MIL-100(Fe) nanofluid-enhanced ultrasonic cavitation for coalbed methane extraction from low-permeability coal

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Low-permeability coalbed methane recovery is restricted by dense pore-fracture structures and the limited cavitation efficiency of conventional water media. This study introduces MIL-100(Fe) nanofluid as an active ultrasonic cavitation medium to regulate cavitation nucleation and strengthen coal pore-network reconstruction. Two coal samples were treated by water- and nanofluid-mediated ultrasonic cavitation and characterized using in-situ X-ray nano-CT, SEM, FTIR, XRD, BET/FHH analysis, nanoindentation, and high-pressure methane adsorption. Compared with water treatment, nanofluid-mediated ultrasonic cavitation more effectively weakened aromatic C–H adsorption sites, increased hydroxyl-related absorption, and optimized microcrystalline parameters, with d 002 / f a reaching 0.0819 nm/0.146 for BG coal and 0.0805 nm/0.142 for SJZ coal. It increased porosity to 29.10% and 30.10%, raised connected-pore volume fractions to 95.23% and 95.64%, and formed mesopore–macropore structures containing 47.5–59.8% mesopores and 40.2–52.5% macropores. The Langmuir adsorption capacity decreased from 16.77 to 10.48 mg g −1 for BG coal and from 24.52 to 17.70 mg g −1 for SJZ coal, while Young’s modulus decreased from 7.96 to 7.01 GPa and from 6.95 to 6.53 GPa, respectively. These results reveal that MIL-100(Fe) nanofluid enhances ultrasonic cavitation through coupled porous multicenter nucleation, thermal regulation, and particle scouring, thereby converting isolated pores into connected methane desorption pathways.