3.1. Preparation of PA66 nanofibers containing TiO2/CQDs by electrospinning
During the electrospinning process, the flow of the polymer solution at the tip of the needle was steady, and there were no current fluctuations. The resulting fibers were white in appearance and exhibited a high degree of flexibility. For the PA66 fibers, SEM images (
Figure 2A and B) presented homogeneous fibers with an average diameter of 253 ± 55 nm, and no beads or crystallites were observed on the surface of the fabricated fibers. However, SEM images of PA66/TiO
2/CQDs (
Figure 2C and D) revealed that the fibers exhibited increased roughness and greater heterogeneity, with diameters ranging around 300 ± 82 nm, indicating the incorporation of TiO
2/CQDs into the fibers.
EDS analysis (
Figure 3) confirmed the presence of Ti in the PA66/TiO
2/CQDs fibers, distributed uniformly throughout the polymer, corroborating the uniform distribution of TiO
2/CQDs in the fibers.
X-ray diffractogram of PA66/TiO
2/CQDs nanofibers showed a high similarity with the TiO
2/CQDs XRD pattern (
Figure 4), indicating the successful incorporation of the TiO
2/CQDs composite. Both XRD patterns showed the presence of TiO
2 anatase and rutile crystalline phases. The XRD peaks at 2θ = 25.5°, 38.0°, 48.3°, 55.3°, 62.9°, and 70.4°, correspond to the anatase (101), (004), (200), (211), (204), and (220) crystal planes, respectively. The rutile pattern can be observed in the reflections at 2θ = 27.6°, 36.3°, 41.4°, and 54.1°, respectively corresponding to (110), (101), (111), and (211) crystal planes. These results are in accordance with the previous works and with the standard pattern (Joint Committee on Powder Diffraction Standards (JCPDS)) for anatase TiO
2 (card No. 00-021-1272) and rutile TiO
2 (card No. 00-021-1276) [
31,
32]. Moreover, the broad peaks at 2θ angles of 20.9° and 23.5° of the PA66/TiO
2/CQDs nanofibers could be attributed to the presence of the PA66 polymer [
33,
34].
FTIR-ATR spectra of PA66 and PA66/TiO
2/CQDs (
Figure 5) showed the presence of the same peaks at 3296 and 3084 cm
-1 corresponding to the amino group stretching vibrations, 2932 and 2860 cm
-1 from stretching vibrations of CH
2, 1634 cm
-1 attributed to the stretching vibration of the amide carbonyl group, 1538 cm
-1 from bending and stretching vibrations of -NH, and 1464 cm
-1 corresponding stretching vibrations of CH
2 [
35,
36]. Furthermore, the PA66/TiO
2/CQDs presented a broad peak at 666 cm
-1 that can be attributed to the stretching of Ti–O bonds [
30]. These results indicated that the composite was incorporated into the fibers without affecting the polymer formation.
3.2. Photocataytic experiments
In previous works, TiO
2/CQDs composites (500 mg L
-1) evidenced good performance as photocatalysts in the solar-driven removal of antibiotics from water resources [
18,
30]. As mentioned before, these composites have two main setbacks: recovery from treated water and reuse. Therefore, to address these disadvantages, the TiO
2/CQDs composite here produced was incorporated in nanofibers of PA66 with a ratio of 1:2 TiO
2/CQDs:PA66 (
w/w). To maintain the photocatalyst dosage used in previous works (0.5 g L
-1 of TiO
2/CQDs) [
18,
30], 1.5 g L
-1 of PA66/TiO
2/CQDs was used for the photocatalytic experiments.
For each antibiotic, photodegradation kinetics studies (
Figure 6) were carried out in (i) the absence of the photocatalyst (photolysis), (ii) with bare PA66 fibers, and (iii) with PA66/TiO
2/CQDs photocatalyst. The kinetics parameters [rate constants (
k, h
−1), determination coefficient (
R2), and half-life times (
t1/2, h)] for photolysis and photocatalytic experiments carried out in PBS and river water, for both antibiotics, were obtained by applying pseudo-first-order equation fittings (
Table 2).
For AMX, initially, the comparison between photolysis, bare fiber PA66, and PA66/TiO
2/CQDs photocatalyst was performed in PBS. The results showed that the bare fiber PA66 and the photolysis had similar behavior (
Figure 6A), showing almost no AMX removal after 6 h of irradiation, demonstrating that the removal of AMX was not impacted by the polymer. The rate of removal of AMX has greatly increased with the incorporation of TiO
2/CQDs in the PA66 fibers (
Table 2 and
Figure 6A). The application of PA66/TiO
2/CQDs was able to remove AMX from water much faster than only photolysis, increasing the kinetic rate by around 30 and 17 times in PBS and river water, respectively.
