Adsorption of Acid Yellow 17 and Congo Red Dyes by Amino-Silanized Nanocrystalline Zirconia: Kinetics, Isotherms and Thermodynamics Perspective

The study investigates the adsorption of Acid Yellow 17 (AY17) and Congo Red (CR) dyes using amino-silanized nanocrystalline zirconia (NH₂–ZrO₂) as an adsorbent in batch experiments. Nanocrystalline zirconia (ZrO₂) was synthesized via co-precipitation and functionalized with 3-aminopropyltriethoxysilane (APTES) using a grafting method. Characterization techniques including SEM, EDX, XRD, FT-IR spectroscopy, TGA, and BET analysis were employed to analyze the prepared material. Adsorption parameters such as pH, contact time, temperature, adsorbent dosage, and initial dye concentrations were optimized. pH significantly influenced adsorption, with optimal conditions at pH 2 for AY17 and pH 6 for CR. Equilibrium was reached within 75 min for AY17 and 165 min for CR. Temperature also affected adsorption, with maximum removal observed at 313 K for AY17 and 323 K for CR. Kinetic and isotherm models were applied to analyze adsorption kinetics and equilibrium data. The pseudo-second-order kinetics model and Langmuir isotherm model best described the experimental data. Maximum monolayer adsorption capacities (q_m) were estimated as 66.2 mg/g (at 313 K) for AY17 and 105.9 mg/g (at 323 K) for CR. Thermodynamic analysis indicated the spontaneous nature of AY17 and CR adsorption onto NH₂–ZrO₂, with negative values of ∆G° indicating feasibility. The ∆H° values suggested exothermic adsorption for AY17 and endothermic adsorption for CR, with a positive ∆S° indicating increased randomness at the solid–liquid interface during adsorption. Successful regeneration studies over five cycles demonstrated the reusability of NH₂–ZrO₂ as an adsorbent for efficient removal of anionic azo dyes from wastewater.