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Article

Highly Effective Fe-Doped Nano Titanium Oxide for Removal of Acetamiprid and Atrazine under Simulated Sunlight Irradiation

by
Zhanpeng Liu
1,
Junjian Lin
1,
Zhimin Xu
1,
Fangfang Li
2,
Siyao Wang
1,2,
Peng Gao
3,
Guomei Xiong
1 and
Hongbo Peng
1,2,*
1
Faculty of Modern Agricultural Engineering, Kunming University of Science & Technology, Kunming 650500, China
2
Yunnan Provincial Key Laboratory of Soil Carbon Sequestration and Pollution Control, Faculty of Environmental Science & Engineering, Kunming University of Science & Technology, Kunming 650500, China
3
City College, Kunming University of Science & Technology, Kunming 650051, China
*
Author to whom correspondence should be addressed.
Agronomy 2024, 14(3), 461; https://doi.org/10.3390/agronomy14030461
Submission received: 23 January 2024 / Revised: 15 February 2024 / Accepted: 19 February 2024 / Published: 26 February 2024
(This article belongs to the Section Weed Science and Weed Management)
Browse Figures
Versions Notes

Abstract

:
Pesticides are widely detected in large quantities in the environment, posing an ecological threat to the human body and ecology. Semiconductor nanomaterials such as nano-titania (nTiO2) have strong photocatalytic degradation efficiency for pollutants. However, the wide bandgap and limited light absorption range inhibit nano-titania’s practical application. Therefore, nTiO2 was modified by Fe3+ doping using the microwave hydrothermal method to improve its photocatalytic performance in this study. Fe-nTiO2 doped with a 1.0% mass ratio was used due to its high photocatalytic performance. Its maximum degradation efficiencies for ACE and ATZ under a 20 W xenon lamp were 88% and 88.5%, respectively. It was found that Fe3+ doping modification distorted the spatial morphology of nTiO2 and shortened the bandgap to facilitate the photocatalytic reaction. The electron paramagnetic resonance results showed that the reactive radicals (1O2, ·OH) produced by photogenerated electrons (e−) and holes (h+) of Fe-nTiO2 were the main active species in the degradation of ACE and ATZ. Additionally, the application of Fe-nTiO2 significantly enhanced the growth of lettuce under sunlight; the degradation efficiencies of ACE and ATZ in lettuce were 98.5% and 100%, respectively. This work provides new insights into the removal of organic contaminants by photocatalysts under sunlight in agriculture.

