Sludge-derived biochar-supported Mg-Fe-LDH composite enabling singlet oxygen-dominated methylhydrazine degradation

Methylhydrazine (MMH), a highly toxic nitrogen-rich propellant contaminant, is difficult to remove by conventional treatment processes. Herein, waste activated sludge was upcycled into biochar and used as a functional carbon scaffold to construct a Mg-Fe layered double hydroxide/biochar composite (Mg-Fe-LDH@BC) for peroxymonosulfate (PMS) activation. The biochar matrix reconstructed LDH from aggregated plate-like particles into hierarchical microspheres assembled from ultrathin nanosheets, improving pore connectivity, active-site exposure, and mass transfer. More importantly, biochar served as an interfacial electron-transfer mediator, facilitating Fe(II)/Fe(III) redox cycling and steering PMS activation toward a singlet oxygen (¹O₂)-dominated non-radical oxidation pathway. Under optimized conditions, the Mg-Fe-LDH@BC/PMS system achieved 95.8% MMH removal within 60 min and exhibited good tolerance toward common inorganic ions and realistic water matrices. Quenching experiments and EPR analysis strongly suggest that ¹O₂ was the predominant reactive species under the tested conditions, with secondary contributions from SO₄•^-, •OH, and O₂•^-. LC-MS analysis further revealed that MMH degradation proceeded through oxidative bond cleavage, nitrogen-centered oxidation process with decreasing toxicity. Life cycle assessment further indicated that the LDH@BC/PMS system offered a favorable balance between catalytic efficiency and environmental burden. This study establishes a sludge-valorization-enabled catalyst design strategy and highlights structure-electron-pathway coupling as a key principle for efficient and sustainable treatment of refractory propellant wastewater.