###### Abstract  
The conventional Fenton process generates large amounts of Fenton sludge during wastewater treatment. Achieving effective utilization of Fenton sludge and reducing its production remain pivotal challenges. In this study, Fenton sludge biochar catalysts (Cat) were prepared using Fenton sludge via pyrolysis. In addition, chemical oxygen demand (COD) from secondary effluent was removed by Fenton sludge biochar catalysts activated with H\({}_{2}\)O\({}_{2}\)/Fe(VI). Specifically, the removal efficiency of COD could reach 46.2% in the Cat\(-\)2/H\({}_{2}\)O\({}_{2}\)/Fe(VI) system under weakly alkaline conditions. The mechanistic analysis confirmed that high-valent iron, \({}^{\bullet}\)OH, O\({}_{2}\)\({}^{\bullet-}\), and \({}^{1}\)O\({}_{2}\) all participate in the degradation process. Furthermore, a continuous-flow reactor was applied to treat secondary effluent, with COD decreasing from 65 mg/L to 36 mg/L. This study provides new insights into the resource utilization of Fenton sludge and the treatment of complex wastewater. F +
Footnote †: journal: Applied sciences  
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Footnote 1: email: +86-133-11169839 (L.H.) materials. These methods present environmental risks and hinder the efficient recycling of resources [7]. The conversion of Fenton iron sludge into catalysts has significant advantages in terms of resources and the environment. Iron species in the sludge (such as Fe\({}_{2}\)O\({}_{3}\) and Fe\({}_{3}\)O\({}_{4}\)) combine with the heterogeneous carbon matrix to form stable active sites (Fe-O) during pyrolysis, enabling the efficient recovery and utilization of iron. Simultaneously, high-temperature treatment can stabilize heavy metals to reduce the risk of secondary pollution [8]. The obtained iron-carbon composites can enhance electron transfer efficiency and promote the activation of H\({}_{2}\)O\({}_{2}\) to generate -OH radicals, degrading organic pollutants effectively across a wide pH range [9]. Moreover, the pyrolysis process can achieve Fenton sludge reduction (weight reduction rate > 50%) [10] and optimize catalyst performance to meet different wastewater treatment needs [11]. This technology provides a sustainable solution for the high-value utilization of Fenton sludge and wastewater treatment. In recent years, studies have explored the use of Fenton sludge-derived biochar in H\({}_{2}\)O\({}_{2}\)-based Fenton-like systems. [PERSON] [12] synthesize Fe\({}_{2}\)O\({}_{3}\) using Fenton sludge under different pyrolysis temperatures (400, 600, and 800 \({}^{\circ}\)C). The degradation rate of rhodamine B reached 100% within 100 min at pH = 5 in the sludge-based catalyst/H\({}_{2}\)O\({}_{2}\) system. However, the pH of Fenton sludge biochar catalysts/H\({}_{2}\)O\({}_{2}\) system tended to be acidic [13], which could lead to catalyst dissolution, reducing catalytic activity and causing secondary pollution. Therefore, controlling the pH of the system is crucial. Research has shown that high ferrates and H\({}_{2}\)O\({}_{2}\) exhibited a synergistic effect. [PERSON] [14] found that the degradation rate of methylene blue (MB) could reach up to 91% in the Fe(VI)/H\({}_{2}\)O\({}_{2}\) system under weakly alkaline conditions. However, studies on introducing high ferrates into the Fenton sludge biochar catalyst/H\({}_{2}\)O\({}_{2}\) system are still lacking, which is critical for overcoming the limitations of the Fenton sludge biochar catalyst/H\({}_{2}\)O\({}_{2}\) system. This study aims to develop an efficient and stable Fenton sludge biochar catalyst for activating H\({}_{2}\)O\({}_{2}\)/Fe(VI) to degrade organic pollutants. In addition, the removal performance of chemical oxygen demand (COD) from actual secondary effluent by a Fenton sludge biochar/H\({}_{2}\)O\({}_{2}\)/Fe(VI) system was evaluated. First, the structure and activity of the Fenton sludge biochar catalyst under different pyrolysis times were analyzed, and the effects of operational parameters on the system were discussed.