摘要
Persistent Pb(II) ions in soils and aquatic systems diminish soil fertility, degrade water quality, and destabilise ecosystems. Hydroxyapatite nanowires (HAPNWs) are promising adsorbents owing to their chemical stability, biocompatibility, and strong affinity for heavy metals. This study evaluates HAPNWs hydrothermally synthesised between 0°C and 55°C, correlates temperature-induced structural variations with adsorption performance, and elucidates the stepwise Pb(II) uptake pathway. Nanowires synthesised at 35°C displayed the highest aspect ratio and achieved the greatest equilibrium adsorption capacity. The sorption kinetics followed a pseudo-second-order model, whereas the equilibrium data were best described by the Freundlich isotherm. Multi-technique characterisation revealed a two-stage uptake mechanism. Stage I, outer-sphere complexation, involves rapid binding of Pb 2+ to surface -PO 4 and -OH groups via electrostatic interactions without disturbing the apatite lattice. Stage II, near-surface ion exchange accompanied by incipient dissolution-reprecipitation, occurs under continued loading: surface Ca 2+ is partially replaced by Pb 2+ , and Pb 10 (PO 4 ) 6 (OH) 2 nuclei start to form on the HAP surface, progressively reducing active-site availability. Competitive sorption tests demonstrated a pronounced Pb 2+ preference. In equimolar quinary solutions (Pb, Cu, Cd, Zn, Ni), the selectivity sequence was Pb > Cu > Cd > Zn > Ni. The strongest inhibition appeared in the binary Pb-Cu system, where uptake decreased to approximately 40% of the single-ion value, indicating direct competition for identical surface sites. By elucidating the temperature-dependent relationship between structure, mechanism, and performance, this work provides both theoretical insights and a methodological framework for the rational design of high-performance adsorbents for heavy metal remediation in complex environmental systems.
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