Introduction

Nanoscience and nanotechnology, which is rapidly growing and expanding in recent years, produces nanoparticles in the range of 1–100 nm by particle structure and size modification (Jamkhande et al. 2019). Today, the application of nanoparticles has increased in various fields of pharmaceuticals, textiles, medicine, printing, agriculture, environment and industry (Ren et al. 2019; Ardestani et al. 2020). Nanoparticles have characteristics such as large cross-sectional area, very small size, specific network order, crystalline shape, stability and high reactivity. These properties of nanoparticles have caused them to have various applications, one of the most important applications of which is the destruction and removal of toxic contaminates and organic pollutants from aqueous solutions and their transformation into harmless substances (Park et al. 2006; Ealia and Saravanakumar 2017). In the past decade, the use of spinel ferrites with the general formula MFe2O4 has increased, where M in these magnetic nanoparticles is a divalent metal such as Ni, Fe, Mn, Co and Zn (Yelenich et al. 2015). Among these spinel ferrites, NiFe2O4 nanoparticles have attracted the attention of researchers due to their magnetic and catalytic properties, chemical and thermal stability, low cost and easy preparation, low toxicity and high absorption ability. Nickel ferrite nanoparticles due to the mentioned features are used in catalysts, drug delivery, medical diagnostics, information storage, pigments, electronic devices and as absorbents (Zandipak and Sobhanardakani 2016; Lingamdinne et al. 2017; Sanadi et al. 2021). These nanoparticles can be synthesized by various physical, chemical and biological methods, but physical and chemical methods are not very suitable for the synthesis of nanoparticles. Because the reaction is carried out in special and long conditions, and also due to the use of expensive and dangerous chemicals, they are not compatible with the environment (Senapati et al. 2012). Green synthesis of nanoparticles, which is performed using microorganisms (bacteria, fungal and algae) and plant extracts (extracts from leaves, roots, flowers, seeds and barks of different plants), can be an economic and valuable alternative to physical and chemical methods on a large scale (Ying et al. 2022). Plant extracts are commonly used as stabilizing, reducing and capping agents to control crystal growth (Din et al. 2018). Pulicaria gnaphalodes is rich in phenolic compounds and has high antioxidant properties. This plant used for years in traditional Iranian medicine to treat heatstroke and severe diarrhea and as an anti-inflammatory. This plant, which belongs to the Asteraceae family, grows in hot and dry desert areas of Asia, Africa and Europe (Derakhshani et al. 2023). Today, the use of nanoparticles synthesized in a green way using plant extracts is considered as one of the newest methods to remove environmental pollutants.

Pharmaceuticals and personal care products (PPCPs) have wide applications in the fields of medicine, animal husbandry, industry, aquaculture, agricultural activities, cosmetic care and people's daily life and have been considered as emerging water pollutants. These products are a threat to humans and aquatic ecosystems due to their widespread use and poor purification by biological processes of wastewater (Wang and Wang 2016; Barceló and Petrovic 2007). PPCPs include several classifications, among which antibiotics are one of the most widely used categories in the prevention and treatment of infectious diseases in humans and animals (Kovalakova et al. 2020). Antibiotics are stable chemical compounds that after use, a small amount of them changes in the body, and a large percentage of them are excreted from the human and animal body unchanged and enter the environment through viruses or feces (Javadmoosavi et al. 2023). Also, pharmaceutical industry wastewater is one of the important industries in the introduction of antibiotics into the environment (Xian et al. 2010; Qin et al. 2021). The high concentration of antibiotics in the environment causes an increase in antibiotic resistance in pathogenic bacteria and as a result drug resistance in humans (Dimitrakopoulou et al. 2012). The antibiotic penicillin G (PG) (C16H17N2NaO4S), which is soluble in water, is one of the most commonly used antibiotics and belongs to the class of beta-lactam antibiotics (Ma et al. 2018). This antibiotic, which is used against vulnerable organisms, especially Gram-positive bacteria, destroys the cell wall of these bacteria by preventing the production of peptidoglycan (Peterson et al. 2012). Considering that, common processes in water and wastewater treatment are not able to remove and destroy these pollutants, method such as advanced oxidation processes (AOPs) should be used to eliminate them (Naghizadeh et al. 2017). Advanced oxidation processes are one of the practical methods for removing water-soluble pollutants, which is based on the production of hydroxyl free radicals using ozone, H2O2, UV and a combination of them. These free radicals have a high power in destroying strong and toxic pollutants due to having free electron (Akbari et al. 2019; Derakhshani and Naghizadeh 2014). Photocatalytic degradation, as one of the advanced oxidation processes, is a sustainable, clean, environmentally friendly and economical method in energy consumption, and it can be used to remove high concentrations of antibiotics in aqueous solutions (Gusain et al. 2019; Koe et al. 2020). In the present study, green synthesis of NiFe2O4@ZnO nanocomposite using Pulicaria gnaphalodes was examined for photocatalytic degradation of PG in aqueous solution.

