ZHANG Shuang ¹, LI Jie ¹, JIANG Yuwei ¹, TAN Junjun ^{1, 2}

(1. School of Material science and Chemical Engineering, Hubei University of Technology, Wuhan 430068, Hubei, China; 2. Hubei Provincial Key Laboratory of Green Light Industrial Materials, Hubei University of Technology, Wuhan 430068, Hubei, China)

Extended abstract:

[Background and purposes] Oleic acid is commonly used to regulate the morphology of hydroxyapatite nanowires (HANWs) during synthesis, but it also covers surface-active sites and hence suppresses the intrinsic adsorption properties. To overcome this limitation, this study was aimed to introduce an efficient and rapid method for removing surface oleate ligands, using a sodium phosphate‑ethanol/water system. With this approach, the intrinsic surface of HANWs fwas ully exposed, significantly enhancing their adsorption capacity for copper ions in aqueous solutions.

[Methods] HANWs were synthesized hydrothermally at 180 ℃ for 24 h. The surface oleate ligands were subsequently removed via a Na₃PO₄-assisted stripping method to obtain ligand-free HANWs. Freestanding membranes were fabricated by vacuum filtration of aqueous HANW suspensions. The materials were characterized by using XRD, FE-SEM, XPS, BET, FTIR, zeta potential and contact angle measurements. Their Cu²⁺ adsorption performance, membrane filtration efficiency and regeneration capability were systematically evaluated.

[Results] Na₃PO₄-assisted ligand removal strategy effectively stripped oleic acid from HANWs surfaces, preserving their one-dimensional morphology, while fully exposing adsorption sites for enhanced performance. A Cu²⁺ removal efficiency of 95.49% was achieved, representing a 3.36-fold enhancement over untreated HANWs. Adsorption kinetics followed the pseudo-second-order model, while isotherm data fit the Langmuir model, yielding a maximum adsorption capacity of 72.80 mg·g⁻¹ at 45 ℃. According to the thermodynamic analysis results, the adsorption process was endothermic (ΔH^⊖>0) and spontaneous (ΔG^⊖<0). The Cu²⁺ uptake proceeded through ion exchange coupled with surface complexation. In the form of filtration membrane, the HANWs exhibited high Cu²⁺ removal efficiency, although the performance decreased with increasing flow rate and initial Cu²⁺ concentration. After six consecutive adsorption–regeneration cycles, the membrane retained 69.25% of the initial water permeance, demonstrating excellent reusability and practical potential for water purification.

[Conclusions] A facile and efficient Na₃PO₄-mediated method was proposed for complete removal of oleate ligands from hydrothermally synthesized HANWs. The treatment significantly enhances the Cu²⁺ adsorption capacity of HANWs, with the adsorption process that can be well described by using the Langmuir isotherm and pseudo-second-order kinetic models. The ligand-free HANWs retain high Cu²⁺ removal efficiency across a broad pH range and in the presence of competing ions. Moreover, a practical HANW membrane fabricated from the material demonstrates excellent Cu²⁺ treatment capacity, regenerability and effluent meeting WHO safety standards. The adsorption proceeds mainly via ion exchange and surface complexation. This work offers a versatile strategy for reactivating surfactant-capped nanomaterials and establishes a foundation for practical application of HANW-based technologies in water decontamination.

Key words: hydroxyapatite nanowires; adsorption; Ligand removal; copper removal; oleic acid