Rapid and selective adsorption of a typical aromatic organophosphorus flame retardant on MIL-101-based metal–organic frameworks

Abstract

The pollution of aromatic organophosphorus flame retardants (aromatic OPFRs) in aquatic environments has drawn great attention over the last few years. Two MIL-101-based metal–organic frameworks (Cr-MIL-101 and Fe-MIL-101-NH₂) which possess ordered mesoporous cavities (2.9 and 3.4 nm) and aromatic structures were chosen and prepared to selectively adsorb a typical aromatic OPFR [triphenyl phosphate (TPhP)] from aqueous solution. Pore distribution analysis showed that Cr-MIL-101 and Fe-MIL-101-NH₂ had both a mesoporous structure (2–3.5 nm) and microporous structure (1–2 nm), conducive to diffusion and adsorption of TPhP molecules. Compared with Fe-MIL-101-NH₂ as well as commercial activated carbon, Cr-MIL-101 showed rapid and efficient adsorption for TPhP, and its initial sorption velocity (v₀) calculated from the pseudo-second-order model was up to 568.18 μmol g⁻¹ h⁻¹. The adsorption equilibrium of TPhP on the Cr-MIL-101 was almost achieved within 12 h, while the equilibrium time of other adsorbents required more than 48 h. The study of selective adsorption found that Cr-MIL-101 had a higher sorption amount for aromatic OPFRs than alkyl-OPFRs and other aromatic compounds with different chemical structures. Cr-MIL-101 was able to keep a steady selective adsorption for TPhP in the presence of co-existing aromatic compounds. Based on the analysis of K_ow, molecular structure and further density functional theory calculations, hydrophobic interactions may play a dominant role in the selective adsorption process of TPhP, and π–π interactions may be also involved. Cr-MIL-101 exhibits reusability and promising potential to rapidly and selectively remove aromatic OPFR in environmental remediation.