Adsorption of Methylene Blue onto Polydopamine-Functionalized Halloysite Nanotubes: Kinetics and Equilibrium Studies

The elimination of synthetic dyes from industrial effluents represents a persistent environmental challenge. Developing sustainable and effective adsorbents is essential to preserve global water resources. In this study, we introduce an efficient and facile strategy for methylene blue removal from aqueous systems using halloysite nanotubes functionalized with polydopamine. The polydopamine coating generated a dense array of active adsorption sites, as confirmed by X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, thermogravimetric analysis, and zeta potential measurements. Porosity characterization and X-ray diffraction provided complementary insight into the structure and morphology of the hybrid nanotubes. We systematically investigated the influence of contact duration and solution pH on dye uptake by pristine and functionalized nanotubes. Kinetic evaluation revealed excellent agreement with the pseudo-second-order model (R² > 0.999), yielding rate constants of 0.002 and 0.003 g·mg^-1·min^-1 for samples treated with polydopamine for 6 and 24 h, respectively. The equilibrium adsorption data were analyzed using Langmuir and Freundlich isotherms, showing that functionalized nanotubes achieved a maximum adsorption capacity of 86 mg·g^-1 at 25 °C and pH 10 - almost double that of pristine HNTs (47 mg·g^-1). The high adsorption efficiency, comparable to conventional adsorbents such as zeolites, together with the simplicity and environmental compatibility of the functionalization procedure, underscores their suitability for real-world wastewater applications. Additionally, the demonstrated applicability of this modification method to other dye-adsorbent systems highlights its adaptability and broad potential.