ZnO nanophotocatalytic solution with antimicrobial potential toward drug-resistant microorganisms and effective decomposition of natural organic matter under UV light

2.1 Materials used

2.2 Synthesis of ZnO NMs

2.2.1 Green procedure utilizing leaf extracts

The fabrication of ZnO NMs utilized NLE as a natural stabilizing and reducing agent. Fresh neem leaves were collected, washed with deionized water to eliminate surface impurities, and air-dried at room temperature for 48 h. The dried leaves were ground into a fine powder and extracted using the hot water method. 50 grams of neem powder were boiled in 500 ml of deionized water at 80 °C for 30 min to facilitate the release of bioactive compounds, including flavonoids, terpenoids, and polyphenols. The extract was filtered with Whatman filter paper No. 1, and the resulting clear filtrate was stored at 4 °C for later nanomaterial synthesis.

A 0.1 M aqueous solution of zinc nitrate hexahydrate (Zn(NO₃)₂•6H₂O) was prepared for the synthesis of ZnO NMs and heated to 70 °C with continuous stirring. NLE was incrementally introduced to the zinc nitrate solution, thereby commencing the reduction process. A 0.1 M sodium hydroxide (NaOH) solution was added dropwise to control pH and promote nanomaterial formation until the reaction mixture achieved a pH of approximately 10. The reaction was conducted at 70 °C for 2 h with continuous stirring, resulting in the gradual formation of ZnO NMs [27]. The synthesis of ZnO NMs was indicated by the gradual colour transition of the solution from pale yellow to opaque white. The bioactive components in neem extract reduced zinc ions (Zn²⁺) while concurrently stabilising the nanomaterials, thus reducing agglomeration. The reaction mixture was allowed to cool to ambient temperature, and the ZnO precipitate was harvested for further purification and characterization (Figure 1).

Figure 1:

Preparation method of ZnO NMs.

2.3 Characterization techniques

A range of characterization techniques was utilized to assess the crystallinity, shape, elemental content, and optical characteristics of the biosynthesized ZnO NMs. The XRD pattern of the synthesized ZnO NMs displayed distinct peaks that align with the hexagonal wurtzite phase, thereby affirming the elevated level of crystallinity present in the nanoparticles. The pronounced sharpness of the peaks signifies the presence of well-defined crystalline structures, which are crucial for facilitating photocatalytic and antimicrobial functions. The size of the crystallites within the ZnO NMs was ascertained through the application of the Scherrer equation:

Where: ‘D’ represents the size of the crystallite. ‘K’ represents the shape factor, which is valued at 0.9. ‘λ’ represents the wavelength of the X-ray, measured at 1.5406 Å. ‘β’ represents the full width at half maximum (FWHM) of the most significant peak, and ‘θ’ represents the angle of diffraction.

The study of the XRD peak associated with the (101) plane at a diffraction angle of 36.3° substantiates the distinctly defined crystalline structure of the synthesized ZnO NMs, thereby reinforcing their elevated level of crystallinity. Scanning electron microscopy (SEM) elucidated surface shape, particle distribution, and agglomeration traits following gold sputter coating to improve conductivity. Ultraviolet-visible (UV–vis) spectroscopy assessed the optical characteristics, ascertaining the bandgap energy by Tauc’s plot and validating the photocatalytic potential of ZnO. The application of Fourier-transform infrared (FTIR) spectroscopy demonstrated the existence of distinct Zn–O stretching vibrations, thereby validating the successful synthesis of ZnO NMs. The examination further revealed the presence of biomolecules derived from neem extract on the nanoparticle surfaces. The quantitative evaluation of the FTIR spectra indicated that although the biomolecules are present in considerable quantities, their influence on the overall characteristics of the ZnO NMs remains balanced. The intensity ratios observed between the biomolecular peaks and the ZnO-specific peaks clearly illustrate this phenomenon. The results indicate that biomolecules are crucial for the stabilization of ZnO NMs, while simultaneously preserving their photocatalytic activity, thereby ensuring their efficacy in environmental and antimicrobial applications.

