High capacity Hg(II) and Pb(II) removal using MOF-based nanocomposite: Cooperative effects of pore functionalization and surface-charge modulation.

Water pollution by heavy metal ions especially Hg(II) and Pb(II) is one of the most important concerns because of their harmful effects on human health and environment sustainability. Here, we developed Fe3O4@TMU-32 metal-organic framework (MOF)-based nanocomposite by applying pore functionalization and surface-charge modulation strategies. Based on synergic effects of these strategies, Fe3O4@TMU-32 nanocomposite shows very high capacity toward Hg(II) and Pb(II) metal ions. TMU-32 (with formula [Zn(OBA)(DPU)]·2DMF·H2O where H2OBA and DPU are (4,4'-oxybis(benzoic acid)) and 1,3-di(pyridin-4-yl)urea)) is decorated with urea functional groups containing carbonyl and amine groups that can interact with metal ions. As results, TMU-32 show very high capacity toward Hg(II) and Pb(II) ions. To improve the TMU-32 capacity toward Hg(II) and Pb(II) cations, we tried to modulate the surface-charge of TMU-32 as a host-framework. Surface-charge modulation strategy had been conducted through encapsulation of Fe3O4 nanoparticles by TMU-32 in an in-situ synthesis procedure and synthesis of Fe3O4@TMU-32 nanocomposite. Fe3O4@TMU-32 nanocomposite shows improved removal capacity (45 % and 54 % toward Pb(II) and Hg(II)) rather pristine TMU-32 framework because of urea decorated framework and charge modulated surface. Fe3O4@TMU-32 nanocomposite adsorb 1600 mg.g-1 of Pb(II) and 905 mg.g-1 of Hg(II) which extremely rare in the literature. Such improvement can be related to the electrostatic interaction between cationic nature of Pb(II) and Hg(II) and negative charge of the Fe3O4@TMU-32 adsorbent.