Sustainable green synthesis of ZnFe₂O₄@ZnO nanocomposite using Oleaster tree bark methanolic extract for photocatalytic degradation of aqueous humic acid in the presence of UVc irradiation

The presence of humic acid in water raises serious concerns due to its ability to form carcinogenic trihalomethanes. It has an adverse effect on the esthetic water quality and may result in biofouling of pipelines with negative hygienic consequences (Naghizadeh et al. 2013b; Derakhshani & Naghizadeh 2014). The formation of disinfection byproducts (DBPs) due to the presence of humic compounds in water and acute health problems caused by it has increased the need to pay attention to the use of new methods of removing humic acid in water environments (Naghizadeh et al. 2013a; Algamdi et al. 2019). In recent years, several methods have been proposed to remove humic acid, such as surface adsorption using activated carbon, ion exchange resins, membrane separation, advanced coagulation, electromicrofiltration and so on. The use of these methods has limitations, such as low removal percentage, high investment and operation cost, lack of easy access, and production of excess sludge and contaminated wastewater, which increases the need for a very efficient and accessible method (Naghizadeh et al. 2015; Chianese et al. 2020; Tang et al. 2020).

Advanced oxidation processes (AOPs) are suitable and high-efficiency methods for purifying water containing pharmaceuticals, biological toxins, dyes, reactive oxygen species and NOMs and converting them into safe products such as carbon dioxide and water (Matilainen & Sillanpää 2010; Amor et al. 2019). The AOPs are based on the production of hydroxyl free radicals, which have a very high oxidizing power due to having at least one pair of free electrons. To produce these powerful hydroxyl radicals, ozone, H₂O₂, ultraviolet rays, etc. are used. The combination of these materials leads to the production of hydroxyl radicals with strong oxidizing power to decompose a significant number of organic materials (Sarathy & Mohseni 2006; Stasinakis 2008; Babu et al. 2019; Ghernaout & Elboughdiri 2020).

In the photocatalyst process, UV light is used to excite the semiconductor catalyst. In this process, the pollutant is exposed to UV radiation in the presence of photocatalytic particles in the course of surface reaction, in which electrons (e⁻) move from the valence band (VB) to the conduction band. As a result, positive holes (h⁺) are formed and oxidation and reduction reactions are carried out on the surface of the catalyst (Moctezuma et al. 2012).

ZnO is a semiconductor with the characteristics of chemical stability, high activity and environmental friendliness, which is widely used as a catalyst. However, although this semiconductor has excellent photocatalytic activity, due to its wide bandgap (3.2 eV), it causes unfavorable absorption in the visible light region, which reduces its efficiency against visible light. One of the effective solutions to solve this problem is using spill materials with a narrow bandgap such as ZnFe₂O₄ (1.9 eV) including favorable photochemical and magnetic stability characteristics and high performance in the visible light region. ZnFe₂O₄ alone also has weak photogradation activity. Therefore, it is expected that the combination and synthesis of the p-type semiconductor (ZnFe₂O₄) with the n-type semiconductor (Zn) and the ZnFe₂O₄@ZnO nanocomposite eventually, access to the surface of the reactants and as a result the photocatalytic process will increase (Sun et al. 2013; Rameshbabu et al. 2016; Wang et al. 2017; Yadav et al. 2018; Zouhier et al. 2020; Nguyen et al. 2022).

There are various physical and chemical methods for the synthesis of nanoparticles. Meanwhile, the method of green synthesis of nanoparticles using microorganisms, enzymes, plants and plant extracts is a compatible, environmentally friendly and cost-effective alternative to physical and chemical methods. This method causes the synthesis of nanoparticles in a wide range and with less contamination compared to other methods. The synthesis of zinc oxide nanoparticles using the green synthesis method shows the stabilization and high stability of nanoparticles (Geoprincy et al. 2013; Agarwal et al. 2017; Nabi et al. 2018).

In this research, the capability of ZnFe₂O₄@ZnO nanocomposite synthesized by the green synthesis method for the photocatalytic degradation of humic acid under the influence of UVc was studied. Also, the effects of parameters such as solution pH, contact time, nanoparticle dosage and different concentrations of humic acid were discussed and investigated.

In order to investigate the morphology characterization and specific surface of the synthesized nanocomposite, methods such as scanning electron microscopy (SEM) (HITACHI, S4160, Japan), energy-dispersive X-ray spectroscopy (EDS) (Oxford Instruments, United Kingdom), X-ray diffraction (XRD) (Philips, PW1730, Holland), Fourier transform infrared spectroscopy (FT-IR) (Thermo Nicolet, AVATAR, 370 FT-IR, USA) and dynamic light scattering analysis (DLS) (Horiba, Japan) were used.

To determine the zero charge potential of ZnFe₂O₄@ZnO nanocomposite, Erlenmeyer flasks containing 50 mL of distilled water at different pH levels and 0.025 g of nanocomposite were prepared. After stirring for 24 h, the final pH was determined.

Various parameters affecting the photocatalytic process such as pH (11–3), nanocomposite concentrations in the range of 0.1–0.005 g/L, different concentrations of humic acid (2–15 mg/L) during the contact time of 10–100 min were studied. At first, after adding the desired materials to the reactor, it was stirred for 30 min in the dark to reach the absorption–desorption equilibrium, and then the solution was exposed to the light of the lamp until the end. At the mentioned time intervals, 5 mL sampling was done and after separating the nanoparticles from the sample solution, it was determined by a spectrophotometer (UV/VIS spectrophotometer T80 + , PG Instrument Ltd, England) at wavelength 290 nm.

