 rpm) period of 60 min was required. The photodegradation activity was characterized using the following equation:

D = (C₀ − C)/C₀ × 100% = (A₀ − A)/A₀ × 100%

(1)

where A is the absorbency of phenol, and C is the concentration of collected supernatant at a certain time interval.

3. Results and Discussion

3.1. XRD

Figure 1 illustrates the X-ray diffraction (XRD) patterns for NC, ZFO, and the hybrid nanocomposites containing varying amounts of ZFO (0.1, 0.3, and 0.5 g by weight). The peaks in these patterns were distinct and aligned closely with the reference cards JCPDS no. 00-050-2241 and JCPDS no. 22-1012, signifying the crystalline nature of the original nanocellulose [14] and the spinel cubic structure of the pure ZFO nanoparticles [15]. The XRD pattern revealed peaks at approximately 2θ = 17.69°(101), 22.08°(002), and 34.51°(040), indicative of a typical cellulose structure. The corresponding scan data for ZFO demonstrated the formation of well-defined ZFO nanoparticles, as confirmed by JCPDS card no. 22-1012. The diffraction peaks at 29.9, 35.3, 42.8, 53.1, 56.6, and 62.2° were attributed to the (220), (311), (400), (422), (511), and (440) crystal planes, respectively. No additional peaks indicative of impurities were detected, thus verifying the sample’s purity. In the XRD pattern of the nanocomposites, besides the characteristic peaks of NC at 2θ = 22.85° (002), there were also diffraction peaks that aligned with the standard ZFO pattern. With an increase in the amount of zinc ferrite, the intensity of the peaks corresponding to ZFO increased, while the intensity of the peaks associated with nanocellulose decreased, as depicted in Figure 1.

3.2. FT-IR

The FT-IR spectra of ZFO, NC, and ZFO/NC are given in Figure 2. The peak corresponding to the Zn-O bond (569 cm⁻¹) can be assigned to the intrinsic lattice vibration in the spectrum of ZFO [16]. The absorption bands at 1637 cm⁻¹ and 3398 cm⁻¹ can be respectively attributed to water deformation vibrations and -OH vibration modes [17]. The NC absorption peak observed at 1110 cm⁻¹ derived from C–O–C pyranose ring vibration for the curves of NC and ZFO/NC [18]. After doping, the absorption spectrum underwent a red shift, possibly due to changes in the molecular structure and the influence of lattice vibrations.

3.3. UV–Vis

The optical characteristics of the relevant materials were evaluated using UV–vis spectroscopy within the wavelength range of 250 to 800 nm. The absorption spectra for the untreated ZFO, NC, and the ZFO/NC composite are illustrated in Figure 3a. The highest absorption peak for ZFO occurred at 415 nm. The energy band gap (E_g) of each material was determined by applying Tauc’s equation, as shown in Equation (2), where α, hυ, A, and E_g denote the absorption coefficient, proportionality constant, photon energy, and optical bandgap energy, correspondingly. For a direct bandgap semiconductor, n = 1; for an indirect bandgap semiconductor, n = 4. The resulting graphs of (ahυ)² versus (hυ) are shown in Figure 3b. The calculated E_g values were found to be 1.58, 1.83, and 1.74 eV for NC, ZFO, and ZFO-NC, respectively. The incorporation of impurity elements led to lattice strain within the crystal structure, which could be a result of changes in the energy band structure of the doped samples [19].

(α h υ) = {A (h υ - E_{g})}^{\frac{n}{2}}

(2)

