## 4 Conclusions  
The degradation of diclofenac (DCF) in aqueous solution was studied using electron-beam (EB) irradiation after a nanobubbling treatment and compared with the treatments of nanobubbling and EB irradiation alone. The results demonstrated that at lower doses (0.5 kGy), the degradation of DCF was increased by 15%, from 36% to 51%, when using EB radiation after nanobubbling pretreatment in comparison with EB radiation. Nevertheless, at higher doses (5 kGy), the difference between the processes was not so evident, since 96% and 99% of the DCF was degraded using EB treatment and nanobubbling followed by EB radiation, respectively. Thus, the nanobubbling pretreatment seemed to have no significant effect on the degradation of DCF in aqueous solution. Actually, although the inorganic ion formation slightly increased, the results indicated that the COD value markedly increased after the nanobubbling pretreatment in the flow system, which could be related to the amount of organic matter in the sample. Concerning the degradation products, some with an aromatic ring with chlorine were detected and identified in solutions treated by EB radiation after nanobubbling, which can justify the higher COD values on these solutions. Further studies should be performed with increasing radiation doses in order to achieve  
Figure 13: Identified products and proposed mechanisms of degradation of DCF by combination treatment of nanobubbling and electron-beam irradiation in a flow system.  
mineralization of the DCF solutions. The toxicity of the radiolytic by-products should be also assessed to understand the effectiveness of the DCF solution treatments. This work can support the development of strategies to remove pharmaceutical pollutants from wastewaters using ionizing radiation technologies as an alternative process to the conventional ones, which have been demonstrated to be ineffective in the elimination of DCF and other pharmaceuticals.  
Conceptualization, Y.S.; methodology, Y.S., J.M. and S.C.V.; validation, Y.S.; investigation, Y.S., J.M., S.C.V., D.C.-S., M.S., S.B., E.C., G.C.J. and S.W.; resources, A.M.; writing--original draft preparation, Y.S., J.M., S.C.V. and S.W.; writing--review and editing, Y.S., J.M., S.C.V., S.W. and J.W. All authors have read and agreed to the published version of the manuscript.  
This work was funded by the European I. FAST project (grant agreement No. 101004730), the program of the Minister of Science and Higher Education \"PMW\" in the years 2021-2025 (contract no. 5180/H2020/2021/2), the Polish Ministry of Education and Science (statutory task no. III.4), and Erasmus+ KA131 grant. The authors are grateful to the Foundation for Science and Technology (FCT, Portugal) for financial support through national funds FCT/MCTES to C2 TN (UID/Multi/04349/2020), CQE (UIDB/00100/2020 and UIDP/00100/2020), and RNEM (RNEM-LISBOA-01-0145-FEDER).  
Not applicable.  
The data presented in this study are available on request from the corresponding authors.  
The authors would like to express their thanks to [PERSON] (for performing the calibration of the L-alanine solution against alanine pellets) and to [PERSON] (for performing the TOC analysis).  
Author [PERSON] was employed by the company Fine Bubble Technologies. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.