This study developed a heterogeneous catalyst derived from iron slag (IS) waste containing main constituents of FeO, ZnO and SiO2 for Fenton and ozonation processes to remove an azo dye (Reactive Red 24 (RR24)) from aqueous solutions. It was found that heterogeneous catalytic Fenton and ozonation processes with IS catalyst can effectively remove RR24 and degradation kinetic of RR24 followed pseudo-first order kinetic model. The decolorization and mineralization of RR24 were dependent on the solution pH with a maximum efficiency obtained at pH 11 for heterogeneous catalytic ozonation processes (O-3/IS, O-3/H2O2/IS) and at pH 3 for heterogeneous catalytic Fenton (H2O2/IS). The combination of ozone with IS had synergistic effect on RR24 decomposition. An increase the IS dosage from 0.5 g/L to 1.5 g/L for H2O2/IS and O-3/H2O2/IS and from 0.5 g/L to 1.0 g/L for O-3/IS could enhance the decolorization and mineralization of RR24. The decolorization efficiency and mineralization degree of RR24 after degradation evaluated through the COD removal efficiency indicated the following order: O-3/H2O2/IS > O-3/IS > O-3 approximate to O-3/H2O2 > H2O2/IS. The K values (apparent first-order rate constant values) of pseudo-first order model followed the same sequence as mineralization efficiency of RR24, in term of COD removal. The iron slag (IS) was characterized by various techniques, these being: scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). The main degradation mechanism of RR24 by before mentioned treatment systems due to FeO, ZnO and SiO2 constituents of the IS increased O-3 decomposition as well as enhanced reaction rate with H2O2 leading to more hydroxyl radicals (*OH) generation, thus RR24 degradation rate was improved. This study demonstrated that iron slag can be successfully used as the heterogeneous catalyst in Fenton and ozonation processes for effective removal of RR24 from aqueous solutions.