Abstract:Adsorption patterns of Pb2+ on the surface of birnessites different in Mn oxidation were studied with an X-ray absorption fine structure (XAFS) spectroscopy and mechanism of the adsorption explored by comparing the patterns. Results show that birnessites high in Mn oxidation were high in Pb2+ adsorption capacity. Nearby a Pb2+ ion adsorbed on the surface of birnessite existed one Pb-O and two Pb-Mn shells. In birnessites the same in Mn oxidation, when Pb2+ adsorption was low (600 mmol/kg), the coordination number of oxygen atom and Pb2+ in the Pb-O shell was 3.1 and they were 0.227 nm apart, and the coordination number of Mn atom and Pb2+ in the two Pb-Mn shells was 2.8 and 6.1 and they were 0.357 and 0.377 nm apart, separately. When Pb2+ adsorption rose up to 2344 mmol/kg, the coordination environment distorted. According to the fitting, in the Pb-O shell, the coordination number of oxygen atom and Pb2+ decreased to 1.2, and the distance between the two was 0.226 nm, and in the two Pb-Mn shells, the coordination number of Mn and Pb2+ decreased to 1.0 and 2.8, separately, and the distances between the two was 0.356 and 0.375 nm, separately. In birnessites different in Mn oxidation, Pb2+ adsorption patterns were basically the same. The three adsorption patterns commonly found were single-corner-sharing complex formed on side surfaces of layers along u axis, double-corner-sharing complex formed on side surfaces of layers along a or b axis, and triple-corner-sharing complex formed with the octahedral vacant sites. The amount of Pb2+ adsorbed increased with increasing AOS in birnessite, which led to decrease in Pb2+ coordination number between Pb and O in Pb-O shell and Pb-Mn shells due to the distortion of the Pb2+ coordination environment.