In the first section of this research, the multi-walled carbon nanotubes from polyethylene terephthalate plastic bottles waste were synthesized by pyrolysis method. Then, the performance improvement of graphite carbon nitride photocatalyst with multi-walled carbon nanotubes and the green synthesis of silver nanoparticles from eucalyptus plant leaves were investigated to remove methylene blue dye from water environments for the first time. The synthesis of samples was confirmed through FESEM, EDS-MAP, Raman, DLS, BET, and XRD analysis. After that, Design-Expert 13 software was used to design experiments and investigate the effects of independent variables, including pH, methylene blue concentration, photocatalyst dosage, and reaction time on methylene blue removal efficiency. According to the variance analysis results, the maximum experimental and predicted removal efficiency in the optimized parameters (wastewater concentration of 12.6 mg L-1, pH of 9, photocatalyst dosage of 0.52 g L-1, and a reaction time of 231 min) were 98.7% and 99.89%, respectively. Also, the predicted values showed strong agreement with the experimental results with R2=0.95 and Adj-R2=0.93. Moreover, the solution pH significantly affects the photocatalytic degradation of methylene blue compared to other parameters by influencing the surface charge properties of the catalyst. In the second stage, the performance of the synthesized photocatalysts in the removal of tetracycline was studied. The findings showed that the tetracycline removal efficiency of MWCNTs@g-C3N4@Ag, g-C3N4@Ag, and g-C3N4 with a concentration of 0.5 g/L and 120 min was 92.6%, 82% and 78.9%, respectively, which prove the better performance of MWCNTs@g-C3N4@Ag in the tetracycline removal. The experiments presented that the tetracycline degradation increased with an increase in the photocatalysts dosage and reaction time. Finally, life cycle assessment (LCA) of MWCNTs@g-C3N4@Ag photocatalyst by the ReCiPe (H) 2016 method using