ZnFe₂O₄ nanoparticles were synthesized using previous studies with a slight modification. Typically, 8 g of iron salt (FeCl₃·6H₂O) was dissolved in 30 mL of degassed deionized water under vigorous stirring. Then, 3.86 g of zinc salt (Zn(NO₃)₂·4H₂O) was dissolved in 30 mL of deionized water and added to the iron salt solution. In this research, the molar ratio of Fe³⁺:Mn²⁺ was 2:1. The transparent solutions of zinc and iron were mixed together under vigorous stirring for 45 min. Then, 1 g of Sodium Dodecyle Sulfate (SDS) surfactant dissolved in 30 mL of distilled water was slowly added to the mixture. Then, aqueous sodium hydroxide was added to the above mixture to maintain the pH = 12. After 2 h, the zinc ferrite was synthesized after washing with water and ethanol, drying and calcining at 500 °C for 3 h. Secondly, ZnFe₂O₄@ZnO materials were synthesized by the co-precipitation route. First, 0.5 g of obtained ZnFe₂O₄ from the previous step was dispersed in 30 mL of deionized water for 30 min. Then, 1.2 g of Zn(NO₃)₂·4H₂O was added to the solution under vigorous stirring (pH = 12). The mixture was magnetically stirred for 2 h. Finally, the sample was dried and calcinated to 500 °C for 3 h (Shirzadi-Ahodashti et al. 2020; Nguyen et al. 2022).

After collecting the Oleaster tree bark and preparing it, extraction was done by percolation method using methanol. At first, after cleaning and washing, the plant was dried and immersed in methanol from the part of the separating funnel and the methanol was drained after a period of 12 h. We continued this process for 72 h (3 days) and then used a rotary apparatus to separate methanol from the ingredients to separate the desired extract.

In a further step, 1 g of the synthesized nanocomposite was mixed with 50 mL of ethanol and distilled water and dispersed under ultrasonic conditions for 30 min. Then, in another container, 6.25 g of Zn(NO₃)₂·6H₂O was slowly added to the container containing the ZnFe₂O₄ mixture. After 2 h of intense ultrasonic stirring, the final product was centrifuged and washed three times with water and ethanol. Then, to achieve the final product of sediment obtained it was calcined at 350 °C for 2 h (Krishnan et al. 2021).

According to the locations of the catalyst surface and the amount of UVc light transmission, with increasing concentration up to 0.05 g/L, the number of active sites on the nanocomposite surface and the production of hydroxyl radicals and finally the degradation of humic acid increases. Also, the results showed that with the increase in the concentration of nanocomposite, their accumulation in the solution increases, and as a disruptive factor, by preventing the passage of ultraviolet light, the degradation efficiency decreases. Therefore, the concentration of 0.05 g/L of catalyst was chosen as the optimal concentration for the next experiments. In the study of photocatalytic removal of humic acid by MnFe₂O₄@TiO₂ nanocomposite, it was shown that with increasing doses of nanocomposite, the rate of degradation of humic acid increased and then with increasing dose of nanocomposite pollutant degradation decreased again (Derakhshani & Naghizadeh 2022).

The reason for the decrease in efficiency with the increase in humic acid concentration can be that at lower pollutant concentrations, active sites on the surface of the nanocomposite are more available for the absorption of humic acid. Increasing the concentration of humic acid as an intervention led to a decrease in the available sites of the nanocomposite and an increase in repulsive forces. Also, at the beginning of the process, the degradation of humic acid by the nanocomposite happened fast, and after that, the rate of degradation was slower, which can be explained by the fact that in the early stages of degradation, due to the complete availability of nanocomposite sites, the process was carried out faster. After the reduction of the mentioned sites, the degradation rate became a little slower, and on the other hand, the increase in the concentration of humic acid interferes and reduces the production of hydroxyl radicals. Another reason for this is the electron excitation of the catalyst produced by hydroxyl radicals in the early stages. But at the concentration of 15 mg/L, due to the increase in the mass driving force of the humic acid solution, more pollutant molecules were transferred to the surface of the ZnFe₂O₄@ZnO nanocomposite and the degradation increased (Farzadkia et al. 2015; Kamranifar et al. 2019a, 2019b; Nguyen et al. 2022).

In this study, the photocatalytic degradation efficiency of humic acid using ZnFe₂O₄@ZnO nanoparticles was investigated in the presence of UV light. The green synthesized ZnFe₂O₄@ZnO nanoparticles were characterized with different analyses. Also, the effects of different parameters on the photocatalytic degradation process were investigated. The results of this study showed that the highest percentage of humic acid degradation by ZnFe₂O₄@ZnO nanocomposite was at pH = 3, the initial humic acid concentration of 15 mg/L and ZnFe₂O₄@ZnO nanocatalyst of 0.05 g/L. Also, after five stages of saturation and regeneration, this nanocatalyst still had a high capacity in humic acid degradation. Therefore, it can be said that this nanocomposite has a high ability to remove humic acid from water resources.

M. A. wrote the draft of the paper. A. N. was the supervisor of this research project and contributed to conception and design of the work. A. H. and S. M. contributed to acquisition and analysis of the data. A. J. and F. M. contributed to substantively revise the work. All authors read and approved the final manuscript.

The authors declare that no funds, grants or other support were received during the preparation of this manuscript.

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

The authors declare there is no conflict.