Sorption mechanisms of Chromium(III), Copper(II) and Lead(II) on two natural mixed clays from Burkina Faso as determined by extended X-ray absorption fine structure (EXAFS) spectroscopy

摘要

Chromium(III), copper(II) and lead(II) are among the heavy metals produced and released in large amounts by anthropogenic sources worldwide, including Burkina Faso. Previous studies have demonstrated the successful application of domestic natural mixed clays for the removal of these metal ions as a cheap and environmentally friendly method. Qualitative mineralogical characterization of the clays revealed that they consist of kaolinite, illite, orthrose and quartz, and minor quantities of albite and montmorillonite. pHPZC for the clays, as determined by potentiometric titrations, are in the range 6.8 to 7.3. In this study, the interactions of chromium(III), copper(II) and lead(II) ions with these clay minerals were examined by the use of extended X-ray absorption fine structure (EXAFS) spectroscopy. Cr3+ forms tetrameric hydrolysis complexes on the mineral surface with a Cr–O bond distance of 1.98 Å, and two Cr···Cr distances at 3.02 and 3.62 Å. This is indicative of a tetrameric [Cr4(OH)6(H2O)12]6+ entity bound to the clay mineral surface. A distance of 3.17 Å, regarded as a Cr···Fe distance, indicates that one Cr3+ ion in the hydrolyzed tetramer binds to two oxygens in the mineral surface which are bound to either one or two iron(III) ions in the surface. Pb2+ binds two oxygen atoms at an average bond distance of 2.31 Å, with a significant contribution of linear multiple scattering from the PbO2 entity. The EXAFS results of Cr3+ sorption are consistent with the presence of a hydrolysis product of polymeric Cu2+ species with a surface complex or precipitate.