Effective removal of Hg(II) ions from aqueous solutions by functionalized metal–organic frameworks: adsorption models, thermodynamics, and Box-Behnken design optimization

The pollution of groundwater with mercury Hg(II) poses a significant global risk and its elimination is of high importance. In this research, a new MOFs adsorbent was created through the post-functionalization of Th-MOF with tryptophan and used to selectively eliminate Hg(II) ions from aqueous solutions. The identification of the functionalized NH 2 -Th-MOF was achieved through FTIR, FESEM, EDX, XRD, XPS, BET, and point of zero charge (pH pzc ). The absorption characteristics of the recently developed MOF adsorbent were examined through batch experiments. The highest observed adsorption capability was 792 mg/g at the most effective pH of 4. The study measured the impact of pH, dosage, temperature, time of contact and original concentration on the absorption procedure. The adsorption procedure was well suited to pseudo-second-order and Langmuir kinetic and isotherm models, respectively. The mode of adsorption was directed towards monolayer and chemisorption, through an adsorption energy of 33.9 kJ/mol. Additionally, the removal rate exhibited a direct relationship with the square of mercury ions concentration. Interestingly, our research uncovered that the complete adsorption–desorption procedure exhibited both an spontaneous and endothermic nature, as evidenced by the thermodynamic factors (ΔH°), (ΔS°), and (ΔG°). The NH 2 -Th-MOF adsorbent proved to be easily regenerated with a decrease in removal rate of only 15.6% after six consecutive cycles. The chemical stability of the material was crucial for its reusability, as indicated by the indistinguishable XRD patterns before and after regeneration. Additionally, there were no variations in the functional groups and the FT-IR remained stable. The interaction among NH 2 -Th-MOF and Hg(II) was thoroughly investigated, taking into account factors like H-bonding, π-π interaction, and electrostatic contact. Furthermore, compared to other competing metal ions, NH 2 -Th-MOF exhibited exceptional discerning adsorption for Hg(II) ions. The Box-Behnken Design was used to optimize the adsorption findings.