## 4 Conclusions  
In conclusion, in this study, we have reported, for the first time, the photocatalytic activity of CZTS NPs in relation to the degradation of wastewater and primary water micropollutants, showing remarkable results. We chose DCF as a benchmark primarily because it is a known and worrying emerging micropollutant due to its natural degradation of harmful and even more toxic carbazoles through photolysis. Secondarily, its main peak in UV-vis spectroscopy at 276 nm allowed for the study of the degradation kinetics. We have demonstrated that CZTS NPs are an active photocatalyst suitable for DCF degradation and that they prevail over photolysis kinetics when the reaction is performed under UV-vis light irradiation, leading to an outstanding 90% of the DCF being degraded, with no carbazole formation taking place. Compared to the photocatalysts featured in other photocatalytic degradation studies reported in the literature, CZTS is a cheap photocatalyst that shows comparable results in terms of degradation rates, but more importantly, CZTS can operate in only-visible-light conditions, obtaining comparable results. Thanks to the visible-light absorption properties of CZTS, our catalytic system demonstrably performs well when irradiating with a visible-light-only source, providing the same performance as under UV-vis light. This also makes our system suitable for indoor applications with visible light sources. We look forward to the possible industrial implementation of this system in dedicated pipelines, especially seeing as it could facilitate improved results compared to the current DCF industrial degradation rates. These preliminary results are promising and could facilitate new ways for wastewater and primary water treatment against emerging micropollutants such as DCF, considering that, nowadays, the DCF degradation rates in industrial water treatment plants range between 20 and 40%. Further investigations are needed to understand the kinetics mechanism and the nature of the degradation products formed. A further step will be identifying the best possible operating conditions to decrease the amount of catalyst used, identifying the lowest/highest DCF concentration degradable with our system, and testing the recycling of CZTS NPs. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app14219923/s1](https://www.mdpi.com/article/10.3390/app14219923/s1)]([https://www.mdpi.com/article/10.3390/app14219923/s1](https://www.mdpi.com/article/10.3390/app14219923/s1)). Cu2 ZnSnSnSn nanoparticle preparation; Figure S1: 300 W Xenon lamp LSB530 (Quantum Design, San Die-go, CA, USA); Figure S2: Photolysis and photocatalysis setup; Figure S3: Quantum Design (San Diego, CA, USA) producer's emission spectra; Figure S4: Cu2 ZnSnSnSn4 nanoparticles used as a photocatalyst; Figure S5: Filtered reaction solution after 120' of UV-vis and after 120' of UV-vis photocatalysis. Conceptualization, G.T., V.T. and F.G.; methodology, G.T., A.H.H., A.L. and F.G.; validation, G.T., V.T. and F.G.; investigation, G.T., A.H.H. and A.L. ; resources, S.B. and F.G.; data curation, G.T. and F.G.; writing--original draft preparation, G.T., V.T. and F.G; writing--review and editing, G.T., V.T. and F.G.; visualization, G.T. and V.T. ; supervision, G.T., V.T., S.B. and F.G.; project administration, S.B. and F.G.; funding acquisition, S.B. All authors have read and agreed to the published version of the manuscript. **Funding:** This work was supported by a PhD scholarship on Green Issues from action IV.5 of the PON Research and Innovation 2014-2020 \"Education and research for recovery--REACT-EU\" program. This work was also supported by the \"MUSA--Multilayered Urban Sustainability Action\" project, funded by the European Union, via NextGenerationEU, under the National Recovery and Resilience Plan (NRRP) Mission 4 Component 2 Investment Line 1.5: Strengthening of research structures and creation of R&D \"innovation ecosystems\", set up of \"territorial leaders in R&D\".