Unveiling pore evolution mechanism in sludge-based biochar induced by ultra-high temperature carbonization coupled with steam activation

The escalating generation of municipal sewage sludge (MSS) necessitates efficient resource recovery. However, conventional carbonization of high-ash sludge typically yields products with poorly developed porosity and insufficient structural stability. To overcome this bottleneck, this study proposes an integrated strategy combining ultra-high-temperature (UHT) carbonization with steam activation. An L₉(3 ³) orthogonal design identified carbonization temperature as the dominant factor regulating pore evolution. Under optimal conditions (1200 °C, 2 °C/min, and 120 min) followed by activation, the optimal char (SW12a) achieved a specific surface area of 199.72 m 2 /g with a predominantly mesoporous structure (62.68%). Notably, this represents a ∼5- fold increase over conventional high-temperature sludge chars (39.72 m 2 /g, 0.066 cm 3 /g), demonstrating the exceptional pore-forming efficacy of this approach for ash-rich precursors. Mechanism analyses revealed that the inherent inorganic components acted as a rigid structure, effectively preventing the ash-melting-induced pore collapse typically observed at 1200 °C. Despite requiring higher energy input, UHT uniquely promoted the formation of highly stable crystalline carbon and a distinctive “honeycomb” composite structure. Furthermore, steam etching exposed nitrogen-doped functional groups, endowing the material with excellent aqueous dispersion stability (Zeta potential = −30.05 mV). By overcoming the inherent porosity limitations of high-ash sludge and facilitating robust crystalline structures, this UHT-activation strategy effectively offsets its energy demands, upgrading sludge waste into advanced functional materials.