Caesalpinia bonduc seeds assisted biosynthesis of Fe/ZnO photo-catalyst for degradation of methyl orange dye in aqueous medium

doi:10.21203/rs.3.rs-3753152/v1

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Caesalpinia bonduc seeds assisted biosynthesis of Fe/ZnO photo-catalyst for degradation of methyl orange dye in aqueous medium

https://doi.org/10.21203/rs.3.rs-3753152/v1

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The photocatalytic degradation of organic dyes has emerged as an efficient approach for wastewater treatment. In this study, Fe-ZnO nanocomposites (NCPs) were synthesized using extracts from the Caesalpinia bonduc seeds, a renewable and eco-friendly source. The synthesized NCPs were found to exhibit remarkable photocatalytic activity towards the degradation of methyl orange (MO) dye, a commonly used textile dye. Characterization techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), Transmission electron microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS), Thermogravimetric Analysis (TGA), and Fourier-transform infrared spectroscopy (FTIR), to confirm the successful synthesis of the nanocomposites. The photocatalytic degradation of methyl orange dye was investigated under visible light irradiation with a wavelength range of 400–700 nm. The results exhibited an impressive 90% reduction in the concentration of the methyl orange (MO) dye within 120 minutes of reaction time. Thorough investigations were conducted to assess the impact of various reaction parameters such as the catalyst loading, initial dye concentration, and the effect of hydroxyl radical scavengers to investigate the reaction kinetics and the degradation mechanism of MO dye. Moreover, the reusability and stability of the Fe-ZnO nanocomposites were evaluated, demonstrating their capability for practical applications in wastewater treatment. The study highlights the advantageous utilization of Fe-ZnO nanocomposites, synthesized through an environmentally benign green chemistry technique utilizing C. bonduc seed extracts. These nanocomposites serve as efficient and reusable catalysts for the photocatalytic degradation of methyl orange dye, thereby contributing to the advancement of eco-friendly and cost-effective solutions for treating textile wastewater.

Fe-ZnO nanocomposites

Caesalpinia bonduc seeds

Dyes

Water treatment

Green synthesis

The world is experiencing water shortages due to climate change, population growth, and inadequate infrastructure. This crisis affects millions, compromising their health, sanitation, and overall well-being. Efforts are being made to address this issue, including the development of sustainable water management strategies and investments in water purification technologies, but it remains a persistent global challenge. [1–4]. Only 3% of the Earth's water is freshwater; within that small percentage, only 0.2% is suitable for drinking [5]. Multiple sources, including agriculture, industry, households, and leaks, affect water bodies with pollution. On top of these sources, industrial units are the primary source of water contamination. Industrial revolution has adversely impacted the climate and communities by reducing green urban areas while increasing the level of pollution[6]. Industrial effluents contain various pollutants, including pesticides, dyes, herbicides, heavy metals, mineral acids, pharmaceuticals, oil, and grease (Iqbal et al., 2022). The industrial effluents released by the textile industry contain millions of tons of colored dyes that are released into freshwater streams daily[7–9].

The textile industry generates a substantial amount of wastewater contaminated with several types of dyes, including reactive, basic, acidic, dispersed, anthraquinonoid, azo, and neutral dyes[10]. The most commonly used dyes in the textile industry are Azo and anthraquinone dyes [11]. These dyed materials are incredibly hazardous to humans, microorganisms, and the aquatic ecosystem, causing respiratory complications, ulcers, dermatitis, and other health issues [12].

Dyes are complex organic compounds that are highly stable and resistant to biodegradation, making them challenging candidates to remove from wastewater streams [13, 14]. Conventional techniques such as biosorption, ion exchange, adsorption, UV radiation, ozonation, chemical coagulation, electrocoagulation, microbial decolorization, and flocculation have been employed vastly to encounter dye contaminated water [15, 16]. However, these methods have proven to be the least effective due to factors such as cost-effectiveness, complex procedures, and the production of secondary pollution. It is, therefore, necessary to switch to an efficient and environment-friendly approach for treating industrial effluents such as dyes. One such technique is the photocatalytic treatment of dye-contaminated wastewater using metals and metal oxides as catalysts in the presence of a suitable light source. Transition metals and metal oxides have gained particular attention due to their easy availability, reactivity, and low cost. Their electronic properties make them unique from standard metals [2, 17, 18].

