Biosynthesis of MnFe2O4@TiO2 magnetic nanocomposite using oleaster tree bark for efficient photocatalytic degradation of humic acid in aqueous solutions

The presence of humic acid compounds in water resources, as one of the precursors of Trihalomethanes and Holoacetic acids, causes health problems for many communities. The aim of this research study was to investigate the photocatalytic degradation efficiency of humic acid using MnFe2O4@TiO2 nanoparticles which produced by green synthesis method. The synthesis of metal nanoparticles using plant extracts and the study of their catalytic performance is a relatively new topic. Many chemical techniques have been proposed for the synthesis of MnFe2O4@TiO2 nanoparticles, but green synthesis has received much attention due to its availability, simplicity, and non-toxicity. The properties of synthesized nanoparticles were determined by SEM, FT-IR, XRD, EDS, and DLS analysis. The results of the study showed that under optimal experimental conditions (pH = 3, nanocomposite dose = 0.03 g/L, humic acid initial concentration = 2 mg/L, and contact time = 20 min), it is possible to achieve maximum degradation of humic acid. Therefore; MnFe2O4@TiO2 nanoparticles have high efficiency for removing of humic acid from aqueous solutions under UV light.


Introduction
Natural organic matters (NOMs) are macromolecule that exist directly in surface and groundwater resources and are formed through the biological and chemical decomposition of plant and animal tissues (Derakhshani and Naghizadeh 2018). Natural organic matters are divided into two categories: hydrophilic and hydrophobic which humic substances are hydrophobic. Humic substances make up 60 to 90% of natural organic matter. Humic substances are divided into three categories. First and most important is humic acid, which is completely insoluble (Naghizadeh et al. 2015;Derakhshani and Naghizadeh 2018). Removal of humic acid from the water resources is essential before disinfection process because it causes serious problems in the water treatment operation. The most important problem of humic acid presence in water resources is its reaction with chlorine during the disinfection process, which produce disinfection by-products (DBP s ) including trihalomethanes (THM s ) and haloacetic acids (HAA s ). These two types cause serious problems for human health, including toxicity, mutagenicity, carcinogenesis, and damage to the kidneys and bladder (Khodadadi et al. 2020;Akbari et al. 2021). DBP s cause other problems such as adverse effects on taste and color, increased presence of toxic substances and heavy metals, increased growth of microorganisms in water distribution networks and water resources, negative effects on membrane performance, and increased use of disinfectants (Naghizadeh et al. 2017).
Different processes such as adsorption, advanced oxidation process, membrane separation, chemical coagulation, ion exchange, and biodegradation have been studied to removal of humic acid from aqueous solution (Mohammadi et al. 2022). But there are a lot of problems in their application for humic acid removal. One of the problems of using the coagulation process in the removal of humic acid is the production of high sludge and high consumption of coagulants, which increases the costs of operation and maintenance in the water treatment process (Mohammadi et al. 2022). One of the limitations of using ion exchange to remove natural organic compounds is the slow kinetics of the process (Asgari et al. 2020. Nowadays, advanced oxidation processes (AOPs) for treatment of water and wastewater contaminated with hazardous organic substances is the most used advanced technique (Lou et al. 2016). Advanced oxidation processes (AOPs) are based on the production of chemical free radicals such as hydroxyl which have high oxidation power for the degradation of organic molecules of an aqueous solution (Kamani et al. 2021). The mechanism of this process involves the irradiation of ultraviolet radiation into a semiconductor material, and as a result of this radiation, electron excitation occurs from the capacitance band to the conduction band, and this electron excitation causes the production of hydroxyl radicals in aqueous solution (Kord Mostafapour et al. 2016;Shirzadi-Ahodashti et al. 2020b). Among advanced oxidation processes, photocatalytic process due to their high degradation potential are widely used in water treatment (Maleki et al. 2016). In the photocatalytic process, ultraviolet (UV) irradiation, visible light, and metal catalysts are used as semiconductors (Kamranifar et al. 2019;Khodadadi et al. 2018). In recent years, the use of metal semiconductor photocatalyst is very important due to their easy to use in conditions of environmental and the ability to complete mineralize the pollutants, in different environmental fields including water and wastewater treatment processes (Vaiano et al. 2015). Photocatalysts are materials which a pair of electron-holes are created by artificial light or sunlight on their surface so that the free radicals resulting from this action have the oxidizing properties of the material. In fact, photocatalysts only provide the conditions for reactions (Bora and Dutta 2014;Shirzadi-Ahodashti et al. 2020a). Recently, the use of magnetic nanocatalysts in various fields of research has developed because nanocatalysts can be recycled (Wang and Astruc 2014;Sharma et al. 2018). Nanoparticles with a diameter between 1 and 100 nm have special properties such as regular morphology (Shirzadi-Ahodashti et al. 2020a). Nanoparticles are very important because of their very small size and large surface to volume ratio, which cause to physical and chemical differences in their properties (Iravani 2011). There are various methods to produce nanoparticles such as ultrasonic wave reduction, chemical reduction, microwave synthesis, photocatalytic reduction, electrochemical reduction, nanoparticle vapor phase synthesis, radiation reduction, and green synthesis (Yang et al. 2020;Zhou et al. 2021;Choi et al. 2021). Using nanoparticles synthesized by green methods has several advantages such as environmental compatibility, simplicity, low cost, and one-step without the using of chemical coatings agent and reduction agent. Green nanotechnology refers to the synthesis of nanoparticles through biological pathways such as the use of plants, microorganisms, and viruses using various biotechnological tools (Parveen et al. 2016). Most recently, the green synthesis of nanoparticles has received special attention of researchers, due to being natural and low cost (Mortazavi-Derazkola et al. 2021;Rana et al. 2020).
In this work, the production of the MnFe 2 O 4 @TiO 2 magnetic nanoparticle with the green synthesis method using oleaster tree bark was investigated. The properties of nanoparticles were recorded by SEM, FT-IR, XRD, EDS, and DLS analysis. The prepared MnFe 2 O 4 @TiO 2 nanoparticles have been studied for their catalytic activity in the photocatalytic degradation of humic acid from aqueous solutions. In the photocatalytic process, UVc light was used to degradation of humic acid. Finally, the effects of different variables on the photocatalytic degradation process of humic acid including contact time, pH, concentration of humic acid, and nanoparticle dosage were investigated.

