Adsorption of organic compounds from aqueous solution by pyridine-2-carboxaldehyde grafted MIL-101(Cr)-NH2 metal-organic frameworks

Metal-organic frameworks (MOFs) are of particular interest in the adsorption of organic pollutants. This study investigated the adsorption of aniline (ANI), furfural (FUR), and bisphenol A (BPA) in aqueous solution using MIL-101(Cr)-X (X = -H, -NO2, -NH2 and, -N-2-pyc), MIL-100(Cr, Fe), HKUST-1, and ZIF-8. For the first time, an imino-functionalized MOF was used to adsorb organic compounds in water. MOFs were characterized by XRD, N2 adsorption-desorption, SEM, TEM, elemental analysis, and FTIR. MIL-101-N-2-pyc demonstrated the highest adsorption capacity of 460.8, 126.5, and 100.7 mg/g towards 500 mg/L BPA, ANI, and FUR respectively. The adsorption isotherms better suited the Langmuir model than the Freundlich model. Importantly, 98% of BPA (100 mg/L) was removed and the kinetic data correlated well with the pseudo-second-order model (R-2 > 0.998). Binary adsorption of FUR and acetic acid (AA) were determined for industrial relevance. MIL-101-N-2-pyc outperformed commercial activated carbon and the other MIL-101s with an impressive co-adsorption of 66.1 mg/g FUR and 62.5 mg/g AA. Overall, the adsorption mechanism was due to hydrogen bonding, 7C-7C interaction, and electrostatic attraction. Moreover, MIL-101-N-2-pyc exhibited remarkable structural stability and less than 0.01 wt% Cr leached into water. The adsorbent regeneration was improved after 4 cycles in methanol.