Characterization of mercury sorption on hydroxylapatite: batch studies and microscopic evidence for adsorption

Although previous studies have investigated Hg sorption on various common minerals, there has been limited study of Hg interaction with apatite. In this study, systematic experiments regarding Hg sorption on HAP were performed over a wide range of physicochemical conditions. In the sorption edge experiments, Hg uptake by HAP exhibits a maximum sorption (∼90%) at pH 6.0, which rapidly decreases at pH>6.0. Sorption isotherms are fitted well by Freundlich equations, and the distribution coefficient (KD) increases in the order of pH 5>7>9. In both the sorption edge and isotherm experiments, sorption patterns and quantities are minimally influenced by variations in the ionic strength. The results from the kinetic experiments are in good agreement with the pseudo-second-order rate law. The initial sorption rate at pH 9 is much slower than that at pH 5 and 7. During desorption, ∼90% of the sorbed Hg is retained at both pH 6.0 and 9.0, which indicates strong bonding of Hg to the HAP surface. Our results suggest that adsorption plays an important role in controlling the initial stage of interactions between Hg and HAP.