2.4 Photocatalytic experimental set up

2.5 Measurement and analysis

2.6 Antibacterial/antifungal potential of biosynthesized ZnO NMs

The biosynthesized ZnO NMs were assessed for their bactericidal and fungicidal properties through the disk diffusion method against Gram-negative E. coli (ATCC 25922) and C. albicans (ATCC 10231). There were five replicates for each treatment taken in to consideration. A standardized bacterial culture was uniformly distributed on Mueller-Hinton agar plates, prepared by neutralizing the pH with 1 M NaOH and distilled water to create a 1 l solution. Three wells were constructed in each agar plate and filled with ZnO NM solutions at concentrations of 0.05 mg/ml and 0.1 mg/ml, with a control included on each plate. The plates were sealed and incubated at 37 °C for 24 h to facilitate interaction between the nanomaterials and microbial cells. Following incubation, inhibitory zones were quantified to assess the antibacterial effectiveness of ZnO NMs. The antibacterial efficacy assessed by the mean inhibition zone diameter. Increased doses resulted in bigger inhibitory zones, demonstrating a dose-dependent relationship. The results illustrate the effectiveness of ZnO NMs against bacterial and fungal infections, suggesting their potential application in antimicrobial treatments.

3.1 Characterization of ZnO NMs

The SEM picture of ZnO NMs is directly correlated with the study’s results about the photocatalytic breakdown of NOM under low-energy UV light. The pronounced aggregation of nanomaterials and the porous surface topology depicted in the image suggest an increased active surface area, which is essential for attaining effective photocatalytic activity [32] (Figure 2a). The study demonstrates a 92 % degradation of NOM within 180 min utilizing 3 g/l of ZnO, suggesting that the synthesized nanomaterials exhibit significant photocatalytic efficiency. The efficiency is affected by the nanoscale structure, crystallinity, and defect sites in ZnO particles, which collectively enhance charge separation and the generation of reactive oxygen species in photocatalysis. The SEM image demonstrates that the porous and uneven shape of ZnO results in an extensive surface area, hence augmenting the adsorption of NOM molecules before photocatalytic degradation. The irregular morphology and nanomaterial clusters may facilitate light absorption and multiple scattering effects, thereby improving the efficiency of UV-induced electron-hole pair generation. The study reports a 59 % removal of OCD, indicating high mineralisation efficiency that corresponds with the anticipated performance of biosynthesized ZnO NMs. The minimal carbon residue observed on the ZnO surface after the reaction indicates that the nanomaterials possess self-cleaning properties, likely resulting from the effective oxidative degradation of organic pollutants, which results in a reduced presence of surface-adsorbed byproducts. The study underscores the significance of biosynthesised ZnO in sustainable water treatment, highlighting its potential to contribute to global environmental objectives. The morphological characteristics observed in the SEM image, including high particle density, strong interparticle interactions, and aggregated nanomaterials, indicate that biosynthesis techniques may have contributed to defect-rich ZnO surfaces, enhancing photocatalytic efficiency. The structural features likely enhance hydroxyl radical formation, facilitating more rapid and thorough degradation of NOM. The XRD pattern of the synthesized Zno NMs displays clear peaks indicative of the hexagonal wurtzite structure, consistent with JCPDS card No. 36–1451. The diffraction peaks observed at approximately 31.8° (100), 34.4° (002), 36.3° (101), 47.5° (102), 56.6° (110), 62.9° (103), and 68.0° (112) validate the crystalline structure of ZnO. These peaks’ distinct and sharp features indicate a high degree of crystallinity, suggesting that the employed biosynthetic technique produced well-structured nanomaterials with minimal amorphous content [33] (Figure 2b). The prominence of the (101) peak at roughly 36.3° signifies the thermodynamically stable wurtzite phase of ZnO. The absence of supplementary peaks in the diffraction pattern verifies the phase purity of ZnO NMs, indicating the lack of substantial impurities such as Zn(OH)₂ or metallic Zn [34]. The average crystallite size can be calculated using Scherrer’s equation, which use the full width at half maximum (FWHM) of the most prominent peak and the Cu Kα wavelength (0.154 nm) [35]. This assessment confirms that the synthesized ZnO NMs are within the nanometre scale, hence validating the effectiveness of the green synthesis technique. The broadening of peaks at lower angles indicates a possible occurrence of lattice strain or size-induced broadening, phenomena commonly seen in nanoscale materials. The distinct diffraction pattern signifies a low defect density and few dislocations, enhancing structural stability. The hexagonal wurtzite phase of ZnO demonstrates enhanced optical and photocatalytic characteristics, making it suitable for environmental applications such as water filtration and pollution degradation. The XRD examination reveals that the biosynthesized ZnO nanomaterials exhibit good crystallinity, phase purity, and nanoscale dimensions, making them suitable for the photocatalytic degradation of NOM in water treatment applications. UV–vis spectroscopy offers essential information regarding the optical properties, band gap, and light absorption characteristics of ZnO NMs, which directly affect their photocatalytic efficiency (Figure 2c).

