Adsorption of Safranin O on halloysite nanotubes: a mechanistic case

Abstract
Industrial efuents, laden with various organic dyes, are extremely hazardous to the environment and all living organisms. Among these, Safranin O, a reddish brown, water-soluble, synthetic, azine-based cationic dye is extensively used in various industries and biological laboratories. As a possible dye removal strategy, clay materials have emerged as a potential earth�abundant low-cost adsorbent owing to their conducive surface properties for favorable dye adsorption. However, system�atic mechanistic investigation with relatively unexplored clay materials is required to fnd the “best” adsorbate–adsorbent combination. Herein, halloysite nanotube, a unique nanoaluminosilicate, is thoroughly characterized and evaluated as an adsorbent for removal of Safranin O using batch adsorption studies and thermodynamic and kinetic analyses. Under the optimized condition, 98% removal efciency was achieved within 6 h at room temperature (100 g halloysite in 10 mL of dye solution with conc. of 100 mg mL−1). The corresponding adsorption efciency reaches 37.518 mg g−1, and the process follows Langmuir isotherm and pseudo-second-order kinetics. The experimental results coupled with several advanced characterization techniques indicate that adsorption proceeds via various difusion processes, followed by a combination of coulombic, hydrogen bonding, and hydrophobic interactions between adsorbent and adsorbate, by replacing the surface�bound solvents. Subsequent optimization by response surface methodology validates the feasibility of the process, and the material can be reused for at least three cycles. The present fndings ofer generalized protocols to elucidate the mechanism of adsorption for similar adsorbate–adsorbent combinations and help to develop scalable inexpensive sustainable wastewater remediation technologies