1. Introduction

Pesticides are widely used, and thus, they are detected in large quantities in water and soil environments. The abuse of pesticides such as herbicides and insecticides has caused environmental problems such as farmland non-point source pollution, decreases in soil quality, and pollution [1,2]. Therefore, the efficient removal of these pesticides from soil environments has become a key scientific issue. At present, photocatalysis degradation is an important way to remove pesticides. After the surface treatment of a photocatalyst, it was found that the performance was adjustable, and pesticides could be directly decomposed and mineralized to CO2/H2O [3]. Therefore, solving pesticide pollution by photocatalysis is of great significance.
At present, the photocatalytic degradation method, especially the photocatalytic degradation of organic contaminants by nanometal oxides, has attracted wide attention. These include nano zinc oxide, nano titanium dioxide, nano titanium oxide (nTiO2), and manganese dioxide [4]. As the first discovered semiconductor material, nTiO2 has strong photocatalytic activity, but it has defects. The natural light absorption wavelength of nTiO2 is below 387.5 nm [5]. Moreover, the utilization efficiency of visible light is not more than 1%, and the photogenerated quantum yield is less than 20%, which seriously inhibits its actual application [6]. Therefore, the improvement in the utilization efficiency of nTiO2 in natural light has become an important research topic.
To improve the photocatalytic activity of nano metal oxides, modification methods such as metal/non-metal ion doping [7], noble metal precipitation [8], and the semiconductor composite method [9] have been typically used. Among them, metal ion doping introduces new energy levels, and thus, the energy bandgap can be narrowed, which enables better photocatalytic activity, especially in the visible light spectrum [10]. In addition, doped ions also introduce charge-trapping sites, which inhibit the recombination of electrons and holes [11]. Currently, the direct modification of nTiO2 is mostly achieved via doping by a single species of metal ions [12]. The photocatalytic performance of nTiO2 has been enhanced by reducing the charge recombination, inducing the bandgap displacement, and enhancing photocatalytic activity under visible light irradiation [13]. In particular, the photocatalytic performance of TiO2 could be enhanced by doping metal ions such as Cu2+, Ni2+, Zn2+, and Fe3+, which generate doping states in the bandgap of TiO2 and increase the absorption range of TiO2 in the visible light region [14,15].
In addition, nTiO2 has energy-level-defect intermediate states; the intervention of metal ions led to the formation of intermediate energy levels and thus reduced the bandgap width. Moreover, doped metal ions were involved in the electron transfer process in forbidden bands, which reduced the energy required for the excitation of nTiO2 [16,17,18]. For example, Fe3+ (0.79 Å) and Ti4+ (0.75 Å) have equivalent ionic radii; Fe3+ was shown to be easy to dope into the lattice of Ti [19,20,21,22], and the photocatalytic performance of nTiO2 may be enhanced by doping Fe3+. However, these studies did not investigate the photocatalytic mechanisms of pesticides by doping nTiO2 with Fe3+, nor did they show how they were related to their change properties and reaction conditions. The potential of doping nTiO2 with Fe3+ under solar light to photocatalytically degrade pesticides in the growth of plants has significant practical implications in agriculture, and the actual effects of photocatalysts on pesticide degradation and plant growth should be explored.
We selected two model pesticides, acetamiprid (ACE) and atrazine (ATZ), to investigate their photocatalytic degradation. ACE poses risks to crops and soil environments when it is used in high dosages and with unscientific methods [23]. ATZ is a new generation of triazine organochlorine herbicide that easily produces physiological toxicity in plants and animals because of its high dosage and stable structure [24]. The primary aims of this study are to investigate (i) the changes in the physicochemical properties of nTiO2 caused by Fe3+ doping, (ii) the photocatalytic degradation mechanisms/pathways of ACE and ATZ by Fe3+-doped nTiO2, and (iii) the effects of Fe3+-doped nTiO2 on the growth of lettuce and the degradation of ACE/ATZ under sunlight irradiation. This work provides new insights into the degradation mechanisms and environmental fates of ACE and ATZ as affected by Fe3+-doped nTiO2, which provides a scientific basis for the rational and scientific use of pesticides.

2. Materials and Methods

2.1. Chemicals

Tetrabutyl titanate (C16H36O4Ti) was purchased from Shanghai McLean Biochemical Technology Co. Anhydrous ethanol (C2H5OH), urea (CO(NH2)2), ferric chloride (FeCl3.6H2O), acetamiprid (C10H11ClN4) and atrazine (C8H14ClN5) were purchased from Aladdin’s Reagent (China). The purity of the above reagents was analytically pure (AR). The detailed physicochemical properties of ACE and ATZ are presented in Table S1.

2.2. Synthesis of nTiO2 and Fe3+-Doped nTiO2

Synthesis of nTiO2 was as follows: the precipitant solution was made by dropping hydrochloric acid solution into anhydrous ethanol. The precipitant solution was added dropwise to the tetrabutyl titanate solution, a light blue transparent gel was obtained and then left to precipitate at 25 ± 1 °C. The lower layer of light blue gel was dried in an oven to obtain the titanium source precursor, which was subsequently ground into powder, calcined in a muffle furnace at 500 °C for 2 h, cooled, and the solid particles ground into powder after being passed through a 100-mesh sieve to obtain nTiO2 powder.
Synthesis of Fe3+-doped nTiO2 was as follows: to prepare the Fe3+-doped nTiO2 photocatalysts with different mass ratios, FeCl3·6H2O was accurately weighed to 0.0844, 0.1688, 0.3376, and 0.6752 g, respectively. The FeCl3·6H2O power was slowly poured into ultrapure water and completely dissolved into an iron source solution. Two hundred mg of nTiO2 powder was added into the ultrapure water and sonicated to ensure it was in full contact with the iron source solute, then it was placed into a microwave disintegrator at 130 °C for 1 h. After cooling, the precipitate was filtered, cleaned, dried, ground into powder, and passed through a 100-mesh sieve to obtain Fe3+-doped nTiO (Fe-nTiO2).