Materials and methods

Materials

At the first, penicillin G powder was purchased from Sigma-Aldrich, and then, penicillin G stock solution with a concentration of 500 mg/L was prepared using double-distilled water. Other chemicals used in this study include iron nitrate (Fe (NO3)3·9H2O), nickel chloride salt (NiCl2.6H2O), sodium dodecyl sulfate (SDS) surfactant, zinc nitrate salt (Zn (NO3)2.6H2O), sodium hydroxide (NaOH), hydrochloric acid (HCl), methanol and ethanol were purchased from Merck (Germany). UV-C lamp (Philips, Poland) with a power consumption of 9 watts, radiation intensity of 282–294 W/m2, wavelength of 253.7 nm, diameter of 2 cm and length of 22 cm was installed and used in the center of the reactor.

Preparation of methanolic plant extract

For the synthesis of NiFe2O4@ZnO nanocomposite, the Pulicaria gnaphalodes plant was collected from around the Birjand city (the capital of South Khorasan Province, Iran) and transported to the laboratory. This plant was washed three times with distilled water and then placed at room temperature to dry. To prepare the extract, some of the dried powder of the plant was contacted with methanol solution, and then, the solution was passed through Whatman No. 42 paper. A rotary vacuum evaporator (Hei zbad WB eoc, Germany) was used for methanol concentration and plant extract separation. The schematic of methanolic extract of Pulicaria gnaphalodes is shown in Fig. 1.

Fig. 1
figure 1

Schematic of methanolic extract of Pulicaria gnaphalodes

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Biosynthesis of NiFe2O4@ZnO nanocomposite

First, for the synthesis of nanocomposite NiFe2O4, 5 g of iron nitrate hexahydrate (Fe (NO3)3·9H2O) was added to 70 mL of deionized water and dissolved in water under nitrogen gas for 1 h. In another container, SDS surfactant (1:1 molar ratio to iron salt) was dissolved in 30 mL of distilled water and added to iron salt as a coating agent. The reaction was continued for 30 min under vigorous stirring at room temperature. Then, 1.47 g of nickel chloride salt (NiCl2.6H2O) was added to the reaction solution and 2 M sodium hydroxide to increase the pH of the reaction to 12. After 2 h, the sediments obtained were washed and dried after centrifugation at room temperature for 24 h. NiFe2O4 nanocomposite was obtained by calcining the deposits at 600 °C for 3 h.

A molar ratio of 1:1 was used for the synthesis of NiFe2O4/ZnO nanoparticles. For this purpose, 0.5 g of NiFe2O4 nanoparticles was dispersed in 20 mL of distilled water for 30 min under ultrasonic conditions. Then, 1 g of zinc nitrate salt (Zn(NO3)2·6H2O) was dissolved in 10 mL of distilled water and slowly added to the reaction solution together with the Pulicaria gnaphalodes plant extract at 45 °C. After 30 min, sodium hydroxide solution (2 M) was slowly added to the reaction (pH 12). The reaction continued for 2 h under magnetic stirring. Then, after washing and drying, the obtained precipitate was calcinated at 500 °C for 2 h. The schematic of NiFe2O4@ZnO green synthesis is shown in Fig. 2.

