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[Display omitted] • A new approach to degrading antibiotics does not rely on oxidants or light. • Excellent degradation performance of doxycycline was achieved in a wide pH range. • The mechanism of direct catalytic degradation of doxycycline was deciphered. • A detailed degradation pathway of doxycycline via dark catalysis was proposed. Direct heterogeneous catalysis is conducted to decontaminate antibiotic polluted water for the first time. Perovskite-structured composites La x Sr (1- x) CoO 3-δ (LSC) are synthesized by a green mechanical milling method (as it involves no organic solvent, generates no wastewater, and is viable for industrial-scale use) and applied for the efficient catalytic degradation of doxycycline (DC) via singlet oxygen without the addition of chemical reagents or illumination. The oxygen vacancy produced during ball milling stores numerous electrons. These electrons and singlet oxygen exert a synergistic effect on the catalytic reaction, thereby improving the catalytic degradation of DC. LSC can effectively catalyze the degradation of DC in the dark. The total organic carbon removal efficiency reaches 89.64%, indicating that most DC is eventually mineralized to CO 2 and H 2 O. The catalytic degradation of DC follows two principal degradation pathways involving 13 intermediates. Toxicity prediction indicates that the toxicity of the products initially increases and then decreases with the degradation process and that the final products are not toxic. This study not only allows a facile synthesis route for catalyst preparation but also promotes the development of a green method for wastewater treatment. [ABSTRACT FROM AUTHOR]

• The recycled Fenton sludge showed a high activity for the activation of persulfate. • The existence of Cu(II) can increase the oxygen vacancies of the recycled sludge. • Tetracycline can be decomposed through six possible pathway processes. • The catalytic mechanism by the recycled catalyst is deciphered. • It is a cost-effective and environmentally friendly way to gain active catalysts. Fenton reaction has been widely applied as a unique oxidative method in the treatment of industrial wastewater. However, it generates a massive amount of Fe sludge, which mostly contains other hazardous heavy metal waste that can harm the environment and humans. In this study, we proposed a sustainable method to eliminate the side effect of the Fenton reaction and turn the Fenton sludge containing Cu into an efficient catalyst in the activation of peroxymonosulfate (PMS) to degrade tetracycline (TC). The existence of Cu, which can increase the oxygen vacancies of the sludge, is attributed to the activation efficiency compared with Fe sludge devoid of Cu. X-ray diffraction and X-ray photoelectron spectroscopy results show that Fe is present in the form of α- Fe 2 O 3 and γ- Fe 2 O 3 , whereas Cu mostly exists in the valence state of Cu(II). The influence of catalyst and PMS dosage, pH, and recycling times on the PMS reaction is illustrated. OH , SO 4 − O 2 – and 1O 2 are the critical reactive oxygen species. TC can be decomposed into CO 2 and H 2 O through six possible pathway processes with these free radicals. Given its sustainable treatment and high catalytic efficiency, dealing with Fe sludge that contains heavy metals serves as a modal practice for harmful waste treatment and can be popularized for a broader application. [ABSTRACT FROM AUTHOR]