2.3. Structural Characterization of Fe-nTiO2

The morphology of the Fe-nTiO2 was examined using a field emission scanning electron microscope (SEM, Nova-Nano 450, FEI, Lincoln, NE, USA). The X-ray diffraction (XRD, Empyrean diffractometer, Malvern Panalytical, Malvern, UK) patterns of the prepared Fe-nTiO2 were detected by the D8 Advance X-ray diffractometer at a voltage of 40 kV. The changes in the functional groups for Fe-nTiO2 were detected by a Fourier transform infrared scattering spectrometer (FTIR, NicoLET iS50, Thermo Scientific, Waltham, MA, USA). The elemental composition of Fe-nTiO2 was analyzed by X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Scientific, USA). Electron paramagnetic resonance (EPR, A300, Bruker, Mannheim, Germany) spectroscopy was performed on the photocatalytic experiment using a Bruker A300 micro-spectrometer. The absorption range and response intensity of the Fe-nTiO2 were determined by UV–visible absorption spectroscopy (UV-VIS DRS, Shimadzu UV-3600i Plus, Kyoto, Japan).

2.4. Degradation of ACE and ATZ by Fe-nTiO2

The photocatalytic degradation experiments were carried out in a photodegradation reactor equipped with a cooling water circulation system and a 20 W xenon lamp light source system. Fifty mL of the ACE and ATZ solution were placed in the photodegradation reactor with 50 mg of Fe-nTiO2, respectively. The degradation of ACE and ATZ was carried out at 25 ± 1 °C, and 0.3 mL of the solution in the photoreaction was extracted from the sampling well with a syringe at the time points of 1, 2, 3, 4, 5, 6, 8, 10, and 12 h, respectively. The solution was transferred to a liquid phase vial after passing through a 0.45 μm filter membrane. The concentration of ACE and ATZ was detected by high-performance liquid chromatography (HPLC, Shimadzu LC-2060C). HPLC was performed with a UV detector, and a C18 reversed-phase column (5 μm, 4.6 × 150 mm) was used. The detection wavelength of ACE was 248 nm, and the ratio of the mobile phases was 55% ultrapure water and 45% acetonitrile. The detection wavelength of ATZ was 230 nm, and the mobile phase ratios were 60% methanol, 30% ultrapure water, and 10% acetonitrile. The injection volume was 10 μL, and the flow rate was 1.0 mL/min for ACE and ATZ. In addition, the degradation byproducts of ACE and ATZ were analyzed using a liquid chromatograph-mass spectrometer (LC-MS, Agilent 1100, Santa Clara, CA, USA).
The effect of pH = 3, 7, and 11 on the photocatalytic degradation of ACE and ATZ was explored. The degradation efficiency of ACE and ATZ was calculated by the following equation:
Degradation efficiency (%) = (C0 − Ct)/C0
C0 is the initial concentration of ACE and ATZ, and Ct is the residual concentration in aqueous solution at t h (t: 1, 2, 3, 4, 5, 6, 8, 10, and 12 h) after degradation.

2.5. Potted Application Experiment of Fe-nTiO2

The pot experiment address was located in the greenhouse (24°85′ N, 102°85′ E) at Kunming University of Science and Technology, Yunnan, China. The soil (60-mesh sieve) of 0–20 cm in the cultivation layer in the greenhouse was selected for the pot experiment. The whole experiment was completed in natural conditions. The specific experimental operation is shown in Text S1.

