As previously described, Fe and Cu leaching was monitored during the runs. Fe and Cu releasing along the reaction time are shown in Fig. 4 for slag/H2O2, the behavior of slag/H2O2/UV was similar. According to the graphic, leaching of copper and iron species from slag grain surface was made as a gradual process. The two metals leaching had different behavior. Although, Cu releasing is observed since the first minutes of reaction (10 min), its leaching rate was very slow, favoring a constant Cu concentration in the medium around 3 mg L−1 during the whole treatment time. In the case of iron, its presence in the aqueous phase was only detected after 50 min of reaction. However, its higher releasing rate promoted the system reaches concentration near 25 mg L−1 at the end of the treatment (224 min). Although the photo degradation of contaminants catalyzed by copper slag promotes an important Fe leaching, at the reaction time need for the contaminant removal (30 and 90 min for slag/H2O2/UV and slag/H2O2, respectively) its concentration is lower then the values reported in other studies using Fenton's reaction, where iron concentration is around 10–25 mg L−1. However, the treatment time should be carefully controlled to avoid excessive Fe leaching in the effluent. On one hand, it is important to point out that as Fe was not detected during the first minutes of treatment, Cu should be responsible for the catalytic activity on the slag/H2O2/UV system for the removal of phenol (30 min). On the other hand, slag/H2O2 system occurred with the participation of both metals as catalysts (90 min). When compared to regular homogeneous or heterogeneous Fenton/photo-Fenton, the advantages of using copper slag are the reuse of an industrial residue as catalyst, which can protect the environment an reduce the costs due to the large availability of this waste in mining areas.