Iron belongs to the transition metal family that has recently been extensively used as a photocatalyst. Iron and iron oxide have particularly found applications in various fields of science, such as gas sensors, electro-catalysis, and wastewater treatment[19–23]. Iron oxide exists in various oxidation forms, such as Fe₂O₃, Fe₃O₄, and FeO [24, 25], each with unique properties and applications. Among them, Fe₃O₄ has gained significant attention due to its high stability, superior magnetic properties, and comparable catalytic activity. Iron oxide is used as a heterogeneous catalyst in various chemical reactions such as oxidation, reduction, and hydrolysis. The low cost, easy availability, and excellent catalytic performance in various chemical reactions have credited iron oxide as a promising prospect for application in the field of catalysis and related fields[26, 27].

The combination of metal oxides and semiconductors like Fe and ZnO has been shown to enhance the photocatalytic ability by shortening the band gap energy of ZnO[28]. The addition of metal ions to the zinc oxide produces defects in the crystal lattice of the latter, which not only increases its surface area but also facilitates the absorption of a broader range of electromagnetic radiations, resulting in more efficient photocatalytic reactions[29].

Green synthesis techniques are substituting traditional techniques for synthesizing metal nanoparticles owing to increasing environmental concerns. These eco-friendly techniques employ nontoxic materials such as plants and microbes instead of conventional hazardous chemicals. The green synthesis technique has many advantages, such as cost-effectiveness, reproducibility, and enhanced surface properties. Moreover, these green methods are energy-efficient, generate negligible waste, and produce a low carbon footprint. As a result, green synthesis methods have gained significant attention in the recent past. This technique has been explored for the synthesis of various metal nanoparticles, including metals such as Fe, Co, and Ni, and semiconductors like Zn and Ti [30–37].

Caesalpinia bonduc (C. bonduc) is a medicinal plant known for various applications in traditional medicine for treating bronchial asthma, bronchitis, fever, hydrocele, joint pain, and pleurisy [38]. Phytochemicals such as flavonoids, tannins, terpenoids, and alkaloids have been discovered in various parts of the C. bonduc plant. These phytochemicals possess antioxidant, anti-inflammatory, and antimicrobial properties [38, 39]. These phytochemicals have shown to be excellent reducing and capping agents for synthesizing various metal nanoparticles [40]. In one of our recent studies, we successfully synthesized Ag/Bi/Sno₂ nanocomposite material using seed extracts of C. bonduc [41]. We herein report the green synthesis of Fe-ZnO NCPs material for the first time using seed extracts of the C. bounduc plant. The synthesized material is used as a photocatalyst to mineralize the modal pollutant methyl orange in aqueous medium.

The biomolecules present in the seed extract of C. bonduc confer a coating on the NCPs, which increases their stability, prevents accumulation, and helps enhance the biological activity of the synthesized material. The free amino groups are believed to interact and bind with the NCP surfaces. It is also assumed that the cell wall enzymes provide negatively charged carboxyl groups, which assists in the electrostatic interaction between the NCPs and the biomolecules. Furthermore, the nucleophilic ions on the metal surface supply additional electrons for reducing metal ions, creating nanoparticles with a narrow size distribution [42, 43].

To the best of our knowledge, there is no report on the green synthesis of Fe-ZnO NCPs using the seed extracts of C. bonduc plant and its application as a photocatalyst for the degradation of MO dye in aqueous solution.

2.1. Materials

All the chemicals used in this project were of the highest purity. Zinc acetate dihydrate (Zn (CH₃COO) ₂.2H₂O), Iron (III) chloride hexahydrate (FeCl₃.6H₂O), ethanol, and ammonia solution (NH₄-OH) were purchased from Merck Chemical GmbH International.

2.2. Preparation of Seed Extract

Fresh C. bonduc plant seeds were purchased from the local market. The seeds were washed and tap-dried in folds of clean tissue paper. The seeds were then crushed, and 10 g of the pulverized seeds were added to 100 mL of deionized water. The mixture was heated to 80°C and stirred for 180 min to obtain a pale green solution. The solution was allowed to cool at room temperature, filtered, and conserved in an airtight container for further use.

