Few-layered metal-organic framework nanosheets as a highly selective and efficient scavenger for heavy metal pollution treatment

Current techniques trialed for the sequestration of heavy metals (HMs) are typically metal ion specific. The deployment of adsorbents as amenable choices for the simultaneous elimination of several HMs remained a big challenge. Herein, few-layered CoCNSP nanosheets with periodically aligned sulfur species were exfoliated from layered metal-organic frameworks (MOFs) through a facile-operating method. Laboratory scale experiments revealed that CoCNSP nanosheets showed selective uptake in the order of Co(II), Zn(II), Cd(II), Ni(II) ≪ Cu(II) < Pb(II) < U(VI) < Hg(II) in both single-component and multi-component systems. Attractively, CoCNSP nanosheets showed high distribution coefficients (K_d ~ 10⁶–10⁷ mL/g), fast capture dynamics (<40 min), and ultrahigh uptake capacities (716, 661, 534, and 325 mg/g for Hg(II), U(VI), Pb(II), and Cu(II), respectively). Further, CoCNSP nanosheets could tolerate the influence of ionic strength (up to 100 mM), maintained high adsorption efficiency over a wide pH range, and showed good recycle performance for three times. X-ray photoelectron spectroscopy and theoretical density functional theory calculations demonstrated that the superior adsorption ability dominantly originated from the inner-sphere coordination between sulfur species and HMs. Finally, the traits of simple synthetic method, low-cost and non-toxic raw ingredients, as well as remarkable capture performance, make CoCNSP nanosheets superior to other scavengers. These findings reflect the important synthetic achievement in the deployment of MOFs for alleviating the environmental problems caused by HMs.