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Original Article
2025
:18;
3452025
doi:
10.25259/AJC_345_2025

Preparation of trifunctional magnetic ZnO-based biochar for pollutions removal from wastewater

, ,
aSchool of Coal Engineering, Shanxi Datong University, Datong, Shanxi, 037009, China
bCollege of Biological and Chemical Engineering (College of Agricultural Sciences), Panzhihua University, Panzhihua 617000, China
cSchool of Surveying and Land Information Engineering, Henan Polytechnic University, Jiaozuo, 454003, China

*Corresponding author: Email address: lihypzhu@163.com (H. Li,)

Received: , Accepted: ,
© 2025 Arabian Journal of Chemistry - Published by Scientific Scholar
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This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-Share Alike 4.0 License, which allows others to remix, transform, and build upon the work non-commercially, as long as the author is credited and the new creations are licensed under the identical terms.

Abstract

The trifunctional magnetic Fe3O4/ZnO-based biochar (ZF-BC) was prepared for the removal of levofloxacin and Cr6+ from wastewater in single and binary contaminant systems. The physical and chemical properties of the ZF-BC were characterized and analyzed. The adsorption amounts of levofloxacin and Cr6+ for ZF-BC are 332.11 and 34.40 mg/g in a single contaminant system based on Hill model calculation, respectively. Levofloxacin slightly contributes to Cr6+, while levofloxacin adsorption on ZF-BC is restrained at high Cr6+ concentration in the binary contaminant system. The levofloxacin and Cr6+ adsorption mechanism were investigated and analyzed, combined with density functional theory calculations. ZF-BC also has excellent reusability and stability for levofloxacin and Cr6+ adsorption. Using the photocatalytic degradation system, 97.60% of levofloxacin and 79.43% of Cr6+ were removed. The photocatalytic degradation mechanism analysis indicates that levofloxacin is first adsorbed on ZF-BC and then used as the hole sacrificial agent to release more electrons/holes to participate in the Cr6+ redox reaction.

Keywords

Adsorption mechanism
Binary contaminant system
Cr6+
Levofloxacin

1. Introduction

With the development of science and technology, and increasing focus on medicine and healthcare, many factories and pharmaceutical companies are rapidly expanding [1,2]. This has resulted in the production of large amounts of wastewater, which generally contains many toxic pollutants like antibiotics [3,4]. Levofloxacin is a new type of antibiotic among fluoroquinolones used for treating humans and in aquaculture [5]. Levofloxacin in wastewater is a potential threat to natural organisms due to its properties of facile bioaccumulation and difficultly in natural degradation [6]. Meanwhile, Cr6+ is one of the most toxic heavy metals, usually a by-product of the electroplating and battery manufacturing industries [7,8]. Wastewater containing levofloxacin or Cr6+ directly discharged into rivers and lakes without treatment is harmful to the ecosystem [9].

Adsorption is a promising method for treating polluted wastewater, owing to its advantages of simple operation, high effectiveness, and wide range of application [10,11]. Until now, adsorbents such as graphene oxide, activated carbon, and molecular sieves have been used for wastewater treatment [12]. Graphene oxide can be used as filter media for the treatment of wastewater with levofloxacin, showing an adsorption amount of 257 mg/g [13]. Abushawish et al. (2022) used coconut shells as feedstock to prepare the nitrogen-doped activated carbon for Cr6+ removal with an adsorption capacity of 15.15 mg/g [14]. Although the above adsorbents have good adsorption performance, they are difficult to use in industries due to the expensive production cost. Therefore, it is necessary to develop easy-to-produce, low-cost, and highly efficient adsorbents for pollutant removal.

Biochar is a common adsorbent for wastewater treatment due to its low price [15]. Biochar is well recognized for its distinctive physical properties (e.g., surface area, surface charge, high porosity, and water holding capacity) and chemical characteristics [16]. Maged et al. (2021) prepared coffee bean waste biochar for levofloxacin removal with an adsorption amount of 110.7 mg/g [17]. Yao et al. (2021) used fruit-based biochar for levofloxacin treatment with an adsorption capacity of 115 mg/g [18]. However, the adsorption capacity of original biochar for pollutants is limited, which should be improved [19].

