Green synthesis of zinc oxide nanoparticles from Salvadora persica leaf extract: Characterization and studying methyl orange removal by adsorption

Zinc oxide (ZnO) is a versatile metal oxide with various desirable properties. ZnO is safe, and biocompatible with unique properties like optical, electrical, semiconducting, and chemical sensing. ZnO nanoparticles (NPs) are being used as promising nanomaterials as cosmetic additives, antibacterial, additives, chemical absorbents, catalysts, and polymer additives because of their long life span, specific surface area, and high pore volume. Due to these properties, it has been used in various industrial applications, such as solar cells, pharmaceuticals, cosmetics, photocatalysts, and gas sensors (Yang & Park 2008; Khorsand Zak et al. 2011; Salahuddin et al. 2015; Modi et al. 2022).

Different methods are currently used to synthesize ZnO NPs including sol-gel, thermal decomposition, solvothermal, chemical vapour synthesis, laser ablation, and precipitation strategies (Yedurkar et al. 2016). These methods are labour intensive, costly, and potentially harmful to living organisms and the environment. Consequently, the development of cost-effective and green synthesis methods received high attention from material scientists in the field of biotechnology due to low cost, environmental friendly nature, simplicity of implementation, and dependability. The green synthesis technique does not employ toxic chemicals but rather uses natural and easy-to-get resources such as enzymes, microorganisms, and plant extracts (Basnet et al. 2018). The facile green synthesis method is stable and produces metal NPs with well-defined sizes and shapes from their metallic salts. However, NP activity might be affected by shape, size, and surface chemistry. Many biological organisms including plants (Suresh et al. 2018; Iqbal et al. 2022; Modi et al. 2022; Pathania et al. 2022; Sharma et al. 2022), bacteria (Lengke et al. 2007), yeast (Kowshik et al. 2002), and fungus (Rautaray et al. 2003) have been demonstrated to have the ability to use reductive properties of their proteins and metabolites to transform inorganic metal ions into metal NPs. The capability of green synthesis and capping has been demonstrated in extensive research with plants from many taxonomic groups (Rautaray et al. 2003; Joglekar et al. 2011; Das et al. 2013; El-Rafie et al. 2013). Plant extracts have phytochemicals such as alkaloids, flavonoids, amino acids, alkaloids, vitamins, and alcohol that act as stabilizing and reducing agents for metal ions (Nava et al. 2017). These reductive agents can react with zinc salts to form ZnO NPs using different capping agents (Xu et al. 2021). Several researchers have synthesized ZnO NPs from plant extracts like Moringa oleifera extract (Matinise et al. 2017), Brassica oleracea L. var. italica extract (Sendal et al. 2022), peeled extract of Nephelium lappaceum L. (Karnan & Selvakumar 2016), etc.

Rapid industrialization and urbanization have resulted in the discharge of many chemical contaminants such as heavy metals and dyes into natural water bodies (Garg et al. 2020). Water pollution is considered a global issue, and various treatment methods have been developed. Dyes are the most extensive source of colour in leather goods, footwear, paper, pharmaceuticals, clothing, food, printing industries, etc. (Pathania et al. 2022). The presence of dyes in water has a negative influence on various aquatic organisms because they can generate cancer and mutagens (Mezohegyi et al. 2012). Azo dyes and inks account for more than half of the dyes and inks used in the textile industry owing to their colour range and low cost. Methyl orange (MO) is an azo-derived chemical compound with weak acidic properties and the potential to function as a carcinogenic agent (Vargas et al. 2021). Therefore, the removal of MO dye was investigated by various methods (Peerakiatkhajohn et al. 2021). However, successful methods or combinations of processes are yet to be explored, which can be inexpensive and environment friendly for complete removal.

