Selectivity Adsorptive Mechanism of Different Nitrophenols on UIO-66 and UIO-66-NH 2 in Aqueous Solution

UIO-66 and UIO-66-NH 2 were synthesized by a solvent thermal method, and their textures were characterized. The adsorption selectivity, kinetics, isotherms, and mechanism of the two MOFs in a single solute system for phenol, 4-nitrophenol, 2,4-dinitrophenol, and 2,4,6-trinitrophenol were studied. At pH 4.0, the two MOFs exhibited a greater adsorption capacity for 2,4-dinitrophenol than for other nitrophenols; however, at lower substrate concentrations ( C 0 < 100 mg/L), the adsorption capacity of UiO-66-NH 2 for 2,4-dinitrophenol was greater than that of UiO-66. The study of the adsorption mechanism showed that the electrostatic attraction is the main way in which UIO-66 and UiO-66-NH 2 adsorb nitrophenols. Moreover, there was π-electron accumulation at a certain angle between the benzene ring of the adsorbents and the benzene ring of nitrophenols, whereas UiO-66-NH 2 could also combine the nitrophenols through the hydrogen bonds originating from the amino groups. The fitting results of adsorption data showed the pseudo-second-order model, and the Langmuir isotherm model could well represent the adsorption kinetics and isotherm adsorption processes, respectively. In the mixed solution consisting of 4-nitrophenol, 2,4-dinitrophenol, and 2,4,6-trinitrophenol, the competitive adsorption ability of UIO-66 and UIO-66-NH 2 for 2,4-dinitrophenol were greater than for the other two nitrophenols, showing that the size of the substrate and the surface area of the adsorbent were also factors that affect the adsorption efficiency.