Mechanistic understanding of the enhanced adsorption of diclofenac on a heat-treated and acid-leached halloysite

Diclofenac (DCF), a common nonsteroidal anti-inflammatory drug, is frequently detected in aquatic environments, causing serious threat to aquatic organisms and humans through bioaccumulation, persistence, and toxicity. Using eco-friendly clay-based adsorbents, samples of halloysite (H) processed at 600 degrees C (H600-0N) and then HCl-leached with 3 N concentration (H600-3N) were prepared, characterised by Inductively Coupled Plasma (ICP), N-2 adsorption - desorption, and infrared spectroscopy with Fourier transform (FTIR), and used in DCF adsorption. H600-3N exhibited a substantial surface area increase from 63 m(2) g(-1) for H to 434 m(2) g(-1) due to an elevated SiO2/Al2O3 ratio of 23.83 against 1.72 for H. In this context, H600-3N and H adsorbed 165 mg g(-1) and 37.9 mg g(-1) of DCF, respectively. The isotherms of H600-xN((x = 0 or 3)) were adequately adjusted to the Langmuir-Freundlich model, while the kinetic data were suitably described by the pseudo-second order equation. Through cross-checking the results of characterisation, DCF adsorption and the FTIR investigation between DCF and H600-3N, a mechanism has been suggested that includes two main components: hydrogen bonding between the silanol's hydrogen atom and the negatively charged carboxylate anion and hydrophobic interactions between the - Si - O - Si - entities and the DCF aromatic rings. The elucidation of intermolecular interactions between organic contaminants and 1:1 clay minerals is essential to develop the application of these abundant and low-cost materials in wastewater treatment.