Efficient and comparative adsorption of trinitrotoluene on MOF MIL-100(Fe)-derived porous carbon/Fe composite adsorbents with rod-like morphology: Behavior, mechanism, and new perspectives

Porous carbon (PC) materials containing nanoscale zero-valent iron (nZVI) often exhibit higher removal efficiency for water pollutants due to the strong influence of nZVI reactivity. However, despite the advantages of nZVI, its presence can sometimes affect the adsorptive properties of the PC materials by blocking the pores and masking adsorption sites, thereby inhibiting efficient adsorption. Herein, we synthesized and used an iron-containing rod-like metal-organic framework, MIL-100(Fe), as a precursor to prepare PC rods with and without surface nZVI, namely, Fe-PCR and PCR, respectively. The prepared materials were characterized, and their adsorption properties for 2,4,6-trinitrotoluene (TNT) – an explosive contaminant – were studied. TNT adsorption onto Fe-PCR and PCR occurred rapidly within 10 min and reached equilibrium after 60 min. Moreover, the results from the kinetics studies showed that TNT adsorption process well-fitted the pseudo-second order (PSO) model due to the higher correlation value (R2 > 0.99), indicating a chemosorption process. The results from the adsorption isotherm and thermodynamic investigations revealed that TNT adsorption onto the prepared adsorbents occurred via a single-to-multilayer adsorption process in which exothermic chemisorption adsorption was dominant. The adsorption capacity of PCR reached 409.38 mg/g, which was significantly higher than Fe-PCR (249.97 mg/g) under ambient conditions. This revealed that the presence of surface-abundant nZVI in Fe-PCR limited the adsorption of TNT compared to PCR. Moreover, analysis of the TNT-loaded adsorbents revealed that TNT was not altered by nZVI but adsorbed onto the PC-rods surfaces via π-π EDA interactions, hydrogen bonding, and physical adsorption via pore filling. Thus, the improved structural properties of PCR due to Fe removal (SBET: 720 m2/g; Vpore: 1.76 cm3/g) compared to Fe-PCR (SBET: 408.72 m2/g; Vpore: 0.762 cm3/g) provided greater accessibility to adsorption sites, thereby resulting in better adsorption performance.