Efficient adsorptive removal of two anionic dyes (methyl orange and methyl red) with MOF-525: Adsorption isotherms and kinetics studies

Adsorption is widely used to remove anionic and cationic dyes from wastewater because of its economical and high efficiency. Metal-organic frameworks (MOFs), as a kind of adsorbent, have attracted great attention due to the large surface area, adjustable pore size, and high stability. In this study, a Zr-MOF, named MOF-525, was synthesized by using solvothermal method, and the adsorption performances of MOF-525 on two anionic dyes (methyl orange, MO and methyl red, MR) were investigated. Results showed that the obtained MOF-525, with the specific surface area of 2,613.91 m² g⁻¹, the total pore volume of 1.20 cm³ g⁻¹, and the average pore size of 1.84 nm, exhibited high adsorption capacities for both MO and MR. The optimal pH value for MO and MR adsorption were 3.0 and 4.0, with the adsorption capacity of 854.30 and 629.67 mg g⁻¹, respectively; in addition, the adsorption capacities for both MR and MO were increased with the increasing of SO₄²⁻ concentration. The adsorption kinetics, isotherms, and thermodynamics results indicated the pseudo-second-order model and Langmuir (as well as Freundlich) could describe the adsorption processes of MOF-525 well, and the adsorption processes for both MO and MR were spontaneous exothermic reactions. Besides, the reusability experiment demonstrated that MOF-525 had significant adsorption capacity and good reusability for MR. In conclusion, the possible adsorption mechanisms of anionic dyes by MOF-525 involved hydrogen bonding, electrostatic attraction, and π-π interaction. These results illustrated that MOF-525 might be a potential adsorption material for anionic dyes removal from wastewater.