Sorption of selenite (SeO32-) on hydroxyapatite: An exchange process

The sorption of SeO32- on hydroxyapatite surface was investigated in batch experiments over a range of pH and SeO32- concentrations in the absence and presence of additional Ca and PO4. The sorption is pH dependent with a max imum observed at pH values generally encountered in natural waters. While t he presence of phosphate lowers SeO32- sorption by direct competition, the presence of calcium enhances it. In order to identify the mechanism of sorp tion and the nature of the surface sites, microscopic observation and spect roscopic methods such as X-ray diffraction and X-ray photoelectron spectros copy were used. Surface complexation, coprecipitation, and precipitation pr ocesses were ruled out. Localization of sorbed selenium in the crystallogra phic sites where phosphorus is normally located shows that selenite is sorb ed on the apatite by an anionic exchange with phosphate groups. Although th e exact equilibria involved could not be established, the stoichiometry of the exchange is close to 1:1. According to kinetics experiments and X-ray d iffraction analyses, it seems that selenium is not exclusively located at t he surface but diffuses slightly in a thickness of a few nanometers. (C) 20 00 Academic Press.