Efficient, selective, and reusable metal–organic framework-based adsorbent for the removal of Pb(II) and Cr(VI) heavy-metal pollutants from wastewater

Abstract

Water containing excess heavy metals poses a health risk to humans but can be purified effectively using adsorbents. As such, identifying novel and efficient adsorbents to remove heavy metals such as Pb(II) and Cr(VI) is vital. We report a new metal-organic framework (MOF), {[Cd1.5(btc)(bibp)·2H2O]·H2O}n, capable of capturing Pb(II) and Cr(VI) from water. The adsorbent has a high density of exposed active sites, with maximum adsorption capacities of 537.634 mg/g for Pb(II) (pH = 7) and 787.402 mg/g for Cr(VI) (pH = 6). Adsorption equilibrium was reached after 60 and 120 min with Pb(II) and Cr(VI), respectively, and the adsorbent was regenerated effectively using an ethanol-acetic acid eluent. Analysis of the adsorption kinetics using Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy showed that the abundant –OH groups (hard base) and the Pb(II)/Cr(VI) (soft acid) behave according to the hard-soft acid-base principle. In addition, electron pairs are shared between the imidazole N atoms and Pb(II)/Cr(VI). This study provides new insights into the design of three-dimensional MOFs for removing pollutants from industrial wastewaters, with the potential to mitigate health risks to humans and natural ecosystems.