Materials and characterization
Humic acid with purity of 54% was purchased from ACROS Company (USA). The stock solution with concentration of 500 mg/L was prepared using double distilled water. The required solutions were prepared via dilution by using double distilled water to the desired concentrations. Other chemicals such as iron(III) chloride hexahydrate: FeCl3.6H2O, manganese(II) chloride: MnCl 2 .4H 2 O, tetra-n-butyl orthotitanate: (CH 3 CH 2 CCH 2 CH 2 O) 4 Ti, sodium hydroxide: NaOH, and ammonia: NH 3 , methanol, ethanol were purchased from Merck company(Germany).

Preparation of oleaster tree bark extracts
An alcoholic extract of oleaster tree bark was used for the green synthesis of MnFe 2 O 4 @TiO 2 magnetic nanocomposite. The oleaster tree bark were collected from the South Khorasan province of Iran and transferred to the laboratory. First, the oleaster tree bark was crushed into small pieces and then, to remove the dust, oleaster tree bark was washed with sterile double-distilled water completely. After washing, it was left at room temperature to dry. For alcoholic extraction, 20 g of dried oleaster tree bark was contacted with 400 ml of methanol for 3 days. Then, the obtained solution was passed from the Whatman paper No. 42 and rotary apparatus was used for methanol separating.