Figure 2:

Morphological and structural analysis of ZnO NMs (a) SEM image, (b) XRD analysis, (c) UV-Visible Spectrum and (d) FT-IR spectrum.

The analysis utilizing FTIR was conducted to examine the functional groups linked to the biosynthesized ZnO NMs through NLE, which served as a medium for the environmentally friendly synthesis (Figure 2d). The FTIR spectrum exhibited multiple significant absorption bands that offer important information regarding the structure and functionality of the ZnO NMs. An observable absorption band in the range of 500–600 cm⁻¹ is indicative of the Zn–O stretching vibration, thereby affirming the successful synthesis of ZnO NMs. This peak is indicative of the Zn–O bond and confirms the successful synthesis of the ZnO NM. The extensive band noted in the 3,400 cm⁻¹ region is ascribed to O–H stretching vibrations, which signify the presence of surface hydroxyl groups. The presence of hydroxyl groups is crucial for the formation of hydroxyl radicals (•OH) in photocatalytic reactions, which are integral to the process of pollutant degradation. Observations in the 2,800–3,000 cm⁻¹ range correspond to C–H stretching vibrations, signifying the existence of alkyl or methylene groups derived from the NLE. These groups play a crucial role in stabilizing the ZnO NMs by inhibiting agglomeration. Furthermore, a band within the range of 1,000–1,300 cm⁻¹ is indicative of C–O stretching, a characteristic feature of phenolic compounds and flavonoids present in the NLE. This observation implies that these biomolecules are engaging with the ZnO surface, thereby enhancing and stabilizing its photocatalytic and antimicrobial attributes. The spectrum additionally reveals peaks within the 1,400-1,600 cm⁻¹ range, which can be ascribed to C–H bending and O–H bending vibrations, thereby reinforcing the presence of terpenoids and polyphenols on the ZnO surface. The relative intensity of the phenolic group around 3,400 cm⁻¹ was determined in comparison to the Zn–O stretching vibration (500–600 cm⁻¹), suggesting a moderate presence of biomolecules on the surface of the ZnO NMs. This interaction plays a vital role in ensuring the stability of the nanomaterials and improves their photocatalytic efficiency, as the biomolecules probably facilitate effective electron transfer throughout the photocatalytic process. Moreover, the lack or reduction of peaks associated with C=O, C–H, and various organic functional groups after the photocatalytic process implies that the ZnO nanomaterials demonstrate efficacy in the degradation of organic compounds, signifying successful photocatalysis. The FTIR analysis conclusively demonstrates the synthesis of ZnO NMs and offers comprehensive evidence regarding the interaction between biomolecules derived from the NLE. The interactions involved play a significant role in enhancing the stability, photocatalytic activity, and antimicrobial characteristics of ZnO NMs, thereby establishing them as a viable and sustainable choice for environmental remediation and antimicrobial purposes.

