Synthesis of Fe-doped ZnO/biochar nanocomposites for acid orange 7 photodegradation and antibacterial activity under visible light region

In the present study, different molar ratios of Fe-doped ZnO/biochar (FZBC) were synthesized using a coprecipitation method. ZnO nanoparticles with a size of approximately 30 nm were well dispersed on the biochar matrix. The bandgap energy decreases from 3.11 to 3.08 eV as the Fe-ion concentration increases from 1 to 7 mol%. Photocatalytic studies demonstrated that FZBC exhibited photodegradation toward acid orange 7 (AO7). FZBC exhibited a photodegradation efficiency of AO7 at a concentration of 10 mg. L−1 (93.17%), which was nine times greater than that of pure ZnO (10.30%). Furthermore, the FZBC materials also demonstrated effective inactivation of E. coli, with an antibacterial rate reaching 98%. Overall, these materials are expected to be cost-effective and suitable for photocatalytic degradation and antibacterial activities.


Introduction
In wastewater, various bacterial species, including pathogenic bacteria, are often present and need to be monitored and controlled during wastewater treatment [1].In most cases, wastewater treatment criteria focus on coliform organisms, including Escherichia coli [2].Escherichia coli is a pathogenic microorganism that can cause waterborne diseases such as diarrhea, cystitis, and pyelonephritis [3].Its presence in the aquatic environment can disrupt the ecological balance and pose a threat to human health even at low concentrations [4].Antibacterial agents are essential tools for treating bacterial infections.However, the misuse of antibiotics has led to the emergence of antibiotic-resistant bacterial strains.A 2015 report predicted that antibiotic-resistant bacterial infections will cause 10 million deaths annually by 2050 [5].Therefore, continued research to develop new antibacterial agents through cost-effective and environmentally friendly nanotechnologies is of paramount importance.
In recent decades, environmental pollution caused by organic compounds, such as dyes, has gained widespread attention worldwide [6].Acid orange 7 (C 16 H 11 N 2 NaO 4 S), also known as Orange II, is an anionic azo dye that is extensively used in industries such as textiles, leather, cosmetics, paper, agriculture, and food [7].The presence of dyes in wastewater, even at very low concentrations, is highly noticeable and undesirable [8].Furthermore, due to the complexity and diversity of dyes in wastewater, it is challenging to find a single treatment method that can completely remove all types of dyes [9].Therefore, the removal of these colored compounds from industrial wastewater is highly important.
As mentioned above, the world is currently facing a shortage of clean water sources due to growing industry and the release of organic compounds directly into the environment.Organic dyes and bacteria are the main agents of water pollution and cannot be degraded or inactivated under common environmental conditions [10,11].Therefore, many methods have been applied, such as chemical oxidation, adsorption, and photocatalysis.Among these methods, photocatalysis is a powerful technique for environmental detoxification and has emerged as a promising solution due to its cost-effectiveness and environmental friendliness [12,13].Various metal oxides, such as TiO 2 [14], ZnO [15], CuO [16], WO 3 [17], Fe 2 O 3 [18], and SrTiO 3 [19], are known for their ability to efficiently photocatalyze organic compounds and effectively inactivate bacteria via photocatalytic chemical reactions.Zinc oxide (ZnO), a group II-VI semiconductor material, is often considered one of the most promising photocatalytic agents due to its nontoxic nature and physical stability [20].It exhibits excellent antibacterial activity owing to its hydrophilicity and high oxidative capacity [15,20].However, there are some limitations to photocatalysis associated with this semiconductor, such as low quantum efficiency, photocorrosion, and limited light absorption due to rapid recombination of photogenerated charge carriers [15].To overcome these limitations, researchers have explored alternative methods to enhance carrier mobility.There are various ways to modify the optical and other properties of metal oxide structures [21].The introduction of a dopant into the structure is one such method [22].The doping of transition metal ions into the ZnO crystal lattice is a promising technique for enhancing photocatalytic activity by introducing new energy levels within the bandgap, primarily improving the electron-hole pair separation efficiency by forming electron traps [23].Among the transition metals, iron (Fe) is preferred as a dopant because its d-orbitals easily overlap with the valence band of ZnO [24].When Fe is doped onto ZnO nanostructures, improved antibacterial and magnetic properties have been observed [25].Doping with Fe atoms can also reduce the particle size and increase the number of active catalytic sites, which is highly beneficial for adsorption in photocatalytic reactions [26].Furthermore, the absorption spectrum redshifts in the wavelength range of 320-800 nm due to Fe doping, which results in strong optical absorption in the visible light region [27], and chemical precipitation arrays of Fedoped ZnO show quenching or absorbing properties, indicating their antibiotic properties [28].Additionally, Fe-based nanomaterials were effective for antibacterial effects, and their degradation was found to be faster.Furthermore, the ionic radius of Fe 3+ (0.06 nm) is smaller than that of Zn 2+ (0.074 nm), creating a charge imbalance and thus enhancing photocatalytic and antibacterial activity [29].It is clear from these reported results that the influence of Fe atoms on the ZnO defect luminescence band in nanostructures strongly depends on the specific defect structure of the sample and on the growth process used [21].
Recently, to overcome the limitations of ZnO, ZnO nanostructures have been incorporated into various carbonaceous substrates, such as graphene, graphene oxide, and carbon nanotubes [30].However, these substrates are expensive and involve complex synthesis procedures [30].Therefore, there is a need to develop cost-effective and environmentally friendly materials that can replace these expensive carbon materials for synthesizing carbon-based photocatalytic materials.In recent years, biochar (BC) has garnered significant attention as a carbonaceous support to enhance the photocatalytic performance of metal oxide composites [31].BC is a stable, cost-effective product containing carbon obtained through the pyrolysis of biomass materials under oxygen-free conditions [32,33].BC serves as an excellent carrier or support material for synthesizing metal oxide composite materials due to its high porosity, large surface area, and presence of functional groups on its surface, providing excellent electron conductivity support for photogenerated electron transfer [33,34].Phragmites australis, commonly known as common reed, grows predominantly around lakes, rivers, and streams [35].In Vietnam, Phragmites australis grows widely year-round in rural areas, especially in wetland regions.This plant is primarily composed of high-quality biomass materials, including cellulose, hemicellulose, and lignin, making it a potential source for synthesizing BC [35].Chen M used ZnO/biochar nanocomposites derived from jute fibers for efficient and stable photodegradation of methylene blue dye [36].Alves et al reported that ZnO/biochar composites exhibited more effective antibacterial activity than did pure ZnO [37].Many studies have shown that the physicochemical properties of synthesized biochar depend largely on the type of raw material and the operating parameters (heating rate, temperature, atmospheric conditions, retention time, etc.) used to form biochar [38], thereby affecting the quality of the resulting photocatalytic composite.Therefore, research into biochar-based composites produced from specific raw biomass sources is necessary.To date, studies related to the synthesis of Fe-doped ZnO/Phragmites australis stem-derived biochars for the application of both photocatalytic and antibacterial agents have been limited.This study represents the first use of biochar derived from Phragmites australis stems as an environmentally friendly carbon support material for the production of Fe-doped ZnO/biochar composite materials with photocatalytic degradation and antibacterial properties.This material exhibits significant potential as a photocatalyst in environmental treatment applications.