Multiple scholars reported various studies for improving the adsorption capacity of original biochar by modification [20-22]. Luo et al. (2024) prepared fungus chaff modified biochar with a levofloxacin adsorption amount of 199 mg/g, which is about 7.78 times larger than the original biochar (25.69 mg/g) [23]. Liang et al. (2019) reported that phoenix tree leaves modified by FeCl3 have good Cr6+ adsorption performance with an adsorption amount of 55.0 mg/g compared with original biochar [24]. Therefore, the modification method improves the adsorption capacity of the original biochar for levofloxacin and Cr6+ removal. ZnCl2 is a commonly used modification agent for improving the adsorption performance of original biochar [25]. Wu et al. (2022) compared the levofloxacin adsorption performance of pharmaceutical sludge biochar modified by KOH (PKBC800), ZnCl2 (PZBC800), and CO2 (PCBC800), indicating that the levofloxacin adsorption capacity of the PZBC800 was 123.40 mg/g, i.e., larger than that of PKBC800 and PCBC800 [26]. Ding et al. (2021) prepared a ZnCl2-modified biochar for Cr6+ removal, which exhibits excellent Cr6+ adsorption performance with an adsorption amount of 236.81 mg/g [27]. Therefore, biochar modified by ZnCl2 can be used to produce a highly efficient adsorbent for pollution removal from wastewater. ZnO is generated on biochar by ZnCl2 decomposition at the desired temperature [28]. However, researchers often use HCl to wash the impurities (e.g. ZnO) from the surface of biochar to improve its surface area, which ignores the photocatalytic properties of ZnO on biochar in previous studies [29-31].

In this work, the pine sawdust (PS), a typical waste biomass, was used to prepare the ZF-BC in one pot, using ZnCl2 and Fe(NO3)3 as modification agents for levofloxacin and Cr6+ removal. The ZnCl2 and Fe(NO3)3 formed the ZnO and Fe3O4 on the surface of biochar after heat treatment, which imparted ZF-BC multiple functions of adsorption, photocatalysis, and magnetic recovery [32]. The main work is: (1) to investigate the physicochemical properties of ZF-BC, (2) to analyze adsorption and photocatalytic performance of ZF-BC in the single and binary contaminant system, (3) to analyze synergy effect on levofloxacin and Cr6+ removal in single and binary contaminant systems, (4) to analyze involved adsorption and photocatalytic mechanisms in single and binary contaminant systems.

2. Materials and Methods

2.1. Material

PS was obtained from Jiangsu province, China. Zinc chloride (99.9%) was purchased from Tianjin Hongyan Reagent Co., Ltd. Ferric nitrate (99.99%) and citric acid (99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Levofloxacin hydrochloride (98%) was ordered from Aladdin Chemistry Reagent Co. Ltd.

2.2. Preparation process

PS was mixed with ZnCl2 (20 g) and Fe(NO3)3 (10 g) in 250 mL distilled water. Then, the mixture was dried, and a fixed mass of dried sample was placed into a microwave furnace at 800°C for 30 min in an N2 atmosphere. After heating, the residue in the microwave furnace is ZF-BC. The characterization of the sample has been detailed in Supplementary material.

2.3 Adsorption experiment

The adsorption experiment has been detailed in the Supplementary material. Adsorption kinetic and adsorption isotherm models are listed in the Table S1 and Table S2, respectively.