3. Results

3.1. Characterization of TiO2 and Fe-nTiO2

NTiO2, prepared by the sol–gel methoed, showed an irregular spherical block structure; the average diameter of the particulate spheres was about 1150 nm (Figure 1A), which is consistent with other results [25]. After the Fe3+ was doped into the internal space of nTiO2 by the microwave hydrothermal method, the nTiO2 was aggregated (Figure 1B); this may be due to the much smaller radius of Fe3+ than that of nTiO2 with the crystal size. When Fe3+ occupied the internal space, defects and dislocations were formed in the structure of nTiO2. The contact area between Fe3+-nTiO2 and light increased as the crystal size decreased, thus improving its light absorption efficiency. At the same time, small-sized crystals exposed more surface defects and active sites.
The microscopic appearance of nTiO2 changed after the Fe3+ loading, with a slight agglomeration but no change in the crystalline form according to the SEM images (Figure 1A,B). The XRD diffraction peak showed that the synthesized Fe-nTiO2 was the crystal structure of anatase nTiO2 (JCPDF Card No: 21-1272) (Figure 1C). In addition, the intensity of the diffraction peaks of Fe-nTiO2 was significantly decreased and shifted to the left, which may be caused by the distortion of Fe3+ into the inner part of nTiO2. No diffraction peaks of Fe were on the surface of the Fe-nTiO2 according to the XRD diffraction pattern; Fe3+ may enter into the nTiO2 in the form of Fe3+. The FTIR spectra of Fe-nTiO2 with different Fe3+ doping amounts are shown in Figure 1D. The absorption peak at 3450 cm−1 was -OH [26]; this peak of Fe-nTiO2 was higher than that of nTiO2, probably caused by the water crystallization in the Fe-doping process. The peak of the 1635 cm−1 center in all samples comes from the vibration of the hydroxyl group of water molecules, and the peak size is the same [27]; a broad absorption peak at 625 cm−1 was the vibrational peak caused by Ti-O [28]. These three peaks of Fe-nTiO2 were stronger than those of nTiO2. The FTIR studies showed that the incorporation of Fe in TiO2 replaced the Ti ions, resulting in oxygen vacancies in the lattice.
The elemental composition and surface electronic state of Fe-TiO2 were determined by XPS spectra. C, O, Ti, and Fe were contained in Fe-TiO2 (Figure 2A), and the high-resolution Fe2p spectra of XPS were integrated into six curves, with peaks at the binding energies of 709.26, 713.30, 717.38, 722.38, 726.62, and 730.87 eV (Figure 2B). The fitting peaks at 717.38 and 730.87 eV were assigned to the satellite peaks of Fe2p3/2 and Fe2p1/2 [29]. The Fe2p peaks at 709.26 and 713.30 eV correspond to Fe2p3/2 (Fe3+ and Fe2+ peaks), and the Fe2p peaks at 722.38 and 726.62 eV correspond to Fe2p1/2 (Fe3+ and Fe2+ peaks). In addition, the two peaks of O1s at 531.14 and 530.08 eV (Figure 2C) were attributed to the Fe-O bond of Fe-TiO2 and its own Ti-O bond [30].
The maximum Ti2p3/2 peak was at 457.36 eV (Figure 2D), and the peak height was symmetrical, indicating the absolute coordination of Ti4+ [31]. The peak at 463.08 eV corresponded to the 2p1/2 core level of Ti4+ in TiO2. There was no shoulder peak related to Ti3+ at 456.8 and 462.5 eV in the 2p3/2 and 2p1/2 core levels of Ti3+, respectively. It was generally believed that Ti3+ was easily oxidized by appropriate oxidants such as O2 in the air or dissolved oxygen in water, and surface Ti3+ was rapidly consumed [32]. These results indicated that the surface of Fe-TiO2 was dominated by Ti4+, except for the presence of Ti3+. Overall, these results indicated that Fe-TiO2 was successfully synthesized. Moreover, Fe enters the nTiO2 interior in the ionic form rather than distributing on the surface of the material, which is consistent with the analysis results of SEM and XRD.

3.2. Photocatalytic Activity of Fe-nTiO2

The optical properties of Fe-nTiO2 were measured by UV–visible diffuse reflectance spectroscopy. Fe-nTiO2 showed a good absorption capacity between 200–400 nm (Figure 3A), indicating that it has excellent light absorption ability. Compared with TiO2, the absorption capacity of Fe-nTiO2 in the visible light band was significantly increased. In the UV–vis spectrum of Fe-nTiO2, the new absorption peak centered at 450–600 nm was attributed to the d-d transition of Fe3+ spin-allowed (6A1g → 4A1g + 4 Eg (G)) [33]. The bandgap energy (Eg) of semiconductor catalysts was an important optical parameter for the evaluation of the photochemical processes. The Eg value of Fe-nTiO2 was determined by the Kubelka–Munk equation (Equation (2)) and Tauc relation (Equation (3)).
F (R) = (1 − R)2/2R
(ahv)2 = A (hv − Eg)
where R is the reflectance, F (R) is the transformed reflectance according to Kubelka–Munk, hv is the photon energy, and A is a material constant and represents the absorption coefficient, which is equivalent to F (R).
The optical absorption region of Fe-nTiO2 showed a significant red shift, resulting in a decrease in the bandgap of Fe-nTiO2 from 3.30 eV to 3.15 eV (Figure 3B). The decreased bandgap of Fe-nTiO2 increased the light absorption range and enhanced the photocatalytic performance after the Fe3+ doped nTiO2, which benefitted generating more photogenerated electrons in the photocatalytic process and thus improved its catalytic performance.

