Background & objectives: Dyes beside having negative effects on environmental aesthetic sight have serious negative biological and chemical effects on environment. The propose of this study is to remove photocatalistics of acid red 18 using ZnO nanoparticle synthesized with hydrothermal method discontinuous system.

Methods: In this experimental study first the source reactor and radiation were designed and constructed. Experiments were performed under stable conditions in pH (4, 7, and 10) with a contact time (10, 20, 30, 45 and 60) to determine the optimum pH. Then pH was fixed and effect of the initial concentration of nanoparticles (0.25, 0.5, 1, 2 and 3) grams per liter and various concentrations of Acid Red 18 (10, 25, 50, 100 and 200) milligrams per liter were studied.

Results: The results showed that the red dye removal efficiency of the process under study has directly increased with degree of acidity, so in acidic and alkaline efficiency was 83% and 1/59. It was also found that by lowering the initial dye concentration andincreaseing the contact time and concentration of nanoparticles, the removal efficiency was increased.

Conclusion: Photocatalytic process coupled with ZnO nanoparticles synthesized by hydrothermal method in removal of Acid Red 18 has done best and the efficiency is seemed to be considerable. we� suggest that the efficiency of the removal of dyes and other pollutants needs to be further investigated.