Synthesis of Fe-doped ZnO/biochar
A total of 3.0 grams of raw P. australis biomass was placed in a porcelain boat and heated under nitrogen gas saturation conditions at 400 °C for 2 h (heating rate: 3 °C min −1 ).The product obtained after heating was first washed with 1.0 M HCl solution and then rinsed several times with distilled water until the filtrate became neutral.The washed product was dried at 105 °C to a constant weight and then stored in a moisture-proof container, which served as the raw biochar material (abbreviated as BC).
In the next step, 7.437 grams of Zn(NO 3 ) 2 •6H 2 O were dissolved in 80 ml of distilled water and stirred for 10 min.Subsequently, Fe(NO 3 ) 3 •9H 2 O with Fe/Zn molar ratios of 1, 3, 5, and 7% was added to the above solutions.In each case, 0.50 grams of BC was added to the respective salt mixed solution and stirred for 12 h.Then, 50 ml of 1.0 M NaOH solution was added dropwise to each beaker, and the mixture was hydrolyzed at room temperature for 4 h and allowed to settle for 1 h.The mixture was then filtered and rinsed several times with distilled water until the filtrate became neutral.The washed product was dried at 105 °C for 12 h, ground into fine particles and heated under nitrogen gas saturation conditions at 450 °C (heating rate: 3 °C min −1 ) for 2 h (at this heating temperature, the synthesized composite materials had the highest photodegradation efficiency of AO7), resulting in the Fe-doped ZnO/biochar composite products, denoted FZBC1 to FZBC7, respectively.For comparison, ZnO/biochar (ZBC) and pure ZnO samples were also synthesized under the same conditions as described above in the absence of Fe(NO 3 ) 3 and both Fe(NO 3 ) 3 and biochar, respectively.

Characterization of materials
X-ray diffraction (XRD) patterns were measured using a Bruker D8 Advance X-ray diffractometer (Bruker, Germany).The scanning angle (2θ) ranged from 10 to 80°with a step size of 0.03°.Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images were acquired using a JEOL JEM-1010 transmission electron microscope (USA) and an FEI Nova NanoSEM 450 scanning electron microscope (USA), respectively.Elemental analyses were conducted through EDX mapping spectroscopy using a TEAM Apollo XL EDS system (USA).Fourier transform infrared (FT-IR) spectra of the samples were recorded with an IR Affinity-1S spectrophotometer (Shimadzu).UV-Vis-DRS (ultraviolet-visible diffuse reflectance spectroscopy) spectra were obtained using a UV-2600 instrument from Shimadzu.Nitrogen adsorption/desorption isotherms (BETs) were determined using a Quantachrome TriStar 3000 V6.07A adsorption instrument.Photoluminescence (PL) spectra were obtained with a Horiba Fluorolog 3 FL3-22.

Photocatalytic degradation of AO7
In this study, a beaker containing 200 ml of 10 mg.L −1 AO7 solution suspension and 0.1 grams of the photocatalyst were placed under a lamp holder.The light source used was a Compact 50 W-220 V lamp (Dien Quang) equipped with a wavelength cutoff filter (λ 420 nm, d = 77 mm).Prior to illumination, the solution was stirred in darkness using a magnetic stirrer for 60 min to ensure adsorption equilibrium.After a certain interval, 5.0 ml of the solution was withdrawn and centrifuged to remove the solid photocatalyst.The concentration of AO7 in the supernatant was analyzed by UV-Vis spectrophotometry (Spectro UV-2650, Labomed, USA).The adsorption efficiency of AO7 (A%) was calculated using the following equation [8]: is the initial concentration of AO7, and C 0e (mg.L −1 ) is the concentration of AO7 at sorption equilibrium time t (min).
The photodegradation efficiency (D%) of the photocatalyst was calculated using expression (2) [40,41]: e t e 0 0 = ẃhere C 0e (mg.L −1 ) and C t (mg.L −1 ) are the AO7 concentrations at the sorption equilibrium time and at an irradiation time of t (min), respectively.According to the Langmuir-Hinshelwood kinetics model, the photocatalytic degradation of AO7 dye can be expressed by the following equation [42]: where k is the pseudo-first-order rate constant, C 0e (mg.L −1 ) are the AO7 concentrations at sorption equilibrium.

Antibacterial activity assessment
The disk diffusion method was employed to assess the antibacterial activity of pure ZnO, ZBC, FZBC1, FZBC3, FZBC5, FZBC7, and pristine EMB (as a control sample).In a typical procedure, 5.0 grams of beef extract and 10.0 grams of peptone were added to a 1000 ml container of distilled water.Subsequently, 100 μl of E. coli broth was added to the solution.This mixture was placed in an incubator at 37 °C for 24 h.The E. coli broth was diluted to a concentration of 106 CFU mL −1 , after which 0.05 g of the photocatalyst material was dispersed into 200 ml of this solution.The mixture was stirred for 2 h at 150 rpm under irradiation with a Compact 50 W-220 V lamp.After solidification of the EMB agar, 10 μl of the photocatalytic degradation solution was evenly inoculated onto a Petri dish and placed in a specialized bacterial incubator at 37 °C for 24 h.The number of E. coli colonies in the Petri dish was counted using a colony counter.The antibacterial rate (η%) was determined using the following formula [43]: Y represents the number of E. coli colonies grown on EMB agar and X is the number of E. coli colonies grown on agar containing the photocatalyst material.

