The synthesis of an efficient composite photo-Fenton nanocatalyst was investigated using acid leaching processes of copper slag (CS) with two types of citric acid (EC = edible and AR = analytical-grade reagent). For this purpose, citric acid at room temperature or at 40 °C was used to lixiviate the elements from CS, and then, the leachates were subjected to a thermal treatment (350 °C). The formation of maghemite (γ-Fe