Comparative Evaluation of the Adsorptive Affinity of Zn-H2¬BDC/H3BTC Framework for Cationic and Anionic Dyes: Equilibrium, Kinetic and Thermodynamic Approach

This study reports the adsorption selectivity of the mixed-linker metal-organic framework Zn-H2BDC/H3BTC toward cationic and anionic dyes represented by malachite green (MG) and acid orange 7 (AO7), respectively. The MOF was synthesized via a solvothermal method and characterized using FTIR, PXRD and TGA/DTG analyses, which confirmed the successful formation of the framework, high crystallinity and adequate thermal stability for wastewater treatment applications. Batch adsorption experiments were performed by varying initial dye concentration, contact time, pH and temperature. The adsorption capacity increased with increasing dye concentration, while AO7 adsorption was favoured under acidic conditions and reached equilibrium within 60 min, whereas MG adsorption increased gradually under alkaline conditions. Equilibrium studies showed that both Langmuir and Freundlich isotherm models adequately described the adsorption process, indicating the coexistence of monolayer and heterogeneous adsorption mechanisms. The Langmuir maximum adsorption capacity (Qm) for AO7 (1.848 mg/g) was higher than that for MG (1.469 mg/g), confirming the greater adsorption affinity of Zn-H2BDC/H3BTC toward the anionic dye. Elovich isotherm parameters, including Elovich constant (KE), further indicated higher adsorption affinity and faster uptake of AO7 relative to MG. Kinetic studies revealed that pseudo-second-order kinetics governed AO7 adsorption, whereas intraparticle diffusion significantly influenced MG uptake. Thermodynamic analysis demonstrated that the adsorption processes were spontaneous, exothermic and predominantly controlled by weak physisorption interactions. The results demonstrate that Zn-H2BDC/H3BTC possesses selective adsorption behaviour with stronger affinity toward AO7 and may serve as a promising adsorbent for dye-contaminated wastewater treatment.