The integration of XRD, SEM, UV–vis, and FTIR characterization demonstrated that the biosynthesized ZnO NMs exhibit high crystallinity, structural integrity, and stability, alongside superior optical properties. Their elevated surface area, regulated morphology, and defect-enhanced photocatalytic activity render them highly effective for the degradation of NOM in water purification applications. The findings indicate that green-synthesized ZnO NMs provide a sustainable and efficient method for environmental remediation.

The specific surface area of the ZnO NMs was meticulously assessed through nitrogen adsorption-desorption at cryogenic temperatures (77 K), a recognized method that measures the material’s ability to hold nitrogen gas when subjected to exposure. This approach is vital for precisely evaluating the surface area accessible for chemical reactions, which plays a significant role in ascertaining the efficacy of nanomaterials in photocatalytic and antimicrobial applications. The BET analysis indicated that the ZnO NMs exhibit a mesoporous structure characterized by pores of ideal dimensions – neither overly small nor excessively large. The particular distribution of pore sizes is crucial for the materials’ capacity to effectively engage with and sequester pollutants or microorganisms, thereby improving their efficacy in environmental and biological applications. As depicted in (Figure 3a), the pore volume analysis reveals a significant abundance of pores within the 2–20 nm range, which is essential for the material’s improved engagement with organic pollutants and bacterial organisms. The mesoporosity of ZnO NMs enhances the adsorption of contaminants and promotes microbial inhibition, rendering them suitable for photocatalytic degradation and antibacterial applications. In-depth examination of the pore size distribution, illustrated in (Figure 3b), indicates that ZnO produced with 1 ml and 4 ml of NLE displays a pore size peak near 48 nm, while the sample synthesized with 2 ml NLE presents a more distinct peak at 60 nm. The observed variation in pore size suggests that the quantity of NLE employed has a direct impact on the distribution of pore sizes. This highlights the critical need for meticulous regulation of synthesis parameters to enhance adsorption capacity and facilitate better interactions with pollutants and microbial cells. The surface area of the ZnO NMs was determined to be 13 m²/g for samples synthesized using 2 ml and 4 ml of NLE, as illustrated in (Figure 3c). This value significantly surpasses the 10 m²/g recorded for the sample produced with 1 ml NLE. An expanded surface area is essential for enhancing photocatalytic activity, as it amplifies the quantity of accessible active sites for light absorption. The augmentation of surface area consequently results in a heightened production of ROS, which play a crucial role in the effective degradation of organic pollutants. The increased surface area facilitates more effective interactions between the ZnO NMs and bacterial cells, thereby enhancing their capacity to engage with and inhibit or eliminate the microorganisms. The distinctive interplay of enhanced surface area and mesoporosity renders ZnO NMs exceptionally suitable for various environmental and antimicrobial applications. Moreover, the integration of NLE markedly improves the structural and functional characteristics of the ZnO NMs, rendering them a superior option for sustainable environmental remediation and antimicrobial applications [36], 37].

Figure 3:

BET surface area analysis for ZnO NMs (a) pore volume, (b) pore size and (c) surface area.