3.4. XPS

An XPS analysis was used to understand the development of additional ZFO/NC. The surface content and valence states of the produced composites are shown in Figure 4. The XPS analysis indicated the origin of component elements with a favorable reaction. The Fe 2p3/2 and Fe 2p1/2 peaks in Figure 4a were located at binding energies of 711.1 and 725.2 eV [6,20], respectively, where the binding energies presented a upshift compared to that of Fe 2p in ZFO. This may be attributed to the migration of the electron from ZFO to NC [21], which can provide definite evidence for charge transport of the ZFO/NC heterojunction. For ZFO/NC, the peak between 725.8 and 713.5 eV was a satellite peak, which is characteristic of Fe³⁺. For the Zn 2p spectra, the 1023.3 and 1045.0 eV peaks were assigned to Zn 2p3/2 and Zn 2p1/2 for ZFO/NC, which are characteristic of Zn²⁺ [22,23]. The Zn 2p peaks of ZFO/NC also showed a slight upshift compared to those of pure ZFO, which further provides evidence for electron transfer from ZFO to NC. Figure 4c shows the XPS O 1s spectra of ZFO and ZFO/NC, and the O 1s peak of ZFO/NC also showed a slight upshift compared to that of pure ZFO. The spectrum of NC shows three splitting peaks for C1s at 284.39 eV and 286.66 eV related to C(-C, H) and O-C-O, respectively [24,25].

3.5. TEM and SEM

TEM is the most effective tool used for understanding the morphological structure and size of prepared nanostructures. The TEM images (Figure 5a) revealed that NC was poly-disperse and had smooth rod shapes with an average length of 95 nm and average diameter of 15 nm. On the other hand, ZFO nanocomposites formed in different shapes, such as hexagonal, spherical, and cluster or star-like, with an average size range of 45–150 nm. As shown in Figure 5b, there was good incorporation between ZFO and the rod-like NC to form hybrid ZFO/NC with an average size of 37–88 nm, with numerous shapes found depending on the components. It was observed that the mean particle sizes of the ZFO/NC and NC composites were 85 and 30−50 nm, with cloud-like and rod structures, respectively [26,27].

The surfaces of NC and ZFO/NC were investigated using SEM micrographs at high resolution (Figure 5c,d). As can be seen from Figure 5c, NC appeared as rod-like structures, while ZFO/NC exhibited irregular forms that agglomerated into cluster structures (Figure 5d). When ZFO was dispersed on the cellulose matrix (Figure 5d) to form ZFO/NC nanocomposites, the same shapes of ZFO were observed.

The samples were subsequently analyzed with energy-dispersive X-ray spectroscopy (EDX) to assess the ratio of divalent cations within the nanoparticles, providing further confirmation of the presence of mixed ferrites. The elemental makeups of the pristine nanocellulose and the bare ZnFe₂O₄ are depicted in Figure 6a. The analysis showed that NC contained carbon, oxygen, and trace amounts of impurities, which were likely derived from the sodium hydroxide used in the production process. The hybrid nanocomposites exhibited material compositions that closely matched the intended ratios, as illustrated in Figure 6b. While minor discrepancies might be attributed to technical inaccuracies, the overall results suggest the successful creation of tertiary ferrites with a precisely defined composition. EDS mapping (Figure 6c) confirmed a uniform distribution of Zn, Fe, and O elements in the samples.

3.6. Photocatalytic Properties

To study the effect of a magnetic field on the photodegradation performance, ZFO/NC photocatalysts with different ZFO contents were used as a model. As can be seen in Figure 7a,b, 0.1ZFO/NC, 0.3ZFO/NC, 0.5ZFO/NC, and 0.7ZFO/NC showed significant improvements compared to ZFO and NC. Among them, 0.3ZFO/NC exhibited the best degradation performance in the absence of a magnetic field. However, the reaction efficiency of the 0.5ZFO/NC photocatalyst was greater than those of 0.3ZFO/NC and 0.7ZFO/NC after a magnet was placed near the photocatalytic reaction system, as shown in Figure 7b. In order to visually compare the promotion of the photocatalytic degradation of phenol by magnetic fields, Figure 7c depicts a comparison of the photocatalytic reaction efficiency without and with magnetic fields. All data were fitted with pseudo-first-order kinetics and followed the formula −ln(C_t/C₀) = kt. It can be seen that the photodegradation k-value of ZFO to phenol was the same with or without an external magnetic field, indicating that the presence of a magnetic field had no effect on NC’s degradation of phenol. However, the k-values for the degradation of phenol by xZFO/NC were 0.007, 0.009 min⁻¹ (x = 0.1), 0.012, 0.014 min⁻¹ (x = 0.3), and 0.011, 0.015 min⁻¹ (x = 0.5), respectively, before and after magnetic field placement. The results show that the ZFO/NC photocatalyst exhibits electron spin polarization, which can be controlled by the external magnetic field, and the photocatalytic efficiency was improved to varying degrees after the addition of a magnetic field. A comparison of the efficiency improvements in the ZFO/NC photocatalytic degradation of phenol is shown in Figure 7d. Among all these ZFO/NC samples, the degradation efficiency of phenol was increased by about 35% with the addition of a magnetic field to 0.5ZFO/NC. These results indicated that the photocatalytic degradation efficiency increased with an increase in the ZFO doping concentration and showed the strongest improvement at x = 0.5.