BiosynthesisofMnFe 2 O 4 @TiO 2 nanocomposite
First, 50 mL of distilled water was exposed to nitrogen gas for 30 min. Then, 10.81 g of FeCl 3 was added to the distilled water container under vigorous stirring. After 20 min, 2.87 g of MnCl 2 was dissolved in 30 mL of distilled water and added to the iron solution. Afterwards, 4 M NaOH was used to increase the pH. After adjustment of the pH, the solution was stirred vigorously for 60 min. The solution containing MnFe 2 O 4 nanoparticles centrifuged at 6000 rpm for 4 min and then dried at 70 °C for 24 h. Finally, MnFe 2 O 4 nanocomposites were prepared after calcination at 600 °C for 3 h.
Next, for the synthesis of MnFe 2 O 4 @TiO 2 nanocatalyst, 1 g of as-obtained MnFe 2 O 4 was dissolved in 20 mL of ethanol and 50 mL of deionized water under ultrasonic waves for 30 min. 0.25 g of extract of oleaster tree bark was dissolved in 15 mL of deionized water under ultrasonic waves for 15 min. After MnFe 2 O 4 nanoparticles were dispersed in water and ethanol, the solution was stirred vigorously and 1.46 mL of tetra-n-butyl orthotitanate was added slowly to the solution. Then, the oleaster tree bark extract was added to the solution, and after 30 min of vigorous stirring of the solution, ammonia was used to increase the pH. After pH adjustment, the solution was stirred vigorously for 120 min. The solution containing MnFe 2 O 4 @TiO 2 nanoparticles centrifuged at 6000 rpm for 4 min and then dried at 70 °C for 24 h. Finally, MnFe 2 O 4 @TiO 2 nanocomposites were prepared after calcification at 600 °C for 3 h.

Photocatalytic experiments
Photocatalytic reaction experiments were performed using MnFe 2 O 4 @TiO 2 magnetic nanocomposites in a 300-mL photochemical reactor. The reactor used in this study consists of two main chambers (main reaction chamber and recirculation water chamber). The schematic of the reactor used for the photocatalytic degradation of humic acid is shown in Fig. 1. All testing steps are performed in the main chamber. The secondary chamber is located around the main chamber and reduces the temperature of the UV-C radiation chamber by continuously circulating water flow. A lamp was installed as a UV-C radiation source in the center of the main reactor, which had a power consumption of 18 W, a generated wavelength of 253.7 nm, a radiation intensity of 282-294 W/m 2 , and a length of 22.5 cm. The removal performance of humic acid in the photocatalytic process using MnFe 2 O 4 @TiO 2 magnetic nanocomposite was tested as a function of different solution pH values (3,5,7,9,11), initial HA concentrations (2, 5, 7, 10, and 15 mg/L), nanocomposite dosages (0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.1 g/L), and reaction times (10, 20, 40, 60, and 100 min). The flasks were kept in the dark position for about 30 min before starting of the photocatalytic process under UV-C light to determine the adsorption/desorption equilibrium. At various irradiation times, 5 mL of the suspension was carefully taken from the reacted HA solution, then centrifuged at 6000 rpm for 8 min to completely separate the nanocatalyst particles from the solution. The initial and residual concentrations of HA in the samples were measured using a UV spectrophotometer (T80, UV/Visible, UK) at a spectral peak of 254 nm. The removal efficiency of humic acid (%R) was determined using the following equation: where C 0 is the initial concentration of HA (mg /L) before the photocatalytic process and C t is the residual concentration of HA after the process.