Characterization of photocatalysts
The crystalline structures of the ZnO, BC, ZBC, and FZBCx (x = 1, 3, 5, and 7) samples were revealed through X-ray diffraction (XRD) (figure 1(a)).The ZnO sample exhibited characteristic diffraction peaks at 2θ = 31.7°,34.4°, 36.2°,47.5°, 56.6°, 62.8°, 66.4°, 67.9°, 69.1°, 72.6°and 76.9°, corresponding to the (100), (002), ( 101), (102), ( 110), ( 103), ( 200), ( 112), ( 201), (004), and (202) crystal planes of the hexagonal wurtzite-phase ZnO structure, respectively (JCPDS card no.01-075-9743) [20,36].The broad peak of biochar at 2θ ∼ 23°was attributed to the characteristic diffraction peak of amorphous carbon [44].In the ZBC sample and the FZBC samples, characteristic peaks of pure ZnO were also observed, indicating the formation of composites on the biochar substrate.However, no characteristic peak of amorphous carbon was observed in the composite samples.This might be due to the strong overlap of the diffraction peaks of the ZnO hexagonal wurtzite crystal on the characteristic peak of amorphous carbon.Moreover, the peaks of various types of iron oxides or Znferrite were not detected in the diffraction pattern of the FZBCx samples, possibly because the amount of Fe impurities was very small compared to the amount of ZnO.Therefore, this small amount of Fe impurity is mostly embedded in the ZnO lattice and cannot be observed because it falls below the detection limit of the instrument.However, figure 1(b) shows that loading ZnO nano particles onto BC has slightly shifted the 2θ reflection angle of the ZBC sample towards lower values compared to the pure ZnO sample, while the FZBC samples have slightly higher 2θ values compared to the ZBC sample (table 1), indicating that the presence of Fe impurities tends to slightly increase the 2θ angle values of ZnO in the FZBC composite.The average crystal sizes of the ZnO, ZBC, FZBC1, FZBC3, FZBC5, and FZBC7 samples were calculated using the Scherrer equation for the (101) reflection, yielding values of 37.0 nm, 28.0 nm, 29.0 nm, 31.0 nm, 31.7 nm, and 33.2 nm, respectively (table 1).The variation in particle size indicates that biochar strongly inhibited the crystalline growth of ZnO, while the Fe impurities tended to slightly promote the growth of ZnO crystallites in the ZBC composite within the range of Fe/Zn molar ratios from 1% to 7%.
FTIR was employed to identify the organic and inorganic functional groups present in the samples.Figure 1(c) shows the FTIR spectra of the pure ZnO, BC, ZBC, and FZBCx (x = 1, 3, 5, and 7) samples within the wavenumber range of 400-4000 cm −1 .For all the samples, two broad peaks at approximately 3436 and 1635 cm −1 were observed, attributed to the O−H stretching of hydroxyl groups and O−H bending of H 2 O, respectively [37].The peak at 2350 cm −1 is related to C−O stretching in carbon dioxide molecules, which are adsorbed on the sample surface [8,45].For the ZnO samples, the peaks at 445 cm −1 and 492 cm −1 are associated with the stretching vibrations of oxygen-metal-oxygen (O−Zn−O) and metal-oxygen (Zn−O) bonds (figure 1(b)) [46], respectively.In the case of biochar, the peaks at 2924 cm −1 and 1619 cm −1 are attributed to the asymmetric stretching of C−H and the sp 2 character of C=C, respectively [47,48].The peak at 1462 cm −1 can be assigned to the carboxylate group (−COO − ) stretching vibration [49].Peaks in the range between 900 cm −1 and 1300 cm −1 may be related to single C−O bonds, such as those in ethers, and C−OH groups [48].The peak at 1385 cm −1 is attributed to the vibrational modes of O−H, C−C bending, or C−H bending [50].In the ZBC composite samples, characteristic peaks of both the ZnO and BC components were observed but with lower intensities; in particular, the vibrational modes of Fe-O and Fe-OH could not be identified in these spectral ranges.This result demonstrates the good substitution ability of Fe 2+ and Fe 3+ ions in the matrix of ZnO [51] and shows the successful formation of ZnO/BC heterojunctions in the ZBC composites [52].
The optical properties of the ZnO, BC, ZBC, and FZBCx (x = 1, 3, 5, and 7) samples were assessed via UV-Vis absorption spectroscopy within the range of 200 to 800 nm (figure 2).BC was nearly transparent in the wavelength range longer than 300 nm [53].Pure ZnO exhibited an optical absorption edge at a wavelength of approximately 387 nm, indicating that its optical absorption ability is primarily in the ultraviolet region [33].The ZBC sample exhibited an optical absorption edge shifting to approximately 398 nm.Compared to that of pure ZnO, a significant, observable increase in absorption was observed in the visible light range (from 400 to 600 nm) for all FZBCx samples.This suggests that Fe 3+ ions were successfully doped into the ZnO crystal lattice, possibly creating more active defect sites [28,54].Moreover, the increased visible light absorption through these defect sites might have led to the enhanced photocatalytic activity of the ZnO crystallites doped in the visible light region.The redshift of the Fe 3+ impurity is due to the sp-d exchange interaction between electrons in the   [26].However, this shift might also be due to the Burstein-Moss effect, which creates additional energy levels for Fe 3+ /Fe 2+ below the conduction band of ZnO as the visible light absorption capability increases with increasing amounts of the Fe impurity component [28,51].To calculate the band gap values, the Tauc plot relation was used [51]: where α is the absorption coefficient, h is the Planck constant, v is the wavenumber, A is a constant and E bg is the energy band gap.The forbidden energy gap value from the absorption data was calculated by plotting (αhν) 2 against the photon energy E bg = hν (figure 2(b)) [51].The calculated E bg values for the ZnO, ZBC, FZBC1, FZBC3, FZBC5, and FZBC7 samples were 3.20 eV, 3.11 eV, 3.15 eV, 3.10 eV, 3.09 eV, and 3.08 eV, respectively (figure 2(b) and table 1).These results indicate that Fe doping into the ZnO lattice and its dispersion on the BC support may help narrow the band gap of ZnO in FZBC composites.A decrease in the band gap width, along with an increase in the Fe impurity content, has also been observed in previous research [26,51,54,55].
