Characterization
Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS)
Figure 1 shows the micrographs of pure ZnO and ZnO doped with 1% (ZnOCe1) and 10% (ZnOCe10) cerium (Ce) in molar concentrations. The pure ZnO exhibited a granular morphology with larger and aggregated particles, in contrast to the doped materials, which showed smaller and more dispersed particles. The occurrence of smaller particles indicates that the doping process of ZnO with the rare earth element inhibits crystal growth, likely due to induced lattice distortions by the dopant ions (Ahmad Wani et al. 2023).
Elemental composition of the materials was analyzed by Energy Dispersive Spectroscopy (EDS), and the results are displayed in Figs. 1d-f. The EDS spectrum of pure zinc oxide showed peaks corresponding to the elements Zn and O. Additionally, a potassium peak was observed, originating from residual reagents used in the synthesis. The normalized mass of Zn and O in the pure zinc oxide is shown in Fig. 1d. The semi-quantitative result of atomic percentage of elements exhibited an almost equimolar composition between Zn and O, consistent with the chemical formula of zinc oxide (ZnO).
The EDS spectra of ZnO doped with Ce are presented in Figs. 1e and 1f. Although no Ce-related peak was observed in the EDS spectrum of ZnOCe1, a small amount of the element was detected in the sample, as shown in the table in Fig. 1e. On the other hand, well-defined peaks for the Ce element were identified in the spectrum of ZnOCe10. The semi-quantitative EDS analysis revealed that the amounts of the dopant element Ce were below the theoretical values proposed for the materials' synthesis, especially in the case of ZnOCe1. Nevertheless, the synthesis of the obtained materials was considered satisfactory as it allowed the production of zinc oxides with varying Ce contents and potentially different levels of doping. Additionally, residual potassium was also detected in the doped oxide samples, identified by the energy peak in the EDS spectra of ZnOCe1 and ZnOCe10 (Figs. 1e and 1f).
X-Ray Diffraction (XRD) analysis
The X-ray diffraction patterns for the oxides are presented in Fig. 2. All three materials exhibited diffractograms consistent with the POWCOD database (PDF 000-65-3411), showing the characteristic reflections of the (100), (002), (102), (110), (103), and (112) planes, indicative of the wurtzite crystal structure of ZnO (Figure S1) (Altomare et al. 2017). The presence of cerium did not cause significant alterations in the X-ray diffraction patterns of zinc oxide for the ZnOCe1 material. However, in the case of ZnOCe10, in addition to the ZnO signals, reflections from the (111) and (200) planes were observed, suggesting the formation of the CeO2 crystalline phase, in accordance with the POWCOD reference (PDF 00-721-7887), as depicted in Figure S1(Altomare et al. 2017). This behavior was also reported by Cerrato et al. (2018).
The crystal size was calculated using the Debye Scherrer equation (Eq. 1), where D represents the crystal size in nm, K is a constant that varies with the crystal shape (adopted as 0.94), λ is the wavelength of the Cu Kα radiation line (0.154 nm), β is the full width at half maximum (FWHM) of the highest intensity reflection, and θ is the Bragg angle (Manikandan et al. 2017). The calculated values were 25.26 nm for ZnO, 24.87 nm for ZnOCe1, and 22.13 nm for ZnOCe10.
$$D\left(nm\right)=\frac{K.\lambda }{\beta .cos\theta }$$
1
Doping rare earth elements into semiconductor networks like ZnO is a relatively complex process due to the difference in charges and ionic radii between Zn2+ and Ce4+ ions. However, the decrease in crystal size with the increase in cerium concentration may indicate the presence of this element in the ZnO lattice, leading to a distortion in the oxide matrix that reduces nucleation and growth. The zinc oxide matrix contains tetragonal voids resulting from oxygen vacancies, facilitating the insertion of cerium ions (Ahmad Wani et al. 2023). In the case of ZnOCe10, as the doping was carried out at a higher concentration, it is believed that some of the cerium was not incorporated into the zinc oxide matrix, leading to the formation of a secondary phase of CeO2.
