Adsorption of Methylene Blue by HKUST-1 MOFs Synthesized Using Room Temperature and Solvothermal Methods

Abstract

Two Cu-based metal-organic frameworks (MOFs), HKUST-1(RT) and HKUST-1(ST), were synthesized using room-temperature and solvothermal methods, respectively. Structural characterization revealed optical band gaps of 3.61 eV for HKUST-1(RT) and 3.58 eV for HKUST-1(ST). HKUST-1(RT) exhibited higher porosity and small, irregularly shaped layered sheets, whereas HKUST-1(ST) displayed reduced porosity and larger layered sheets. Zeta potential measurements at neutral pH indicated negatively charged surfaces (−5.64 mV for HKUST-1(RT) and −3.75 mV for HKUST-1(ST)), enhancing the electrostatic adsorption of positively charged methylene blue (MB) molecules. The adsorption kinetics disclosed that pseudo-second-order kinetic model explains the experimental data and has high correlation coefficients of 0.9831 for HKUST-1(RT) and 0.9816 for HKUST-1(ST) at a 0.50 g/L adsorbent dose. The outcome of the intraparticle diffusion model revealed an initial fast adsorption of the dye molecules followed by a slower diffusion inside the pores of HKUST-1. The impact of experimental factors viz dye concentration (0.02–0.14 g/L) and adsorbent dose (0.25–0.50 g/L) were systematically studied. By modeling the adsorption isotherms using Langmuir and Freundlich equations, it was found that the Langmuir model provided a superior fit for both materials. These findings highlight the potential of HKUST-1(RT) and HKUST-1(ST) for efficient dye adsorption applications.