## 1 Introduction  
In recent years, antibiotics have been used extensively in treating bacterial infections and in the animal husbandry, agriculture, and pharmaceutical industries [1]. Speculatively, the consumption of antibiotics will rise by 200% from 2015 to 2030 [2]. However, the excessive use of antibiotics causes bacterial drug resistance and antibiotic resistance [3]. In addition to the potential harm caused by antibiotic-resistant bacteria, the one-time or continuous intake of antibiotics beyond the maximum dosage for the human body also brings direct adverse effects and even toxicity to the human body [4]. Nowadays, antibiotic residues in the environment have become a global concern. Sulfamethoxazole (SMX), a well-known antibiotic, is used frequently to treat or prevent a wide range of illnesses. A serious hazard to human health and aquatic ecosystems is posed by the widespread use of SMX, which is found in surface water regularly [5]. In recent years, scholars have developed various technologies to remove antibiotic residues, such as membrane separation, physical adsorption, biodegradation, and photocatalytic degradation, etc. [6; 7; 8; 9]. However, most of these methods have drawbacks, such as high energy consumption, low mineralization capacity, and ineffective purification [6]. Persulfate-based advanced oxidation processes are gaining attention due to their excellent performance in the removal of pollutants [10]. In comparison to the conventional hydroxyl radicals (\({}^{\bullet}\)OH), sulfate radicals (SO\({}_{4}\)\({}^{\bullet-}\)) are more efficient for the degradation of refractory pollutants due to their longer half-life, higher oxidation potential, etc. Moreover, due to their asymmetric structure, PMS is easily activated [11]. Various strategies have been utilized for PMS activation, including thermal treatment, ultrasound, ultraviolet, radiation, alkaline, transition metal, and carbon material activation [12]. Carbon materials are generating more and more interest as catalysts for PMS activation due to their tunable pore structure, larger specific surface area (SSA), lack of secondary pollution, lack of leaching of toxic metal ions, and thermal stability [13]. One of the carbon materials, graphitic carbon nitride (g-C\({}_{3}\)N\({}_{4}\)), has been reported to be used for PMS activation [14]. However, single g-C\({}_{3}\)N\({}_{4}\) has poor catalytic capacity for activating PMS [15]. In order to improve the PMS activation capacity of g-C\({}_{3}\)N\({}_{4}\), researchers modified it by heteroatom modification [16]. For example, the PMS activation performance of g-C\({}_{3}\)N\({}_{4}\) has been improved by Fe doping, resulting in an increase in phenol degradation rate from 10% (g-C\({}_{3}\)N\({}_{4}\)) to 35% (Fe-doped g-C\({}_{3}\)N\({}_{4}\)) within 60 min [17]. Oxygen doping has been found to significantly enhance the PMS activation performance of g-C\({}_{3}\)N\({}_{4}\) due to the shift in electron density [18]. Additionally, metal oxide-doped g-C\({}_{3}\)N\({}_{4}\) was proved to be effective in activating PMS to degrade pollutants [19; 20; 21]. For example, the CoO\({}_{\text{v}}\)@C/g-C\({}_{3}\)N\({}_{4}\)/PMS system and the LaCoO\({}_{3}\)/g-C\({}_{3}\)N\({}_{4}\)/PMS system exhibited superior tetracycline degradation performance with removal rates of more than 80% at 60 min and 30 min [19; 20]. SMX has been degraded completely in the Fe-Co-O-g-C\({}_{3}\)N\({}_{4}\)/PMS system within 30 min [21]. Research has shown that Bi\({}_{2}\)Fe\({}_{4}\)O\({}_{9}\) can activate PMS effectively [22]. It is speculated that g-C\({}_{3}\)N\({}_{4}\) co-doped with Bi-Fe oxide may increase the active sites on the surface of g-C\({}_{3}\)N\({}_{4}\), thereby improving its catalytic activity. However, the PMS activation ability of Bi-Fe oxide co-doped g-C\({}_{3}\)N\({}_{4}\) has not been verified, and the mechanism of Bi-Fe oxide co-doped g-C\({}_{3}\)N\({}_{4}\) to activate PMS for pollutant degradation is not clear.