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. 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 A, and two Cr···Cr distances at 3.02 and 3.62 A. This is indicative of a tetrameric [Cr4(OH)6(H2O)12]6+ entity bound to the clay mineral surface. A distance of 3.17 A, 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 A, 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.
Highlights
This study investigates the local structural environment of Cr3+, Cu2+, Pb2+ ions sorbed on natural clay surfaces KOR and SIT without any pretreatment as function of pH using extended X-ray absorption fine structure (EXAFS) to deduce the mechanism of the sorption process
The Fourier transforms (FTs) show that Cr3+ binds to six oxygens at mean bond distance of 1.98 Å, in octahedral configuration, and the corresponding CrO6 core multiple scattering was observed at 3.93 Å (Table 2)
The Cr···Cr bond distances in the hydrolyzed tetrameric Cr3+ indicate that one chromium in the hydrolyzed tetramer binds to the surface through two oxygens to one or two iron(III) ions in the mineral surface
Summary
Copper(II) adsorption onto magnetite surface was found to be a fast process following pseudo-second order kinetics (Adewuyi and Pereira, 2016). It was successfully adsorbed on amino-functionalized magnetic nanoparticle surfaces with ion exchange-surface complexation being the main adsorption mechanism (Li et al, 2013) and found to be adsorbed on the upper layer of the crystalline structure of activated carbon prepared from Azadiracta indica bark by means of physisorption (Balakrishnan et al, 2010)
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