Abstract

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. pH_PZC 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.

Key words: Sorption, extended X-ray absorption fine structure (EXAFS) spectroscopy, clay, precipitation, Burkina Faso.