Two Co(II)-Based Metal Organic Frameworks for Highly Efficient Removal of Azo Dyes from Aqueous Environment: Synthesis, Selective Adsorption and Adsorption Mechanism

Two novel Co(II)-MOFs synthesized under solvothermal conditions, namely [Co-3(L)(2)(bimb)(2)(H2O)(2)](n) (OUC-1) and [Co-2(L)(OH)(H2O)(2)](n) (OUC-2) (bimb = 1,4-bis(lmidazol)butane, H3L = 3-(3,5-dicarboxylphenoxy)-5-carboxylpyridine), were used to remove Congo Red and Orange IV from aqueous environment, respectively. The influential parameters of the temperature and pH of solution, initial dye concentration, the structure and charge of dye on the adsorption performance were investigated. It is found that the dye with proper structure (i.e. organic groups favorable for adsorption, such as -NH2, -SO3H, etc, proper distance between the groups, linear structure and more benzene rings), richer negative charges, smaller molecular weight facilitates faster adsorbed. The intermittent operation experiments were adopted to make the adsorption isotherm and kinetics data of OUC-1 and OUC-2 accurately described by Langmuir isotherm and pseudo second-order kinetics. The intra-particle diffusion model indicates the adsorption process is controlled by the intra-particle diffusion and other adsorption stages. Two adsorbents release azo dyes easily through simple organic solutions and maintain high adsorption capacity after 8 cycles. Moreover, OUC-1 and OUC-2 also exhibit high adsorption performance in artificial seawater. The mechanism of selective adsorption involves pore filling, pi-pi stacking interaction, hydrogen bond and electrostatic interaction.