Efficient removal of methylene blue dye using magnetized titanate nanotubes

Abstract

This study investigates the removal of methylene blue (MB) dye, a model organic pollutant, using sodium titanate nanotubes (Na/TNTs) and magnetized titanate nanotubes (Mag/TNTs) as efficient nano-adsorbent materials. Mag/TNTs consist of cobalt ferrite (CoFe₂O₄) nanoparticles supported on Na/TNTs. Both adsorbents were synthesized via a simple hydrothermal method and characterized using SEM, HRTEM, XRD, BET, zeta potential analysis, and FT-IR techniques. Multiwalled open-ended nanotubes were formed, as demonstrated by TEM images. Their typical lengths ranged from 100 to 150 nm, and their average outer diameters were 12.46 and 12.88 nm for Na/TNTs and Mag/TNTs, respectively. Batch adsorption experiments were conducted to evaluate the effects of pH, adsorbent dose, initial dye concentration, temperature, contact time, and adsorbent regeneration on the removal efficiency of MB dye. Among five isotherm models (Langmuir, Freundlich, Langmuir–Freundlich, Sips, Dubinin–Radushkevich), Langmuir–Freundlich and Sips provided the best fits, indicating heterogeneous surface adsorption. Kinetic data were best described by pseudo-second-order and mixed 1,2-order models among the four tested models (pseudo-first-order, pseudo-second-order, mixed 1,2-order, Avrami), suggesting combined physisorption and chemisorption. The maximum adsorption capacities for Na/TNTs and Mag/TNTs increased with temperature, reaching 178.98 mg g⁻¹ and 180.50 mg g⁻¹ at 328 K, respectively. Thermodynamic parameters confirmed that the process is spontaneous and exothermic. Monte Carlo simulations revealed that while CoFe₂O₄ exhibits stronger individual interactions with MB, the higher surface area of Na/TNTs leads to greater overall removal. The study demonstrates that Mag/TNTs are efficient, separable (using an external magnet), and reusable adsorbents for cationic dye removal from wastewater.