Surface speciation of Cd(II) and Pb(II) on kaolinite by XAFS spectroscopy

New insights into the sorption mechanism of cadmium on red mud

“…According to the proposed surface complexation model and the LCF results, it is reasonable to conclude that Cd 2þ sorbed on the RM o as a mixture of outer-sphere complexes (about 65% Cd 2þ ) and polynuclear complexes analogous to XCd(OH) (about 35%) which is assumed to be inner-sphere complexes (Gräfe et al, 2007). In addition, similar XANES spectra between goethite and red mud sorption samples (Fig.…”

“…Thus, the XANES spectra of the sorption samples were fitted using a least-squares algorithm to obtain linear combinations of edge spectra from the three spectra. Cd(OH)Cl was chosen as a reference compound because many studies have proposed that Cd(OH)Cl-like compound was formed on clay minerals during Cd sorption (Lackovic et al, 2002;Gräfe et al, 2007;Serrano et al, 2009). Additionally, the experimental spectrum shape of the compound was well reproduced by FEFF calculation: intensities and positions of the features are satisfactory (Fig.…”

“…In addition, Cd reference compounds of oxide, hydroxide, nitrate and carbonate phase, and hydroxychloride (Cd(OH)Cl) and Cd sorption samples on goethite and Fe(OH) 3 (similar to the preparing procedures for red mud) were also prepared for XANES fingerprint analysis. Cd(OH) 2 and CdCO 3 were prepared following the methods of Gräfe et al (2007), and Cd(OH)Cl was synthesized according to Cudennec et al (2000). Three or more scans were averaged for each sample and reference compound.…”

“…It was proposed that Cd could be sorbed by specific adsorption via inner-sphere complexation and/or non-specific adsorption via outer-sphere complexation, depending on its surface loading and sorbent types (Brummer et al, 1988;Gräfe et al, 2007). In many cases, these reaction mechanisms are proposed based on macroscopic data, lacking direct microscopic evidence (Serrano et al, 2009).…”

“…Recently, X-ray absorption spectroscopy (XAS) has been increasingly applied to investigate the sorption mechanisms and speciation of trace elements in mineral and plant samples (Randall et al, 1999;Waychunas et al, 2003;Gräfe et al, 2007;Isaure et al, 2010). Particularly, X-ray absorption near edge structure (XANES) spectroscopy is expected to meet the need of the detection limit and provide the information of complexation and speciation of Cd in environmental samples (Waychunas et al, 2003;Gräfe et al, 2007;Isaure et al, 2010). To our knowledge, it has not been used so far to investigate Cd sorption reactions in red mud probably due to the limits of high energy required by K-edge and low sensitivity at L III -edge of Cd absorption spectra.…”