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Different pre-conditioning treatments were evaluated in order to stabilize red mud, a waste product from bauxite processing, for obtaining heterogeneous catalysts (named as B1–B3) that can be employed as suitable activators of sodium persulfate (SPS) for the degradation of sulfamethoxazole (SMX), a model antibiotic, in water. The presence of Fe₃O₄ in the composition of the catalysts was found to be a key factor for a suitable activation of SPS, according to the XPS measurements. The oxidation of SMX was successfully fitted to a pseudo-first-order kinetic model (r² > 0.96), obtaining a 68% removal after 180 min when 0.8 mg/L of SMX was oxidized with 2 g/L of SPS and 2 g/L of catalyst B3. The presence of organic and/or inorganic constituents in the water matrix significantly hindered the degradation rate of SMX, the apparent kinetic constants being from 2 to 3 times lower than that determined in ultrapure water …