Mercapto-decorated Zn-based metal-organic framework embedded nanofibrous membrane for oxo-anions treatment in aqueous solution

Abstract

Metal-organic frameworks (MOFs) and nanofibrous MOF-embedded membranes (NMOM) have drawn significant attention in recent times towards wastewater treatment. We report the fabrication of nanofibrous membrane (NMOM) embedded with a newly designed and developed Zn(II)-MOF {[Zn(L)(dmbpy)]n using ligands 1-(4-carboxyphenyl)-5-mercapto-1H-tetrazole (L) and 5,5′-dimethyl-2,2′-bipyridine (dmbpy)} bearing free mercapto and methyl groups on its surface. While the methyl group introduces hydrophobicity to the MOF, resulting in its good water stability, the free peripheral mercapto-functionality acts as interacting site for the foreign substrates. The NMOM was prepared by mixing Zn-MOF into a polyacrylonitrile (PAN) solution and extruding composite nanofibre via electrospinning technique. Both, Zn-MOF and Zn-MOF-based NMOM were used as selective adsorbents for the treatment of oxo-anion contaminated water. With these adsorbents, the selective adsorption, as well as separation of permanganate (MnO4−) ions from the mixture of permanganate (MnO4−) and dichromate (Cr2O72−), was achieved. The synthesized Zn-MOF was also utilized for the photocatalytic reduction of Cr(VI) to less toxic Cr(III) ions under UV light irradiation with good efficiency. The double faceted nature of Zn-MOF was further explored in a one-pot sequential selective adsorption of MnO4− ions from an aqueous mixture of both MnO4− and Cr2O72−, followed by photocatalytic reduction of Cr2O72− ions. Further, the developed NMOM was examined for a large-scale adsorption process. The pristine Zn-MOF showed a maximum adsorption capacity of 937.32 mg/g for the adsorption of MnO4− ions, whereas NMOM showed a higher adsorption capacity of 1077.56 mg/g. The enhanced hydrophobicity and surface area can be attributed to the increased adsorption capacity of the membrane. Thus, induced hydrophobicity of the membrane coupled with hydrolytic stable Zn-MOF showcases the potential of NMOM in treatment of oxo-anion contaminated wastewater.