Adsorption of aqueous Cd²⁺, Pb²⁺, Cu²⁺ ions by nano-hydroxyapatite: Single- and multi-metal competitive adsorption study

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Being an inexpensive but efficient adsorbent, hydroxyapatite is extensively used for decontaminating wastewater and soils polluted by heavy metals. However, its solubility and grain size can affect its remediation effectiveness. This study investigated the ability of nano-hydroxyapatite (nano-HAP) to adsorb aqueous Cd, Pb and Cu ions from single-metal and multi-metal ions reaction systems. Langmuir and Freundlich isotherm equations were employed to study the sorption constants. Based on the sum of squares errors (SSE), results showed that the Langmuir isotherm better fits sorption data than the Freundlich equation. The sorption affinity of nano-HAP for Pb(II) is always higher than that for Cu(II) and for Cd(II); the sorption maxima for the Cd, Pb and Cu follow the order Pb²⁺ > Cu²⁺ > Cd²⁺. This could be inversely proportional to the hydrated ionic radii as Pb²⁺ (4.01 Å) > Cu²⁺ (4.19 Å) > Cd²⁺ (4.26 Å). The measured selectivity coefficients in multi-metal (Cd-Pb-Cu) reaction systems shows that Pb has the highest sorption selectivity on nano-HAP among the metals investigated. This sorption selectivity coincided well with the sorption affinity order in mono-metal reaction systems. The pH of the solution is an important parameter in controlling Cd, Pb and Cu ions sorption on nano-HAP. Indeed, the nano-HAP sorption capacity increases with increasing pH up to a value of 6.25. This implies that the removal of metals from the solution is recommended for pH ≈ 6.25 or below, during remediation using nano-HAP as a sorbent.