According to the results of testing the photocatalytic activity of the ZnO, ZBC, and composite samples FZBC1, FZBC3, FZBC5 and FZBC7 (content in section 3.2), we found that the FZBC3 composite achieved the greatest photodegradation performance of AO7 compared to the above samples.Therefore, we focused on analyzing the morphology, elemental composition, photoluminescence and textural properties of sample FZBC3, as presented below.
The morphology and microstructure of the ZnO, BC, ZBC, and FZBC3 samples were observed using SEM and TEM.The ZnO sample (SEMfigure 3(a) and TEM -figure 3(b)) appeared as aggregated spherical particles with an average diameter ranging from approximately 30 nm to 40 nm. Figure 3(c) shows an SEM image of BC in the form of smooth sheets.The ZBC composite sample (SEMfigure 3(d) and TEM -figure 3(e)) comprised both spherical ZnO particles and sheet-like BC, with the ZnO particle size in the ZBC composite ranging from approximately 20 nm to 30 nm, which is smaller than that of pure ZnO.This suggests the inhibitory role of BC in the crystalline growth of ZnO.The FZBC3 sample exhibited large spherical particles (with diameters ranging from 30 to 35 nm) composed of individual smaller nanosized particles that coalesced together and appeared to be successfully attached to the BC surface (SEM-figure 3(f) and TEM-figure 3(g)).The high-resolution TEM (HRTEM) image (figure 3(h)) displayed lattice fringes with a spacing of d (002) (0.26 nm) characteristic of wurtzite ZnO [56].
EDX-elemental mapping of a selected area on the FZBC3 sample is presented in figure 4. The element Fe was evenly distributed on the surface of BC in the FZBC3 composite (figure 4(d)).In contrast, Zn, O, and C were densely distributed within the FZBC3 network (figures 4(e)-(g)).
The presence of Fe, Zn, O, and C in FZBC3, as well as their integrated images (figure 3(c)), confirms the distribution of these elements in FZBC3.EDX analysis further indicated the presence of Fe, Zn, O, and C in FZBC3 (as depicted in the inset of figure 4(b)).These results demonstrate that Fe-doped ZnO nanostructures were successfully dispersed on BC, indicating that the synthesized FZBC3 composite was free of impurities.
The PL was recorded at room temperature with an excitation wavelength of 368 nm. Figure 5(a) shows that all three samples, namely, ZnO, ZBC and FZBC3, exhibit obvious PL signals with similar curve shapes.The peak   at approximately 409 nm (3.03 eV) corresponds to the transition from the conduction band (CB) to deep holes trapped above the valence band (VB) [57], and the peak at 434 nm (∼2.85 eV) is attributed to the electronic transition between the excitonic level and interstitial oxygen (O i ) [58].The peak at 460 nm (∼2.70 eV) corresponds to the direct recombination of a conduction electron in the CB and a hole in the VB [59].The blue emission peak at 460 nm (2.69 eV) may occur due to the donor-level Zn i to acceptor-level zinc vacancy (V Zn ) [60].The green emission peak in the range of 500-600 nm (2.48-2.06eV) is attributed to defects related to oxygen vacancies because of the recombination of photogenerated holes and electrons trapped by singly ionized oxygen vacancies (Vo) [61].When Fe 3+ ions were incorporated in ZnO and loaded onto a biochar substrate, the fluorescence intensity decreased significantly (figure 5(a)), leading to a decrease in charge carrier recombination, which indicates high electron-hole separation and increased photocatalyst activity.
The porous properties of the ZnO, BC, ZBC, and FZBC3 samples were studied using nitrogen adsorption/ desorption isotherms (figure 5(b)).According to the IUPAC classification, the BC sample exhibited type II isotherms with an H3 hysteresis loop, indicating narrow slit-shaped pores [62].H3 hysteresis is typically observed for nonrigid materials composed of plate-like particles that create slit-shaped pores [63].The surface characteristics of BC included a relatively high specific surface area and a well-defined pore structure, with a specific surface area of 22.80 m 2 /g and an average pore diameter of 12.4 nm (table 2, figure 5(c)).The structural characteristics of BC can provide a good platform for the loading and dispersion of ZnO, and the porous nature of BC is favorable for the formation of active adsorption sites [62].The ZnO, ZBC, and FZBC3 materials exhibited type IV isotherms with an H4 hysteresis loop, confirming the presence of a mesoporous structure in the prepared materials [64,65] (see figure 5(c)).The specific surface area, pore volume, and average pore diameter of the ZnO, BC, ZBC, and FZBC3 samples are listed in table 2. The specific surface areas of ZnO, BC, ZBC, and FZBC3 were 5.5 m 2 /g, 22.8 m 2 .g−1 , 10.3 m 2 .g−1 , and 13.4 m 2 .g−1 , respectively (table 2).The biochar matrix significantly enhanced the specific surface area of ZnO in the ZBC and FZBC3 composites.A larger surface area can provide more active sites.Therefore, the larger BET surface area of the ZBC and FZBC3 composites compared to that of pure ZnO could be one of the reasons for their higher catalytic activity [66].Furthermore, the formation of a ZnO/biochar heterojunction structure can enhance visible light absorption and efficiently separate photoexcited electron-hole pairs, thus further improving photocatalytic efficiency [67].

