**4. Conclusions**

The low-cost mechanochemical combustion method was used to synthesize CuO/ZnO nanocomposites with various Cu contents. The best photodegradation efficiency was obtained with 5 wt % CuO/ZnO. The values of MB degrading with un-doped ZnO and CuO/ZnO were measured to be 81% and 98% respectively, after 2 h of solar irradiation. MB photodegradation with CuO/ZnO under sunlight occurred mainly with •O2 <sup>−</sup> and •OH radicals, while the h<sup>+</sup> radical showed a minor role in the process. The decrease of TOC values of MB indicated the mineralization in the photocatalytic process. Therefore, sunlight photodegradation technology may be a more effective technique for textile wastewater treatment than photodegradation with artificial and expensive Hg-Xe lamp.

**Author Contributions:** M.A.I.M. conceived and designed the experiments and wrote the paper. A.A.M.S. performed the experiments. S.M.M., J.H., and R.I. analyzed the results.

**Funding:** This research was funded by the University Grants Commission of Bangladesh, grant number 6(76)UGC/S&T/Chemistry-02/2018/3577.

**Acknowledgments:** The authors are grateful to the Centre for Advanced Research in Sciences (CARS), University of Dhaka, Bangladesh for providing partial analytical support.

**Conflicts of Interest:** The authors declare no conflict of interest.