The histogram depicting the particle size distribution of green-synthesized ZnO NMs indicates an av. particle-size of 66.22 nm with a standard deviation of 36.76 nm (Figure 4a). The distribution reveals a polydisperse characteristic, with the majority of particles situated in the 20–100 nm range. The presence of particles exceeding 100 nm indicates a certain level of aggregation. The curve overlay indicates a near-lognormal distribution, which is a prevalent characteristic in nanomaterial synthesis. The small size and extensive distribution of ZnO NMs indicate a high surface area, rendering them suitable for photocatalytic applications, including the degradation of natural organic matter in water treatment. Although a significant decrease in the bandgap of the biosynthesized ZnO NMs was noted through Tauc Plot (2.9 eV in contrast to the standard 3.37 eV for bulk ZnO), the size of the nanoparticles (∼66 nm) is excessive to exhibit considerable quantum confinement effects, which are generally seen in particles that are smaller than 10 nm. Consequently, the observed decrease in the bandgap is presumably attributable to various factors, including the existence of surface defects, oxygen vacancies, or modifications in the crystal structure of ZnO that are instigated by the green synthesis methodology. The aforementioned factors may lead to the emergence of localized states within the bandgap, resulting in a reduction of the bandgap and an improvement in photocatalytic activity. The reduction in bandgap enhances photocatalytic efficiency by enabling the absorption of light across a wider spectrum of the UV-visible range. The observed shift in the absorption edge towards longer wavelengths may suggest the presence of oxygen vacancies or defect states, which are recognized for enhancing visible-light absorption and increasing photocatalytic activity. The observed characteristics indicate that the decrease in the bandgap is primarily influenced by structural and electronic alterations occurring during the green synthesis process (Figure 4b), as opposed to quantum confinement effects. The biosynthesized ZnO NMs exhibit improved optical and electronic properties, making them especially appropriate for environmental applications, including water purification and pollutant degradation. The study illustrates the efficacy of low-energy UV irradiation in facilitating photoexcitation in ZnO, resulting in the generation of electron-hole pairs (e⁻ – h⁺) that play a crucial role in the degradation of NOM. The capacity of these ZnO NMs to absorb and activate in response to low-energy UV light further substantiates their potential utility in sustainable photocatalytic applications. Green synthesis of ZnO NMs has gained significant attention due to its environmentally friendly nature and its ability to produce nanomaterials with enhanced properties. Recent studies have demonstrated the use of plant extracts, such as neem, to synthesize ZnO NMs with improved photocatalytic and antimicrobial activities, highlighting the sustainability and efficiency of these approaches. Recent research has further demonstrated the benefits of green-synthesized ZnO in a range of applications, including wastewater treatment and antimicrobial treatments [38], [39], [40].

Figure 4:

Size distribution and optical properties of ZnO NMs. (a) Particle size distribution and (b) Tauc plot.

3.2 Photocatalytic degradation of natural organic matter

3.2.1 Impact of ZnO dosage on degradation

The influence of ZnO NM dosage on the degradation efficiency of natural NOM was examined by adjusting the ZnO concentration from 0.4 to 2.2 g/l, with all other parameters held constant (Figure 5A). The findings indicated a notable enhancement in degradation efficiency with increasing ZnO dosage up to 1.7 g/l, after which the efficiency stabilized. The maximum OCD removal of 79.2 % was attained at a concentration of 1.7 g/l within 180 min, suggesting the optimal loading of the photocatalyst (Figure 5A). At lower dosages (1.61 g/l), the degradation rate was diminished due to insufficient active sites for the breakdown of NOM (Figure 5B).

Figure 5:

Photodegradation study. (A) Degradation efficiency vs ZnO dosages, (B) time equilibrium study and (C) error bar graph of degradation efficiency vs ZnO dosages.

At a concentration of 2.2 g/l, the efficiency experienced a slight decline, presumably attributable to increased nanomaterial aggregation, which reduced the effective surface area and light penetration. The results highlight the importance of optimizing ZnO dosage to enhance photocatalytic performance while reducing material consumption and operational expenses. To ensure the reliability and robustness of the data, all photocatalytic experiments were performed in triplicate. Each measurement was repeated independently three times to account for experimental variability, and the data presented reflect the average of these three trials. Error bars, representing the standard deviation, are included in all figures to visually illustrate the variability and consistency of the results across replicates. These error bars provide insight into the degree of experimental variability and the precision of the data (Figure 5C). Statistical analysis was performed to assess the significance of the observed differences. The comparison of photocatalytic efficiencies at different ZnO concentrations revealed a significant trend, with p-values less than 0.05 for all comparisons. Specifically, the increase in efficiency with ZnO concentration up to 1.7 g/l and the slight decrease observed at 2.2 g/l were statistically significant. This indicates that the optimal ZnO dosage is approximately 1.7 g/l, beyond which no further significant improvement in photocatalytic efficiency is observed.

