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Red mud was modified by impregnation with Co element loading. The Co-RM catalyst was characterized using scanning electron microscopy (SEM), X-ray powder diffraction (XRD), X-ray fluorescence (XRF), and UV full-band scanning. The results showed that the modified Co-RM catalyst successfully loaded the Co element and formed an irregular pore structure on the surface, thereby increasing the number of active sites of the red mud catalyst and effectively improving the degradation efficiency of tetracycline. Under the optimal conditions of a catalyst dosage of 0.3 g/L, a persulfate dosage of 3 g/L, a reaction temperature of 50 °C, and a pH value of 7, a removal rate of 50 mg/L of tetracycline can be achieved: 89.5% after 90 min. The effects of common anions and humic acids in water, as well as radical quenchers (anhydrous ethanol and tert-butanol), on the degradation of tetracycline were investigated. The results showed that Cl−, CO32−, HCO3−, H2PO4−, NO32−, HPO42−, and humic acids showed inhibitory effects on the degradation of tetracycline, while SO42− showed a promoting effect on the degradation of tetracycline. The free radical quenching experiment showed that the most important free radicals that can degrade tetracycline in the system are sulfate radicals. [ABSTRACT FROM AUTHOR]