Urea-Based Metal-Organic Frameworks as High and Fast Adsorbent for Hg²⁺ and Pb²⁺ Removal from Water

Water pollution by various toxic heavy-metal ions, particularly mercury and lead, has been considered as a global problem with serious threat to public health. Nowadays, a variety of adsorbents have been used extensively for the removal of heavy-metal ions from water. In this work, two Zn-based metal-organic frameworks (MOFs), named TMU-31 and TMU-32, were successfully synthesized by a solid-state method named mechanosynthesis, which can be considered as a rapid, green, convenient, solventless, and low-cost process. These two MOFs are densely decorated with urea functional groups, which results in rapid, efficient, and selective removal of diverse heavy-metal ions, such as Hg²⁺ and Pb²⁺ ions, from wastewater. The sorption capacities of 476.19 mg g^-1 in less than 15 min for Hg(II) ions and 909 mg g^-1 for Pb(II) metal ions in less than 5 min were obtained effectively by these two structures. In addition, adsorption isotherms of Hg²⁺ and Pb²⁺ metal ions are in good agreement with Langmuir model, and also, the experimental adsorption data are well fitted with pseudo-second-order kinetic model ( R² = 0.99). This work is the first example of urea-functionalized MOFs as an effective adsorbent for Pb²⁺ and Hg²⁺ ions, which shows the best and high efficiency for the removal of heavy metals in less than 15 min.