In fact, the
t1/2 decreased from 60 ± 1 h (in the absence of photocatalyst) to 1.98 ± 0.06 h when the PA66/TiO
2/CQDs photocatalyst was applied in PBS (pH 8.06). Furthermore, the results showed a good AMX removal even in river water, reducing the t
1/2 for the AMX removal to 3.52 ± 0.09 h. In this case, the photodegradation of AMX in river water was lower when compared to PBS, which can be attributed to the matrix complexity or the pH. River water presented a higher content of organic matter than PBS (
Table 1 shows higher values of TOC than PBS), thus, the photocatalyst could be used to degrade organic matter present in this matrix at the same time as AMX, having a competitive behavior in the photodegradation. Moreover, the literature reports that the photodegradation of AMX is influenced by the matrix pH, being faster at higher pH values [
6].
A control was performed during the photocatalysis experiments, where the same experimental conditions were applied, but the tubes were maintained in complete darkness. These controls confirmed that all the antibiotic removal can be attributed only to photodegradation, excluding other processes like adsorption and thermal decomposition.
As for AMX, the SDZ removal was studied for all the different conditions in PBS, and then, the photocatalyst performance was tested in river water (
Figure 6B and
Table 2). The bare fiber PA66 showed a slower kinetic rate of photodegradation of SDZ than photolysis, indicating that the PA66 fiber exerts a filter effect that inhibits the photodegradation of SDZ. However, the application of the PA66/TiO
2/CQDs photocatalyst was not affected by this phenomenon, as verified by a significant increase in the removal of SDZ. In fact, the
t1/2 of SDZ decreased from 5.4 ± 0.1 h to 1.87 ± 0.05 h with the application of the photocatalyst in PBS. SDZ removal was faster in river water than in PBS, with a
t1/2 decrease from 1.87 ± 0.05 h to 1.33 ± 0.05 h. This evidenced that the matrix slightly affects SDZ photodegradation, which is in accordance with literature where the same behavior was observed [
37]. As for the results of the dark controls of SDZ, it was confirmed that all the observed removal can be attributed to photodegradation, as was found for AMX.
Table 3 presents some studies reported in the literature regarding the photocatalytic degradation of AMX and SDZ using TiO
2-based catalysts and simulated solar light irradiation. It is important to observe that a direct comparison between these photocatalysts is complex since different parameters were used, such as antibiotic concentration and matrix pH. For example, Gao et al. [
38] obtained one efficient photocatalyst for the degradation of AMX (5 mg L
-1) in ultrapure water, with a constant rate of 0.0614 min
−1, preparing a composite based on TiO
2, graphite carbon nitride (g-C
3N
4), and silver nanoparticles. Meanwhile, Thi et al. [
39] coupled TiO
2 to ZnO nanoparticles to remove AMX (50 mg L
-1) by photocatalysis in the presence of O
3, obtaining a
k ranging from 0.0032 to 0.0198 min
-1 when the pH varies from 3 to 11. Regarding SDZ removal, Louros et al. [
27] prepared TiO
2/CQDs composites (by an in situ method), obtaining a rate constant of 0.71 ± 0.02 h
-1 for the photocatalytic removal of this antibiotic (10 mg L
-1) in PBS (pH 8.3), under solar irradiation. Also, Silva et al. [
18] prepared similar composites using TiO
2 P25, achieving better photocatalytic performance with a
k of 4.81 ± 0.06 h
-1, in PBS pH 8.6.
As a proof of concept, some recovery and reuse studies were performed, using PA66/TiO
2/CQDs in subsequent cycles for the removal of SDZ from river water (
Figure 7). The results showed that until the 3rd cycle of reuse, the same removal of SDZ was obtained. In fact, even after 3 cycles, it was possible to remove 88% of SDZ in river water in 4 h, a slightly higher value than in the 1st cycle. Thus, the results here presented evidence the efficiency and stability of the PA66/TiO
2/CQDs photocatalyst and the potential to be applied to remove antibiotics from water environments. Comparatively to the literature (
Table 3), for AMX, both Gao et al. [
38] and Thi et al. [
39] performed reuse cycles for antibiotic removal. On one hand, Gao et al. tested until the 4th cycle with a gradual small loss of efficiency at each cycle, concluding that the photocatalyst maintained high catalytic activity after four cycles and, therefore, has excellent stability in visible light photodegradation reactions. On the other hand, Thi et al. also tested the photocatalyst for 4 cycles of reutilization, the wash and thermic treatment of the photocatalyst after each cycle, and verified a small drop (5%) in AMX mineralization efficiency. Thus, the authors conclude that the photocatalyst used maintained its stability and catalytic activity for the removal of AMX from wastewater. For SDZ, only Louros et al. [
41] performed the recovery and reuse of the materials up until the 5th cycle. In this case, the authors found no significant differences between
t1/2 obtained for the successive cycles so the results showed after-use recovery photocatalysts, excellent photostability, and reusability of this photocatalyst.
The PA66/TiO2/CQDs photocatalyst developed in this study presented an average performance when compared with others found in the literature. However, these nanofibers have the potential to be easily reused, allowing their use as sustainable photocatalysts for water treatment and contributing to water circularity.