Adsorptive removal of boron by zeolitic imidazolate framework: kinetics, isotherms, thermodynamics, mechanism and recycling

Seven water-stable metal-organic frameworks (MOFs) were prepared and tested for boron removal for the first time. XRD, SEM, nitrogen adsorption/desorption isotherms and thermogravimetric analysis were employed to confirm the structures. All the seven MOFs exhibit good boron adsorption capacities, especially ZIF-8 with an extraordinarily high capacity of 247.44 mg·g−1 at 45 °C. The adsorption kinetics, isotherms, thermodynamics, mechanism and recycling of ZIF-8 were further investigated in batch adsorption process. Boron adsorption on ZIF-8 is a spontaneous endothermic process controlled by entropy change, reaches equilibrium in 1 h. Kinetic fitting of data indicates chemisorption is significant in the process and intraparticle diffusion is not the only rate-limiting step. Freundlich model describes the isotherms well, also suggesting chemisorption plays an important role in boron adsorption by ZIF-8. Characterization on exhausted ZIF-8 with 11B MAS NMR and XPS reveals that the three interactions happen mainly between boron and ZIF-8 at the Zn sites. The adsorption capacity does not decrease obviously after four cycles. ZIF-8 exhibits excellent boron adsorption capacity than all the previous materials, which makes it a promising agent for boron removal.