Photodegradation of AO7 with different catalysts
The photocatalytic activity of the pure ZnO, ZBC, and FZBCx (x = 1, 3, 5, and 7) samples was studied through experiments involving the degradation of AO7 under visible light irradiation conditions (λ > 420 nm), as depicted in figure 6(a).No photocatalytic degradation of AO7 was observed after 2 h of visible light irradiation, indicating that AO7 is stable under visible light irradiation in the absence of a photocatalyst [68,69].The ZnO sample exhibited a relatively low AO7 removal efficiency (10.30%) due to adsorption.The composite ZBC showed a significantly greater AO7 degradation efficiency (72.75%) than did pure ZnO, with a 7.06-fold increase.Notably, the FZBC3 composite exhibited the highest photocatalytic efficiency (93.17%) among all the samples, including ZnO (10.30%),ZBC (72.75%),FZBC1 (68.99%),FZBC3 (86.90%), and FZBC7 (78.83%).AO7 degraded 93.17% within 120 min.Figure 6(b) illustrates the gradual degradation of AO7 on FZBC3 over time, with AO7 nearly completely degrading at 120 min.The UV-Vis spectra of the treated AO7 solution are presented in figure 6(b).AO7 possesses four absorption peaks at 229, 260, 311, and 485 nm and a shoulder peak at 407 nm.The peak at 485 nm is attributed to the hydrazone form, whereas the shoulder at 407 nm is attributed to the azo group [70].In general, the peaks at 229-311 nm correspond to the π-π * transitions in the benzoic and naphthalene rings of AO7, respectively [71].The decrease in the intensity of these peaks during the reaction indicates the decomposition of the aromatic moieties and color groups of the dye [70,71].
To gain a better understanding of the reaction kinetics of the process of AO7 degradation, experimental data were fitted using a first-order kinetic model (equation ( 3)).As shown in the inset in figure 6(c), the photocatalytic degradation curves in all cases were well fitted to the first-order apparent kinetic model.Furthermore, significant differences in photocatalytic activity were observed for ZnO, ZBC, and FZBCx (figure 6(c)), implying that the combination of ZnO with BC or the combination of Fe-doped ZnO with BC significantly enhanced the photocatalytic activity of ZnO in the composites.Figure 6(d) shows that FZBC3 exhibited the highest degradation rate constant.The rate constant values for ZnO, ZBC, FZBC1, FZBC3, FZBC5, and FZBC7 were  3, reveals that the catalytic activity of the FZBC3 composite is relatively high compared to that in previous reports.

