Factors influencing the removal of Pb(II) ion by means of hydroxyapatite

The reaction behaviour of synthetic hydroxyapatite, Ca-5(PO4)(3)OH (HAP) toward the Pb(II) ion is investigated for the Pb/Ca molar ratio range 1-0.01, at different temperature values, surface areas of HAP, pH values and solid/liquid ratio by means of ion concentration and pH measurements, powder diffractometry and I.R. spectroscopy. The Pb-containing solid phase is invariably Pb-5(PO4)(3)OH (PbAP) and in addition, at pH>6, the presence of 3PbO . H2O is also verified. The rate of the reaction depends mainly on pH, Pb/Ca molar ratio and HAP surface area and is due to PbAP precipitation alternated with to HAP dissolution. At a Pb/Ca molar ratio of 0.01 the adsorption Pb(ll) on the HAP surface should also occur. At Pb/Ca molar ratios in the range 1-0.6 the reaction is quantitative only at a high surface area. At Pb/Ca molar ratios in the range 0.2-0.01 the reaction is quantitative irrespective of the surface area. For the molar ratio Pb/Ca = 0.01 the EPA limit level of 0.015 mg/l is reached after 2 days by using HAP with a surface area of 57.80 m(2) g(-1)