Removal of Pb 2+ from synthetic aqueous solution using hydroxyapatite and hydroxyapatite@AD37 composite materials

Abstract

In this paper, a simple method was proposed to obtain hydroxyapatite (HA) and hydroxyapatite/partially hydrolysed polyacrylamide (HA/AD37) composite materials which where applied to lead retention from aqueous solution by means of the batch method. The characterization of the materials verified that the presence of AD37 created interconnected porosity in the composite HA/AD37 giving it a good swelling properties that conducted to an easy separation of the material from aqueous solutions.

Retention experiments carried out by varying the dose of lead and the contact time between adsorbent and adsorbate showed that the maximum adsorption capacity (Q_max) obtained for 2072.2 mg/L as initial concentration of Pb2 + was equal to 984.63 mg/g for HA and 924.50 mg/g for HA/AD37. Furthermore, AD37 used alone cannot retain Pb2 + ions. Indeed, the calculated Q_max of AD37 part of the composite was of 806.57 mg/g. The obtained Q_max values was elevated more than the reported values in many literatures.

Based on the correlation coefficient, the kinetic study proved that pseudo-second order model agrees well with the obtained experimental data for Pb²⁺ retention by both HA and HA/AD37. Also, isotherm study explored that adsorption of lead was best fitted by Langmuir model for HA and Temkin model for HA/AD37.

At last, the mechanism of retention was probed by characterizing the adsorbents after contact with lead ions by XRD and SEM. The results showed the transformation of calcium-hydroxyapatite to different structures of lead hydroxyapatite confirming the presence of ion exchange mechanism between Ca²⁺ and Pb²⁺.