方解石表面Cd2+与Pb2+类质同象的AFM研究

摘要

借助原位液槽原子力显微镜(in situ AFM)的观察,通过Cd2+,Pb2+替代方解石最外层晶格Ca2+生长模式的实验研究,探讨了Cd2+与Pb2+作用下方解石表面溶解与结晶行为.在液体反应槽中,分别将含不饱和Cd2+与Pb2+溶液流经方解石{10(-1)4}解理面,结果发现:(1)Cd2+的存在不影响方解石沿＜(-4)41＞晶向台阶的溶解,而Pb2+的存在则强烈阻碍了方解石沿＜441＞+晶向台阶的溶解;(2)停止输入溶液含Cd2+,Pb2+溶液后,随着方解石表面与溶液达到平衡,溶解过程逐渐转变为结晶过程.结果显示在Cd2+存在时,单分子生长层具有方解石原有的定向性,而在Pb2+存在时的生长则不具任何定向性.尽管有此差异,但( Ca,Cd) CO3和(Ca,Pb) CO3固溶体都受控于单分子层外延生长这一结晶机理.含Cd2+和Pb2+溶液对方解石溶解动力学的作用与选择性吸附的阳离子半径大小、吸附复合体的几何形状及其结晶学取向有关.Cd2+离子倾向于优先进入更狭小的＜(-4)41＞-晶向的微台阶上,而Pb2+则倾向于形成扭曲的八面体络合物吸附在更开阔的＜441＞+晶向台阶上.因此,Pb2+存在下方解石表面生长方向无序可认为是白铅矿和方解石结构差异的原因.%The behavior of calcite surfaces in the presence of Cd2+ and Pb2+ is investigated by in situ fluid cell Atomic Force Microscopy (AFM) to determine the differences between Cd/calcite and Pb/calcite interactions and the growth mode of surface precipitate.Experiments are conducted on {10(-1)4} cleavage faces exposed to Cd- and Pb-bearing solutions that are undersaturated with respect to calcite,otavite,and cerussite.Two observations are made:(1) The presence of Cd2+ does not affect the relative step kinetics along ＜(-4)41＞ steps (i.e.vs+＞vs-) during calcite dissolution,whereas Pb2+ strongly retards the retreat of the obtuse ＜441＞+ steps to reverse the relative step kinetics; (2) The initial dissolution is followed by a development of surface precipitation after the flow-through input is turned off to allow the surfaces to equilibrate with the solutions.The surface precipitate grows in a layer spreading mechanism showing clearly defined directions in the presence of Cd but not Pb.The kinetic effects of cadmium and lead on calcite dissolution are suggested to be related to a selective cation adsorption in accordance with the cation size,the geometry of adsorption complexes,and the crystallographic direction dependent step geometry.The Cd2+ ions,having a slightly smaller ionic radius than Ca2+,are thought to have a stronger presence along the acute ＜(-4)41＞_ steps that are geometrically more confined.In contrast,the much larger Pb2+ ions are believed to form distorted octahedral surface complexes to adsorb predominantly at the more open ＜(-4)41＞+ steps.The structural incompatibility between cerussite and calcite is speculated to account for the lack of step directions for the layer growth on calcite surfaces in the presence of Pb2+,The occurrence of layer-by-layer growth indicates that epitaxial growth plays an important role in the formation of (Ca,Cd)CO3 and (Ca,Pb)CO3 solid solutions.