3.3. Degradation Mechanisms of ACE and ATZ

Fe-nTiO2 has the strongest degradation ability for ACE and ATZ when the doping amount of n (Fe3+):n (nTiO2) was 1.0%, and thus 1.0% Fe-nTnO2 was used in the following experiments (Figure S1). The photocatalytic degradation of ACE by Fe-nTiO2 under xenon lamp illumination reached 17.6 µg/mg at 720 min, which was enhanced by about 30% when compared with nTiO2 (13.6 µg/mg) (Figure 4A). Similar results were also observed for the photocatalytic degradation of ATZ; its degradation amount reached 17.7 µg/mg at 720 min, which was higher than that of nTiO2 and FeCl3 (Figure 4B).
Distinct DMPO and TEMPO characteristic signal peaks were detected after 10 min of the reaction (Figure 5), demonstrating the production of hydroxyl radical (·OH) and singlet state oxygen (1O2) in the reaction system.
The degradation amount of ACE and ATZ at pH = 11.0 was lower than that of pH = 3.0/7.0, and there was no difference between pH = 3.0 and 7.0 (Figure 6A,B). The degradation of ACE by Fe-nTiO2 reached equilibrium, and its degradation amount was 13.2 µg/mg after 720 min at pH = 11.0. The degradation amount of ACE was 18.0 µg/mg and 17.6 µg/mg at pH 3.0 and 7.0, respectively (Figure 6A). For ATZ, the degradation amount was 16.5 µg/mg and 17.7 µg/mg at pH 3.0 and 7.0, respectively. The degradation of ATZ was lowest when compared with pH 3.0 and 7.0 (Figure 6B).

3.4. Photocatalytic Degradation Ways and Intermediate Products of ACE/ATZ

The degradation intermediates of ACE by Fe-nTiO2 according to liquid chromatography coupled with mass spectrometry (LC-MS) are summarized in Table S2. The possible degradation pathways for the photocatalytic degradation of ACE by Fe-nTiO2 (Figure 7) were as follows: ACE captured hydrogen ions (H+) to produce α-aminohydroxyl and H2O. O2 was reduced to O2·− by α-amino -OH. ACE was finally decomposed into 6-chloro-9- (tetrahydro-2-pyranyl) purine (m/z = 238), which was further oxidized to produce 6-chloronicotinaldehyde (m/z = 141), 6-chloropyridine-2-carboxylic acid (m/z = 557), and N-cyano-N’-methylacetamide (m/z = 97) (Figure S4).
In addition, 4-chloro-1,3-benzenediol, 4-chlorocyclopent-1-enecarboxylic acid, 3-hydroxycyclopentacarboxylic acid, and 6-chloropiperidine-3-carboxylic acid with mass-to-charge ratios of 144,146 and 130,163 were also detected during the degradation process of ACE. The N atom in the pyridine ring of the ACE molecular structure was destroyed by -OH and separated from the pyridine ring to form NO2 or NO3−, while the C atom in the pyridine ring was attracted to each other to form 4-chloro-1,3-benzenediol. It was further reduced by H2O2 to form 4-chlorocyclopent-1-ene carboxylic acid. The active Cl atom and C=C were replaced by -OH and reacted with H2O2 to form 3-hydroxycyclopentanecarboxylic acid.
The degradation of ATZ included both hydrolysis and photolysis; the degradation intermediates of ATZ are summarized in Table S3. The possible degradation pathways for the photocatalytic degradation of ATZ by Fe-nTiO2 (Figure 8) are as follows: C-H on the central C atom of the isopropylamine group in the side chain of ATZ was oxidized by ·OH to replace the H+ and formed 2-chloro-4-ethylamino-6-propanolamine-1,3,5-triazine (m/z = 230), which was oxidized by ·OH to form 2-chloro-4-ethylamino-6-amino-1,3,5-triazine (m/z = 174); or the C-Cl and alkyl in the ATZ side chain were destroyed by ·OH to form 2-hydroxy-4-amino-6-isopropylamino-1,3,5-triazine (m/z = 170) (Figure S5).
When the side chain Isopropylamine group of ATZ was first oxidized and dehydrogenated by ·OH, 2-chloro-4-acetamide-6-isopropylamino-1,3,5-triazine (m/z = 230) was formed and then oxidized by ·OH to form 2-chloro-4-amino-6-isopropylamino-1,3,5-triazine (m/z = 188) and was then finally oxidized and decomposed into 2-chloro-4,6-diamino-1,3,5-triazine (m/z = 146) [34]. After further oxidation by ·OH, 2-chloro-4-amino-6-nitro-1,3,5-triazine, 2-chloro-4,6-dihydroxy-1,3,5-triazine, and 2-hydroxy-4,6-diamino-1,3,5-triazine (m/z = 128) formed with mass-to-charge ratios of 176,148, and 128, respectively. It was further oxidized to 2-hydroxy-4,6-dinitro-1,3,5-triazine (m/z = 190) or cyanuric acid monoamide (m/z = 128), and finally degraded to cyanuric acid (m/z = 128). Subsequently, the triazine ring was further oxidized and decomposed into inorganic small molecules such as H2O and CO2 by ·OH.

