Thiol- and Amine-Incorporated UIO-66-NH2 as an Efficient Adsorbent for the Removal of Mercury(II) and Phosphate Ions from Aqueous Solutions

In this study, the novel adsorbent UIO-66-IT was synthesized to extract mercury and phosphate ions from contaminated water. The synthetic strategy involved the preparation of the metal–organic framework (UIO-66-NH2) followed by post-synthetic modification using the chelating ligand 2-imino-4-thiobiuret to form the UIO-66-IT adsorbent. The structure and the morphology of the adsorbent were investigated by a variety of analytical techniques including Fourier transform infrared, X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, and Brunauer–Emmett–Teller surface area measurements. The adsorption of mercury and phosphate was optimized by studying the effect of pH, initial concentration, contact time, dose, temperature, and competitive ions. The results revealed exceptionally high adsorption capacities toward mercury and phosphate ions of 580 and 178 mg/g, respectively, at pH = 5.5 and an initial concentration of 1500 and 1000 mg/L. The adsorption isotherms are in excellent agreement with the Langmuir isotherm model, indicating the formation of a monolayer on the surface of UIO-66-IT. The kinetics of adsorption fit well with the pseudo-second-order kinetics model, which suggests the chemical adsorption of mercury ions via the nitrogen and sulfur functional groups of the adsorbent and the physical adsorption of phosphate anions by protonated functional groups on the surface of the UIO-66-IT adsorbent. Selectivity studies showed removal efficiencies of 98.9% Hg(II) from a solution containing a mixture of metal ions at 25 mg/L. Regeneration studies showed that the adsorbent can be recycled several times by using nitric acid for mercury removal and sodium chloride for phosphate removal. Removal efficiencies were higher than 99% for both regenerations. Due to the simple synthetic strategy via cost-effective starting materials, unique chemical structure, rapid adsorption kinetics, and high surface area, which lead to excellent removal efficiency, stability, and excellent regeneration, UIO-66-IT is introduced as a unique adsorbent for the selective removal of mercury and phosphate ions to remediate polluted water.