Table S1

Table S2

2.4. Photocatalytic experiments

For preparing the sample, 0.01 g ZF-BC was mixed with 0.2 L of levofloxacin solution at different concentrations. Then, 0.07 g ZF-BC was mixed with 0.2 L Cr6+ solution (80∼120 mg/L). To eliminate the adsorption effect of ZF-BC, the mixture solution was stirred in dark conditions. After this, the mercury lamp was switched on and the specimen was extracted from the mixture solution for a certain amount of time. The photocatalytic efficiency (α, %) of levofloxacin or Cr6+ was obtained using the following Eq. (1):

(1)
α = C 0 − C t C 0 × 100 %

3. Results and Discussion

3.1. Characterization of ZF-BC

3.1.1. X-ray diffraction (XRD) analysis

ZF-BC has several characteristic peaks of ZnO (Figure 1). The 2θ = 30.0°, 35.4°, 43.0°, 53.3°, 56.9°, and 62.5° correspond to the (220), (311), (400), (422), (511), and (440) of the Fe3O4 in the standard PDF card (PDF#19-0629). The 2θ = 31.77°, 34.42°, 36.25°, and 47.54° correspond to the (100), (002), (101), and (102) of ZnO in the standard PDF card (PDF#36-1451). The ZnO and Fe3O4 XRD spectra analysis were added, indicating that the ZnO and Fe3O4 characteristic peaks were consistent with the ZF-BC (Figure S1). These results indicate that ZnO and Fe3O4 were generated on ZF-BC by the decomposition of ZnCl2 and Fe(NO3)3 with high crystallinity. ZnO loading on the surface of ZF-BC imparts it with photocatalytic degradation property, and Fe3O4 gives ZF-BC magnetic separation capacity.

Figure S1
XRD pattern of the ZF-BC.
Figure 1.
XRD pattern of the ZF-BC.

3.1.2. Microstructure analysis

Figure 2(a-c) presents the scanning electron microscopy (SEM) micrographs and energy-dispersive X-ray (EDS) images of ZF-BC. ZF-BC shows flourishing pore structure, which effectively improves the surface area of ZF-BC, providing plenty of adsorption sites for levofloxacin/Cr6+ removal (Figure 2a). The ZnCl2 can be used as a chemical agent to produce the pores on ZF-BC during the preparation process. ZF-BC exhibits particulate matter (Figure 2b). EDS is employed to test the elemental composition of ZF-BC in the white square box from Figure 2. It can be seen from Figure 2(c) that ZF-BC has C, O, Zn, and Fe elements. The results show that ZnCl2 and Fe(NO3)3 can form metal compounds, which are loaded on the surface of biochar after heat treatment. The particulate matter includes ZnO and Fe3O4, which appear on ZF-BC on XRD analysis. .

(a-b) SEM micrographs and (c) EDS of ZF-BC, (a) 3000x, (b) 5000x.
Figure 2.
(a-b) SEM micrographs and (c) EDS of ZF-BC, (a) 3000x, (b) 5000x.

3.1.3. Pore structure analysis

Figure 3(a) shows the nitrogen adsorption isotherm of the ZF-BC. The N2 adsorption amount of ZF-BC significantly increases at P/P0 <0.1, which is consistent with a Type Ⅱ isotherm [33]. The surface area of ZF-BC is 1248.51 m2/g, contributing to pollution removal. This result indicates that ZF-BC shows a flourishing pore structure. The pore size distribution is an important parameter for adsorbents, which determines their application. ZF-BC is a kind of mesoporous material due to the main pore sharpest peak ranging from 0 nm to 5 nm (Figure 3b). The average pore diameter of the ZF-BC is 2.33 nm, which is enough for pollution adsorption [34].

(a) N2 adsorption isotherm and (b) pore size distribution of the ZF-BC.
Figure 3.
(a) N2 adsorption isotherm and (b) pore size distribution of the ZF-BC.

3.1.4. Fourier transform-infrared (FT-IR) analysis

The peak at 3436 cm-1 is ascribed to the -OH group (Figure 4). The C=C/C=O groups appeared at 1562 cm-1, and the C-O band was observed at 1130 cm-1 [35,36]. The characteristic peak of the aromatic structure of the -CH peak appeared at around 878 cm-1 [37]. Besides, the peak at 570 cm-1 was attributed to the metal-O group, indicating that the metal compounds exist [38]. The FT-IR spectra of the ZnO and Fe3O4 have been shown in Figure S2, which shows the Zn-O and Fe-O groups. These results demonstrate that ZF-BC has oxygen-containing groups, contributing to pollutant removal from wastewater.