3.5. The Effect of Fe-nTiO2 on the Growth of Lettuce

The combined effect of Fe-nTiO2 (F-T) and ACE/ATZ on the growth of lettuce was investigated. Fe-nTiO2 promoted growth indexes such as plant height, fresh weight, and leaf number of lettuce. Compared to the control group, the plant heights of the FTC (F-T (ACE)) and ACE groups increased by 77.3% and 5.5% after 30 d of lettuce planting (Figure 9A). The fresh weights of lettuce in the F-T and ACE groups were 102.9% and 7.9% (Figure 9B). The number of lettuce leaves in the F-T and ACE groups was 125.6% and 63.3%, respectively (Figure 9C). Although the application of ACE significantly increased the growth index of lettuce, the cumulative risk of pesticides in plants could not be ignored. The plant heights of the FTZ (F-T (ATZ)) and ATZ groups were reduced by 11.0% and 42.5% when compared with the control group after spraying ATZ. The fresh weights of the FTZ and ATZ groups were reduced by 44.9% and 362.0%, respectively. The number of leaves in the FTZ and ATZ treatment groups increased by 33.3% and decreased by 8.4% compared with the control group. The FTZ group significantly enhanced the growth and development of lettuce compared with the ATZ group.
At the same time, the concentrations of ACE and ATZ in the growth of lettuce were determined (Figure 9D). The results showed that Fe-nTiO2 could effectively reduce the residue of ACE in lettuce tissue, which was 83.0% lower than that of the control, and the ACE residue in root tissue was higher. The degradation efficiency of ATZ in lettuce tissue reached 100%. It reduced the cumulative risk of plants caused by the application of pesticides.

4. Discussion

The local coordination properties of Fe-doped nTiO2 were further elucidated according to the EPR results. TiO2 itself has some oxygen vacancies; however, the intensity of the oxygen vacancies of the carriers was significantly reduced after microwave compounding (Figure S2), which may be mainly due to the coordination of free iron adsorbed on the oxygen vacancies of TiO2, which was consistent with other research [35]. Therefore, the surface vacancies of TiO2 may be the main sites for Fe3+ binding.
The increasing amount of Fe3+ doping gradually increased the hydroxyl group (-OH) on the surface of nTiO2 (Figure 1D and Figure S3); both the degradation efficiency of ACE and ATZ were positively correlated with the content of -OH. Therefore, -OH played a crucial role in the production of active species and the degradation efficiency of these two pesticides. In addition, -OH was the typically reducing functional group, and it was more likely to capture an oxidizing hole (h+) during the photoreaction process. This capture inhibited the recombination of electron (e−) and h+, promoted the formation of high-density e−, and led to the creation of superoxide [36]. It is important to note that O2.− typically served as a crucial precursor for the generation of ·OH and 1O2. Therefore, Fe3+ could effectively prevent the recombination of e− and h+ and benefit the formation of ROS.
In addition, superoxide anion radical (O2·−), a common reactive species in photocatalytic processes, was detected by EPR. The DMPO-O2.− signals were not observed, indicating that O2.− was not the main reactive species (Figure S3). It is worth noting that compared to nTiO2, the characteristic peaks of 1O2 (Figure 5A) and ·OH (Figure 5B) for Fe-nTiO2 significantly enhanced, demonstrating that more 1O2 and ·OH formed. Therefore, the degradation of ACE and ATZ was enhanced by Fe-nTiO2 when compared with nTiO2, which contributed to the 1O2 and ·OH.
Overall, the doping modification of Fe3+ for nTiO2 significantly enhanced the photocatalytic degradation of ACE and ATZ. Other studies have shown that TiO2 doped by Fe3+ reduced the bandgap energy, enhanced the light response capability, produced photogenerated e- more quickly, accelerated the transfer of e- in a more timely manner, and effectively slowed down the recombination of e- and h+ pairs; therefore, the catalytic performance was better [37].
The valence band of Fe3+ was higher than that of nTiO2, which led to the transfer of e- and h+ to the surface of Fe-nTiO2. The e- on the surface of Fe-nTiO2 combined with dissolved oxygen molecules (O2) to produce superoxide radical anions (O2·−) (Equation (4)). The positively charged h+ of Fe-nTiO2 reacted with H2O to generate ·OH (Equation (5)) [38]. Moreover, Fe3+ could also be used as a ‘trapper’ for e- (Equation (6)), but Fe2+ was unstable and easily regenerated Fe3+, so this reaction was reversible. In this process, Fe2+ could be re-oxidized by O2 in the solution to form O2− (Equation (7)); O2− and its unstable substances further combined with H2O to generate ·OH (Equation (8)) and O2− was further generated to 1O2 (Equation (9)) [38]. Photogenerated h+, the reactive radicals (O2−, ·OH, and 1O2) produced by Fe-nTiO2 were responsible for the degradation of ACE and ATZ (Equation (10)).
e- + O2 → O2−
h+ + H2O → ·OH
Fe3+ + e- → Fe2+
Fe2+ + O2 → Fe3+ + O2−
O2− + h+ + H2O → OH· + OH−
O2− + h+ → 1O2
O2− + ·OH + 1O2 + e− + h+ + ACE/ATZ → CO2 + H2O
The degradation mechanisms of ACE and ATZ by Fe-n TiO2 are represented in Figure 10; there were two degradation pathways in this process. The doping of Fe3+ distorted its spatial structure and shortened the bandgap, resulting in more e−–h+ pairs under light conditions. Firstly, e− and O2 generate O2− and combine with H2O to generate ·OH, which further generates 1O2. Secondly, h+ transferred to the -OH and adsorbed on the surface of Fe-nTiO2 to form ·OH in the aqueous environment, which destroyed the structure of ACE and ATZ to achieve the purpose of degradation.
The neutral and acidic conditions were more beneficial to the photocatalytic degradation of ACE and ATZ by Fe-nTiO2 than those of alkaline conditions. This may be due to the photogenerated electron transport to the surface of Fe-TiO2 under xenon lamp irradiation, resulting in the generation of photogenerated holes and the degradation of ACE and ATZ. Studies have shown that at a high pH, the surface of Fe-TiO2 is negatively charged (TiO-), and electrostatic repulsion occurs with the same negatively charged dye (MO-), resulting in a decrease in photocatalytic degradation activity [39].
It is feasible that the application of Fe-nTiO2 in combination with ACE and ATZ can be utilized in agricultural cultivation. The Fe-nTiO2 enhanced the growth indexes, such as plant height, fresh weight, and leaf number of lettuce, while it also reduced the residues of ACE and ATZ in lettuce.

