Advanced theoretical interpretations of tetracycline adsorption on halloysite nanotubes via a monolayer model

This study explores the adsorption of tetracycline (TTC) onto three halloysite-based adsorbents: raw halloysite (H), thermally modified halloysite at 600°C (H600), and thermally and HCl treated halloysite (H600-5N). The adsorption mechanisms are analyzed through theoretical parameters derived from isotherm fitting using a monolayer adsorption statistical model. The adsorption parameter n reveals the orientation and aggregation of TTC on the adsorbents, showing that raw halloysite (H) exhibits the highest site affinity and favored angled adsorption. Modified adsorbents (H600 and H600-5N) determine a less molecule aggregation and a transition to mixed orientations by increasing temperature. The effective receptor site density and maximum adsorption capacity indicated that H600 has superior performances, attributed to an increase in active site density due to thermal effects and improved surface properties. Adsorption energy calculations confirm that the adsorption mechanism is primarily physical, dominated by van der Waals forces and hydrogen bonding, with adsorption energy (ΔE) below 40 kJ/mol. This reversible adsorption process highlights the potential of halloysite-based adsorbents, particularly H600, for efficient TTC removal in environmental and industrial applications.