ZnO NMs are well-known for their photocatalytic capabilities, but comparative analyses have revealed that titanium dioxide (TiO₂), silver-doped ZnO (Ag/ZnO), and graphitic carbon nitride (g-C₃N₄) have distinct performance characteristics. TiO₂, a widely studied photocatalyst, typically requires high-energy UV light (UV–C) for effective degradation, while ZnO exhibits greater efficiency under lower-energy UV light. The high recombination rates of charge carriers in TiO₂ can limit its photocatalytic efficiency, while ZnO shows enhanced electron mobility and reduced recombination rates due to its nanostructure. Silver doping in ZnO (Ag/ZnO) can improve photocatalytic performance by facilitating charge separation, but it introduces additional complexity, cost, and raises environmental concerns due to potential metal leaching in aqueous environments, making the sustainability of Ag/ZnO questionable. Graphitic carbon nitride (g-C₃N₄) offers high photocatalytic stability but suffers from low charge-carrier mobility, restricting its overall efficiency. Green synthesis methods using NLE offer a cost-effective and sustainable route to produce ZnO nanoparticles, which demonstrate superior photocatalytic degradation of natural organic matter (NOM) under low-energy UV irradiation while avoiding toxic chemicals. The performance of green-synthesized ZnO compares favorably with TiO₂, Ag/ZnO, and g-C₃N₄ in terms of efficiency and environmental sustainability. Green-synthesized ZnO achieved a 92 % degradation of NOM within 180 min. The use of plant extracts like neem aligns with green chemistry principles, minimizing hazardous chemicals and enhancing eco-friendliness. Furthermore, green synthesis provides a more biocompatible and economically viable approach compared to traditional methods, with various plant extracts showing adaptability in producing ZnO nanoparticles with enhanced photocatalytic activity. The morphology and crystal structure of these nanoparticles can be confirmed using techniques like X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Surface modification of ZnO nanoparticles with Ag can further enhance their photocatalytic performance by promoting efficient charge separation. ZnO composites, such as ZnO/TiO₂ hetero-structures, also show promise for improving light absorption and charge separation efficiency. Overall, green-synthesized ZnO nanoparticles present a sustainable and efficient alternative for photocatalytic applications, and further research in this area can lead to the design of highly effective and environmentally friendly photocatalytic materials [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53]. The results of the current investigation indicate that ZnO attained a 92 % degradation of NOM within a time frame of 180 min, demonstrating a commendable performance when evaluated against these materials in relation to efficiency and environmental sustainability. Table 1 presents a comprehensive details associated with a range of photocatalysts, such as TiO₂, ZnO, Ag/ZnO, g-C₃N₄, and green-synthesized ZnO NMs, accompanied by pertinent references.

Table 1:

Comparison of photocatalytic properties of different materials.

Aspect	Details	Ref.
Photocatalytic capabilities	ZnO nanoparticles are well-known for their photocatalytic capabilities, with TiO2, Ag/ZnO, and g-C3N4 showing distinct performance characteristics	[41], 42]
Efficiency under UV light	TiO2‚ typically requires higher-energy UV-C light, while ZnO works better under lower-energy UV light	[41]
Charge recombination rates	TiO2‚ suffers from high recombination rates, reducing its photocatalytic efficiency, while ZnO exhibits enhanced electron mobility and lower recombination rates	[42], 43]
Silver doping in ZnO (Ag/ZnO)	Ag doping improves photocatalytic performance but introduces environmental and cost concerns due to metal leaching	[44]
Graphitic carbon nitride (g-C3N4)	g-C3N4 has high photocatalytic stability but suffers from low charge-carrier mobility	[42]
Green synthesis using neem leaf extract	Green synthesis methods using neem leaf extract provide a sustainable way to produce ZnO nanoparticles	[45]
Performance comparison with TiO2, Ag/ZnO, g-C3N4	Green-synthesized ZnO performs better than TiO2, Ag/ZnO, and g-C3N4 in terms of efficiency and environmental sustainability	[44], 45]
Degradation efficiency (NOM)	Green-synthesized ZnO achieved 92 % degradation of NOM within 180 min	[46]
Use of plant extracts for synthesis	Plant extracts like neem align with green chemistry principles by minimizing the use of toxic chemicals	[48], [49], [50]
Biocompatibility and sustainability	Green synthesis is biocompatible and economically viable, reducing environmental contamination	[48], 51]
Surface modification of ZnO with Ag	Surface modification with Ag nanoparticles can enhance ZnO’s photocatalytic performance by promoting charge separation	[46], 52]
ZnO composites (ZnO/TiO2‚ heterostructures)	ZnO/TiO2 composites improve light absorption and charge separation efficiency for photocatalytic applications	[38], 53]