Result and discussion
CharacterizationofMnFe 2 O 4 @TiO 2 nanocomposites XRD analysis XRD analysis results (Fig. 1) (Fig. 2b), the MnFe 2 O 4 @TiO 2 nanocomposites showed sharp peaks related to TiO 2 as well as some additional peaks which were assigned to anatase TiO2

Scanning electron microscopy
The scanning electron microscopy was applied to characterize the mean particle size and morphology MnFe 2 O 4 @ TiO 2 nanocomposites. Figure 3 shows the SEM images of MnFe 2 O 4 @TiO 2 nanocomposites synthesized by co-precipitate route at various magnifications. Due to the SEM images, spherical shapes are observed that are heavily aggregated. In addition, it can be seen in Fig. 3 that MnFe 2 O 4 @TiO 2 nanocomposites composed from nano-scale clusters with diameter between 100 and 120 nm; however, the clusters are agglomeration of very fine nanoparticles with diameter of 80 ± 10 nm.

DLS and zeta potential analysis
Dynamic light scattering (DLS) and zeta potential analyses show the hydrodynamic particle size and the surface charge of the MnFe 2 O 4 @TiO 2 nanocomposites. Figure 4a indicates the DLS histogram of MnFe 2 O 4 @TiO 2 nanocomposites, according to which the hydrodynamic diameter of MnFe 2 O 4 @TiO 2 nanocomposites are about 100-160 nm. The surface charge in nanomaterials plays a key role in preventing the aggregation and increasing the stability of materials. The zeta potential of the prepared MnFe 2 O 4 @TiO 2 showed that the negative surface charge was − 30.16 mV.

FT-IR analysis
In order to get the direct evidence of TiO 2 deposition on the surface of MnFe 2 O 4 , Fourier-transform infrared (FTIR) spectroscopy of MnFe 2 O 4 -TiO 2 has been carried out (Fig. 5). The MnFe 2 O 4 -TiO 2 spectrum shows characteristic peaks at 3308.12, 1363.91, and 554.08 cm −1 . As shown, the main absorbance bands of MnFe 2 O 4 -TiO 2 appeared at 554.08 cm −1 that can be related to the stretching vibrations of M-O bonds (Fe-O and Mn-O). The peak at 3308.12 cm −1 is attributed to the stretching vibrational mode of O − H bond. The weak band at 1363.91 cm −1 is ascribed to the anti-symmetric stretching vibration of nitrate, arising from the residual nitrate (Raju. and Murthy., 2013).

EDS analysis
The energy-dispersive X-ray spectroscopy (EDS) analysis was applied to determine the elements of different area. Figure 6 shows that the elements in MnFe 2 O 4 @TiO 2 were Mn, Fe, O, and Ti. The EDS result shows that MnFe 2 O 4 @TiO 2 is pure and confirms the XRD results. After that, the EDS mapping analysis was performed to show how the elements are located throughout the nanoparticles (Fig. 7). As can be see, the elements of magnesium, titanium, iron, and oxygen were distributed uniformly. Overall, due to the above EDS results, it is reasonable to believe that such MnFe 2 O 4 @TiO 2 can be successfully synthesized using green method.

Influence of solution pH on photocatalytic efficiency
pH is one of the most important parameters which affect the photocatalytic degradation of humic acid in aqueous solution. To evaluate the pH effect on photocatalytic degradation of humic acid, pH values of 3, 5, 7, 9, and 11 were selected. Figure 8 shows the effect of various pH on photocatalytic decomposition of humic acid by MnFe 2 O 4 @ TiO 2 nanocomposites. As this diagram shows, the highest Humic acid has the highest rate of degradation in an acidic environment (pH = 3) because there are more hydrogen ions (H + ) in the acidic environment and the nanoparticle surface becomes positively charged due to H + ions. Due to the presence of carboxylic and phenolic compounds, humic acid compounds have negative charges on the surface. In an acidic environment, the surface charges of nanoparticles and humic acid molecules are opposite. Thus, nanoparticles can react more effectively with hydroxyl groups of humic acid in solution and as a result, the removal of humic acid increases (Moein et al. 2020;Mohammadi et al. 2022). The conducted study by Kim et al. showed that the rate of photocatalytic degradation of humic acid is higher at low pH (Kim et al. 2013). Also, Joolaei et al. concluded that the HA degradation at an acidic pH is greater that of the neutral and alkaline pH values. That is, degradation increases when pH decreases and decreases when pH increases (Joolaei et al. 2017).