To assess the photostability of phenol when exposed to UV–visible light, a photolysis test (a control reaction) was conducted without the presence of the photocatalyst. The UV–vis spectra of the phenol solution prior to and following photolysis are depicted in Figure 8a. Following 15 min of illumination, no significant reduction in the peak of maximum absorption for phenol was detected. The peak at 271 nm corresponds to the chromophoric group responsible for the dye’s coloration. The minimal decrease in this peak after the photolysis process indicates a discoloration of approximately 3.57%. Figure 8b shows the absorption spectrum of phenol degradation under the condition of a magnetic field with the progression of time for 0.5ZFO/NC. The intensity of the maximum absorption peak of phenol gradually decreased with the progression of visible light irradiation. When the irradiation time reached 120 min, the intensity of the maximum absorption peak of phenol had decreased by 76%, indicating that 0.5ZFO/NC destroyed the chromophoric group of phenol. Additionally, when compared with Figure 8a, no new absorption peaks appeared, suggesting that the phenol was degraded.

The sustainability and stability of photocatalysts play a crucial role in practical applications. In order to achieve the purposes of economy and environmental protection, the photocatalytic activity of the photocatalyst is usually required to remain unchanged as much as possible after multiple degradation cycles. To test the sustainability and stability of 0.5ZFO/NC, cyclic degradation experiments were performed under a magnetic field using 0.5ZFO/NC as a photocatalyst. As shown in Figure 9, each degradation was performed under the same conditions, indicating that the photodegradation performance of 0.5 ZFO/NC in the magnetic field did not decrease significantly after three reaction cycles.

3.7. Photogenerated Carrier Dynamics Analysis

Photocurrent responses can be used to determine the separation and transfer kinetics of photogenerated carriers during photocatalytic processes [28,29,30]. In this study, during photocurrent measurements, a magnetic field was applied to the photoelectrodes to analyze its effect on the photocatalytic performance. Figure 10a shows the photocurrent density–time curves of 0.5ZFO/NC in the absence of a magnetic field and in the presence of a magnetic field. The photocurrent density increased sharply when the visible light was turned on and then rapidly returned to zero immediately after the light was turned off. Upon the application of the magnetic field, the photocurrent density of 0.5ZFO/NC increased to 4.3 μA/cm², which was higher than the photocurrent density without a magnetic field. According to these experimental results, the magnetic field plays a significant role in enhancing the photocatalytic activity of ZFO/NC, i.e., the separation and transfer of photogenerated carriers can be made quicker with the help of a magnetic field.

In addition, the transport capacity of the carriers was evaluated by employing electrochemical impedance spectroscopy (EIS). Electrochemical impedance testing is a commonly used electrochemical characterization method to measure the change in material impedance by giving a fixed sine wave frequency and to analyze and study the electrode dynamics of the surface of photocatalytic materials. In general, the smaller the arc radius of the EIS, the better the carrier capacity. As shown in Figure 10b, the arc radius of 0.5 ZFO/NC under the action of a magnetic field was the smallest, indicating that its carrier transport capacity was the best. This result is consistent with the conclusions of the photocurrent and degradation experiments.