Figure S2
FT-IR spectra of ZF-BC.
Figure 4.
FT-IR spectra of ZF-BC.

3.2. Adsorption of the levofloxacin/Cr6 + in single contaminant system

3.2.1. Effect of pH

The Cr6+ exists in several forms at different pH values. Cr6+ exists as HCrO4- and Cr2O72- at pH 3-7 [39]. It also exists as CrO42- at pH >6.8. Figure 5(a) shows that ZF-BC adsorbs a large amount of Cr6+ at pH=3-4. The reason is that the Cr6+ solution contains plenty of H+. ZF-BC has a positive charge, which could capture Cr6+ by electrostatic interaction [40]. The zero potential charge of ZF-BC is about 6.2 (Figure S3). The potential of the ZF-BC is negatively charged at pH > 6.2. The H+ concentration generally decreases as pH increases. Therefore, the Cr6+ adsorption amount generally decreases at pH =3-6. Levofloxacin can form different molecule forms at different pH values. Levofloxacin forms the cation (pH < 6.02), anion (6.02 < pH < 8.15), and molecule (pH > 9.7) [18]. Figure 5(b) indicates that the levofloxacin adsorption amount of the ZF-BC generally increases at pH=4-6. It can be explained that the electrostatic repulsion occurs between levofloxacin and ZF-BC at pH = 4 due to the existence of lots of H+ in the aqueous solution. The electrostatic repulsion generally becomes weak with increasing pH value. ZF-BC exhibits a high levofloxacin adsorption capacity at pH=6. Levofloxacin species is an anion at 6.02 < pH < 8.15. The electrostatic repulsion occurs at pH > 6.2 owing to the negatively charged potential of ZF-BC. Therefore, the levofloxacin adsorption amount decreases at pH > 6.2 due to electrostatic repulsion.

Figure S3
Influence of the pH on (a) Cr6+ and (b) levofloxacin adsorption on ZF-BC.
Figure 5.
Influence of the pH on (a) Cr6+ and (b) levofloxacin adsorption on ZF-BC.

3.2.2. Adsorption kinetics study

Pseudo-first order, pseudo-second order, and intraparticle diffusion models were employed for analyzing the levofloxacin and Cr6+ adsorption process [15,41]. Table S3 summarizes the analysis results. The pseudo-second order model has a high correlation coefficient (R2) value for fitting levofloxacin and Cr6+ adsorption data compared to other adsorption kinetics models (Table S3). Besides, the qe,cal value is close to the experimental qe value. Therefore, the levofloxacin and Cr6+ adsorption kinetic process can be better analyzed by the pseudo-second order model, demonstrating chemical adsorption of levofloxacin and Cr6+ (Figure 6) [42]. The average adsorption rate constants (k2) for levofloxacin and Cr6+ are 0.00025 and 0.0021, respectively (Table S3). The intraparticle diffusion model was employed to further investigate the rate-limiting step of levofloxacin and Cr6+ adsorption on ZF-BC. The C values calculated from the intraparticle diffusion model are not zero, proving that levofloxacin and Cr6+ adsorption on ZF-BC aren’t only controlled by intraparticle diffusion (Table S3) [43].

Table S3
(a) The levofloxacin and (b) Cr6+ adsorption data fitting the Pseudo-second order.
Figure 6.
(a) The levofloxacin and (b) Cr6+ adsorption data fitting the Pseudo-second order.