2.3. Synthesis of Fe-ZnO Nanocomposites

A one-pot synthesis technique was used to synthesize 1–3% Fe-ZnO NCPs. The environmentally friendly synthesis of Fe-ZnO nanoparticle composites (NCPs) was achieved by utilizing C. bonduc seed extract as a reducing and capping agent. The process began with the preparation of iron chloride and zinc acetate solutions in separate flasks, each with concentrations ranging from 1–3% iron content relative to the zinc. The salt solutions were then stirred with 100 mL of seed extract using a magnetic stirrer for an hour at 80 ⁰C for 3 hours. A change in the color of the solution from pale green to dark brown indicated the formation of Fe-ZnO NCPs. 10 mL 25% ammonia solution was added to the reaction mixture to ensure the completion of the reaction. The obtained material was washed multiple times with an ethanol-water mixture, dried at 105°C for 24 hours, and then subjected to calcination at 450°C, maintaining this temperature for 4 hours. The obtained material was again crushed in a mortar and pestle, sieved through US standard mesh size number 325, and stored for characterization and application as a photocatalyst.

The C. bonduc seed extract serves a dual role in the synthesis process, acting as a reducing and capping agent. Its biomolecules, such as polyphenols and proteins, facilitate the reduction of metal ions from the iron chloride and zinc acetate precursors [44]. The biomolecules in the seed extract also function as capping agents, controlling particle size and preventing agglomeration[42]. This dual action of the seed extract creates a favorable environment for the formation of Fe-ZnO nanocomposite material. The reduction potential of the biomolecules initiates the nucleation and growth of these NCPs in the solution[45]. Their binding to the NCPs, surfaces helps maintain stability and uniform dispersion, resulting in the synthesis of the Fe-ZnO nanoparticle composite.

2.4. Characterization of Fe-ZnO catalysts

X-ray diffraction (XRD) analysis was conducted using a JOEL JDX X-ray diffractometer (Model-3532, Japan). The powdered sample was examined within a 2θ angle range from 20 to 90 degrees, employing a fixed Cu-K α wavelength of 1.54059 Å. The thermogravimetric analysis (TGA) was performed using a PerkinElmer USA 6300 analyzer, heating the sample from 10 to 800°C at a ramping temperature of 5 degrees per minute under a nitrogen flow. Morphological analysis was conducted using the JEOL-JSM 5910 scanning electron microscope equipped with energy-dispersive X-ray spectroscopy. X-ray photoelectron spectroscopy (XPS) analysis of the synthesized products was conducted using a Nexa XPS analyzer provided by Thermo Scientific UK, employing non-monochromatized Mg Kα X-ray excitation. While TEM (Transmission Electron Microscope) analysis was conducted using a JEOL JEM-2100 TEM analyzer at an acceleration voltage of 200kV. Fourier transform infrared (FT-IR) spectra were recorded on Shimadzu Varian4300 spectrophotometer in KBr pellets.

2.5. Reaction Setup

To assess the photocatalytic efficiency of ZnO and Fe-ZnO NCPs, methyl orange dye was employed as a target pollutant in an aqueous medium with visible light radiation as the energy source. The experiments were conducted using a 100 mL Pyrex glass beaker, which served as a batch reactor. The reactor was kept in a water bath assembly to control the reaction temperature effectively. A 12 W white LED bulb, having wavelength ranging 400–700 nm, was positioned 15 cm above the reaction mixture, serving as the visible light source for the reaction. 50 mg/L of the dye solution and 0.04 g of the photocatalyst were stirred for 30 min without light irradiations. A 2 mL reaction sample was drawn after 2 minutes and was analyzed using UV-visible spectrophotometry, serving as the initial sample. The reaction mixture was exposed to visible light, and 2 mL samples were collected at 15-minute intervals. All the samples were then diluted in the same proportions (2 mL sample + 2 mL distilled water) using a 2000-microliter pipette, centrifuged, and analyzed using a UV-visible spectrophotometer at λmax 464 nm for further examination. The reaction was monitored for a maximum of 120 minutes. The degradation efficiency was calculated based on the measured absorbance values. The same setup has been reported in the literature, where white LED lights have been effectively used as visible light sources for various chemical reactions [46]. Furthermore, one of the papers has discussed that the visible spectrum of the white LED bulb ranges between 400–600 nm [47].

The degradation efficiency of photocatalytic reaction were estimated by employing the following formula:

$$Degradation Efficiency \left(\%\right)=\left[\frac{\left([{MO]}_{0}-[MO]\right)}{{\left[MO\right]}_{0}}\right]\times 100------\left[1\right]$$

Where [MO]_o is the initial absorbance of the dye solution and [MO] is the absorbance of the solution after a particular reaction time.