3.8. Photocatalytic Mechanism

A schematic of the mechanism of visible-light photocatalytic degradation of phenol by the doped ZFO/NC nanocomposites is shown in Figure 11. Blue light incident on the scaffolds induces the generation of electrons (e⁻) and holes (h⁺), which then migrate to the CB and VB, respectively [31]. The photocatalytic degradation process consists of two stages: the separation and transfer of photogenerated carriers. In the first stage, electrons are excited to the conduction band under the irradiation of light. However, there is a possibility that photogenerated electrons and holes will reunite. Therefore, the density of photogenerated carriers actually participating in the reaction is limited, which is not conducive to the photocatalytic reaction. At this time, by introducing an external magnetic field, the Lorentz force generated by the magnetic field can drive the positively charged holes and negatively charged electrons to move in opposite directions according to the left manipulation, separating the electrons and holes, thereby inhibiting the recombination and increasing the number of free carriers. According to the left rule, the charged particles all experience the Lorentz force perpendicular to the direction of their motion, and the Lorentz force acting on the electrons and holes is in the opposite direction because the electrons have a negative charge and the holes have a positive charge. This effect causes electrons to move in one direction and holes to move in the opposite direction, and it increases the density of carriers involved in the reaction. However, during stirring, the direction of the Lorentz force is constantly changing, resulting in longer transport paths for the separated electrons and holes and a higher probability of recombination. Therefore, in the second stage, the introduction of NC establishes a directional transfer path for the separated holes, so that the holes are attracted to NC, which further inhibits the recombination of electrons and holes. In addition, NC provides a number of active centers that favor redox reactions, further improving the efficiency of photodegradation. During photocatalysis, phenol is oxidized by ·OH to intermediates like terephthalic acid and benzaldehyde, then these are further broken down to produce products like trans-3-hexenoic acid. Macromolecular organic acids are oxidized to short-chain acids like malonic acid, which are ultimately mineralized to CO₂ and H₂O [32]. A comparison of this contribution with recently published results on the photocatalytic degradation of phenol is revealed in Table 1. The ZFO/NC nanocomposite efficiently catalyzes phenol degradation under visible-light irradiation with excellent reusability, demonstrating its practicability in wastewater remediation.

4. Conclusions

In the present study, we reported the fabrication of ZFO/NC nanocomposites and, for the first time, hybrid ZFO/NC nanocomposites through a solvothermal method, and the successful preparation of the composites was demonstrated using characterization. SEM, TEM, and EDS were used to characterize the morphology of the samples, XRD was used to analyze the crystal phase of the samples, FTIR and XPS were used to analyze the chemical state of the samples, DRS was used to analyze the optical properties of the catalyst, and the band gap and energy band position were calculated. All kinds of characterization results confirmed the successful preparation of the ZFO/NC photocatalyst. The photodegradation of phenol revealed the high efficacy of hybrid ZFO/NC in dye removal as compared to pure NC and ZFO nanocomposites in the presence of an MF. The Lorentz force acting on the photocatalyst can inhibit the rapid recombination of carriers. In the absence of any energy input, the photocatalytic degradation performance of phenol was improved by about 27% by placing a magnet next to the reaction device. After the introduction of NC, the photocatalytic reaction’s efficiency was further improved by 30%, indicating that NC helps to transfer separated and accumulated holes. This work provides a new perspective for suppressing recombination in photogenerated carriers and improving their photocatalytic efficiency through the Lorentz force and NC.

Author Contributions

Conceptualization, W.J.; project administration, W.J.; methodology, P.L.; data curation, J.X.; writing—original draft, Z.L.; writing—review and editing, Z.L.; funding acquisition, W.J. and K.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Key R&D Project (Intergovernmental International Science and Technology Innovation Cooperation, Grant No. 2022YFE0122600), the Heilongjiang Province Unveiling Leader Project (Grant No. 2022ZXJ01A02, 2022ZXJ01A01), and the National Natural Science Foundation of China (Grant No. 22378089).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors acknowledge the School of Food Engineering of Harbin University, HE Harbin Power Plant Valve Company Limited, and Harbin Institute of Technology.

Conflicts of Interest

Author Jiao Xu was employed by the company HE Harbin Power Plant Valve Company Limited. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. XRD patterns of NC, ZFO, and hybrid nanocomposites.

