Surface-cleaned hydroxyapatite nanowires for aqueous copper ion removal: performance, adsorption mechanisms and membrane filtration application

Abstract

Ligand-assisted nanoparticle synthesis offers precise size and morphology control but often compromises intrinsic surface properties and hinders further functionalization due to ligand capping. To address this limitation, we demonstrate a simple and efficient strategy for complete ligand removal using oleate-capped hydroxyapatite (HA) nanowires as a model system. Ethanol treatment at 70 °C (denoted ET-70) effectively eliminates oleate ligands from HA nanowire surfaces, as confirmed by FTIR and contact angle analyses. The resulting clean surfaces expose abundant active sites, significantly enhancing copper ion adsorption capacity compared to untreated counterparts. Batch adsorption experiments demonstrated that ET-70 achieved a maximum Cu²⁺ adsorption capacity of 63.92 mg g⁻¹ with a removal efficiency of 12.79% at an initial concentration of 200 mg L⁻¹ and 45 °C. The adsorption process was well-described by the Langmuir isotherm and pseudo-second-order kinetic models. Remarkably, ET-70 maintained high removal efficiency across wide pH ranges and in the presence of coexisting ions. In membrane filtration applications, HA nanowire membranes processed 197.95 L m⁻² of copper-contaminated water (influent: 5 mg L⁻¹ Cu²⁺, flow rate: 0.95 mL min⁻¹) while consistently meeting the WHO safety standard (<2 mg L⁻¹ effluent concentration). Mechanistic studies indicate that adsorption primarily occurs through ion exchange between Ca²⁺ in HA and Cu²⁺ in solution, complemented by surface complexation with hydroxyl groups. This work not only establishes a versatile ligand-removal protocol for unlocking the functional potential of surfactant-capped nanomaterials but also provides foundational insights for implementing HA nanowires in practical water purification technologies.