Fig. 2
figure 2

Green synthesis of NiFe2O4@ZnO with Pulicaria gnaphalodes plant methanolic extract

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Results and discussion

Structural analysis

FT-IR analysis

FT-IR spectrum was used to identify of NiFe2O4@ZnO nanocomposites. FT-IR spectrum of biosynthesized NiFe2O4@ZnO nanocomposites is shown in Fig. 3. The stretching vibration band of Zn–O bonding was observed at 462 cm−1 (Moradi et al. 2018). It can also be seen that the absorption bands at 473 and 582 cm−1 are related to the metal–oxygen (M–O) vibration of the tetrahedral and octahedral sites of the spinel structure (Liu et al. 2013).

Fig. 3
figure 3

The FT-IR spectrum of NiFe2O4@ZnO nanocomposite

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XRD analysis

The X-rays pattern displays the formation of the spinel phase structure of NiFe2O4 without any additional impurity peaks in Fig. 4. The diffraction peaks appeared at the diffraction angles 2θ = 30.295°, 35.686°, 37.330°, 43.375°, 53.820°, 57.377° and 63.015° which could be indexed based on standard (JCPDS card No.74-2081) to (220), (311), (222), (400), (422), (511) and (440), respectively, correspond to spinel structure of NiFe2O4 (Bashir et al. 2020).

Fig. 4
figure 4

The XRD pattern of NiFe2O4@ZnO nanocomposite

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DLS analysis

Dynamic light scattering (DLS) and zeta-potential analysis were utilized to characterize the hydrodynamic size and surface charge of NiFe2O4@ZnO nanocomposites. The DLS graph of biosynthesized NiFe2O4@ZnO nanocomposites is shown in Fig. 5. The DLS results exhibited that the average hydrodynamic size of the NiFe2O4@ZnO nanocomposites was about 90–120 nm. The negative or positive charge on the surface of products prevents the aggregate of samples by repulsion of similar charges. The zeta-potential value was calculated to be − 6.2 mV.

Fig. 5
figure 5

The DLS analysis of NiFe2O4@ZnO nanocomposite

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VSM analysis

Since the magnetic property of a catalyst is an essential parameter in separation and recovery processes (Ataei et al. 2021). The magnetic properties of the sample were analyzed by VSM analysis. The magnetic properties of NiFe2O4@ZnO were measured at room temperature and are shown in Fig. 6. According to the results, the magnetic saturation value for nanocomposite was 47 emug−1. VSM analysis showed that the prepared nanocomposite has paramagnetic properties, and according to this analysis, it can be seen that the studied nanocomposite can be dispersed in water within a few minutes and separated using an external magnetic field. Then, it dispersed again with a small shake (Esmati et al. 2021).

Fig. 6
figure 6

The VSM pattern of NiFe2O4@ZnO nanocomposite

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TEM analysis

In this study, the dimensions and structural characteristics of the NiFe2O4@ZnO nanoparticles were meticulously examined using transmission electron microscopy (TEM), as depicted in Fig. 7. The TEM micrograph reveals distinct spherical and cubic morphologies of the synthesized nanoparticles. Notably, the dopant distribution on the NiFe2O4 substrate is discernible and well-defined. Upon closer scrutiny, the size of the synthesized NiFe2O4@ZnO nanoparticles can be reliably estimated to fall within the range of 75–85 nm. This observation underscores the meticulous control achieved during the synthesis process, confirming the desired morphology and size uniformity of the nanoparticles.

Fig. 7
figure 7

The TEM micrograph of NiFe2O4@ZnO nanocomposite

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EDS mapping analysis

The investigation of the synthesized NiFe2O4@ZnO nanoparticles through scanning electron microscopy included an elemental mapping analysis employing energy-dispersive spectroscopy (EDS). Figure 8 illustrates the EDS mapping spectrum of the NiFe2O4@ZnO magnetic nanoparticles. The elemental mapping of NiFe2O4@ZnO nanoparticles revealed a distinct presence and even distribution of nickel (Ni), iron (Fe) and zinc (Zn) elements, thereby providing conclusive evidence regarding the successful synthesis of magnetic NiFe2O4@ZnO nanoparticles. This uniform distribution underscores the precision and efficiency of the synthesis process. Furthermore, the absence of any detectable impurities in the synthesized nanoparticles, as indicated by the EDS results, further substantiates the purity and quality of the fabricated NiFe2O4@ZnO nanoparticles. The overall findings strongly support the assertion that the synthesis process was indeed successful and devoid of any undesirable contaminants.

