## 3 Results and Discussion
Figure 8 shows the trends of the DCF concentration decrease during photocatalytic experiments in the presence of UV-vis light (yellow line and circles) and visible light only (red line and squares) as a function of time. The two trends are very similar, but the decrease in DCF concentration is more significant when irradiated with UV-vis light. Using the same figure, it is also possible to compare photolysis with photocatalysis. In the case of photolysis driven by only visible light (green line, squares), the concentration of DCF remains stable, indicating that no reaction takes place without the aid of a photocatalyst. On the other hand, in UV-vis-light-driven photocatalysis, the apparent slight increase in concentration is just an indirect effect of the newly formed carbazole photolytic products. These carbazoles bring an absorbance peak at 310 nm (as indicated in Figure 5), which is too close to DCF's unique peak at 276 nm to determine DCF's concentration decrease over time. A linear behavior was observed in tests performed with UV-vis and visible light only, indicating linear kinetic models of the second order. The reaction order of the DCF photodegradation was determined by plotting 1/A (with A being the DCF absorbance at 276 nm) against the irradiation time (min). For UV-vis and visible irradiation-only photocatalysis, the determination coefficient (R\({}^{2}\)) values were 0.9371 and 0.9776, respectively, and the half-life times (t\({}_{1/2}\)) were 2.1 min and 2.7 min, respectively. Figure 7: (a) UV-vis absorbance spectra of the photocatalysis of a 10.0 mg/L DCF solution mediated by CZTS NPs (1 g/L) under 1 sun irradiance through 300 W Xenon unfiltered lamp (UV-vis light); (b) UV-Vis absorbance spectra of the photocatalysis of a 10.0 mg/L DCF solution mediated by CZTS NPs (1 g/L) under 1 sun irradiance through 300 W Xenon UV-filtered lamp (visible light only); t = 0, 1, 3, 5, 15, 30, 60, 120 min. By comparing the two kinetics, it stands out that the residual DCF concentration after 120 min for the visible-light-only photocatalysis (Figure 8, specifically the green lines and circles) is 1.9 (\(\pm\)0.2) mg/L, corresponding to a degradation percentage of the micropollutant of 80 (\(\pm\)10)%. On the other hand, using the entire light spectrum, including the UV portion, the residual DCF concentration after 120 min is 0.8 (\(\pm\)0.1) mg/L, corresponding to a degradation percentage of the micropollutant of 90 (\(\pm\)10)%. However, the two degradation percentages obtained are not significantly different (as shown by a \(t\)-test at a 95% confidence level). The starting DCF concentration could explain the faster degradation kinetics in the first few minutes. It could be supposed that the reaction proceeds slower when achieving contact between the catalyst and the water-dissolved DCF becomes more difficult due to the DCF being too diluted by the end of the 120 min. Moreover, slight inhibition, due to by-products, of the degradation could occur (i.e., the possible adsorption effect on the NPs that needs to be further investigated), and follow-up work will be conducted to test the catalyst recycle. Further investigations are needed to understand the kinetics mechanism and the nature of the degradation products formed in both cases. However, preliminary results on the investigation of the main degradation product by the end of the 120 min reaction time indicate a molecule at m/z 201 (ESI\(-\)) and m/z 203 (ESI+) with the formula C\({}_{8}\)H\({}_{10}\)O\({}_{6}\) (Figures 9 and 10). Based on the MS/MS spectrum, the chemical structure of the species is proposed (Figure 11), and it does not belong to the carbazole family. Figure 8: Trends in DCF concentration with respect to the initial value (C/C\({}_{0}\)) during the photolysis and photocatalysis experiments with UV-vis light vs. visible light only. Figure 4.0 (ESI\(-\)) MS/MS spectrum of the main degradation product after 120\({}^{\prime}\) reaction time in visible-light-only photocatalysis mediated by Cu\({}_{2}\)ZnSnS\({}_{4}\) nanoparticles.