The influence of scavengers
As the photocatalytic process primarily involves the formation of electron-hole pairs (e-h + ), superoxide anion radicals (•O 2 - ), and hydroxyl radicals (•OH), it is essential to investigate the activities related to the degradation of AO7 molecules (figure 7).To study these activities related to the photocatalytic process, radical trapping experiments were performed using the FZBC3 catalyst.For this experiment, 10 mM potassium iodide (KI), potassium bromates (KBrO 3 ), tert-butanol (tBA), and ascorbic acid (AA) were added to the DC solution as scavenging agents to capture holes (h + ) [76], electrons (e -) [76], hydroxyl radicals (•OH) [77], and superoxide radicals •O 2 -[78], respectively.First, the addition of AA significantly reduced the degradation efficiency to 80.92%, indicating its ability to inhibit •O 2 -radical formation.Next, the addition of tBA reduced the degradation efficiency to 50.42%, suggesting that tBA interferes with •OH radical formation.Third, the addition of KBrO 3 , a radical scavenger, reduced the degradation efficiency to 26.46%, indicating that the inhibition of electron formation was not significant.Finally, photogenerated holes h + reduced the degradation efficiency to 20.99%, indicating that h + is a secondary active species involved in the process.Hence, it is concluded that the formation of superoxide anion radicals •O 2 -and hydroxyl radicals •OH in the reaction solution plays a primary role in the AO7 degradation process.These two free radicals possess high reactivity, leading to significant antibacterial activity as well [3,25].