3.1. Characterization

3.1.1. XRD analysis

The X-ray diffraction (XRD) patterns of all samples shown in Fig. 1 exhibit good agreement with the wurtzite structure of bulk ZnO, which corresponds with JCPDS Card No. 01-079-0206. However, small peaks corresponding to spinel phase ZnFe₂O₄ were observed in the XRD pattern of ZnO with 3 at. % Fe[48]. Quantitative structural analysis was performed using the Rietveld refinement method. The results showed that the lattice parameters c and a do not depend on the iron content, and their values were found to be a = 0.3249 nm and c = 0.52066 nm. This phenomenon can be elucidated by the slight variations in ionic radii between the iron and zinc crystal lattices. The radius of Fe²⁺ is marginally larger than that of Zn²⁺ (0.60 Å), whereas the radius of Fe³⁺ is comparatively smaller. Therefore, substituting Zn with Fe ions does not strongly affect the hexagonal wurtzite structure. Previous studies on Fe-doped ZnO nanoparticles prepared by the sol-gel method [49] and ZnO particles obtained by low-temperature co-precipitation technique [50] have reported comparable results. In the Fe concentration range of 1–3 at. %, the lattice parameters a and c remain practically unchanged. It is worth mentioning that the c/a ratio of our samples, approximately 1.602, is significantly smaller compared to the ideal stoichiometric wurtzite structure (c/a = 1.633 [51]). This phenomenon suggests the presence of oxygen or zinc vacancies, which are formed to maintain charge neutrality[52, 53].

The crystallite size of the catalysts was determined using the Scherrer equation, which correlates the peak broadening observed in the X-ray diffraction (XRD) pattern to the crystallite size. In this case, the highest peak in the XRD pattern was found at a 2θ angle of 36.25 degrees for all three catalysts. The calculated crystallite sizes were 14.57 nm, 12.10 nm, and 14.41 nm for the 1%, 2%, and 3% Fe-ZnO photocatalysts, respectively. The particle sizes obtained through XRD analysis align with those determined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). This agreement further confirms the consistency of the crystallite size estimated using the Scherrer equation.

In studying Fe-doped ZnO using X-ray analysis, it was found that the material mostly follows a wurtzite pattern, with slight indications of ZnFe₂O₄. Even with different concentration of iron, the lattice parameters remained consistent. A clear change in the c/a ratio hinted at potential vacancies in the crystal lattice. Despite the accurate assessment of crystallite sizes, it's crucial to investigate the method's capability to differentiate between various oxides and regulate the concentrations of Fe in Fe-ZnO. This examination is essential to precisely adjust the material properties for diverse practical applications.

3.1.2. FI-IR Analysis

The FTIR spectra as depicted in Fig. 2 describes the chemical bonding presence in C. bonduc seed extracts and Fe-ZnO NCPs. Several prominent transmittance peaks were observed for both seed extract and Fe-ZnO NCPs. The peaks at 3257 cm^− 1 and 3450 cm^− 1 are attributed to the stretching frequency of O-H bonds, indicating the presence of hydroxyl groups [54]. The peak at 2922 cm^− 1 indicates the presence of C = C-H bonds, while the peaks at 1625 cm^− 1 and 1670 cm^− 1 correspond to the alkene group (-C = C-) [55, 56]. Peaks at 1520 cm^− 1, 1431 cm^− 1, 1501 cm^− 1, and 1443 cm^− 1 represent the stretching vibration of C-C bonds [57]. The peak at 1080 cm^− 1 indicates the presence of a C-OH bond [58], while the peaks at 842 cm^− 1 and 750 cm^− 1 are associated with C-H out plane bending [59]. A strong absorption band was observed at 685 cm^− 1, which can be assigned to the vibration of Fe-O bonds [60]. The band appearing at 444 cm^− 1 is due to Zn-O stretching vibrations [61]. The presence of various organic moieties within the structure of C. bonduc contributes to the successful formation of Fe-ZnO nanocomposite particles.