Fig. 8
figure 8

The EDS analysis and mapping of NiFe2O4@ZnO nanocomposite

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Photocatalyst experiments

pHZPC

Using pHzpc, the surface charge of the catalyst was investigated. To determine the pHzpc of the produced nanocomposite, 11 containers containing 50 mL of distilled water with different pHs (2, 3, 4, 5, 6, 7, 8, 9, 10, 11 and 12) were prepared. Then, a specific dose of nanoparticles was added to the solution. After 24 h, the results were recorded. The diagram of Fig. 9 shows the amount of surface charge of NiFe2O4@ZnO nanoparticles. As a result, at higher pH 6, the absorbent surface has a negative charge, and as a result, ions with the opposite charge (positive ions) are easily adsorbed by nanocomposite. Also, at pH lower than 6, the surface charges of the catalyst become positive and negatively charged ions are absorbed faster.

Fig. 9
figure 9

Diagram of pHzpc of NiFe2O4@ZnO synthesized by Pulicaria gnaphalodes

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This issue can be justified by the fact that at pH lower than pHzpc, the surface charge of the catalyst is positive, so the nanoparticles can react more effectively with the hydroxyl groups in the solution and increase the penicillin G removal efficiency. It can also be explained that in acidic pH, the holes produced by light irradiation on the catalyst surface act as the dominant oxidizing agent, these holes migrate to the catalyst surface in the valence band and interact with water molecules and ions. Hydroxyls react on the surface and produce hydroxyl radicals and hydrogen ions, which lead to the decomposition of pollutants adsorbed on the surface of the photocatalyst (Canle et al. 2017).

Determination of pH effect

The results of the studies show that pH directly affects radical production (Norzaee et al. 2017); as a result, it is considered an important factor in the photocatalyst process. According to pHzpc, for pH values lower than 6, the catalyst surface is often positively charged (h+ ions), and for pH values higher than 6, the catalyst surface is ignited by OH− ions (Naghizadeh et al. 2020; Ghaemi et al. 2014). PG is a weak acid in water-soluble state (pKa = 2.75) (Hossain and Dean 2008). As a result, if the pH of the solution is higher than the pKa value, the antibiotic PG tends to become anion. Also, if the pH is higher than 6, it will have a negative surface charge of the adsorbent and an anionic nature, and we can observe the electrostatic repulsion between the catalyst and PG. It is created to reduce the degradation of PG, but the experiments went against our expectations. According to Fig. 10, despite repeating the experiment, this nanoparticle showed that it has higher removal efficiency in alkaline environment.

Fig. 10
figure 10

Effect of reaction time and pH in photocatalytic degradation of PG by NiFe2O4@ZnO nanocomposite (C0 = 20 mg/L, nanocatalyst dosage = 0.6 g/L, T = 25 °C)

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The pollutant concentration of 20 mg/L and the nanocatalyst dose of 0.6 g/L were considered at room temperature for the pH ranges of 3, 5, 7, 9 and 11, and the results showed a direct relationship between pH and removal efficiency, so that the highest the pollutant removal efficiency was related to pH 11, which is the most alkaline. The reason for this behavior can be explained that in the alkaline environment, there are more hydroxide ions (OH) on the surface of nanoparticles that are easily oxidized and form hydroxyl radicals (OH) with a very high oxidation potential, which increases that pollutants are destroyed. Also, the instability of the β-lactam ring in alkaline environments can be effective in further degrading the pollutant (Elmolla and Chaudhuri 2010).

