The method of producing zinc oxide nanoparticles (ZnO NPs) using red sand includes mixing red sand with water to form an aqueous suspension of red sand, removing the supernatant from the suspension, centrifuging the supernatant and retaining a second supernatant from the centrifuged suspension, dissolving a solution of zinc nitrate in the second supernatant to form a precursor solution, and adding 1M NaOH dropwise to the precursor solution to precipitate the zinc oxide nanoparticles. The precipitate may be washed, dried and calcined to provide the red sand synthesized ZnO NPs. The red sand synthesized ZnO NPs have photocatalytic activity and can be used, for example, to degrade organic dyes in aqueous environments.