Figure 2. FT-IR spectra of NC, ZFO, and ZFO/NC nanocomposites.

Figure 3. (a) UV–visible spectra and (b) Tauc plots for the band gap of NC, ZFO, and ZFO/NC nanocomposites.

Figure 4. XPS survey spectra of ZFO and ZFO/NC: (a) Fe 2p, (b) Zn 2p, (c) O 1s, (d) C 1s.

Figure 5. Characterization of synthesized NC and nanocomposites. (a,b) denote TEM images for NC and ZFO/NC, respectively. (c,d) denote SEM images for NC and ZFO/NC, respectively.

Figure 6. Elemental compositions of (a) ZFO and NC and (b) hybrid nanocomposites; (c) EDS mapping results for ZFO.

Figure 7. (a) Degradation curves of phenol by NC, ZFO, and xZFO/NC (x = 0.1, 0.3, 0.5, and 0.7) in the absence of a magnetic field; (b) degradation curves of phenol by NC, ZFO, and xZFO/NC under magnetic field conditions; (c) comparison of the degradation efficiency of xZFO/NC in the absence of a magnetic field and in the presence of a magnetic field; (d) the percentage increase in the photodegradation rate of xZFO/NC after the addition of a magnetic field.

Figure 8. (a) UV–visible absorption spectra of phenol and phenol under UV–vis; (b) absorption spectra of the degradation of phenol by 0.5ZFO/NC under the condition of a magnetic field as a function of time.

Figure 9. Cyclic experiments on photocatalytic degradation of phenol by 0.5ZFO/NC.

Figure 10. 0.5ZFO/NC in the absence of a magnetic field and in the presence of a magnetic field (MF = magnetic field; NMF = no magnetic field): (a) photocurrent response density; (b) electrochemical impedance spectroscopy (CPE = Constant Phase Angle Element).

Figure 11. Schematic of the mechanism of visible-light photocatalytic phenol degradation by NC, ZFO, and xZFO/NC.

Table 1. Summary of the recent advancements in ZFO and ZFO/NC used for phenol photodegradation.

Photocatalyst	Irradiation Source (Time)	Pollutant Degradation Rate (%)	Pollutant Concentration (ppm)	Reusability	Ref
0.5ZFO/NC	90 W LED light (120 min) under magnetic conditions	76%	15	After 3 cycles: 75%	This study
ZFO	8W UV lamp (150 min)	82%	100	-	[33]
ZFO	under 250 W visible light (60 min)	60%	10	-	[34]

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Li, Z.; Gao, K.; Jiang, W.; Xu, J.; Lushchyk, P. Sustainable Removal of Phenol Dye-Containing Wastewater by Composite Incorporating ZnFe₂O₄/Nanocellulose Photocatalysts. Sustainability 2024, 16, 11023. https://doi.org/10.3390/su162411023

AMA Style

Li Z, Gao K, Jiang W, Xu J, Lushchyk P. Sustainable Removal of Phenol Dye-Containing Wastewater by Composite Incorporating ZnFe₂O₄/Nanocellulose Photocatalysts. Sustainability. 2024; 16(24):11023. https://doi.org/10.3390/su162411023

Chicago/Turabian Style

Li, Zan, Kun Gao, Wenrui Jiang, Jiao Xu, and Pavel Lushchyk. 2024. "Sustainable Removal of Phenol Dye-Containing Wastewater by Composite Incorporating ZnFe₂O₄/Nanocellulose Photocatalysts" Sustainability 16, no. 24: 11023. https://doi.org/10.3390/su162411023

APA Style

Li, Z., Gao, K., Jiang, W., Xu, J., & Lushchyk, P. (2024). Sustainable Removal of Phenol Dye-Containing Wastewater by Composite Incorporating ZnFe₂O₄/Nanocellulose Photocatalysts. Sustainability, 16(24), 11023. https://doi.org/10.3390/su162411023

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Sustainable Removal of Phenol Dye-Containing Wastewater by Composite Incorporating ZnFe₂O₄/Nanocellulose Photocatalysts

Abstract

1. Introduction