Engineering of efficient functionalization in a zirconium-hydroxyl-based metal–organic framework for an ultra-high adsorption of Pb2+ ions from an aqueous medium: an elucidated uptake mechanism

Abstract

Metal–organic framework (MOF) adsorbents have been proven to possess an effective Pb²⁺ uptake property in recent years. However, their abilities are inadequate for industrial and real-life situations due to their ineffective adsorption capacity and lack of modified engineering. To overcome these disadvantages, a new hydroxyl-functionalized Zr-based-MOF, denoted as HCMUE-2, was successfully synthesized through the solvothermal method. The Pb²⁺ adsorption onto HCMUE-2 was completely observed under optimal conditions, including solution pH, MOF content, adsorption isotherms, and kinetics. Consequently, the maximum Pb²⁺ adsorption capacity over the OH-modified HCMUE-2 is 1115.9 mg g⁻¹ at a pH value of 5.5, which is one of the highest values compared with the previously reported MOFs. Additionally, the obtained data for Pb²⁺ uptake is fitted with the Langmuir isotherm and pseudo-second-order models, indicating that the removal of Pb²⁺ onto HCMUE-2 is a chemical adsorption process. Notably, HCMUE-2 can maintain the adsorption efficiency of Pb²⁺ by about 93% after seven consecutive cycles. Furthermore, the Pb²⁺ uptake mechanism is elucidated by systematically incorporated analyses. Accordingly, Fourier transform infrared and Raman spectroscopies, thermogravimetric analysis combined with differential scanning calorimetry, energy-dispersive X-ray, and X-ray photoelectron spectroscopy of Pb⊂HCMUE-2 show that the chemical bonds are formed via the electrostatic interaction and electron sharing between hydroxyl moieties within the MOF architecture and the Pb²⁺ species. It is noteworthy that HCMUE-2 is a potential material for removing Pb²⁺ from wastewater.