3.2.2 Impact of low-energy UV light on decomposition rate

The impact of low-energy UV light on the degradation of NOM was assessed by analysing photocatalytic performance across various irradiation conditions. The experiments were performed under low-energy UV light (365 nm, 30 W) and in dark conditions to distinguish between photolytic and adsorptive removal mechanisms [54] (Figure 6).

Figure 6:

Digital image of degradation of NOMs solution with time.

The results indicated that NOM degradation under low-energy UV light was significantly greater than in dark conditions (Table 2). Under irradiation, approximately 92 % degradation of NOM occurred within 180 min, while in the absence of low-energy UV light, only 42 % removal was achieved. This indicates that adsorption contributed to NOM reduction, but was not the primary mechanism.

Table 2:

OCD removal efficiency across varied conditions.

Condition	OCD removal (%)
ZnO + UV	75
ZnO in dark	30
Control	10

3.3 Kinetic analysis of photocatalytic degradation

Kinetic analysis was conducted to evaluate the efficiency of biosynthesized ZnO nanomaterials in degrading NOM. The photocatalytic degradation process adheres to a pseudo-first-order kinetic model, commonly expressed by the Langmuir-Hinshelwood equation [55]:

where:

1. C _ο = initial concentration of natural organic matter (mg/L)

2. C _t represents the concentration of natural organic matter at time t, measured in mg/L.

3. k = reaction rate constant (min⁻¹)

4. t = irradiation duration (min)

The plot of ln (C₀/C_t) against time exhibited a linear relationship, thereby confirming the presence of pseudo-first-order kinetics. The slope of this line represents the rate constant k.

1. Under low-energy UV light:

   1. Slope = k = 0.012 min⁻¹

   2. This suggests an accelerated degradation process attributed to photoinduced ROS.

2. In low light conditions (adsorption contribution only):

   1. Slope = k = 0.005 min⁻¹

   2. The markedly reduced rate indicates that adsorption alone is inadequate for efficient NOM removal.

The kinetic study demonstrated that photocatalytic degradation followed a pseudo-first-order kinetic model, with a reaction rate constant (k) of 0.012 min⁻¹ under low-energy UV light, compared to 0.005 min⁻¹ in darkness. This suggests that photogenerated reactive oxygen species, especially •OH, were chiefly accountable for the degradation of natural organic matter, rather than mere surface adsorption. The spectral examination of NOM degradation by UV–vis spectroscopy at 254 nm demonstrated a progressive decline in absorbance over time, consequently validating the breakdown of NOM’s aromatic structures. The results validate the effectiveness of biosynthesized ZnO NMs in photocatalytic water treatment, underscoring its potential for significant environmental remediation.

3.4 OCD and mineralization

3.5 Influence of green synthesis on ZnO NMs’ performance

3.5.1 Reusability of biosynthesized ZnO NMs’

The green synthesis of ZnO NMs using neem extract offers notable environmental and functional advantages over traditional chemical methods. These biosynthesized nanomaterials exhibit improved stability, reduced toxicity, and lower energy consumption, making them a more sustainable choice for photocatalytic applications. By using plant-based reducing agents, hazardous chemicals are eliminated, minimizing the environmental impact of nanomaterial production. Additionally, biomolecules from neem enhance the dispersion of ZnO NMs and prevent aggregation, improving photocatalytic efficiency. Reusability studies showed that ZnO retained over 85 % of its photocatalytic activity after five cycles, confirming its long-term stability. No structural degradation was observed, further demonstrating its durability for water treatment. Biosynthesized ZnO is also cost-effective, making it suitable for large-scale wastewater treatment and sustainable NOM degradation. In addition, to assess its long-term stability, the reusability study was extended to seven cycles, revealing that ZnO ZnO NMs maintained over 67 % of their initial photocatalytic activity (Figure 7A). This high reusability is essential for practical applications in water treatment. XRD analysis confirmed no significant changes in the diffraction peaks after seven cycles, indicating the preservation of the hexagonal wurtzite structure (Figure 7B). These results emphasize the excellent reusability, stability, and potential of biosynthesized ZnO for sustainable environmental applications. According to Abdelfatah et al. [59], rGO/nZVI demonstrated exceptional recycling capabilities even after six consecutive cycles, in addition to superior DC adsorption performance. As a result, rGO/nZVI formed a novel composite that is thought to be useful for the adsorption of other environmental pollutants.

