Kerisit Sebastien, Liu Chongxuan
Chemical and Materials Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99352, USA.
Environ Sci Technol. 2009 Feb 1;43(3):777-82. doi: 10.1021/es8016045.
Molecular dynamics simulations were carried out to investigate the effects of confinement and of the presence of the mineral surface on the diffusion of water and electrolyte ions in nanosized mineral fractures. Feldspar was used as a representative mineral because recent studies found that it is an important mineral that hosts contaminants within its intragrain fractures at the U.S. Department of Energy Hanford site. Several properties of the mineral-water interface were varied, such as the fracture width, the ionic strength of the contacting solution, and the surface charge,to provide atomic-level insights into the diffusion of ions and contaminants within intragrain regions. In each case, the self-diffusion coefficient of water and that of the electrolyte ions were computed as a function of distance from the mineral surface. Our calculations reveal a 2.0-2.5 nm interfacial region within which the self-diffusion coefficient of water and that of the electrolyte ions decrease asthe diffusing species approach the surface. As a result of the extent of the interfacial region, water and electrolyte ions are predicted to never reach bulk-like diffusion in fractures narrower than approximately 5 nm. The average diffusion coefficient along the mineral fracture was computed as a function of fracture width and indicated that the surface effects only become negligible for fractures several tens of nanometers wide. The molecular dynamics results improve our conceptual models of ion transport in nanoscale pore regions surrounded by mineral surfaces in porous media.
进行了分子动力学模拟，以研究限制作用以及矿物表面的存在对纳米级矿物裂缝中水分子和电解质离子扩散的影响。长石被用作代表性矿物，因为最近的研究发现，在美国能源部汉福德基地，长石是一种重要的矿物，其颗粒内裂缝中含有污染物。改变了矿物-水界面的几个性质，如裂缝宽度、接触溶液的离子强度和表面电荷，以便从原子层面深入了解离子和污染物在颗粒内区域的扩散情况。在每种情况下，都计算了水分子和电解质离子的自扩散系数与距矿物表面距离的函数关系。我们的计算揭示了一个2.0 - 2.5纳米的界面区域，在这个区域内，随着扩散物种靠近表面，水分子和电解质离子的自扩散系数会降低。由于界面区域的范围，预计在宽度小于约5纳米的裂缝中，水分子和电解质离子永远无法达到类似本体的扩散。计算了沿矿物裂缝的平均扩散系数与裂缝宽度的函数关系，结果表明，只有在几十纳米宽的裂缝中，表面效应才可以忽略不计。分子动力学结果改进了我们对多孔介质中被矿物表面包围的纳米级孔隙区域内离子传输的概念模型。