Antibacterial properties
The antibacterial capacities of pure ZnO, ZBC and FZBC3 were evaluated against E. coli (figure 8).Initially, the number of E. coli colonies was counted using a bacterial colony counter, and then the antibacterial ratio was calculated using equation (4).ZnO exhibited poor antibacterial properties, achieving an antibacterial ratio of   ).This could be attributed to the lack of effective antibacterial agents in the ZnO system.The antibacterial ratio of ZBC against E. coli was 81.25% (figure 8(c)).Upon the addition of Fe, the antibacterial ratio of FZBC3 against E. coli reached 98.00% (figure 8(d)).FZBC3 exhibited antibacterial ratios greater than 70% and 16.75% greater than those of pure ZnO and ZBC, respectively.This could be because FZBC3 has a greater surface area and larger pore size than ZnO and ZBC (table 2), resulting in better adsorption performance of the FZBC3 composite.Additionally, FZBC3, which has a large surface area, provides more active sites for adsorption and photocatalytic reactions, leading to excellent photocatalytic antibacterial properties [67].
Based on the above experimental results, the photocatalytic AO7 degradation and antibacterial mechanism for the Fe-doped ZnO/biochar (FZBC) catalyst under visible light irradiation can be proposed, as shown in figure 9.When ZnO was doped with Fe 3+ ions, new localized energy levels of Fe appeared in the band gap of ZnO.When FZBC is irradiated, photoinduced electron-hole (e -/h + ) pairs are created, and electrons rapidly move from the valence band (VB) of ZnO to local Fe energy levels along with d-d transitions at the energy level of  Fe doping [26].These photoexcited electrons are trapped in the Fe energy levels, while the holes remain in the valence band (VB) of ZnO.Electrons from the CB and holes from the VB of ZnO are transferred to the catalyst surface and then consumed in the production of superoxide and hydroxyl radicals (•O , 2 -•OH) by reacting with O 2 , and H 2 O is adsorbed on the FZBC surface.The generated reactive oxygen species attack bacteria and kill them by rupturing their cell walls [75].The h + hole in the VB of ZnO can directly attack bacteria and decompose AO7 [76].On the other hand, it can be said that Fe 3+ ions increase the ability to capture electrons, improving the photocatalytic performance under visible light for pure ZnO.Indeed, Fe 3+ ions can trap electrons and be converted to Fe 2+ .Because Fe 2+ is unstable, it can react with oxygen, turning back into Fe 3+ and reducing O 2 to the superoxide anion radical •O 2 - [76].Overall, the process described above reflects the absorption of more photons under visible light, effectively enhancing the separation of photogenerated electron-hole pairs and generating electrons much more efficiently (figure 9).