Infrared Spectroscopy
The presence of functional groups in pure and doped zinc oxides was investigated using FTIR-ATR, and the obtained spectra are shown in Fig. 3. The broad band around 3250 cm− 1 can be attributed to the stretching vibrations of hydroxyl groups from water molecules adsorbed on the surface of the oxides. The signals at 1500 cm− 1, 1370 cm− 1, and around 1060 cm− 1 are indicative of residual oxalate, with the first two signals attributed to the stretching modes of the C = O bond and the last one to the stretching vibrations of the C-O bond (Hannachi et al. 2022). This hypothesis is supported by the results of TGA analysis of pure zinc oxide, which exhibits a significant mass loss (38%) initiated near 300°C, associated with carbon species (Figure S2). Furthermore, the bands around 860 cm− 1 and 670 cm− 1 are associated with metal-oxygen bond vibrations (Hannachi et al. 2022). The cerium-doped materials showed a similar band profile to pure zinc oxide.
The region of the FTIR spectrum corresponding to metal-oxygen bonds was deconvoluted into sub-bands with Gaussian shapes to identify the components in this region. Pure zinc oxide showed three sub-bands at 701 cm− 1, 669 cm− 1, and 650 cm− 1, with the first one being the most intense. When the material is doped with cerium, a fourth sub-band appears at around 690 cm− 1, which may be related to Ce-O bonds. Additionally, it is observed that the absorption band at 701 cm− 1 in the pure oxide is shifted to longer wavelengths in the spectra of the doped materials. This change in the band position can be attributed to a disturbance in the zinc oxide lattice caused by the presence of cerium, indicating the integration of cerium ions into zinc sites.
UV–vis diffuse reflectance spectroscopy (DRS)
The band gap energy (Eg) of the materials was determined from the diffuse reflectance spectra (DRS). The spectra were transformed according to Eq. 2, using the Tauc plot of (αhv)2 as a function of hv, with extrapolation of the linear part of the plot to find the intersection on the x-axis (Fig. 4).
$${\left(\alpha h\nu \right)}^{1/\gamma }=\text{B}(h\nu -{E}_{g})$$
2
Where h is the Planck constant, ν is the frequency of the photon, Eg is the band gap energy, B is a constant, and the factor γ depends on the nature of the electronic transition. For zinc oxide, γ is equal to ½ (Makuła et al. 2018).
The determined band gap value for ZnO was 3.23 eV, which is in accordance with the band gap range of 3.1–3.4 eV found in the literature for the wurtzite crystal phase (Subash et al. 2013; Razavi-Khosroshahi et al. 2017). The doping of zinc oxide with cerium did not cause significant differences in the optical band gap of the materials compared to pure zinc oxide. The band gap values found were 3.27 eV and 3.24 eV for ZnOCe1 and ZnOCe10, respectively. Similar results were reported by Pathak et al. (2020), where the optical band gap of Ce-doped ZnO was only slightly altered by the doping. Additionally, the increase in the amount of dopant element (1–5% in Ce molar ratio) did not cause major modifications in the band gap value of the materials, with a variation range of 3.18 to 3.20 eV.
Atrazine photocatalysis
The degradation of atrazine was conducted in a homogenous solution under UV light with air inflow, for 180 min (Fig. 5). Under such conditions, the degradation of atrazine occurred rapidly within the first 30 min, followed by a progressive reduction in catalysis velocity. The most efficient catalysts were doped-ZnO with approximately 70% of ATZ degradation. ZnO and the photolysis presented 59 and 47% (p ≤ 0.05) of degradation efficiency, respectively. The slight increase in atrazine degradation by the ZnOCe10% material may be associated with the smaller particle sizes calculated in the XRD analysis. Smaller catalytic particles provide a larger surface area relative to their volume, which typically results in more efficient catalytic activity. After stabilization, the reaction was left for 24 h to check if there was any additional reduction in Atrazine levels. However, no alteration was observed after that period.
Atrazine photocatalysis intermediate products
The photocatalytic reaction of atrazine yields many products such as DEA, DIPA, DEDIPA, DEA-2OH, DIPA-2OH, DEDIPA-2OH and finally cyanuric acid. These reaction intermediates display different toxicities levels which may be higher or lower to Atrazine itself (Tchounwou et al. 2000). In our work we have investigated if the intermediates formed during the photocatalysis and photolysis are more or less toxic than atrazine itself, as well as, if the final reaction products would pose any harm to the nematodes Panagrellus redivivus. Below are the results of the acute effects of atrazine and its intermediates after 24 h exposure. At each 30 min interval, a small aliquot (110 µL) was taken to be used in the toxicity assay. Toxicity testing was conducted in quintuplicates. At the end of the reaction, all treatments were able to reduce drastically the toxic effects of atrazine, except for photolysis reaction. ZnO, ZnOCe1 and ZnOCe10 were statistically similar to negative control (PBS), followed photolysis reaction. Figure 6 displays the mean difference among the treatments.