3.1.3. UV-Visible/ Energy Band Gap Analysis

The estimation of the optical band gap of the materials was conducted using UV-vis spectroscopy, specifically by analyzing the absorption peak observed around 370 nm in each sample, as shown in Fig. 3a. The corresponding data in Fig. 3b, which depicts Tauc’s plot for band gap estimation, exhibited a decreasing trend as the amount of Fe doping increased. The decrease in the band gap can be attributed to the formation of defect states within the band gap of n-type semiconductors[62]. This phenomenon is explained by the transfer of 3d electrons from the trivalent ions of Fe³⁺ to the conduction band, which facilitates the transfer of electrons that leads to the creation of defect states, causing a reduction in the band gap energy[63, 64]. The UV-vis spectroscopy data serve as additional evidence supporting the successful incorporation of Fe dopants into ZnO material. The band gap energy values for Fe-ZnO photocatalysts with varying Fe concentrations were determined to be 2.98 eV (1% Fe), 2.85 eV (2% Fe), and 2.68 eV (3% Fe). The lower band gap energies demonstrate the enhanced photocatalytic activity of Fe-ZnO compared to pristine ZnO (3.12 eV), establishing it as an up-and-coming candidate for efficient photocatalysis. The decrease in bandgap energy with higher Fe content makes it an efficient photocatalyst for activation in the visible region of the electromagnetic spectrum. The variation in Eg (band gap energy) might be linked to the existence of intermediary stages formed by the Fe positioned between the conduction and valence bands. Additionally, the expansion of crystallite size results in a decrease in the band gap energy and vice versa. Moreover, the optical analysis suggests that by introducing ZnO with an appropriate quantity of Fe ions, the optical bandgap can be adjusted to alter its characteristics, potentially beneficial for enhancing photocatalytic properties under visible light [65].

3.1.4. SEM and TEM Analysis

The morphological properties of the synthesized Fe-ZnO NCPs were analysed using SEM and TEM respectively. The micrographs and the particle size distribution of the NCPs are shown in Fig. 4. It was observed that the nanocomposites had a narrow particle size distribution with hexagonally shaped nanostructures. The average particle size of the synthesized nanocomposite was found to be 16 nm. The TEM image in Fig. 4d suggests the even distribution of iron content on the zinc oxide surface.

3.1.5. TGA Analysis

The thermal properties of Fe-ZnO NCPs and ZnO NPs were assessed using thermogravimetric analysis (TGA). The nanoparticles were heated in nitrogen flow from room temperature and gradually increasing to 800°C at 10°C/min. The TGA curves (Fig. 5) exhibited three distinct weight loss regions. The first weight loss occurred in the temperature range of 0-300°C. The weight loss of approximately 5% was experienced between 0-100°C primarily arises from the elimination of residual water content within the precursor material while the subsequent weight loss of 5% occurring between 101–300°C is primarily attributed to the volatilization and removal of the remaining organic compounds present in the material being analysed. The third weight loss of approximately 25% occurred in the temperature range of 300–350°C and was attributed to the decomposition of Zinc/ Iron hydroxide, leading to the formation of the Zinc-Iron oxides. No subsequent weight loss was observed as the temperature increased from 350 to 800°C, indicating the presence of well-crystalline Fe-ZnO NCPs. It is to be noted that the Fe-ZnO NCPs exhibited a higher weight loss around 100°C compared to ZnO, suggesting their relatively higher thermal instability [66].

3.1.6. XPS Analysis

Figure 6a illustrates the surface composition and chemical valence states of the individual elements found in the synthesized Fe-ZnO nanoparticles. 5a. The spectrum displays a prominent peak at 1021 eV [67], indicating the presence of the Zn 2p state in the Fe-ZnO nanoparticles. Figure 6b, 6c, and 6d present high-resolution spectra of the individual elements, providing further insight into their specific chemical valence states. The analysis of the figure reveals important information about the chemical valence states of the Fe-ZnO nanoparticles. Specifically, the peak observed at 531 eV in the O 1s region confirms the presence of oxygen (O) in an O²- ion deficient region[68]. Additionally, the peak at 711 eV is attributed to the Fe 2p state, indicating the presence of Fe³⁺ ions in the ZnO lattice[69]. These findings suggest that Fe³⁺ ions have been incorporated into the ZnO structure during the nanoparticle preparation [69].

The significance of the lower Fermi energy level due to the inclusion of Fe in the Fe-ZnO nanocomposite is crucial in understanding its electronic structure and related properties. With Fe³⁺ ions being incorporated into the ZnO structure, the alteration of the Fermi energy level is anticipated due to the different electronic configuration of Fe compared to ZnO.