Nanocatalyst dose effect

After determining the optimal pH in the previous step, the effect of catalyst dosage on PG degradation efficiency was tested in the range of 0.1–1.25 g/L, pollutant concentration of 20 mg/L at room temperature. According to the results that can be seen on the graph of Fig. 11 initially, with the increase in the dose of nanocomposite, the amount of pollutant removal has also increased, but for concentrations higher than 0.6 g/liter, the power of nanoparticle destruction has a downward trend. The reason for the upward trend in the graph can be explained by the active sites on the surface of the nanocatalyst, which increases the dose of nanoparticles, increases hydroxyl radicals and increases the pollutant degradation efficiency (Kamranifar et al. 2019) and can be the reason for its downward trend for concentrations higher than 0.6 g/L. The inability to penetrate UV light in the solution is due to the lack of the number of nanoparticles (Daneshvar et al. 2004).

Fig. 11
figure 11

Effect of dose of ZnFe2O4@ZnO nanocomposite in photocatalytic degradation of PG by NiFe2O4@ZnO nanocomposite (C0 = 20 mg/L, pH = 9, T = 25 °C)

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Determination of the effect of PG concentration and contact time

The efficiency of the NiFe2O4@ZnO degradation process can be impacted by two key factors: PG pollutant concentration and contact time. In this study, we investigated the effect of PG concentration (ranging from 10 to 150 mg/L) on its photocatalytic degradation at room temperature (other parameters were fixed in optimized values, pH = 11, catalyst dose 0.6 g/L). The graph of Fig. 12 displays the results, indicating a decrease in nanoparticle removal efficiency as PG concentration increases from 10 to 150 mg/L. As the concentration of pollutants increases, the efficiency of the removal process decreases. This decrease in efficiency can be attributed to several factors. For instance, the formation of OH radicals in the photocatalytic process remains constant for a fixed amount of catalyst dose.

Fig. 12
figure 12

Effect of PG concentration in photocatalytic degradation of PG by NiFe2O4@ZnO nanocomposite (C0 = 20 mg/L, nanocatalyst dosage = 0.6 g/L, pH = 9, T = 25 °C)

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As a result, the available OH radicals may not be sufficient to decompose high concentrations of penicillin G. Furthermore, increased concentration of pollutants leads to the absorption of light by antibiotic molecules, making them less exposed to UV photons (Derakhshani et al. 2024). In a study entitled "Photocatalytic Removal of PG Pollutant from Aqueous Solution with Fe+3–TiO2 Nanoparticles" which conducted by Dehghani et al., the results showed that the efficiency of antibiotic removal increased as the initial concentration of PG decreased (Dehghani et al. 2014). Based on the graphs shown in Figs. 10, 11 and 12, which were used to determine the optimal state of photocatalytic activity of the nanocatalyst, it can be concluded that all experiments had a contact time ranging from 0 to 120 min. The results indicate that there is a direct relationship between the increase in contact time and the amount of removal, as the number of OH radicals and positive generating holes increases with longer contact time. This is a predictable outcome. Although the slope of the graphs is higher in the first 20 min, it may be due to the active surfaces on the nanocatalyst rapidly saturating (Ghauch et al. 2009). A study entitled "Graphitic carbon nitride loaded on powdered mesoporous silica nanoparticles for photocatalytic tetracycline antibiotic degradation under UV-C light irradiation" was conducted in 2023. The study found that the highest removal efficiency occurred when the contact time was 120 min, which was the longest contact time tested in the experiment (Naghizadeh et al. 2023). The schematic of photocatalytic degradation of PG by NiFe2O4@ZnO is shown in Fig. 13.

Fig. 13
figure 13

The schematic of photocatalytic degradation of PG by NiFe2O4@ZnO

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Conclusion

This study investigated the synthesis of NiFe2O4@ZnO nanocomposite using methanolic extract of elder tree bark. Various analyses such as FT-IR, XRD, DLS, VSM, TEM and EDS were performed and reported to identify the structure and elemental composition, which indicates the successful synthesis of magnetic nanocomposites with a diameter of 75–85 nm. Also, the synthesized magnetic nanoparticles were able to remove the penicillin G antibiotic in aqueous solutions under UV light, and the optimal conditions for doing it were also reported in the study.