This study describes, for the first time, a facile green approach for the synthesis of ZnO NPs using an aqueous leaf extract of Salvadora persica for MO removal. Salvadora is a large evergreen tree that is found in arid coastal areas, desert flood plains, and saline areas (Haque & Alsareii 2015). Traditionally, the plant is used for various purposes, including food, oral hygiene, fuel, cosmetics, and medications. Reuben et al. (2011) performed a phytochemical screening of the aqueous extract of S. persica leaves and showed the presence of flavonoids, sterols, saponins, tannins, and flavone aglycones (Reuben et al. 2011). These phytochemicals can be used as precursor agents to extract desired NPs (like silver NPs, Ag-doped ZnO NPs, silver nickel NPs, etc.), which have been reported in various studies. Few studies also reported the biosynthesis of ZnO NPs from S. persica. Those studies used S. persica root and leaf extract to synthesize ZnO NP for characterization and applications like cytotoxic activity, methylene blue dye removal, etc. (Miri & Sarani 2019; Verma et al. 2020). However, the precursors were different in those studies, which made morphologically different NPs of ZnO. Information on the contributing factors of dye removal is also rarely investigated. Very few studies are available on using S. persica leaf extract for the green synthesis of ZnO NPs; however, the removal of an important azo dye, i.e. MO removal by adsorption, and studying its operating factors are yet explored. Therefore, this study aimed to investigate ZnO NP preparation via green synthesis using S. persica leaves for MO removal by adsorption. After the synthesis, the optical and structural properties were analysed by UV-vis spectra, X-ray diffraction (XRD), and Fourier transform infrared (FTIR). Morphological and elemental properties were carried out by Field emission scanning electron microscopy-energy dispersive spectroscopy (FESEM-EDX). The removal study of MO was analysed by UV-vis spectroscopy on various operating factors like solution pH, adsorbent dose, the influence of contact time, and the initial dye concentration on dye removal. Finally, the adsorption isotherm of MO removal was analysed for the newly synthesized ZnO NPs.

Zinc nitrate hexahydrate Zn(NO₃)₂.6H₂O and pure MO (C₁₄H₁₄N₃NaO₃S) dye powders were supplied by Sigma-Aldrich and used as received. Fresh leaves of S. persica were collected from a local farm in Barka, Oman. All analytical grade chemicals were used as received.

The collected S. persica (SP) leaves were washed thoroughly with distilled water and dried under sunlight. The dried leaves were ground to powder using a grinder. S. persica leaves were then used to make boiled (SP) and unboiled leaf extracts (SU). To prepare boiled leaf extracts, distilled water was added to 3 and 9 g of SP powder and then boiled at 60 °C for 1 h with stirring. The plant extracts were cooled to room temperature and then filtered to obtain the clear extract. The synthesized NPs using 3 and 9 g of SP were labelled as ZnO-SP1 and ZnO-SP2, respectively. The unboiled leaf extract (labelled as ZnO-SU) was prepared using the same procedure but without heating.

Preliminary detection of the synthesized NPs was achieved through a visual colour change. Several analytical techniques have been employed to characterize the green-synthesized ZnO NPs. The optical properties of the prepared NPs were monitored by UV-Vis spectral analysis in the range between 200 and 700 nm using a UV-Vis spectrophotometer (Evolution 300-Thermo Fisher Scientific). FTIR spectra were obtained using FTIR spectrophotometer (Shimadzu spectrophotometer (Kyoto, Japan) in the 500–4000 cm⁻¹ region. Patterns of XRD were obtained for the synthesized NPs using an XRD apparatus (PANalytical, XPert PRO, USA) over the range of 2θ = 20–70° and with Cu-Kα (λ = 0.15406 nm) radiation. Elemental analysis was performed using an EDX. The shape and surface morphology of the prepared NPs were determined by field emission scanning electron microscopy (SEM; JSM-7600F-JEOL).

To evaluate the effectiveness of the synthesized ZnO NPs to remove MO, about 20 mg of the synthesized NPs was dispersed in 10 ml of 10 ppm of MO solution at room temperature and with a constant stirring for 30 min to ensure good particle dispersion in the solution. To study the influence of the pH of the solution on MO removal, experiments at various initial pH ranges of 1–7 were conducted for an initial dye concentration of 10 ppm. The influence of the adsorbent amount on dye removal was investigated by varying the NP dose from 0.01 to 0.05 g. To observe the effect of duration time on dye removal, the contact time was varied from 20 to 100 min. In addition, the influence of dye concentration was examined by varying the dye concentration from 5 to 20 mg/l, and the other experimental parameters were kept constant.

