Mechanistic insights into enhanced permanganate activation by biochar derived from aluminum sludge: Critical roles of persistent free radicals and alumina.

Permanganate (Mn(VII)) is widely used as the oxidant in water treatment, yet its relative stability limits its reactivity toward refractory organic contaminants. In this study, we proposed a novel "waste-to-value" strategy by pyrolyzing aluminum sludge from water treatment plants to alumina-biochar composite catalysts (Al2O3@BCs), aimed at enhancing Mn(VII) activation for the degradation of phenolic pollutants. The Al2O3@BC pyrolyzed at 600°C exhibited the highest catalytic performance, achieving 100% phenol (PE) degradation within 15 min. During the Mn(VII) activation process, Al2O3@BC exhibited a dual-function mechanism by simultaneously promoting Mn(III) generation and enhancing the oxidation potential of Mn(VII). The carbon-centered persistent free radicals (PFRs) on the biochar in Al2O3@BC significantly promoted the reduction of Mn(VII) to reactive Mn(III), while the alumina in Al2O3@BC improved the oxidation potential of Mn(VII) from 0.46 V to 0.54 V. These two pathways contributed comparably to PE degradation. Density functional theory (DFT) calculations revealed that the Mn(VII) was reduced to reactive Mn(III) by electron donation from PFRs, in which the type of carbon-centered PFRs exhibited the highest Mn(VII) reduction activity. The Al2O3@BC catalyst exhibited excellent resistance to environmental matrix interference and efficiently degraded various organic contaminants during actual water purification. Furthermore, the continuous-flow system and quantitative analysis based on life cycle assessment (LCA) revealed that the Mn(VII)/Al2O3@BC system possesses significant environmental advantages and technical feasibility. This study highlights the sustainable utilization potential of aluminum sludge and offers new insights into the activation mechanism of Mn(VII) in water treatment.