Adsorption of Methylene Blue and Congo Red by Silica Nanowires

Abstract

Silica nanowires (SiO₂ NWs) were prepared via a PVP‑Na₃Cit‑TMOS system and used as adsorbents for the removal of methylene blue (MB) and Congo red (CR) from aqueous solutions. The adsorption of MB reached equilibrium within 20 min at 25 °C and pH 9.0, with a capacity of 155.0 mg/g, while CR achieved equilibrium within 45 min at 25 °C and pH 2.0, with a capacity of 60.8 mg/g. The adsorption processes followed the pseudo‑second‑order kinetic model and the Langmuir isotherm model, indicating monolayer physical adsorption. Thermodynamic parameters (ΔG < 0, ΔH < 0) confirmed spontaneous exothermic adsorption. pH played a critical role: alkaline conditions favored MB adsorption due to enhanced electrostatic attraction, whereas strongly acidic conditions promoted CR adsorption. The SiO₂ nanowires exhibited significantly higher affinity for the cationic dye MB, demonstrating promising potential for the treatment of cationic dye‑contaminated wastewater.