3.2.3. Adsorption isotherms study

The Langmuir, Freundlich, Hill, and Temkin equations were used to investigate the levofloxacin and Cr6+ adsorption process (Figure 7) [44,45]. The R2 values of levofloxacin and Cr6+ calculated using the Hill model were 0.9911 and 0.9959, respectively. These had a larger R2 value than other adsorption isotherm models (Table S4). The n value of levofloxacin obtained using the Hill model was 1.4720, proving that 1.4720 levofloxacin could be adsorbed by one adsorption site of the ZF-BC. While the n value of Cr6+ is 1.7348, showing that 1.7348 Cr6+ could be adsorbed by one adsorption site of ZF-BC [46]. Levofloxacin and Cr6+ adsorption on the ZF-BC calculated from the Hill model are 332.11 and 34.40 mg/g, respectively.

Table S4
(a) Levofloxacin and (b) Cr6+ adsorption data fitting the adsorption isotherm models.
Figure 7.
(a) Levofloxacin and (b) Cr6+ adsorption data fitting the adsorption isotherm models.

3.2.4. Reusability and stability of the ZF-BC

The recyclability of ZF-BC was analyzed and investigated. Figure S4 shows the levofloxacin and Cr6+ adsorption amount of ZF-BC after regeneration. The analysis results indicate that levofloxacin and Cr6+ adsorption amounts generally decreased in successive cycles. The reason is that some adsorption sites of ZF-BC lose binding capacity after regeneration. However, ZF-BC also has high levofloxacin and Cr6+ adsorption capability after regeneration. Therefore, ZF-BC could be used as a recyclable adsorbent for levofloxacin and Cr6+ removal from wastewater.

Figure S4

3.2.5. The influence of impurity ions

Given the ubiquitous presence of the SO42-, Cl-, Mg2+, Na+, and K+ in various water bodies, the influence of impurity ions on the levofloxacin and Cr6+ adsorption process was analyzed. Figure S5 shows the adsorption amounts of levofloxacin and Cr6+ on ZF-BC under impurity-free conditions as well as in the presence of the above impurity ions at different concentrations. As the concentration of impurity ions increases, the levofloxacin and Cr6+ adsorption amounts exhibit a slight decrease. This can be attributed to the competition between the two for adsorption sites on ZF-BC, which leads to a reduction in adsorption capacity [47].

Figure S5

3.3. Levofloxacin and Cr6 + adsorption in binary contaminant system

The levofloxacin and Cr6+ adsorption performance of the ZF-BC in the levofloxacin and Cr6+ binary system was analyzed. The Cr6+ adsorption capacity, presented in Figure S6(a), increases in the levofloxacin-Cr6+ binary system with different levofloxacin concentrations. The reason is that levofloxacin has nitrogen-containing functional amino and carboxyl groups [48]. These groups can bind with Cr6+ after levofloxacin adsorption on ZF-BC. Besides, the generated complexes between levofloxacin and Cr6+ can also contribute to Cr6+ adsorption through the bridging complexation between Cr6+ and ZF-BC. The levofloxacin adsorption capacity of the ZF-BC shows a rising tendency at low Cr6+ concentrations (Figure S6b). It can be explained that the adsorbed Cr6+ can provide the adsorption sites for levofloxacin adsorption. Cr6+ acts as a “bridge” between levofloxacin and ZF-BC, contributing to levofloxacin adsorption. When the Cr6+ concentration increases to 50 mg/L, the levofloxacin adsorption capacity of the ZF-BC decreases due to competitive adsorption. Therefore, Cr6+ competes with levofloxacin, resulting in low levofloxacin adsorption.

