XAFS studies on surface coordination of Pb2+ on birnessites with different average oxidation states

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 (600 mmol/kg) the two Pb-O shells consisted of 3.1 0 atoms at 0.227 nm and 8.2 0 atoms at 0.399 nm, respectively. The two Pb-Mn shells consisted of 2.8 Mn atoms at 0.357 nm, and 6.1 Mn atoms at 0.377 nm. At high lead loading (2344 mmol/kg) 0 atoms number decreased to respectively 1.2 at 0.226 nm, and 3.0 at 0.397 nm in the two Pb-0 shells, and Mn atoms number decreased to respectively 1.0 at 0.356 nm and 2.8 at 0.375 nm 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-0 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. (C) 2010 Published by Elsevier B.V.