Synthesis of rod-like hydroxyapatite with high surface area and pore volume from eggshells for effective adsorption of aqueous Pb(II)

• RHNPs with large surface area and pore volume were produced from burnt eggshells.
• The RHNP samples produced with 100–400 ml/h were much stable after calcination.
• The RHNP samples showed fast adsorption with high adsorption capacity of Pb2+.
• The adsorption revealed Langmuir isotherms and a pseudo-second-order reaction.

We report the synthesis of rod-like hydroxyapatite nanopowder from waste eggshells (decomposed to CaO) and phosphoric acid at room temperature without pH control. The procedure involves the use of a syringe pump to control the addition rate of the phosphoric acid in order to maintain a stable pH. Results revealed that pure, rod-like, and nano-size hydroxyapatite particles were produced at various addition rates. Moreover, the sample obtained from 200 ml/h addition rate exhibited large surface area (160.1 m2/g) and pore volume (1.0 cm3/g). Consequently, the prepared hydroxyapatite and that previously produced as adsorbents showed fast adsorption rates (10 min) as well as very high maximum adsorption (qm = 994.5 mg/g and 1000.5 mg/g, respectively) for Pb2+ in aqueous solution. Furthermore, the adsorption process revealed a pseudo-second-order reaction model and the equilibrium adsorption could be described by Langmuir model.

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