Influence of nanocatalyst dose on photocatalytic efficiency
For economic removal of humic acid from aqueous solution, it is important to find the optimum amount of nanocatalyst for efficient degradation. The effect of catalyst dose on degradation efficiency of humic acid in the range of 0.005 to 0.1 g/L was investigated. Figure 9 shows the effect of nanocatalyst dose on the degradation process of humic acid. As can be seen in this diagram, with increasing the nanocomposite dose, the degradation efficiency increases to 0.03 g/L and then decreases again. The highest degradation efficiency was observed at the nanocomposite dose of 0.03 g/L. The photocatalytic degradation efficiency of humic acid increases with increasing catalyst dose due to the increase in surface area and the number of active sites on the catalyst surface. So that with ultraviolet radiation, more electron holes are produced, which leads to increased production of oxidizing radicals and further decomposition of humic acid (Eslami et al. 2016).
By increasing the nanocatalyst amount more than 0.03 g/L in solution, turbidity is created and the removal efficiency of humic acid decreases. This turbidity causes complete disruption in the light transmission in the solution. Another reason for the decrease in humic acid removal percentage with increasing nanocatalyst dose is that at higher doses of this nanocatalyst, due to the increase in magnetic properties, the nanocatalyst becomes agglomerated, which this reduces the active sites available to absorb photons on its surface (Khodadadi et al. 2020). The results of the study of  Babel et al. showed that degradation of humic acid increases as the catalyst concentration increases up to 0.4 g/L, and with a further increase of the catalyst concentration, the degradation of humic acid almost reduced (Babel et al. 2017).
In the dark phase (− 30-0 min) when the UV lamp is off, the dominant mechanism is surface adsorption. In this step, humic acid is adsorbed on the porous on the surface of the nanocomposite. But the maximum removal efficiency in optimum adsorption condition was about 20%. When the UV lamp is turned on, the degradation efficiency reaches over 80%. Therefore, the dominant removal mechanism between − 30 and 0 min (dark condition) is adsorption and the dominant removal mechanism over than 0 min (light condition) is degradation process.

Influence of initial humic acid concentration and contact time on photocatalytic efficiency
Initial concentration of humic acid and the contact time are two important factors in the efficiency of MnFe 2 O 4 @TiO 2 decomposition process. To investigate the effect of initial concentration of humic acid on its photocatalytic decomposition using MnFe 2 O 4 @TiO 2 nanocomposites, the concentration of humic acid in the range of 2 to 15 mg/L at different contact times was investigated. As shown in Fig. 3, increasing the concentration of humic acid reduces the decomposition efficiency. At high concentrations of humic acid, the active sites on the catalyst surface are reduced. This is because of more pollutant molecules attach to these active sites. Also, the OH° radicals produced are not sufficient. This is because the number of these radicals is constant because the nanocatalyst dose is equal in all prepared solutions (with different concentrations of humic acid) and as a result, increasing the concentration of this contaminant leads to a deterrent effect in its destruction. The third reason is that at high humic acid values, the photons received by the photocatalyst surface decrease because UV light is absorbed by the humic acid molecules and intermediate products (Esmati et al. 2021). Results of the study of Babel et al. showed that increasing the initial concentration of humic acid reduced its degradation efficiency (Sekartaji and Babel 2016). Reaction time is also an important parameter that affects the decomposition process of pollutants. Therefore, in the present study, the degradation efficiency of humic acid in the range of 0 to 100 min of reaction time at different concentrations of pollutants was investigated. According to Fig. 10, it is obvious that with increasing the reaction time from 0 to 20 min, the degradation efficiency of humic acid increased, and after 20 min, it remained almost constant. In the early stages of humic acid degradation, due to the greater contact of the MnFe 2 O 4 @TiO 2 nanocatalyst with the humic acid solution and the large number of cavities on the nanocatalyst surface, the removal of humic acid is greater. The degradation of humic acid decreases over time. This is because with the gradual occupation of these sites by humic acid, the repulsive force between the contaminant molecules and the MnFe 2 O 4 @ TiO 2 nanocatalyst increases. Eventually, the removal of humic acid decreases compared to the early stages, and after a certain period of time, the amount of degradation reaches an almost constant amount (Oskoei et al. 2016). The results of the study of Oskoei et al. showed that increasing the contact time to 10 min increased the degradation efficiency, and after 10 min, the degradation time was almost fixed (Oskoei et al. 2016).

