Morphology of PAN, ZnO/PAN, and La-doped ZnO/PAN
The morphology of the electrospun PAN, ZnO/PAN, and La-doped ZnO/PAN was visualized by FE-SEM (SEI QUANTA-200) analysis. Nanofibers with an average diameter of 641.98 nm were observed in the case of plain PAN nanofibers, as shown in Fig. 1a. It was noticeable that the fibers exhibited a smooth surface and uniform diameter throughout their length. When the PAN fibers were loaded with ZnO, the smooth surface became coarse. The diameter of the ZnO-loaded PAN nanofibers increased in size compared to that of the plain PAN nanofibers (Fig. 1b). Figure 1c shows the FE-SEM image of the La-doped ZnO/PAN nanofibers, which were obtained by uniformly embedding the surface of the PAN nanofibers with La-ZnO crystals. The average diameter further increased owing to the presence of La-ZnO crystals. Hence, by assessing the morphology of the synthesized nanofibers, it can be concluded that the ZnO seeded PAN and La-doped ZnO/PAN nanofibers possess rough surfaces advantageous for catalytic applications.
Chemical composition of ZnO/PAN
The chemical composition and bonding states of La-doped ZnO/PAN were determined by XPS. The oxidation state in the crystal structure is a feature of the core-level binding energy of metals or non-metals (Sangami and Dharmaraj 2012). Figure 2 shows the XPS binding spectra of Zn 2p, O1s, and La 3d. The spectrum of Zn 2p displayed two peaks due to the spin-orbit splitting at binding energies of 1022 and 1045 eV corresponding to the core levels 2p3/2 and 2p1/2, respectively (Fig. 2A). The energy difference between the two peaks for Zn2+ ions was approximately 23.1 eV, which matches well the standard value of 23.1 eV (Phuruangrat et al. 2014). The binding energies for the O1s state of the ZnO/PAN and La-doped ZnO/PAN nanofibers are shown in Fig. 2B. A peak at a binding energy of 531.5 eV was observed that corresponded to O1s for both ZnO and La-doped ZnO due to the O2− ions in the ZnO lattice (Fig. 2B (a)). However, in the La-doped ZnO/PAN, an additional peak occurred at a binding energy of 533 eV (Fig. 2B (b)), which could be attributed to the oxygen vacancies in the ZnO matrix (Dhara et al. 2014). Figure 2C shows the binding energy spectrum of La 3d, which features a doublet centered at 840 and 855 eV, corresponding to 3d5/2 and 3d3/2, respectively.
Degradation of atrazine under various reaction conditions
Influence of light source
Initially, the light source was varied to find optimal conditions for achieving the efficient degradation of atrazine using La-doped ZnO/PAN nanofibers as catalyst. The maximum degradation was observed under UV radiation, as shown in Fig. 3a. The degradation efficiency was low in the case of reactions carried out under visible light compared to those under UV light source. The results showed that the source played a prominent role in the degradation process.
Influence of pH
The pH of a reaction mixture plays a major role, as it can determine the charge of the contaminants and catalyst. Hence, a change in the pH could influence the adsorption of atrazine on the surface of a catalyst (Zhang et al. 2013). The effect of pH on atrazine degradation is shown in Fig. 3b. The reaction was carried out within the pH range of 3–10 to determine the most effective pH, and the results revealed that pH 7 (neutral) allowed for a higher efficiency (approximately 96%) than other tested pH values. In neutral and alkaline media, OH− radicals are the active oxidizing species. A high concentration of OH− radicals at neutral pH enhances photodegradation (Lee et al. 2016).
Influence of catalyst type
After the optimization of pH, the influence of the catalyst type was investigated by employing different catalytic agents. Figure 3c depicts the change in the degradation efficiency with different catalysts. The La-doped ZnO/PAN nanofibers showed a maximum efficiency of 97% compared to plain PAN and ZnO/PAN nanofibers, which exhibited efficiencies of 35 and 95%, respectively. The large band gap and high recombination rate of the photogenerated electron-hole pairs reduced the degradation efficiency of ZnO. Owing to the presence of the 4f electronic configuration in La, its doping on ZnO reduced the band gap and delayed the recombination of charge carriers. This made the La-doped ZnO/PAN nanofibers very efficient for photodegradation (Sanchez Rayes et al. 2017).
Influence of catalyst quantity
The La-doped ZnO/PAN nanofibers were found to be a potential catalyst for the degradation of atrazine. To explore their effective concentration, reactions were carried out using different catalyst quantities ranging from 10 to 30 mg (Fig. 3d). No reaction occurred in the absence of catalyst, while all reactions carried out in the presence of the catalyst afforded a degradation product, although the percentage of efficiency differed from lower to higher concentrations. A maximum degradation of 98% was obtained with 30 mg of catalyst. A higher amount of catalyst may provide a larger surface area for the reaction to proceed. This in turn could increase the degradation of atrazine as the number of interacting molecules on the surface increased.
Influence of atrazine concentration
The optimal conditions for achieving the maximum degradation of 10 ppm atrazine were determined by tuning various reaction parameters. The effect of atrazine concentration on the degradation efficiency was determined by increasing the concentration to 15 and 20 ppm (Fig. 3e). The results revealed that the degradation efficiency of the proposed photocatalyst considerably decreased upon increasing the toxin concentration. The photocatalytic degradation generally depends on the OH radical concentration. The OH radical formation is limited to the catalyst surface regardless of the type of light source and amount of catalyst. Hence, when the concentration of atrazine increases, it surrounds the catalyst surface and leads to a decrease of degradation efficiency (Wang et al. 2007; Mkhalid 2016).
