In this work, the degradation and mineralization of Orange II solutions (0.1 mM) using catalysts based on pillared saponite impregnated with several iron salts is reported. Oxidation is carried out in a batch reactor, in presence of various hydrogen peroxide concentrations, and in a wide range of temperature and pH values. Twelve samples are prepared, with three iron loads (7.5, 13.0 and 17.0 wt.%) and four iron salts as precursors, namely Fe(II) acetate, Fe(II) oxalate, Fe(II) acetylacetonate and Fe(III) acetylacetonate. The samples are characterized using X-ray diffraction, thermal analysis, infrared spectroscopy, energy dispersive spectroscopy and adsorption of nitrogen at 77 K. The catalytic results show that these solids present good properties for the degradation and mineralization of Orange II solutions, allowing to reach, in the best conditions and after 4 h of oxidation, 99% of dye degradation with 91% of total organic carbon (TOC) reduction (at 70 °C), using only ca. 90 mg of clay catalyst per litre of solution. Nevertheless, 96% of dye removal with 82% of mineralization are also reached at 30 °C. Besides, the amount of iron released into the final solution is lower than 1 ppm, in the worst of the cases, and 0.09 ppm in the best case. Keywords Orange II Fenton's reagent Oxidation Clay-Fe catalysts Heterogeneous catalysis 1 Introduction Chemical process industries, such as oil refineries, petrochemical units, dye and dye intermediate manufacturing industries, textile units, among others, are typical industries that dump toxic organic compounds to the nearer water courses, thus making the water polluted [1] . To overcome this, the use of advanced oxidation processes (AOPs) has been widely proposed, as they operate at temperature near ambient and atmospheric pressure. These processes involve the generation of highly reactive radicals (specially hydroxyl radicals) in enough quantity to effect water purification [2] , and their use is justified by the low organic content of the wastewaters to be treated, besides the low reaction temperatures, thus requiring the presence of very active oxidation agents. In particular, the oxidation using Fenton's reagent (a powerful source of oxidative HO radicals generated from H 2 O 2 in the presence of added Fe 2+ ions [3] ) has proved to be a promising and attractive treatment method for the effective destruction of a large number of hazardous and organic pollutants [4–8] . The generated HO radicals are highly oxidative, non-selective, and able to decompose many organic compounds, including dyes [9] . However, it should be pointed out that the homogeneous Fenton process has a significant disadvantage. Homogeneously catalyzed reactions need up to 50–80 ppm of Fe ions in solution, which is well above the European Union directives that allow only 2 ppm of Fe ions in treated water to dump directly into the environment [10] . In addition, the removal/treatment of the sludge-containing Fe ions at the end of the wastewater treatment is expensive and needs large amount of chemicals and manpower. To overcome the disadvantages of the homogeneous Fenton process, and also considering the possibility of recovering the catalyst, some attempts have been made to develop heterogeneous catalysts, prepared by incorporating Fe ions or Fe oxides into porous supports [11–14] . Other transition metal complexes supported on several surfaces such as metal oxides, resins and mixed (Al-Cu) pillared clays have also been used as potentially active catalysts for the decomposition of H 2 O 2 and for the oxidative degradation of organics [15] . Among the porous solids used as supports for the iron phases, it is worth mentioning the use of silica, alumina, silica-alumina and cation-exchanged resins, which have been used in the degradation and mineralization of dyes [11] . More complex systems have been prepared by modifying a polyacrylonitrile (PAN) fibre by treatment with a mixture of hydrazine and hydroxylamine to introduce chelating functional groups onto the fibre surface. These functional groups are used to coordinate the transition metal cations Fe 3+ , Co 2+ , Ni 2+ and Cu 2+ to the fibre to act as the active catalytic sites for decomposition of the hydrogen peroxide [12] . Using an alternative strategy, other catalytic systems have been prepared by co-intercalation of two natural smectites (Wyoming SWy-1 and Tunisia-Gafsa VI) with Fe-Al polycations, obtained by polymerisation of a mixture of FeCl 3 and chlorhydrol [13] . Tachiev et al. [14] have reported other catalysts, in which Fe(II) and Fe(III) cations are complexed by the ligands DTPA, EDTA, EGTA and NTA. The use of zeolites [16] and carbons [17] to support iron catalysts is also worth mentioning. The mechanism of H 2 O 2 decomposition on homogeneous Fenton's oxidation is not well established, where several oxidising agents have been suggested to be involved in the oxidation reactions, in addition to the HO radicals. For the heterogeneous systems this is still less clear, being a matter of controversy. Some authors suggest an initial step of fast adsorption of the H 2 O 2 molecule on ( Fe(III)) sites [17] and others the adsorption of the organics [18] . Nevertheless, the involvement of the following steps has been suggested in most of the works found in the literature, which correspond to Fe 3+ reduction with generation of less oxidative HO 2 radicals, followed by Fe 3+ regeneration with formation of the hydroxyl radicals: (1) X − F e 3 + + H 2 O 2 → X − F e 2 + + H O 2 + H + (2) X − F e 2 + + H 2 O 2 → X − F e 3 + + O H − + HO where X represents the surface of the catalyst. However, it must be remarked that the radicals can also be generated in the surface of the solid so they are actually “caged” in the solid structure, subsequently reacting with the adsorbed reagent(s) without radicals generation. Obviously, besides the indicated...