Reaction of lead ions with hydroxylapatite granules

In the effort to improve the performance of hydroxylapatite (HA) in removing lead ions from aqueous solutions, millimeter-sized granules with 50 % porosity were synthesized. Such HA particles, after drying at 100°C, or heating at 800°C or 1100°C, exhibited the specific surface areas of 50 m² g⁻¹, 25 m² g⁻¹, and 5 m² g⁻¹, respectively. It was found that heavy metal sorption capacity of HAs can be related to their surface area. Non-calcined granules were difficult to handle and easy to crush. Hardened granules showed heavy metal absorption on their outer surfaces. Absorption capacity of sintered HA particles towards lead was lower but adsorbed lead ions were spread inside the porous structure of HA granules more evenly. Under flow conditions, lead ions were captured by HA at a rate of 0.5 mg g⁻¹ min⁻¹. Small lead phosphate aggregates were released from the HA sorbent together with calcium ions. Size of the aggregates depended on the lead concentration and ranged from 1–50 μm in diameter; the aggregates could be removed by ultrafiltration. Results show that porous hardened HA granules can be used as an efficient phosphate source for the immobilization of lead ions from aqueous media. Organic ligands tend to interfere with the water purification procedure.