5. Conclusions

In this study, the photocatalytic degradation mechanisms of ACE and ATZ by Fe-nTiO2 under xenon lamp light were systematically investigated. Fe3+-doping modification changed the spatial morphology of nTiO2, which shortened the forbidden band and enhanced its light absorption ability. Fe3+ doping produced more photogenerated e−–h+ pairs, and the degradation amount of ACE and ATZ by Fe-nTiO2 reached 17.6 µg/mg and 17.7 µg/mg at 720 min, respectively. The photocatalytic degradation was the strongest when the pH was 7.0. In addition, the photogenerated e− of Fe-nTiO2 interacted with O2 firstly to produce O2·−, then reacted with H2O and H+ to produce ·OH and to further produce 1O2. The h+ transferred to the valence band of nTiO2 and generated a large amount of h+, which reacted with ACE and ATZ to damage their structures and thus led to their degradation. The degradation rates of ACE and ATZ by Fe-nTiO2 under sunlight were 83.0% and 100.0%, respectively, which significantly promoted plant growth and development and provided data support for the application of photocatalytic materials in agricultural cultivation.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agronomy14030461/s1, Table S1. Physico-chemical properties of acetamiprid and atrazine; Table S2. Degradation intermediate products of ACE by Fe-nTiO2 under xenon lamp light; Table S3. Degradation intermediate products of ATZ by Fe-nTiO2 under xenon lamp light; Figure S1. Photocatalytic degradation of ACE (A) and ATZ (B) by Fe-nTiO2 with different Fe3+ doping levels; Figure S2. EPR spectra of oxygen vacancies of nTiO2 and Fe-nTiO2; Figure S3. EPR spectra of O2.- for nTiO2 and Fe-nTiO2; Figure S4. Mass spectrum for the degradation of ACE by Fe-nTiO2; Figure S5. Mass spectrum for the degradation of ATZ by Fe-nTiO2; Text S1. Pot experiment operation.

