In recent years, Metal–Biomass Carbon (M–BC) hybrids have been widely studied as promising, cost-effective, and sustainable catalysts for persulfate activation in the degradation of emerging organic contaminants. M–BC systems offer advantages such as good performance and the sustainable use of biomass waste. Despite the considerable attention they have received, significant uncertainty remains regarding their precise catalytic mechanisms. A primary concern is the inherent complexity of biomass precursors, which frequently render the resulting catalytic structures ill-defined or akin to a “black box”. To address this challenge, this review critically evaluates the current state of mechanistic research, focusing on the debate between radical and non-radical pathways. In this paper, five fundamental challenges to clear mechanistic understanding are identified, including interference of inherent inorganic species, lack of precursors standardization and inherent heterogeneity, ambiguous overlapping active sites, methodological limitations in chemical quenching due to competitive adsorption, and conductivity-related constraints on non-radical pathways. Among these, the interference from inherent inorganic species is of primary concern, as the available evidence suggests it frequently confounds reported synergistic effects. Additionally, the future research directions for improving the experimental standardization and mechanistic understanding of M–BC catalysts are proposed. This review enriches the field by providing a clear path toward rigorous mechanistic understanding and the rational design of M–BC catalysts for water remediation.

Sewage sludge, particularly paper mill sludge, is carbonaceous by nature. It can be employed as a heterogeneous catalyst for Fenton-like reactions to attain high catalytic activity and long-term stability. In this paper, the Co/Fe bimetallic composite catalyst was prepared by taking paper mill sludge as the carrier via impregnation and calcination. This catalyst was employed to catalyze peroxymonosulfate (PMS), to produce SO4•−, which has strong oxidizing power. This study focused on determining the optimal preparation conditions, characterization of the catalyst, and investigating the effects of various inorganic anions on the degradation of Rhodamine B (RhB) in the Co/Fe-PMS system. The optimal catalyst preparation condition was found to be a 0.5 mol/L impregnation concentration, with the best ratio of Co(NO3)2 to FeSO4 solution being 7:3. The presence of Co and Fe in the catalyst was determined through FTIR and XRD analyses. The catalyst is a porous material with magnetic properties and is capable of degrading more than 95% of RhB within two minutes. It was found that H₂PO₄⁻, Cl⁻, and HCO₃⁻ had negative impacts on the degradation of RhB. Low concentrations of NO₃⁻ had a positive effect, whereas high concentrations of NO₃⁻ had a negative effect. This study presents a novel method for preparing highly efficient Fenton-like catalysts and offers an effective approach for sludge utilization.