Figure 7:

Reusability and stability studies (A) Recycle test and (B) XRD analysis after 7^th cycle.

3.6 Antibacterial/antifungal potential

This study examined the antibacterial and antifungal properties of biosynthesized ZnO NMs against the Gram-negative bacterium E. coli and the fungus C. albicans. The diameters of the inhibition zones for both bacteria and fungi were assessed using the disk diffusion method (Figure 8), with the results detailed in Table 3. ZnO NMs demonstrate antimicrobial properties via three main mechanisms: interaction with bacterial cells, the release of Zn²⁺ ions, and the generation of reactive oxygen species (ROS). ZnO NMs, owing to their positive charge, exhibit a strong attraction to the negatively charged bacterial cell walls. The accumulation disrupts the bacterial membrane, modifies potassium channels, and results in cell death. Furthermore, Zn²⁺ ions interact with proteins, compromising bacterial structural integrity. The antibacterial efficacy of ZnO NMs was evaluated at concentrations of 0.05 mg/mL and 0.1 mg/mL utilizing the disk diffusion method (Table 3). Higher concentrations demonstrated increased inhibition. Nonetheless, the presence of lipopolysaccharides in Gram-negative bacteria may inhibit the activity of nanomaterials, thereby diminishing their effectiveness. A direct correlation exists between ZnO concentration and antibacterial potential. With increasing dosage, there was a corresponding rise in ROS, which heightened oxidative stress and resulted in damage to bacterial membranes. The findings (Figure 8) indicate that ZnO NMs demonstrate a dose-dependent antibacterial effect, positioning them as a viable option for antimicrobial applications. With a high concentration of 2 × 108 CFU mL⁻¹ against Gram-negative bacteria (E. coli, Klebsiella pneumonia), graphene oxide nanoparticles demonstrated strong antibacterial activity [60], while zinc oxide nanoparticles demonstrated antifungal activity against C. albicans [61].

Figure 8:

Antibacterial/antifungal studies: E. coli (left) and C. albicans (right).

ZnO NMs exhibit notable antimicrobial properties, effectively targeting microorganisms such as E. coli and C. albicans. This phenomenon is influenced by various mechanisms, with empirical evidence affirming their efficacy. A fundamental mechanism involves the production of ROS, including hydroxyl radicals, which inflict damage on microbial membranes. Upon exposure to ultraviolet or visible light, ZnO NMs generate electron-hole pairs that engage with water and oxygen, resulting in the production of reactive oxygen species. The presence of these highly reactive molecules leads to oxidative stress within microbial cells, resulting in membrane damage, the leakage of cellular contents, and ultimately culminating in cell death. In addition to the production of ROS, the liberation of Zn²⁺ ions from ZnO NMs plays a significant role in their antimicrobial efficacy. The interaction of these ions with microbial proteins and enzymes leads to structural alterations, which disrupt vital biological functions, consequently enhancing the bactericidal and fungicidal effects. Furthermore, ZnO NMs possess the capability to engage directly with microbial cell membranes via electrostatic attraction. The nanomaterials, possessing a positive charge, exhibit an attraction towards the negatively charged cell walls of bacteria and fungi. This interaction facilitates their potential penetration of the cell membrane, leading to destabilization and subsequent intracellular damage. The integration of these actions renders ZnO NMs exceptionally proficient in addressing microbial infections [62], [63], [64], [65].