Growth and Behavior
Body size is a sensitive endpoint to monitor the toxic effects of atrazine and its degradation products. Even at low concentrations (at the last time point of the reaction) with no expressive mortality rates, it is possible to observe the toxic effects of many substances such as heavy metals and pesticides. Here, we have observed that despite the mortality rate of 8% (approximately), the complete photolysis of atrazine in the working conditions was responsible for significant reduced body length (Fig. 7). Besides, from an observational point of view, the moving speed and the pattern of movement seemed impaired when compared to other treatments. The nematodes seemed less active and hesitant while moving. ZnO presented similar behavior to photolysis products, though significant lower. In the other hand, both ZnOCe1 and ZnOCe10, were able to neutralize this toxic effect.
Similar to body size, head trashes are an indicative of wellness. Movement and coordination rely on muscle and neuronal function; thus, we have investigated the possible alterations in this parameter after chronic exposure (4 days). In our work, we have observed that exposure to atrazine resulted in reduced head trashes in comparison to the degradation products formed during photolysis and in reactions catalyzed by ZnO and doped-ZnO (Fig. 8).
Atrazine is a chlorinated pesticide used as a pre- and post-emergent herbicide able to block photosynthesis in broadleaf and grassy weeds. Despite its use, atrazine may reach several other organisms besides undesirable plants in crops. Several studies have shown that atrazine can reach surface and ground waters contaminating aquatic fauna while this route is susceptible to bioaccumulation.
By exposure to sunlight, microorganisms and other chemical substances in the environment, atrazine may be broken down to different metabolites such as diaminochlorotriazine (DACT; 6-chloro[1,3,5]triazine-2,4-diamine), deisopropylatrazine (DIA; 6-chloro-N-ethyl- [1,3,5]triazine-2,4-diamine), and deethylatrazine (DEA; 6-chloro-N-isopropyl- [1,3,5]triazine-2,4-diamine). Moreover, atrazine degradation products can be as equal or even more hazardous as atrazine itself. Another concern is that atrazine is frequently found in groundwater, soil and air causing non-point source pollution. The presence of atrazine and its metabolites in the environment, has been linked to endocrine disruption, hermaphroditism in frogs, DNA damage and miscarriage and birth defects in humans. Therefore, the aim of our research was to study the efficiency of three catalysts in degrading atrazine, as well as testing the degradation products and atrazine in different endpoints on Panagrellus redivivus. We also used photolysis for comparison since this process may occur in the environment, though in a lower extent (Chowdhury et al. 2021).
Advanced oxidation processes (AOPs) has been widely used in the removal of environmental contaminants. AOPs works by generating highly active free radicals to mineralize pollutants. In our work, the photodegradation of atrazine by direct exposure to UV and the catalysts resulted in a significant reduction of atrazine concentration, considering the low UV potency used (0.871 mW cm− 2). In comparison to literature available, our catalysts present a great catalytic efficiency towards atrazine (Navarra et al. 2022; Kashmery and El-Hout 2023). The degradation obtained was able to reduce P. redivivus mortality to negligible levels, as observed for ZnOCe1% and 10%, followed by ZnO and then photolysis. ZnO and its dopped versions, had different outcomes, when comparing degradation rates to mortality rates. We found out that despite their apparent statistical similarity, ZnO mediated intermediates presented more toxic effects to the nematodes. This was observed by lower body sizes, and less head trashes. According to Figs. 3 and 4, these parameters are considered sensitive endpoints to monitor toxic effects of atrazine. Observations pointed out in other articles explain that the formation of certain compounds strongly depends on particular experimental conditions. The results obtained here showed that ZnO might be producing more toxic intermediates in comparison to the ZnOCe1 and ZnOCe10. Further, it was observed that the toxic behavior of atrazine extends beyond its high environmental concentration. Recent studies have revealed a rather intriguing phenomenon—an atrazine dose response that follows a hump-shaped or U-shaped curve (Pathak et al. 2020). This intriguing behavior implies that even at low doses, atrazine can exert more significant adverse effects than expected. Unlike the linear relationship often associated with toxic substances, where higher doses lead to greater harm, atrazine seems to defy this conventional wisdom. Instead, it reminds us that the intricate dynamics of pesticide toxicity are not always straightforward and can vary depending on the concentration, making it a noteworthy subject of investigation in environmental research.