The XPS analysis highlights the chemical valence states and composition of Fe-ZnO nanoparticles, showcasing the incorporation of Fe³⁺ ions into the ZnO lattice. These results are complemented by the FTIR analysis also, which identifies various chemical bonds present in the C. bonduc seed extracts and Fe-ZnO nanocomposite particles, such as O-H, C = C-H, alkene groups, C-C, and C-OH bonds, along with specific bands corresponding to Fe-O and Zn-O vibrations. The presence of these organic moieties from the C. bonduc seed extracts contributes to the successful formation of the Fe-ZnO nanocomposite particles, validating the chemical characteristics and structural features revealed by the XPS analysis.

3.2. Photocatalytic Activity

The photocatalytic activities of synthesized ZnO and Fe-ZnO were investigated for degradation of methyl orange dye as the targeted pollutant under visible light illumination. A 12 W white LED light (wavelength 400-700nm) was used as the visible light source. The λmax (peak of maximum absorbance) of methyl orange dye was determined to be 464 nm based on its visible spectrum. Two control experiments were conducted to assess the photolysis and dye removal due to adsorption.

In the initial control 50 mL of 50 mg/L methyl orange dye was irradiated with visible light for 30 minutes without any catalyst. Samples taken at various intervals for analysis consistently showed no significant changes in absorbance. This observation suggests that there was no occurrence of photolysis of the methyl orange dye under visible light in the absence of a photocatalyst.

The second blank experiment was aimed at assessing dye removal through adsorption. A reaction mixture was prepared by thoroughly mixing 50 mL of MO dye (Initial dye concentration of 50 mg/L) with 0.04 g of Fe-ZnO photocatalyst under dark conditions for 30 minutes. Analysis of a sample taken from the dye solution showed only a minor 6.5% decrease in the concentration of MO dye, suggesting that the removal of the dye through adsorption was not significant. Subsequently, the photocatalytic activities of Fe-ZnO NCPs were evaluated by mixing the dye solution with respective photocatalysts under visible light illumination for 2 hours. The photocatalytic degradation efficiency of Fe-ZnO exhibited a 90% degradation rate in comparison to pristine ZnO when degrading MO dye under visible light illumination in 120 minutes. The photocatalytic efficiency of Fe-ZnO was also evaluated through total organic carbon (TOC) measurements. ZnO resulted in a 26% decrease in TOC, whereas Fe-ZnO exhibited a more substantial reduction of 72%.

In another investigation, Fe-ZnO photocatalysts with different Fe concentrations (1, 2, and 3%) were tested for their efficiency against MO dye. All Fe-ZnO displayed superior photocatalytic activity compared to pristine ZnO. However, the sample containing 3% Fe-ZnO exhibited the highest photocatalytic efficiency and was chosen for further analysis. The presence of 3% Fe in the Fe-ZnO photocatalyst have the potential enhance the mobility of photo-induced charge carriers, hence reducing the rate of electron hole recombination [70]. The observed enhancement in the photonic efficiency of Fe-ZnO can be attributed to the strong interaction and formation of an effective junction between Fe and ZnO nanoparticles[71].

3.3. Effect of Catalyst Loading

The effect of variation of Fe-ZnO photocatalyst loading (0.02–0.04 g) towards the degradation of methyl orange dye was studied under controlled experimental conditions (Fig. 7a). A 50 mL MO dye solution with a concentration of 50 mg/L was subjected to a reaction duration of 60 min for each dosage. The results revealed that increasing the photocatalyst dosage from 0.02-0.04g led to a notable improvement in the photodegradation efficiency, increasing from 49–63%. A decrease in photocatalytic activity was observed when the catalyst dose was further increased beyond 0.04g. The decrease in photocatalytic activity with an increase in the catalyst loading can be attributed to catalyst agglomeration, leading to a reduction in the catalyst's surface area and consequently diminishing its activity." [72]. Another factor could be the turbidity of the reaction under study; the turbidity of the reaction mixture results in poor penetration of light to induce the photocatalytic reactions, thereby producing a lesser number of OH radicals (^•OH), resulting in poor photocatalytic activity[73].

3.4. Recyclability Studies

The recyclability of the Fe-ZnO photocatalyst was evaluated through a series of reactions. The catalyst was recovered after each reaction and directly reused without pretreatment or modification. The results as depicted in Fig. 7b show that the Fe-ZnO photocatalyst preserved its catalytic performance over four cycles. Even after four cycles, the activity of the catalyst only experienced a slight decrease of approximately 10%. This level of stability is remarkable and indicates that the Fe-ZnO photocatalyst is highly stable for degrading MO dye over an extended period. This stability is significant because it highlights the potential of the Fe-ZnO photocatalyst for sustainable applications in dye degradation. It can effectively serve as a recombinant center for the excited species, contributing to efficient and sustainable dye degradation processes.

