Efficient removal of mercury(ii) ions using glutamic acid modified metal–organic frameworks: adsorption models, thermodynamics, and parameter optimization

Abstract

The targeted removal of mercury(II) ions from contaminated aqueous systems presents a significant environmental issue, attributed to mercury's high toxicity, persistence in nature, and potential for bioaccumulation. This research focuses on the synthesis and assessment of a glutamic acid-functionalized silver metal–organic framework (NH₂-Ag-MOF) as an effective sorbent for the elimination of Hg(II) ions from water. The modified framework demonstrated remarkable properties, including a substantial surface area of 1750.49 m² g⁻¹, a mesoporous structure with an average pore diameter of 3.84 nm, and a plethora of nitrogen-rich functional groups that markedly improved its affinity for mercury. Characterization techniques such as XRD, FT-IR, SEM, EDX, BET analysis, and XPS verified the successful functionalization of amine groups while preserving the structural integrity of the parent Ag-MOF. Batch adsorption experiments revealed that the uptake of Hg(II) was significantly affected by parameters including pH, dosage, temperature, and initial concentration, with a maximum adsorption capacity of 638.2 mg g⁻¹ attained. The adsorption behavior was consistent with the Langmuir isotherm model and followed pseudo-second-order kinetics, indicative of monolayer chemisorption, supported by an adsorption energy value of 31.6 kJ mol⁻¹. Thermodynamic analysis showed a positive enthalpy change (ΔH° = 84.9 kJ mol⁻¹) and negative changes in Gibbs free energy (ΔG°), confirming that the adsorption process is endothermic and proceeds spontaneously. Furthermore, Density Functional Theory (DFT) calculations indicated that the binding of Hg(II) is facilitated through CO coordination and electrostatic interactions with –COO⁻ groups present. The adsorbent exhibited efficiency across five regeneration cycles.