Pb^(2+)在不同锰氧化度水钠锰矿表面配位形态的EXAFS研究

STUDY WITH EXAFS OF SURFACE COMPLEXATION OF Pb^(2+) ADSORBED ON BIRNESSITES DIFFERENT IN Mn OXIDATION

采用X-射线吸收精细结构光谱(XAFS)研究了Pb2+在不同锰氧化度的水钠锰矿表面的吸附形态,通过比较其吸附形态的差异,进一步探讨Pb2+在水钠锰矿表面的吸附机理。结果表明,锰氧化度高的水钠锰矿对Pb2+的吸附容量较大。Pb2+邻近存在一个Pb-O配位壳层和两个Pb-Mn配位壳层,对于相同锰氧化度的水钠锰矿,当Pb2+吸附量低时(600 mmol kg-1),Pb-O配位壳层中的氧原子与Pb2+的配位数为3.1,与Pb2+的距离为0.227 nm,两个Pb-Mn配位壳层的锰原子与Pb2+的配位数分别为2.8、6.1,与Pb2+的距离分别为0.357、0.377 nm;当Pb2+的吸附量增大时(2 344 mmol kg-1),其配位环境发生畸变,拟合得到的Pb-O配位壳层中的氧原子与Pb2+的配位数减少为1.2,与Pb2+的距离为0.226 nm;两个Pb-Mn配位壳层的锰原子与Pb2+的配位数分别减少为1.0、2.8,与Pb2+的间距分别为0.356、0.375 nm。Pb2+在不同锰氧化度水钠锰矿表面的吸附形态基本相同,皆存在三种吸附形态,一是与八面体空位形成三齿共角配合物,二是与水钠锰矿层结构u轴方向层边面形成单齿共角配合物,三是与结构a或b轴方向层边面形成双齿共角配合物。

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 Pb^2＋ adsorption was low （600 mmol kg-1） , 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 2 344 mmol kg-1 , the coordination environment distorted. According to the fitting, in the Pb-O shell, the coordination number of oxygen atom and Pb^2＋ 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 Pb^2＋ 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 oc- tahedral vacant sites. The amount of Pb2＋ adsorbed increased with increasing AOS in birnessite, which led to decrease in Pb^2＋ coordination number between Pb and O in Pb-O shell and Pb-Mn shells due to the distortion of the Pb^2＋ coordination environment.