3.5. Kinetics and Mechanism

The improved catalytic activity of Fe-ZnO for degrading MO dye can be credited to the creation heterojunctions between Fe and ZnO. These heterojunctions, when exposed to visible light, generate positive holes in the valence band with the concurrent production of electrons that can subsequently move toward the conduction band. During this process, the photogenerated electrons in the conduction band of ZnO may combine with the photogenerated positive holes in the valence band of Fe moiety. However, the effective holes in the valence band of ZnO and the electrons inside the conduction band of Fe continue to be separated in space[74]. The separation permits a sequence of redox reactions to occur, leading to a higher catalytic performance of Fe-ZnO than ZnO alone (Fig. 9).

To further gain a deeper understanding of the mechanism, scavengers such as EDTA and isopropyl alcohol were used to quench positive holes and ^•OH, respectively. Adding EDTA to the reaction mixture showed no significant degradation activity, suggesting that positive holes are not responsible for the degradation [75]. However, the presence of isopropanol led to a decrease in photocatalytic efficiency, suggesting the importance of hydroxyl radicals in the photodegradation process. These results highlight the crucial role of Z-scheme heterojunctions between Fe and ZnO in facilitating efficient charge separation and promoting the generation of reactive species, resulting in enhanced photocatalytic performance for degrading the methyl orange dye.

The following series of redox reactions can explain the photocatalytic mechanism under consideration.

$Fe-ZnO+hv\to Fe-ZnO \left({h}^{+}\right)+Fe-ZnO \left({h}^{-}\right)$	[2]
$Fe-ZnO \left({h}^{+}\right)+{H}_{2}O\to {OH}^{\star }$	[3]
$Fe-ZnO \left({h}^{-}\right) + {O}_{2} \to {O}_{2}^{-}$	[4]
${O}_{2}^{-}+ {H}_{2}O \to {OH}^{\star }$	[5]

$MO+ {OH}^{\star }\to Degraded products$ [6]

$MO+ {O}_{2}\underrightarrow{Fe-ZnO+hv} Degraded products$ [7]

The net rate of the reaction based on the above mechanism can be written as

$-\frac{d \left[MO\right]}{dt}={k}_{r}[MO{]}_{aq}{O}_{2}$ [8]

But at a constant concentration of molecular oxygen, the equation can be modified as

$-\frac{d \left[MO\right]}{dt}={k}_{App}[MO{]}_{aq}$ [9]

Where k_appa is the apparent rate constant, considering the concentration of oxygen as a constant, on integrating Eq. (08) we get,

$ln\frac{ {\left[MO\right]}_{t}}{{\left[MO\right]}_{0}}=-{k}_{app}t$ [10]

${\left[MO\right]}_{t}={\left[MO\right]}_{0}.\text{exp}(-{k}_{Ap}t)$ [11]

Where [MO]_o and [MO]t represents the initial and final concentration of methyl orange dye in mg. L^− 1.

The photodegradation data given in Fig. 8 were analyzed by the nonlinear method according to the rate law given in Eq. 11. The apparent rate contents for ZnO were found to be 0.055 compared to 0.013 for Fe-ZnO photocatalyst for the degradation of MO dye, as shown in Fig. 10. The Fe-ZnO photocatalyst was three (3) times more effective than ZnO pristine.

3.6. Effect of concentration of dyes

The catalyst's performance in degrading dyes is significantly affected by the concentration of dyes. Using initial concentrations of 30, 40, and 50 mg/L of MO dye, a series of degradation experiments were conducted to evaluate this effect (Fig. 11). Each dye solution was treated with 0.04 g of Fe-ZnO catalyst under visible light irradiation, and reaction samples were periodically collected and analyzed using a UV-visible spectrophotometer. The resulting data were analyzed using a nonlinear method based on the rate law Eq. 11 to determine the apparent rate constants. The obtained rate constants were 0.0182, 0.0145, and 0.00688 per minute for initial dye concentrations of 30,40 and 50 mg/L, respectively.

