Adsorption of methylene blue and malachite green from aqueous solution by sulfonic acid group modified MIL-101

A metal-organic framework (MOF) with sulfonic acid group (denoted as MIL-101-SO3H) was directly synthesized without post-synthesis method and applied to remove methylene blue (MB) and malachite green (MG) from aqueous solution. Characterizations of MIL-101-SO3H were achieved by XRD, TEM, XPS, nitrogen adsorption-desorption analysis and elemental analysis. MIL-101-SO3H mainly possessed mesopore, high surface area, big pore volume and anionic group which are benefit for the adsorption of cationic dye. The adsorption isotherm, thermodynamic parameters and kinetics characteristic of dye adsorption onto MIL-101-SO3H were investigated. The experimental isotherm data were analyzed using Langmuir isotherm equation and the results indicated the Langmuir isotherm was suitable to describe the adsorption behaviors. Thermodynamic parameters calculated by the Gibbs free energy function, confirm the adsorption process was spontaneous, endothermic and entropy-driven. The pseudo-second-order and intraparticle diffusion, were employed to obviously describe the adsorption mechanism. The dye adsorption capacities of MIL-101-SO3H were found to increase as the initial pH values in the experimental ranges. These results suggested the π-π interactions between the aromatic rings of MB and MG with MIL-101-SO3H may be mainly interactions in low pH values and electrostatic interactions can gradually increase with increasing initial pH values.