Metal-Organic Framework UiO-66 as an Efficient Adsorbent for Boron Removal from Aqueous Solution

Water-stable metal-organic framework (MOF) UiO-66 was studied in boron removal from water for the first time. XRD, SEM, nitrogen adsorption/desorption isotherms, and thermogravimetric analysis (TG) were employed to confirm the structure. The boron adsorption kinetics, isotherms, thermodynamics, mechanism, and recycling on UiO-66 were further investigated in batch adsorption process. UiO-66 exhibits great adsorption performance of 10.59 mmol g(-1) at 45 degrees C, and the adsorption process reaches equilibrium rapidly in 1 h. Pseudo-second-order model, intraparticle diffusion, and Boyd model are employed for kinetic analysis. The process is a spontaneous endothermic process controlled by entropy change rather than enthalpy change, which suggests intensive chemisorption. The adsorption capacity does not decrease obviously after four cycles. Characterization on exhausted UiO-66 with B-11 MAS NMR reveals not solely interaction between boric acid and UiO-66. The XPS patterns suggest interaction with an Zr site, which is confirmed by the adsorption simulation with quantum chemistry. Higher boron adsorption capacity than conventional boron removal adsorbents and easy regeneration make UiO-66 a promising agent for boron removal from aqueous solution.