Atrazine degradation reaction kinetics
Figure 4 shows the degradation kinetics plot, which indicates that atrazine degradation followed second-order rate kinetics under the optimized conditions. The rate constant for second-order kinetics is given by the equation:
[K = 1/t × x/ a(a-x)] dm3 mol− 1 min− 1
where ‘a’ is the preliminary concentration of atrazine and ‘x’ is the concentration of atrazine after time ‘t’. The correlation coefficient ‘R2’ and rate constant ‘K’ were calculated based on the above equation. The value of R2 was 0.997, while that of K was 0.046 dm3 mol− 1 min− 1.
Mineralization of atrazine
The mineralization degree of atrazine using the La-doped ZnO/PAN nanofiber catalyst was analyzed by TOC measurements (Fig. 5). The measurements were performed at regular time intervals of 10 min over 40 min, and 68% of the organic carbon was found to be reduced. Complete mineralization could not be achieved, leading to the deposition of the intermediates in solution. According to the literature, the opening of the triazine ring by an advanced oxidation process (AOP) is difficult; this can be the reason for the incomplete mineralization of atrazine (Yang et al. 2014).
UV absorbance spectrum of atrazine under optimized conditions
A UV-visible spectrum was recorded at regular time intervals of 10 min under optimized conditions using 30 mg of La-doped ZnO/PAN nanofibers as catalyst at pH 7 for degrading 10 mg/L of atrazine (Fig. 6a). Atrazine exhibited a strong absorption peak at 222 nm. A gradual reduction in the intensity of the peak with time was observed, indicating the degradation of atrazine.
Synthetic wastewater analysis of atrazine
The degradation of atrazine was performed to identify the efficiency of the synthesized material at the laboratory scale under optimized conditions for 150 min using a photoreactor. The formation of the degradation products was confirmed by GC-MS and quantified using HPLC. The absorbance peak of the control at a wavelength of 222 nm disappeared in the degradation sample (Fig. 6b). This signifies that the material could completely degrade the toxin, even at a large scale.
Quantification of atrazine using HPLC analysis
Figure 7a and 7b show the liquid chromatogram of pure atrazine and the degraded sample obtained from HPLC analysis, respectively. The retention time (RT) of atrazine was 12.883 min, while in the degraded sample the peak corresponding to atrazine diminished along with the formation of a few more peaks at RT 9.7, 9.1, 5.3, 4.3, and 2.3 min. The emergence of these additional peaks confirmed that atrazine broke down into various metabolites with the intensity of the peak at 5.3 RT being higher compared to other peaks. Hence, this may be the main hydroxylation product of atrazine degradation with an efficiency of 94.9%.
GC-MS analysis to confirm the mass of the degradation product
The degradation of atrazine was confirmed by GC-MS analysis (Fig. 8a). The major characteristic spectra of atrazine corresponded to m/z = 215, 200, 187, 173, 158, 104, 68, 58, and 43. The molecular ion peak was at m/z 215, which completely disappeared in the degradation product, as shown in Fig. 8b. The mass spectrum of the degradation product exhibited a molecular ion peak at m/z = 187. Based on existing literature, the degradation compound was identified as 2-chloro-4-(isopropylamino)-6-amino-S-triazine (Yang et al. 2014). The mass difference between atrazine and the degradation product was 28, indicating the loss of an ethyl group. The GC-MS results suggested that atrazine degraded into an intermediate compound.
Degradation pathway
Based on the GC-MS analysis, a possible degradation pathway was proposed, as shown in Fig. 9. The main methods to achieve atrazine degradation proposed in early reports were dealkylation, alkyl chain oxidation, and dechlorination. In this study, only one product was obtained, as confirmed by GC-MS. Hence, the only plausible mechanism through which the product was obtained is alkyl chain oxidation. The abstracted hydrogen of the photogenerated free radical from the amine alkyl group leads to the formation of an organic free radical. This radical can be oxidized to a carbonyl group, and eventually, the acetyl group undergoes degradation from the core to form 2-chloro-4-(isopropylamino)-6-amino-S-triazine (Qin et al. 2010).
Photocatalytic mechanism
Figure 10 displays the general mechanism of atrazine photodegradation, according to which UV light irradiation of the catalyst excites electrons from the valence band (VB) to the conduction band (CB). This results in the formation of electron-hole pairs in the system, which are the main reason for the degradation to occur. The electrons in the CB reduce the dissolved oxygen into oxygen radicals, while the holes in the VB oxidize water into hydroxyl radicals. These radicals may initiate the degradation of pesticides into various nontoxic fragments.
Catalyst stability and recyclability
The stability and reusability of the La-doped ZnO/PAN nanofibers were tested over three cycles of photodegradation (Fig. 11). The catalyst was separated by centrifuging the solution mixture and washing thoroughly with water to remove impurities. Then, it was dried and used for the next reaction. For each cycle, the efficiency was measured at a 10 min time interval. The results revealed that the catalyst exhibited 90% efficiency even after three cycles. Hence, the catalyst was stable for more than three cycles and could be easily recycled for further use.