ZnO NPs were obtained using fresh leaf extract of S. persica, which contains stabilizing and reducing agents such as terpenoids and flavonoids (Aumeeruddy et al. 2018). The formation of ZnO NPs was confirmed by the visual examination. The addition of Zn(NO₃)₂.6H₂O to the leaf mixture has changed the colour to yellow, indicating the formation of ZnO NPs. After calcination, a yellowish-white powder was formed (Figure 1). This observation agreed with the results obtained by Khan et al. (2019).

The green-synthesized zinc NPs were analysed using FTIR to detect the phytochemicals that are stabilizing and capping the NPs. The FTIR spectra of the plant and synthesized ZnO NPs are presented in Figure 3(b). The highly intense peak located at 3,000–3,500 cm⁻¹ denoted the presence of -OH groups. The peaks located at 2917.17, 2849.13, and 2360.25 cm⁻¹ were attributed to the C = C alkyne stretching. The bands observed at 1,614, 1,427, and 1,085 cm⁻¹ were assigned to C = O stretching in polyphenols, C-N stretch in primary amines, and C-O stretching in amino acids, respectively (Alharthi et al. 2020).

The interaction of the functional groups with the ZnO NPs was identified as the cause of a slight change and shift in the position and intensity of some associated peaks in the produced nanoparticles' FTIR spectra. The extract's primary biomolecules were bound or capped to the ZnO NPs' surface (Alamdari et al. 2020). The obtained results confirmed the contribution of phenols, polyphenols, and primary amines in capping and stabilizing the formed ZnO NPs. The spectra shown in Figure 3(b) reveal the presence of a new band at 875 cm⁻¹, which was assigned to the ZnO stretching band (Jan et al. 2020). The FTIR spectra confirmed the successful capping of biomolecules on the synthesized NPs.

The size and crystalline nature of ZnO NPs were determined using XRD analysis. The XRD pattern provided peaks at 2θ from 0 to 70° in which the highest relative intensities of the ZnO NPs were observed at 2θ = 31.61, 34.28, 36.12, 47.39, 56.42, 62.69, and 68.90°. The sharp peaks are related to the (100), (002), (101), (102), (110), (103), and (112) crystallographic phase of a hexagonal wurtzite structure of zinc oxide (Kahsay et al. 2019). Similar patterns were reported using the leaf extracts of S. persica (Alharthi et al. 2020), Euphorbia sanguinea (Ekennia et al. 2021), and Costus woodsonii (Khan et al. 2019). The hexagonal wurtzite structure is more stable under ambient conditions than the cubic zinc blend. The XRD pattern contained only zinc oxide peaks, which confirms the purity of the synthesized ZnO NPs.

The shape of the ZnO NPs became clear and with a well-defined shape as the amount of plant extract increased. The active components increased with the increase of plant amounts which resulted in changing the morphology of the obtained NPs. It has been reported by Andres et al. (1990) and Pacholski et al. (2002) that the precursor concentration significantly affects the shape of ZnO nanostructures. In the study by Lakshmeesha et al. (2014), ZnO NPs with flaky flower, rose-like, hexagonal, bud, and a bell shape microstructure were found to form with increasing the amount of Neriumoleander leaf extract. In addition, many factors such as heating, plant extract, and concentration have been found to influence nanorod formation. In this study, the temperature at which that plant extract was prepared was found to play a major role in determining the morphology of ZnO NPs. In this regard, at 60 °C, rod-shaped NPs were obtained (Figure 4(a) and 4(b)), and at room temperature, hexagonal particles were formed (Figure 4(c)). From the microscopic analysis, it can be concluded that the extract preparation method plays a major role in obtaining different morphologies of ZnO. Boiling Salvadora leaf extract yielded a mixture of hexagonal and nanorod ZnO. However, hexagonal ZnO NPs were formed when Salvadora leaf extract was prepared without heating.

The EDX peaks (Figure 4) confirm the presence of zinc and oxygen, which clearly indicates the formation of ZnO NPs. The peaks showed that SB-1 had 40.8% Zn and 26.1% O₂, SB-2 had 58.2% Zn and 18.9% O₂, and SU had 62.3% Zn and 18.4% O₂. Minor elemental traces of aluminium, calcium, chloride, potassium, and magnesium, which were derived from the plant material, were also present.

