Contribution of Hydroxyapatite and Ferrihydrite in Combined Applications for the Removal of Lead and Antimony from Aqueous Solutions

In this study, lead (Pb) and antimony (Sb) sorption experiments were conducted to elucidate the mechanisms of Pb and Sb sorption by combined applications using single or combined applications of hydroxyapatite (HAP) and ferrihydrite (FH), to evaluate the contribution of each material in Pb and Sb sorption, and to assess the chemical stability of the sorbed Pb and Sb. In the combined application, isotherms of Pb sorption and Sb sorption were well fitted to Langmuir and Freundlich isotherm models, respectively. The Pb and Sb amounts sorbed in the combined application were the same levels as the summed totals of those sorbed in the single applications, indicating that in the combined application, Pb sorption and Sb sorption were not suppressed. Pb was mainly sorbed on HAP in the combined application, at a 90% level of the total adsorbed Pb. The HAP and FH contributions to Sb sorption were 32 and 68 % of the total adsorbed Sb, respectively, and Sb was sorbed on each material independently even in the combined application. The percentages of both Pb and Sb dissolved from the sorbed materials in the combined applications at pH 4 and 6 were the same levels as those in the single applications. However, the percentages of Sb dissolved in both combined and single applications were high at an alkaline pH. These results suggest that HAP and FH in a combined application would be useful for simultaneous Pb and Sb immobilization in soil with acidic to neutral pH, but not in soil with an alkaline pH.