Assessment of halloysite clay nanotubes as adsorbents for removing tetracycline hydrochloride from contaminated aqueous systems via experimental study and SQM calculations

Abstract

Anthropogenic waste disposal from household, hospital, and large scale industrial use has generated a serious environmental problem. This article evaluates the adsorption process for mitigating the antibiotic tetracycline hydrochloride (THC) using halloysite clay mineral nanotubes. The use of this material for THC removal in aqueous medium is considered an efficient, eco-friendly and low-cost method. The clay mineral was applied both in its natural and acid-treated forms, and the samples were characterized by XRF, XRD, SEM-EDS, TEM, FTIR, TGA, XPS, and N₂ adsorption/desorption for assessing their properties and main active sites. Response surface methodology (RSM) applied to the results of a D-optimal DoE indicated that halloysite treated with 1 mol L⁻¹ HCl (Hal-A1) has the highest adsorption capacity, achieving an elimination rate >98% in 50 min. The high fit of the adsorption data to the pseudo-second order kinetic model and the adsorption enthalpy of 49.50 kJ mol⁻¹ suggest that the mechanism for this process is based on chemisorption, which was further confirmed by semi-empirical quantum mechanical calculations (SQM) (−63.2 kcal mol⁻¹ < E_ads < −50.3 kcal mol⁻¹). The maximum adsorption capacity calculated from the Langmuir equilibrium isotherm (R² = 0.9978) was 9.63 mg g⁻¹ for Hal-A1. The nanotubular clay mineral halloysite showed high efficiency as an adsorbent for the fast removal of THC from aqueous solution, standing out for being a natural, low-cost and environmentally friendly material.