Conclusions
The Fe-doped ZnO/biochar composite was coprecipitated with Phragmites australis stem biomass-derived biochar.The Fe/ZnO nanoparticles and their dispersion on the biochar matrix shifted its light absorption to the visible-light region, resulting in visible-light-driven photodegradation of both the AO7 dye and photodeactivation of E. coli.The Fe-doped ZnO/biochar exhibited much greater photocatalytic activity toward AO7 dyes as well as deactivation of E. coli.than those of the individual components.These results suggest that the Fe-ZnO/biochar photocatalyst can be considered a method for the treatment of dye-polluted wastewater and that the photocatalytic antibacterial process using this catalyst will be highly useful for systems with high requirements for cleanliness and surface sterility.

Figure 7 .
Figure 7. Effect of different scavengers on the photodegradation of AO7 over FZBC3 samples.

Figure 8 .
Figure 8. Antibacterial activities of the a) without catalyst, b) ZnO, c) ZBC, and d) FZBC3 samples under visible light irradiation.

Table 1 .
The average crystalline size (D), reflection angle 2θ and band gap energy (E g ) of the synthesized samples.
conduction band of ZnO and the d electrons of Fe 3+

Table 2 .
Textural properties of ZnO, BC, ZBC and FZBC3..0159min, 0.0158 min, 0.0372 min, 0.0335 min, and 0.0231 min, respectively.A comparison of the apparent first-order rate constants of the FZBC3 catalyst with those in previously published literature, as listed in table

Table 3 .
Comparison of the rate constant of the present catalyst with the literature values for AO7 degradation.