The uptake of pollutants onto adsorbent materials is influenced by various parameters, including the concentration of the dye, the contact duration, the solution pH, temperature, and the dosage of adsorbent. The influence of some of these parameters on the MO removal by ZnO NPs was investigated. Different NP structures have different degrees of reaction sites. Sharma (2016) investigated the influence of different ZnO NP structures on Alizarin Red S dye removal (Sharma 2016). Among the studied structures, the nano-flower was found to be the most efficient photocatalyst.

The pH of the solution is critical for pollutant removal from wastewater. The influence of pH on MO removal using ZnO NPs was studied in batch experiments with a pH range of 1–7. The dye removal percentage versus the solution pH is presented in Figure 6(b). Figure 6(b) shows a steady increase in the MO percentage removals from pH 1.0 to 5.0 after which a slight decrease up to pH 7 was obtained. The maximum dye removal of 92% was observed with sample SB2 at the solution pH of 5.0. The decrease in dye removal after pH 5 is attributed to the interionic repulsion of negatively charged dye molecules and adsorbate surface. This result agrees with the study reported by Mokhtari et al. (2016), where the solution pH 5.0 showed the maximum MO removal by copper sulphide. The pH 4.0 was chosen for the following experiments due to its resemblance with real dye-polluted water.

ZnO NP dose is essential to determine the synthesized ZnO NPs' capacity to remove a given concentration of MO. To investigate the effect of ZnO NP dose on MO removal, the adsorbent dose was varied from 0.01 to 0.05 g. Figure 6(c) shows the influence of sample dosage on the MO removal. MO removal percentage was observed to increase markedly with the increase of ZnO NP amount due to the increased adsorption sites in the lattice, and hence, the removal was enhanced. The maximum removal of MO was found to increase rapidly from 36 to 64% with increasing SB2 from 0.01 to 0.05 mg. The same trend was observed with other samples. The maximum removal for SB1 and SU was found to be 56 and 54%, respectively, using a 0.05 mg sample.

Figure 6(d) illustrates the relationship between the primary MO concentration and the dye removal percentage. The MO removal decreased with the increase in MO concentration. Increasing dye concentration from 5 to 20 mg/l decreased the dye removal from 67.9 to 25.7%, 52.0 to 28.8%, and 43 to 20% using SB2, SB1, and SU, respectively. The active sites of ZnO NPs get saturated beyond a particular concentration, which may decrease in dye removal with the increasing initial dye concentration. The same trend was reported by Suresh et al. (2015); Cai et al. (2016); and Silva et al. (2019).

The removal process of the used NPs fitted well with Langmuir. This isotherm revealed monolayer dye absorption on the adsorbent surface (Ezekoye et al. 2020). This result is consistent with the study reported by Zhang et al. (2016) and Zafar et al. (2019).

Different studies investigated the green synthesis of ZnO and its application from S. persica. Table 2 summarizes a few of the reported studies with their synthesis and performance information.

It was obvious that the method of synthesis, synthesis agents, temperature condition, and extract source resulted in different outcomes including shape, size, and performance of ZnO NPs extracted from S. persica.

The study shows a green and promising synthesis process to synthesize ZnO NP with the use of S. persica leaf as a reducing and capping agent. The present study shows the effectiveness of the synthesized ZnO NPs for the removal of MO. The UV-Vis, EDX, and XRD showed a successful preparation of ZnO NPs. FTIR predicts the presence of phytochemicals for the effective formation of ZnO NPs. The SEM morphology presented a hexagonal and rod shape of ZnO NPs. The structural shape of ZnO NPs was highly influenced by the extract preparation method. The weight percentage of 58.2% Zn and 18.9% O₂ showed the maximum 80% MO removal at equilibrium removal. The MO removal was found to be influenced by various factors such as the shape of the synthesized ZnO NPs, contact time, initial concentration of dye, and solution pH. However, the shape of the synthesized ZnO NPs has a major influence on the removal of MO. The obtained results were analysed by the Langmuir, Freundlich, and Temkin isotherm models. The experimental data presented excellent fits for the isotherm models in the order: Langmuir > Freundlich > Temkin based on its correlation coefficient values. This study describes, for the first time, that different morphologies of ZnO can be obtained using the aqueous leaf extract of S. persica, and MO removal was found to be highly influenced by the structure of ZnO NPs.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.