Figure S6

3.3.1. Density functional theory (DFT) analysis of levofloxacin and Cr6 + adsorption in binary contaminant system

DFT calculation is used to analyze the levofloxacin and Cr6+ adsorption process in a binary contaminant system. ZF-BC has -OH, -COOH, -Zn-O, and -Fe-O groups. Therefore, simpler biochar-like molecules with -OH, -COOH, -Zn-O, and -Fe-O groups could act as a ZF-BC model. The adsorption energy of levofloxacin and Cr6+ was -711.47-682.48 kcal/mol. This result demonstrates that the levofloxacin adsorption Cr6+ process is feasible and spontaneous, owing to the abundant surface functional groups of levofloxacin (Figure S7). It also proves that levofloxacin/Cr6+ can form a complex in a binary contaminant. The optimized structure of “d” has low adsorption energy compared with other optimized structures (Figure S7). Therefore, the optimized structure of “d” of the levofloxacin and Cr6+ was used as the candidate for further analysis. The ZF-BC adsorption of the levofloxacin/Cr6+ complex has four kinds of adsorption optimized structures Figure 8(a-d). The adsorption energy of ZF-BC for levofloxacin and Cr6+ was low (Table S5). This result demonstrates that the complexation of levofloxacin/Cr6+ was easily adsorbed.

Figure S7

Table S5
(a-d) Four kinds of the adsorption optimized structure of the ZF-BC adsorption complexes with levofloxacin/Cr6+.
Figure 8.
(a-d) Four kinds of the adsorption optimized structure of the ZF-BC adsorption complexes with levofloxacin/Cr6+.

3.4. Adsorption mechanism

Pseudo-second-order equations could analyze the levofloxacin and Cr6+ adsorption process, which shows that chemisorption controls the adsorption of levofloxacin and Cr6+ onto ZF-BC [49]. Some new peaks were detected after levofloxacin and Cr6+ adsorption (Figure 9). The -CH bending vibration appeared at 878 cm-1 [50]. However, it moved to 801 cm-1 after levofloxacin adsorption. This result proves that π-π interactions contributes to levofloxacin adsorption. The stretching vibration of O-H at 3436 cm-1 migrated to 3378 and 3374 cm-1 after levofloxacin and Cr6+ adsorption, respectively. It can be explained by the formation of hydrogen bonding between ZF-BC and levofloxacin/Cr6+ in the adsorption process [51]. The metal-O at 570 cm-1 migrated to 551 cm-1 and 440 cm-1 after levofloxacin and Cr6+ adsorption, respectively. This result shows that metal oxides of Zn or Fe may be involved in the adsorption process of levofloxacin and Cr6+ by surface complexation [52]. The intensities and positions of C-O (1130 cm-1) and C=O/C=C (1564 cm-1) changed after levofloxacin and Cr6+ adsorption. These results demonstrate that C-O and C=O/C=C groups involve levofloxacin and Cr6+ by surface complexation [53].

FT-IR spectra of the ZF-BC before and after levofloxacin/Cr6+ adsorption.
Figure 9.
FT-IR spectra of the ZF-BC before and after levofloxacin/Cr6+ adsorption.

ZF-BC before and after adsorption levofloxacin/Cr6+ were analyzed by X-ray photoelectron spectroscopy (XPS) spectra to investigate the adsorption mechanism. Figure 10(a-b) shows the C1s spectra analysis of ZF-BC before and after levofloxacin/Cr6+ adsorption. The C-O peak area decreases from 29.11% to 24.77% after levofloxacin adsorption. The peak area of C=O decreases by 2.78% after levofloxacin adsorption. The binding energy of C-O/C=O groups changed after levofloxacin adsorption. These observations prove that C-O/C=O groups participate in levofloxacin adsorption [54]. The C-O peak area decreased from 29.11% to 21.62% after Cr6+ adsorption (Figure 10c), demonstrating that the C-O group is involved Cr6+ adsorption. However, the peak area of C=O increased from 13.37% to 14.98%. The C-O group acted as an electron-donating group and was then oxidized into the C=O group during the Cr6+ adsorption process [55]. Therefore, Cr6+ could be reduced to Cr3+. Cr2p spectrum was divided into four peaks (Figure 10d). Peaks at 577.55 and 586.26 eV corresponded to Cr3+, and peaks at 579.49 and 587.89 eV corresponded to Cr6+. The existence of Cr3+ indicated that Cr6+ removal includes adsorption and reduction [56]. Cr3+ can be adsorbed on ZF-BC by complexation [57].