Author Contributions

Conceptualization: Z.L. and J.L.; methodology: Z.X.; validation: S.W., F.L. and Z.L.; formal analysis: G.X.; investigation: J.L.; writing—original draft preparation: Z.L.; writing—review and editing: J.L.; visualization: P.G.; supervision: H.P.; project administration: F.L.; funding acquisition: H.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by Yunnan Fundamental Research Projects (grant No. 202101BE070001-063), Yunnan Talent Support Plan Projects, the Recruitment Program of Highly-Qualified Scholars in Kunming University of Science and Technology (grant NO. KKKP201823026), Yunnan Major Scientific and Technological Projects (grant No. 202202AG050019), and the National Scientific Foundation of China (grant No. 42167030).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. SEM images of nTiO2 (A) and Fe-nTiO2 (B); XRD patterns of nTiO2 and Fe-nTiO2 (C); FTIR spectra of Fe-nTiO2 with different Fe3+ doping amounts (D).
Figure 1. SEM images of nTiO2 (A) and Fe-nTiO2 (B); XRD patterns of nTiO2 and Fe-nTiO2 (C); FTIR spectra of Fe-nTiO2 with different Fe3+ doping amounts (D).
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Figure 2. XPS spectra of Fe-TiO2. Survey spectrum (A), O1s (B), Fe2p (C), and Ti2p (D).
Figure 2. XPS spectra of Fe-TiO2. Survey spectrum (A), O1s (B), Fe2p (C), and Ti2p (D).
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Figure 3. UV–vis diffuse reflectance absorption spectra (A) and bandgap relationship (B) of nTiO2 and Fe-nTiO2 samples.
Figure 3. UV–vis diffuse reflectance absorption spectra (A) and bandgap relationship (B) of nTiO2 and Fe-nTiO2 samples.
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Figure 4. Photocatalytic degradation kinetic curves of ACE (A) and ATZ (B) by Fe-nTiO2 under xenon lamp illumination.
Figure 4. Photocatalytic degradation kinetic curves of ACE (A) and ATZ (B) by Fe-nTiO2 under xenon lamp illumination.
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Figure 5. EPR spectra of 1O2 (A) and . OH (B) in nTiO2 and Fe-nTiO2.
Figure 5. EPR spectra of 1O2 (A) and . OH (B) in nTiO2 and Fe-nTiO2.
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Figure 6. Effect of pH for the photocatalytic degradation of ACE (A) and ATZ (B) by Fe-nTiO2.
Figure 6. Effect of pH for the photocatalytic degradation of ACE (A) and ATZ (B) by Fe-nTiO2.
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Figure 7. Possible photocatalytic degradation pathways of ACE by Fe-nTiO2.
Figure 7. Possible photocatalytic degradation pathways of ACE by Fe-nTiO2.
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Figure 8. Possible photocatalytic degradation pathways of ATZ by Fe-nTiO2.
Figure 8. Possible photocatalytic degradation pathways of ATZ by Fe-nTiO2.
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Figure 9. The height (A), fresh weight (B), and leaf number (C) of lettuce after spraying Fe-nTiO2. Residual ACE and ATZ concentrations (D) in lettuce after Fe-nTiO2 combined with ACE and ATZ application.
Figure 9. The height (A), fresh weight (B), and leaf number (C) of lettuce after spraying Fe-nTiO2. Residual ACE and ATZ concentrations (D) in lettuce after Fe-nTiO2 combined with ACE and ATZ application.
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Figure 10. Energy band and the photocatalytic degradation mechanisms of ACE/ATZ by Fe-nTiO2.
Figure 10. Energy band and the photocatalytic degradation mechanisms of ACE/ATZ by Fe-nTiO2.
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Liu, Z.; Lin, J.; Xu, Z.; Li, F.; Wang, S.; Gao, P.; Xiong, G.; Peng, H. Highly Effective Fe-Doped Nano Titanium Oxide for Removal of Acetamiprid and Atrazine under Simulated Sunlight Irradiation. Agronomy 2024, 14, 461. https://doi.org/10.3390/agronomy14030461

AMA Style

Liu Z, Lin J, Xu Z, Li F, Wang S, Gao P, Xiong G, Peng H. Highly Effective Fe-Doped Nano Titanium Oxide for Removal of Acetamiprid and Atrazine under Simulated Sunlight Irradiation. Agronomy. 2024; 14(3):461. https://doi.org/10.3390/agronomy14030461

Chicago/Turabian Style

Liu, Zhanpeng, Junjian Lin, Zhimin Xu, Fangfang Li, Siyao Wang, Peng Gao, Guomei Xiong, and Hongbo Peng. 2024. "Highly Effective Fe-Doped Nano Titanium Oxide for Removal of Acetamiprid and Atrazine under Simulated Sunlight Irradiation" Agronomy 14, no. 3: 461. https://doi.org/10.3390/agronomy14030461

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