Interestingly, contrary to the general trend where the reaction rate increases with increasing concentration, the results revealed a decrease in the reaction rate as the concentration of dyes increased [75]. This observation can be attributed to several factors. Dyes are highly pigmented substances, and even a tiny amount of dye imparts a strong color to the solution. Concentrated solutions of dyes exhibit pronounced coloration as the dye concentration increases. This intense color poses a hindrance to the penetration of radiation required for the formation of ^•OH, consequently reducing the rate of reaction[76].

Moreover, as the concentration of dyes increases, the molecules tend to adsorb onto the catalyst surface, effectively covering the active sites and impeding the catalytic activity. This adsorption phenomenon further contributes to the decline of the reaction rate. Hence, the decrease in catalytic performance associated with higher dye concentrations can be attributed to the limited penetration of radiation and the hindrance of the catalyst surface by the dye molecules. It is essential to consider these factors when evaluating the influence of dye concentration on catalyst performance in the context of dye degradation processes[77].

A cost-effective and highly efficient visible-light-driven photocatalyst. The successful synthesis of Fe-ZnO was achieved by employing a green chemistry approach utilizing C. bonduc seed extracts. The synthesized catalysts underwent thorough characterization using various analytical techniques such as XRD, UV-vis, SEM, TEM, FT-IR, TGA, and XPS analysis, confirming the successful fabrication of respective photocatalysts. The photocatalytic performance of synthesized catalysts was evaluated by studying their ability to degrade MO dye under visible light exposure. The results revealed that the bimetallic catalyst Fe-ZnO exhibited significantly enhanced photodegradation ability compared to the monometallic ZnO catalyst alone. Specifically, within a reaction time of 120 minutes, Fe-ZnO achieved complete degradation (90%) of the MO dye, while pristine ZnO only achieved 50% degradation. These findings highlight the promising potential of Fe-ZnO as a catalyst for applications involving the elimination of organic pollutants. The significance of this work lies in successfully synthesizing a highly efficient photocatalyst through a straightforward and cost-effective green chemistry technique, using C. bonduc seed extracts as a reducing and capping agent. The fabrication of respective photocatalysts (ZnO and Fe-ZnO) opens new possibilities for their practical utilization in organic pollutant removal, providing an environmentally friendly approach.

Author Contribution

1. Mohsin Siddique: Resources, Methodology, Writing original draft, Supervision. 2. Sherzod Abdullaev: Data Visualization, Analysis. 3. Muhammad Altaf: Data Visualization, Analysis.4. Patrik Viktor: Review & Editing, Funding acquisition. 5. Monika Fodor: Writing - Review & Editing, Funding acquisition.6. Raheel Naeem: Methodology, Analysis, Writing original draft. 7. Murtaza Sayed: Data visualization, Analysis.8. Jibran Iqbal: Analysis, Writing - Review & Editing. 9. Noor S. Shah: Data visualization, Writing - Review & Editing.

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Caesalpinia bonduc seeds assisted biosynthesis of Fe/ZnO photo-catalyst for degradation of methyl orange dye in aqueous medium

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Abstract

Figures

1. Introduction

2. Experimental

2.1. Materials

2.2. Preparation of Seed Extract

2.3. Synthesis of Fe-ZnO Nanocomposites

2.4. Characterization of Fe-ZnO catalysts

2.5. Reaction Setup

3. Results and Discussions

3.1. Characterization

3.1.1. XRD analysis

3.1.2. FI-IR Analysis

3.1.3. UV-Visible/ Energy Band Gap Analysis

3.1.4. SEM and TEM Analysis

3.1.5. TGA Analysis

3.1.6. XPS Analysis

3.2. Photocatalytic Activity

3.3. Effect of Catalyst Loading

3.4. Recyclability Studies

3.5. Kinetics and Mechanism

3.6. Effect of concentration of dyes

4. Conclusions

Declarations

Author Contribution

References

Additional Declarations

Status:

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\(Fe-ZnO+hv\to Fe-ZnO \left({h}^{+}\right)+Fe-ZnO \left({h}^{-}\right)\)	[2]
\(Fe-ZnO \left({h}^{+}\right)+{H}_{2}O\to {OH}^{\star }\)	[3]
\(Fe-ZnO \left({h}^{-}\right) + {O}_{2} \to {O}_{2}^{-}\)	[4]
\({O}_{2}^{-}+ {H}_{2}O \to {OH}^{\star }\)	[5]