XAFS studies on surface coordination of Pb²⁺ on birnessites with different average oxidation states
2011
Zhao, Wei | Tan, Wenfeng | Feng, Xionghan | Liu, Fan | Xie, Yaning | Xie, Zhiliu
The adsorption of Pb²⁺ on birnessites was investigated by X-ray absorption fine structure (XAFS). Two Pb–O and two Pb–Mn shells were detected in the birnessites with Pb loading. For the same birnessite at low Pb loading (600mmol/kg) the two Pb–O shells consisted of 3.1 O atoms at 0.227nm and 8.2 O atoms at 0.399nm, respectively. The two Pb–Mn shells consisted of 2.8 Mn atoms at 0.357nm, and 6.1 Mn atoms at 0.377nm. At high lead loading (2344mmol/kg) O atoms number decreased to respectively 1.2 at 0.226nm, and 3.0 at 0.397nm in the two Pb–O shells, and Mn atoms number decreased to respectively 1.0 at 0.356nm and 2.8 at 0.375nm in the two Pb–Mn shells. This is indicative of the distortion of the Pb²⁺ coordination environment with increased loading of Pb. The amount of Pb²⁺ adsorbed increased with increasing values of the average oxidation state (AOS) of birnessite, also in this case the distortion of the Pb²⁺ coordination environment led to a decrease in the Pb–O coordination numbers in both the Pb–O shells and in the Pb–Mn coordination numbers in both the Pb–Mn shells. Based on the XAFS analyses, three bonding mechanisms of Pb²⁺ adsorbed on the birnessites with different Mn AOS were proposed: a single-corner-sharing complex on particle edges along the u axis of MnO₆ layers, a double-corner-sharing complex on particle edges along the a or b axis, and a triple-corner-sharing complex in the interlayer above or below vacant sites.
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