Effects of synergistic ultrasonic–NaClO treatment on coal microstructure at different liquid saturations

Efficient coalbed methane (CBM) extraction is important for safe coal mining and for utilizing this gas as a relatively clean energy resource. Low–permeability coal is generally characterized by a compact matrix and poorly connected pore–fracture networks, which restrict methane desorption, diffusion, and flow. Combined ultrasonic–NaClO treatment provides a physicochemical approach to coal modification, but the role of liquid saturation in this process remains unclear. Low–temperature N 2 adsorption–desorption, FTIR, and SEM were used to characterize the pore structure, functional groups, and surface morphology of coal samples subjected to combined ultrasonic–NaClO treatment at different liquid saturations. The results suggest a synergistic interaction in which ultrasonic cavitation enhanced NaClO transport within the pore–fracture system and exposed new reactive surfaces, while NaClO–induced oxidation–dissolution weakened the coal matrix and pore walls, increasing their susceptibility to cavitation and thereby promoting pore opening and microfracture development. The most pronounced pore–structure modification was observed at 50% liquid saturation. Relative to untreated coal, the BET specific surface area and pore volume increased by 36.85% and 24.39%, respectively. FTIR results were consistent with cleavage or rearrangement of aliphatic side chains and bridging bonds, reduced condensation and connectivity of aromatic units, and a less compact local macromolecular structure. SEM revealed more evident lamellar exfoliation, interparticle pores, and microfractures, together with clearer local pore–fracture connections. These findings provide a theoretical basis for synergistic physicochemical modification of low–permeability coal and enhanced CBM recovery.