Photocatalytic degradation mechanism of theMnFe 2 O 4 @TiO 2
In the semiconductor photocatalytic process, oxidative decomposition reactions require the availability of three basic components, including a light-sensitive catalytic surface (usually a semiconductor), a photon energy source, and a suitable oxidizing agent. The mechanism of this process includes the activation of a semiconductor by artificial light or sunlight.
The main characteristic of a semiconductor is having valence and conduction bands, and the area between these two bands is called the band gap. Absorption of photons with energy higher than the energy of the band gap leads to the transfer of an electron from the valence band to the conduction band, with the simultaneous production of holes in the valence band (Eq. 1).
These holes have a very high oxidation potential, which is enough to produce hydroxyl radicals from water molecules and hydroxide ions adsorbed on the semiconductor surface (Eqs. 2 and 3).
The formed electrons can react with the absorbed oxygen molecule and reduce it to the superoxide radical (O 2 − ), which in turn reacts with protons to form peroxide radicals (Eqs. 4, 5 and 6).
(1) Catalyst  Also, hydrogen peroxide acts as an electron acceptor and produces additional hydroxyl radicals according to Eq. 7.
In general, produced radicals (hydroxyl radicals, superoxide, etc.) destroy organic or inorganic pollutant molecules in polluted water through some secondary reactions. Degradation of water pollutants can also occur through the direct transfer of produced electrons or holes from the surface of the catalyst to pollutant molecules.

Conclusion
The present study aimed at investigation of photocatalytic degradation of humic acid in aqueous solutions using MnFe 2 O 4 magnetic nanocomposite coated with semiconductor TiO 2 (MnFe 2 O 4 @TiO 2 ). In this study, the production method of the MnFe 2 O 4 @TiO 2 magnetic nanoparticle with the green synthesis method using oleaster tree bark was investigated. Also, the effects of various parameters including pH, initial humic acid, contact time, and synthesized nanoparticle dose were surveyed. The results of the study showed that it is possible to achieve maximum efficiency under optimal experimental conditions (pH = 3, nanocomposite dose = 0.03 g/L, humic acid initial concentration = 2 mg/L and contact time = 20 min) and MnFe 2 O 4 @ TiO 2 nanoparticles have high photocatalytic activity and can effectively remove humic acid from aqueous solutions under UV light.
Author contribution All authors contributed to the study conception and design. Material preparation, data collection, and analysis were performed by Elham Derakhshani, Ali Naghizadeh, and Sobhan Mortazavi. The first draft of the manuscript was written by Elham Derakhshani, and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.
Funding We received financial support from the Birjand University of Medical Sciences, Iran (No. 5720).
Data availability All data generated or analyzed during this study are included in this published article.

Declarations
Ethical approval This article does not contain any studies with human participants or animals performed by any of the authors.

(7)
Catalyst(e − ) + H 2 O 2 → Catalyst + HO − + HO ⋅ (8) Catalyst h + + RX ads → Catalyst + RX ⋅+ ads Consent to participate This section is "not applicable" for this study, as the study does not involve any human participants or their data or biological material.

Consent for publication
All the authors mentioned in the manuscript have agreed to authorship, read, and approved the manuscript, and given consent for submission and subsequent publication of the manuscript.

Competing interests
The authors declare no competing interests.