The detail survey of the (a) C1s before and (b) after adsorption levofloxacin, (c) Cr6+, (d) detail survey of the Cr2p after adsorption Cr6+.
Figure 10.
The detail survey of the (a) C1s before and (b) after adsorption levofloxacin, (c) Cr6+, (d) detail survey of the Cr2p after adsorption Cr6+.

3.5. Photocatalytic activity

3.5.1. Removal levofloxacin and Cr6 + in the single system

The ZF-BC was used as the photocatalyst to investigate the photocatalytic degradation for levofloxacin and Cr6+ at different concentrations. The Levofloxacin and Cr6+ solution was mixed with the ZF-BC in dark conditions for eliminating the adsorption effect of ZF-BC. Figure 11 show the adsorption and photocatalytic process of levofloxacin and Cr6+. As Figure 11 shown, the levofloxacin and Cr6+ removal are 30.50-44.82% and 9.22-20.35% at 15-25 mg/L of levofloxacin and 80-120 mg/L of Cr6+, respectively. This result can be explained by the flourishing pore structure of ZF-BC.

(a) Levofloxacinand (b) Cr6+ adsorption and photodegradation process of ZF-BC.
Figure 11.
(a) Levofloxacinand (b) Cr6+ adsorption and photodegradation process of ZF-BC.

Levofloxacin removal was 90.39-97.60% at 15-25 mg/L. This result is attributed to the existence of the ZnO on ZF-BC, which is photoexcited under UV light. Subsequently, the holes and electrons are separated, and h+, e-, •OH and •O2- are produced [58]. Then, active species attack levofloxacin, and it is eventually degraded into CO2 and H2O. The adsorption and photocatalytic processes of ZF-BC can remove almost 100% of the levofloxacin. Therefore, ZF-BC is a promising adsorbent-photocatalyst for levofloxacin wastewater treatment. While, the Cr6+ removal is only 12.83-23.67% at 80-120 mg/L within 80 min under UV irradiation (Figure 11b). This result indicates that the Cr6+ isn’t removed from wastewater under UV irradiation. The reason might be that there are no available the e- in the solution. Therefore, the Cr6+ can’t be reduced into Cr3+ by the e- [59]. Improving the Cr6+ photocatalytic degradation efficiency of the ZF-BC is a pressing issue.

3.5.2. Removal levofloxacin/Cr6 + in the in binary contaminant system

If the levofloxacin is added into the Cr6+ solution, can it improve the Cr6+ photocatalytic degradation? The influence of levofloxacin on the photocatalytic degradation of Cr6+ has been shown in Figure 12. As Figure 12 shows, the Cr6+ removal generally increased in the presentence of levofloxacin under UV irradiation. Besides, the first-order kinetic constants (k) of the Cr6+ degradation also prove that ZF-BC has a high k value (0.0099 min-1) in the binary contaminant system, which is approximately 18.33 times higher than that of the single system (0.00054 min-1). Besides, levofloxacin removal also increases as irradiation time increases in the binary contaminant system. However, the levofloxacin removal deceased in the binary contaminant system compared to the single system. The k value in the binary contaminant system (0.0081 min-1) was lower than that of the single system (0.0094 min-1). The above analysis demonstrates that the existence of levofloxacin in a binary contaminant system contributes to Cr6+. Therefore, the synergy is achieved in the removal process of Cr6+ and levofloxacin. The levofloxacin photocatalytic degradation and Cr6+ reduction degradation performances of various photocatalysts have been listed in the Table S6. The analysis result proves that the ZF-BC has promising potential in levofloxacin and Cr6+ wastewater treatment.

Table S6
The levofloxacin and Cr6+ removal in the in binary contaminant system.
Figure 12.
The levofloxacin and Cr6+ removal in the in binary contaminant system.

3.5.3. Photocatalytic mechanism analysis

ZnO on ZF-BC can be photoexcited under UV light irradiation. Subsequently, plenty of holes and electrons are generated [60]. The adsorbed oxygen molecules on ZF-BC can react with the electrons (e-) to generate •O2- [61]. While, the adsorbed H2O reacts with h+ to form the h+. Firstly, the levofloxacin is adsorbed on ZF-BC in the dark adsorption stage. The adsorbed levofloxacin accumulates on ZnO. Then, the photodegradation reactive species conduct the photocatalytic process. The generated photodegradation reactive species, such as H+, •OH, and •O2-, attack levofloxacin [62]. Finally, levofloxacin generates a series of intermediates, which decompose into CO2 and H2O [63].

Zhang et al. (2022) pointed out that the photoinduced electrons (e-) contribute to the photocatalytic reduction of Cr6+ [64]. Undoubtedly, the photocatalytic reduction of Cr6+ is restrained in an O2 atmosphere. The O2 reacts with the e- and generates •O2-. The NaBrO3 is used as the e- scavenger, which is added into the binary contaminant system. As shown in Figure 13(a), Cr6+ removal generally increases, indicating the important role of e- in Cr6+ removal process. However, the Cr6+ removal is also a process of capturing e-. Therefore, the Cr6+ removal cannot be totally restrained by the addition of the O2 and NaBrO3. Interestingly, when the Triethanolamine (TEOA) and methyl alcohol are added into the Cr6+ single system, the Cr6+ removal increases compared to the blank experiment (Figure 13b). The TEOA and methyl alcohol can be used as the H+ scavenger or electron donors in the photocatalytic process [65]. The TEOA and methyl alcohol can consume a mass of H+. Therefore, the photocatalytic system will provide a good deal of available e-, which can involve Cr6+ removal. However, the addition of TEOA has significantly contributed to the Cr6+ removal compared to methyl alcohol. It can be explained that TEOA has good H+ removal efficiency [55]. The plausible hypothesis is that levofloxacin can also be used as the electron donor or H+ scavenger to generate the available e-, involving Cr6+ removal.

(a) The quenching experiments of ZF-BC on the reduction of Cr6+ in the coexistence system, and (b) the effect of TEOA and MeOH on the reduction of Cr6+ in the single system.
Figure 13.
(a) The quenching experiments of ZF-BC on the reduction of Cr6+ in the coexistence system, and (b) the effect of TEOA and MeOH on the reduction of Cr6+ in the single system.

4. Conclusions

ZF-BC was successfully prepared for levofloxacin and Cr6+ removal from wastewater. Levofloxacin and Cr6+ adsorption amounts are 332.11 and 34.40 mg/g, respectively. The existence of levofloxacin slightly improves Cr6+ removal, while the levofloxacin adsorption process is restrained at high Cr6+ concentration in the binary contaminant system. The possible adsorption mechanism of levofloxacin and Cr6+ was analyzed. ZF-BC demonstrated excellent reusability and stability for levofloxacin and Cr6+ adsorption. The levofloxacin removal achieves 97.60%, and Cr6+ removal reaches 79.43% in levofloxacin under UV irradiation. The photocatalytic degradation mechanism analysis demonstrates that the existence of levofloxacin contributes to the separation of photo-generated carriers on ZF-BC and substantially enhances the synergistic reduction efficiency of Cr6+.

Acknowledgment

The authors would like to express their gratitude to the Specialized Research Fund for the Fundamental Research Program of Shanxi Province (202403021211071) and Ph.D. Research Start-up Funding Project of Panzhihua University (bkqj2021009) for financial support.

CRediT authorship contribution statement:

Chao Lv: Writing, Revision, Investigation. Haoyu Li: Methodology, Investigation, Supervision. Xiaojing Qin: Writing-review & editing, Methodology.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Data will be made available on request.

Declaration of Generative AI and AI-assisted technologies in the writing process

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Supplementary material to this article can be found online at https://dx.doi.org/10.25259/AJC_345_2025.

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